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Industrial ProcessControl Valves

Key components for plant safety and economic efficiency

++Arca_5.0_englisch 13.06.2008 11:38 Uhr Seite 1

Contents

Designed for maximum performance 4

Plant function and process integration 6

Basic functions of process control valves (6) – Working range of a control valve (8) – Valve capacity (flow coefficient) (9) – Transfer function and valve characteristic (10) – Nominal valve size (12) Integration into the pipe network (13) – Valve actuators (13)Positioner (15) – Instrumentation (18)

Significance for economics and plant safety 19

Influencing factors on the overall economics (19) – Plant safety and SIL classification (28) – Plant safety by means of explosion protection (30)

Specification and selection criteria for control valves 33

Data sheet for control valves (33) – Calculation of flow coefficients, selection of valve trim and nominal valve size (34) – Selection of the valve design (41) – Selection of materials for valve housing and trim(44) – Selection of materials for sealing elements (48)

Emissions 50

External tightness (50) – Internal tightness (54) – Sound emissions (56)

Modern solutions for special areas of application 60

Control valves in power generation (60) – Control valves for pressure swing adsorption plants (62) – Anti-surge control valves on turbocompressors (63)

Process valves terminology 66

The company behind this book 71

This book was produced with the technical collaboration of ARCA Regler GmbH.

Translation: PTS GmbH, Luxembourg

Editorial team: Johannes Fliegen, Lothar Grutesen, Johannes Herzwurm, Joachim Lukoschek, Heinz M. Nägel, Andreas Pomsel

© 2007 All rights reserved withsv corporate media GmbH, D-80992 Munich, Germanywww.sv-corporate-media.de

First published in Germany in the seriesDie Bibliothek der TechnikOriginal title: Industrielle Prozessregelventile© 2006 by sv corporate media GmbH

Illustrations: ARCA Regler GmbH, TönisvorstTypesetting: abavo GmbH, D-86807 BuchloePrinting and binding: Sellier Druck GmbH, D-85354 FreisingPrinted in Germany 889518


Designed for maximum performance 5

economics. The focus of the presentation is onthe group of control valves that are mostwidely used – the globe valves. The readerwill find a compact overview that should aidhis understanding of these high-performancevalve modules.

4

Designed formaximum performanceWith its maximum speed of 330 km per hour,the German high-speed train ICE 3 has a brak-ing power of more than 8200 kW provided bythe electrical brake. A fascinating value – butonly a fraction of what control valves, so tosay the “brakes” in any industrial process, areable to achieve. A steam conditioning valve,for example, used in a typical bypass stationfor an auxiliary turbine (Fig. 1) with a nominalsize of DN 400 throttles 71.6 t/h of steam from an inlet pressure of 38.5 bar to 0.9 barabsolute pressure. The calculated power loss atthe valve is more than 21100 kW. So thiscontrol valve, at DN 400 not particularly large,controls 2.5-times the extraordinary brakingpower of an ICE 3. On the one hand this con-stitutes the fascination of process controlvalves, but on the other hand it also representsa great challenge for the valve manufacturer.At points in the industrial process where somuch power must be controlled, any mistakesin design and operation are extraordinarilycost-intensive.Regardless of whether in power stations, in thechemical, petrochemical, foodstuff industries,or in gas supply and water supply and dis-posal, control valves are decisive points of intervention in process cycles. They ensure themanufacture of products of optimal qualityunder economically advantageous conditions.The intention of this book is to demonstratethe significance of control valves as importantinterfaces within complex industrial processesand as key components for plant safety and

Fig. 1:Steam conditioningvalve (angle-style)

2.5-times thepower of theICE brakingsystem

Decisive pointsof interventionin the process


Basic functions of process control valves 7

flow (Fig. 2). In times of model-assistedprocess control and digital control technology,the control valve itself is only inadequatelyrepresented, however, in the field of controltechnology. Essentially, one is dealing with apipe system component that interacts in acomplex manner with the process-techno-logical sequences. In the energy balance ofeach process, the pressure difference across thevalve that is necessary for the control functionappears as a performance loss; often, however,

6

Plant function andprocess integrationIn process circuits, control valves are the mostcommon continuously operating elementsused to influence and control the processes ina targeted manner. They are the connectingelements and the interfaces between electroniccontrol technology and the process medium.At the same time, they function as connectingcomponents between the individual processphases, thereby controlling the continuousprocess flow and balancing out the differentpressure levels. Other control valves that areused, for example, in heating and coolingcircuits, intervene indirectly in the process.

Basic functions of process control valvesThe process control valve is on the one hand aparticipant in the flow of digital data and onthe other hand an executive arm in the process

Fig. 2:Control valveinterfaces andcomponents

Safety position

End position signals

Stroke

Pneumatic actuator

Control valveProcess flow

Positioner

Setpoint

iy

Auxiliary power(compressed air)

Slotted nut for securing the actuator

Guide bushingPacking set/stuffing box

Bonnet

Valve stem

Retainer

Parabolic plug

Clamped seat Seat seal

Bonnet seal

Valve housing Fig. 3:

Essential com-ponents of a control valve

Intermediaryfunction

Pipe systemcomponent


Valve capacity (flow coefficient) 98 Plant function and process integration

the controlled pressure drop is the primarytask of the valve, for example, when depres-surising a pressure vessel.Figure 3 shows the design of a modern controlvalve in terms of its essential components. Inthe design of a control valve, and independentof the actual task it has to fulfil in the process,there are four essential aspects to consider:

• process-technological design of the neces-sary working points and operating condi-tions

• safety design and response to malfunctions• process-control-related concept with com-

munications path and actuator philosophy• process-control-related concept with actu-

ation dynamics and time response.

Working range of a control valveThe primary, and at the same time the mostcomplex task in the design of a control valveis the determination of its working range.Here, accurate design and calculation reallypays off, since it is essentially in this phasethat the installed performance of the processcomponents, pipe sizes and valve dimensionsare determined. At the same time, the com-ponents installed upstream and downstream ofthe control valve must be taken into accountup to the extent that allows a fluid mechanicaldecoupling of the parts of the plant.A control valve is always primarily a flowcontrol valve, and the pressures that occurahead of and behind the valve are the result ofthe upstream and downstream system com-ponents and their characteristics. What appliesgenerally in control technology also applieshere, namely that an optimised controlled loopis the most efficient and also the most eco-nomical controller.

Main design aspects

Decisive phaseof project design

Flow control asbasic function

Taking accountof extreme situations

Flow rate understandardisedconditions

Besides the requirements of the process op-erating within its normal limits, special andextreme situations must also be taken intoaccount in the control valve design. Amongstthese is the behaviour in the event of a con-trolled shutdown (sealing function) and, inthe same way, the behaviour in the event ofsignal interruption or the failure of auxiliarypower. Furthermore, the leakage require-ments for the closed valve must be estab-lished. Here, a limitation to requirementsthat are actually necessary is recommended,since the control and sealing functions are, if anything, contradictory demands from the design point of view and can thereforeonly be achieved using resource-intensivemeasures.

Valve capacity(flow coefficient)

Calculation methods for the flow coefficients– defined by the Kv value – are described inmany publications. In contrast to the ζ valueused in the analysis of pipe systems, which de-scribes the specific flow resistance of eachpipe element, the Kv value used for controlvalves defines the flow rate of a control valvein m3/h under standard conditions (water at 20°C and 1 bar pressure difference). TheKvs value (nominal flow coefficient) is the Kv value of the control valve at the nominalstroke. In the American standards, the Cv value is commonly used for the flow rate;this describes the flow rate in gpm (gallons per minute) at 1 psi (lb/in2) pressure difference.The conversion factor between the two sys-tems is given by: Cv 1 = Kv 0.865. Today, con-venient calculation programs are almost exclu-sively used for such calculations. The empir-


Transfer function and valve characteristic 11

characteristics interact in each case. The char-acteristics of the spray nozzles and the pumpcan be seen in Figure 4b, while Figure 4cshows the influence of the valve characteris-tics (linear and equal percentage) on the plantcharacteristic. System limits are the tank

10 Plant function and process integration

ical formulae given in the section on “Processvalves terminology” provide rough estimates(see p. 68).

Transfer function and valve characteristic

The transfer function of the valve, which de-scribes the relationship between input and out-put signals, is important in the control design.But what in fact is the output signal of a con-trol valve? The process parameters to be con-trolled such as pressure, level or temperatureare in the final analysis only a result of theflow control provided by the valve, and this inturn is determined by the throttling process.Thus the static transfer function describes therelationship between input signal and thethrottling area. The fundamental intermediateparameter in this signal sequence is the strokeof the valve, or in other words, the position ofthe valve plug. The valve plug has a contourthat, according to the stroke setting, exposes amore or less large throttling area – startingfrom a controllable initial value up to a max-imum value. The quotient of the initial valueand maximum value is called the rangeability.This fundamentally defines whether the re-quired process conditions can be achieved. Thebehaviour between these values is describedby the valve characteristic. The choice of char-acteristics is the subject of many theories,while in practice the choice is usually onlybetween linear and equal-percentage character-istics. The objective is basically to achieve aconstant control amplification over the wholeworking range. Figure 4 shows, for the example of a sprayheader control used to cool crude steel in acontinuous casting plant (Fig. 4a), how three

Fig. 4 (opposite):a) Spray header

controlb) Spray nozzles

and pump characteristic

c) Valve and plantcharacteristic for linear and equal-percentagecharacteristic

Control valve

Flow rate controller

Nozzles

Pump

FCa)

Nozzles

Pump

00

5

10

15

20

5 10 15

Pressure difference/head [bar]b)

c)

Volu

met

ric fl

ow r

ate

[m3 /

h]

20 25 30

00

5

10

15

20

20 40 60

Stroke [%]

Vol

umet

ric fl

ow r

ate

[m3 /

h]

80 100

Linear valve characteristic

Plant characteristic for linear valve characteristic

Equal-percentage valve characteristic

Plant characteristic for equal-percentage valve characteristic

Pressure difference across the control valveValve stroke as

an important intermediate parameter


Valve actuators 13

as well as for fluids at temperatures close tothe boiling point which vaporise when thepressure is lowered.

Integration into the pipe networkThe usual forms of connection of the valveinto the pipe system are flanged, welded, orscrewed connections. Flanged connectionsare far and away the most common form ofconnection, while welded connections areused primarily in high-pressure lines ofwater/steam circuits. The advantages of adirectly welded connection lie in the hermeticand permanent sealing. A significant disad-vantage on the other hand is the limitedrepair capability, since for modern valves, an easy replacement of worn-out componentson site is required. Welded connections onvalves are more expensive as a rule, becausesteel pipe stubs, so-called spool pieces, arewelded on to the cast housing body in thefactory in order to obtain an unproblematicalpairing of materials at the interface with thepipe.In general terms, a control valve should beseen not only as a valve but also as anelement of the pipe system. For this reason, itmust be ensured by means of bypass andshut-off valves that in the event of a possiblefailure of the control valve, the latter can beextracted out of the circuit for maintenanceand repair without interrupting plant oper-ation.

Valve actuatorsValve actuators serve to position the valveplug according to the requirements of thecontrol system. Three types of actuator designare well-established in process technology:


level, assumed to be constant, and the ambientpressure behind the nozzles.If there is no clear technical priority for anequal-percentage characteristic, for globevalves a linear characteristic should bechosen for economic reasons. There is a tendency for control valves with an equal-percentage characteristic to be selected witha larger Kvs value. Perforated plugs in mostcases require a larger seat diameter or alarger valve stroke for an equal-percentagehole pattern, and therefore require higheractuation forces.

Nominal valve sizeThe required flow rate coefficient and the de-sign of the valve trim – i.e. plug shape (one-or multi-stage configurations) and character-istic – essentially determine the space re-quirement, i.e. the nominal valve size. In add-ition to the controlling stages, the housingmay also possibly have to accommodateuncontrolled but fixed throttling stages (per-forated discs, perforated cages, flowlabyrinths). Besides these spatial requirements there arealso mechanical aspects to be taken into ac-count. As a rule, the nominal valve size shouldnot be greater than the nominal pipe size; onaccount of bending and torsion momentswithin the pipe system, however, it should alsonot be smaller than half the pipe size. Further-more, the flow velocities at valve entry andexit must also be taken into account. Media containing solids require a particularlylarge amount of space. For these kinds ofmedia, angle-style control valves are usedwhich allow an unhindered outlet flow towardsthe valve exit. This type of design is also suit-able for the rapid expansion of gaseous media

Standard: linearcharacteristic

Nominal size asa function of theKvs value

Mechanical aspects

Flanged orwelded connections

Maintenanceduring operation


Positioner 15

• ambient conditions (protection class, tem-perature, corrosion)

• manual actuation for emergencies (requiredor not required).

PositionerPositioners serve to convert the standard signalsusual in control technology with pressures of 0.2 to 1.0 bar or current levels of 4 to 20 mAinto an actuation pressure (in the context of the compressed air supply available in the plant)that can be used for the valve actuator (usually a pneumatic actuator). Together with theactuator, the positioner thus forms a controlcircuit that is subordinate to the processcontrol circuit.Microprocessor-controlled positioners (“intel-ligent positioners” or “smart positioners”) pro-vide the possibility of adjusting many param-eters both on site and also via the communica-tions system. The link to the process controlsystem is made via a bidirectional data ex-change that goes beyond the normal controland feedback signals. The equipment usedincludes the Highway Addressable RemoteTransducer (abbreviated to HART®), wherethe status information is modulated onto theanalogue control signal in the form of a digitalsignal, as well as the “real” fieldbuses,Profibus® (PA) and Foundation FieldbusTM,with which both the actuation signal and alsothe status information are digitally transmitted.

Positioner mountingFor mounting and fastening the positioners tothe actuator, the Namur guidelines weredeveloped by the chemical industry; thesehave also been internationally converted into IEC 60534-6-1. These guidelines make itpossible to interchange the positioners of


• pneumatic• electric• electrohydraulic.

Pneumatic valve actuators are cost-effective,can be used in potentially explosive areaswithout any problems, have low actuatingtimes, a constant sealing force, as well assafety positions that can easily be imple-mented. They are therefore the first choice inprocess technology. Electrical valve actu-ators are dynamically stable and precise;direct control by means of the process con-troller is possible. Although the primaryenergy source is more cost-effective, a higheroutlay is required for safety functions andfor use in hazardous areas. Moreover, elec-trical actuators are relatively slow. Electro-hydraulic actuators may be distinguished byvery good dynamics and stability, by speedcombined with high actuation forces, as wellas by flexible safety functions, however,their disadvantage lies in the fact that theyare expensive and resource-intensive tomanufacture.Depending on the type of valve selected andthe requirements of the process, the followingcriteria must be taken into account in theselection of the actuator:

• on/off or control application (what controlaccuracy is required?)

• primary energy source available (com-pressed air or electric power)

• safety position• actuation force required• stroke required (are adjustable end stops

required?)• requirements for the stroke stiffness of the

actuator• permissible deviation from linearity of the

actuator characteristic

Pneumatic …

… electrical, and also …

… electro-hydraulic actuators

Criteria for ac-tuator selection

Conversion intoa usable actu-ation pressure

Bidirectionaldata exchange

Namurguidelines


Positioner 17

users. The “open-system” concept allows thepositioner to be mounted both on single- ordouble-acting actuators; in addition, a con-necting face for the direct mounting of a so-lenoid valve is generally present.

Integration of intelligent positioners intothe control and/or maintenance systemIncreasing levels of functionality and growingparameterisation options for field devices aswell as the simultaneous reduction of operat-ing personnel require that an extensive level ofcontrol of all field devices is ensured, eithercentrally from the process control system orfrom a maintenance panel. Fieldbuses such asProfibus® and the Foundation FieldbusTM,and also modern HART® systems, enable indi-vidual addressing and data exchange with eachfield device.Over and above pure data exchange, however,it must be guaranteed that the superordinatesystem “knows” the properties and setting op-tions for all field devices connected, and canalso display these visually to the operator. Inthis respect, with Electronic Device Descrip-tion (EDD) and Field Device Tool/DeviceType Manager (FDT/DTM), two opposingphilosophies have developed.In the case of integration by means of EDD,one is dealing with a standardised text filewhich describes in detail the signals and prop-erties of the field device. Here, the user inter-face is prescribed by the superordinate systemand thus is not specifically matched to thedevice in question. Future versions of EDDshould, however, compensate to a large extentfor the disadvantage of this design feature.The superordinate system using FDT/DTMtechnology provides FDT software (e.g.PACTware®, FieldCare®), in which as manyDTMs of the field devices as required are inte-


different manufacturers. With the advent ofintelligent positioners, however, the Namurmounting has proved to be no longer suffi-ciently robust. As a result, a trend towards“direct” or “integrated” positioner mountingsystems has developed. Here, the factorystandards of well-known valve manufacturershave become established to an increasingextent. In almost all cases, these standardsalso include a “pipe-less” air supply betweenpositioner and actuator. A technically very interesting alternative forthe positioner mounting is represented by theVDI/VDE 3847 guideline (Fig. 5) developedby a working group of manufacturers and

Fig. 5:ARCAPRO®

positioner mountingaccording toVDI/VDE 3847

Direct orintegratedmounting

Central assistmanagement ofthe field devices

Possibilities ofvisualisation

EDD concept

FDT/DTM technology


19

Significance foreconomics andplant safetyThe economics and safety of plants are veryclosely linked. Control valves assume keyroles with regard to both criteria since theyhave a decisive influence on the quality of theprocesses and the availability of the plant.Life-cycle costs, safety for people and theenvironment, as well as explosion protectionin plants, are essential factors in the assess-ment of control valves.

Influencing factors on the overall economicsIncluded in the life-cycle costs of a controlvalve are not only the costs for selection andplanning and the investment itself, but also thecosts for installation and training of the person-nel. During operation, costs accrue for main-tenance, wear and possible failure, repair, spareparts, and energy, as well as costs for possibleenvironmental damage. Also to be consideredare the costs for disposal and recycling. The individual design of a control valve andthe configuration of its actively moved partshave a significant influence on the consider-ation of life-cycle costs. With modern solutionssuch as:

• multi-spring diaphragm actuators• valve seat quick-exchange systems• reliable stem sealing systems• intelligent positioners with diagnostic func-

tions and• piezo-controlled positioners with negligible

air consumption


grated. In addition to information concern-ing the field device, the DTM also provides an individual representation of each fielddevice. One can envisage the DTM as an equip-ment driver for computers which is also inte-grated into the operating system and which in-cludes a special representation of each device.Today’s generation of digital positioners offersFDT/DTM technology as an alternative to theEDD concept. The DTM developed on thebasis of FDT specification 1.2 facilitates espe-cially the use of the expanded diagnostics.

InstrumentationBesides positioners, further instrumentation is necessary in individual cases. This can include:

• position switches and position indicators (incertain applications independent of the po-sitioner)

• solenoid valves for safety-related valve set-tings (mainly on/off)

• power amplifiers (also called boosters) forreducing the actuating times of actuatorswith large air volumes

• pneumatic lock-up valves, which in theevent of instrument air failure freeze thecurrent valve position

• pneumatic filter regulators for preparation ofthe instrument air

• pneumatic quick-exhaust valves for rapidmovement into safety positions.

Furtherinstrumentation

Key role

Essential costs

Optimisation solutions


Influencing factors on the overall economics 21

The pipe-less air connection between positionerand diaphragm chamber of the actuator guidesthe actuation air via internal passages throughthe actuator yoke in a functionally reliable man-ner. External pipework or screwed connectionsare not required for the air supply. Any bendingof pipes or leakages at the screwed connectionsis thus excluded, as are any functional failuresarising from these defects.In conventional diaphragm actuators, the airdisplaced by the stroke movement of the actu-ator is balanced out by ambient air intaken bymeans of a breather opening. If the atmos-phere is loaded with aggressive constituents,the air sucked into the actuator can generate asignificant amount of corrosion within a shortperiod of time by forming condensates. This isavoided in the case of modern actuators wherethe instrument air is not only used to operatethe actuator, but is subsequently (as exhaustair from the positioner) used to ventilate thespring chamber and thus to keep it dry andfree of corrosion (Fig. 7).

20 Significance for economics and plant safety

it is possible to ensure that the quality ofprocess control and thus the quality of theproduct manufactured is at the highest level, itis also possible to minimise operating andmaintenance costs, and thus to optimise theoverall economics of the industrial processes.

Compact multi-spring diaphragm actuatorModern spring-loaded diaphragm actuators(Fig. 6) are not only configured in a particu-larly compact manner, but they also offer anumber of design details which enhance oper-ational security and durability and thus theeconomics. Features to be named include, forexample:

• pipe-less positioner mounting• ventilation system that utilises the “used”

instrument air to protect the spring chamberfrom ambient effects

• rolling diaphragms with diaphragm clamp-ing in the force bypass.

Fig. 6:Modern multi-springdiaphragm actuator

Fig. 7:Positioner mountingwith air purge ofspring chamber

Air supply

Exhaust air Air purgeDelivery air

Ventilation and bleed plugs

Springs

Actuator stem

Integrated yoke

Central attachment with guide and sealing

Diaphragm

Actuator housing

Pipe-less airconnection

Avoidance ofcorrosion



higher pressure drop and larger nominalvalve size, is shaft-guided. The advantages of this configuration can be summarised asfollows:

• A seat ring without a thread can be easilymanufactured from special materials.

• The integrated form-fit alignment betweenbonnet and seat ring, and thus between stemguide and seat ring, ensures the prescribedseat leakage.

• The “floating” bearing support of the clampseat prevents the transfer of pipe forces ontothe seat ring via the valve housing. For leak-age class V, this is an important advantagesince in the case of screwed seats, the nega-tive effect of pipe forces on the valve seatcan be proven.

• The seat retainer guarantees an even outflowof the medium.

Valves with a clamped seat (Fig. 9) are easierto maintain than valves with a screwed-in seatring (Fig. 10) since the clamped seat ring canbe very quickly and reliably replaced withoutany special tools. It is therefore clearly su-perior to the screwed-in seat, since special toolsare required to ensure the tightening torquesrequired for the permanent avoidance of by-


At the heart of each diaphragm actuator is thediaphragm. Its service life is dependent notonly on the quality of its material but also to alarge extent on the kind of mechanical load-ing, in particular in the area of the diaphragmclamping. An uncontrolled clamping of the dia-phragm and the resultant formation of creasestogether lead to a diaphragm movement with

Fig. 9:Control valve withclamped seat

Fig. 8:Diaphragm clampingin force bypass

an increased flexion action which significantlyreduces the service life. Clamping with a de-fined pressure in the force bypass (Fig. 8)ensures a defined diaphragm movement andthus provides the basis for high availabilityand a long service life.

Valve seat quick-exchange systemThe valve plug and seat ring are the mosthighly loaded components of a control valve.The degree of wear and the sealing function,as well as durability and ease of servicing ofboth components, are determined by optimaldesign and the correct choice of materials. Asa simple, universally applicable and cost-effective solution, the control valve with thereplaceable, clamped seat ring has proved tobe effective; in this design, the valve plug isstem-guided in a conventional manner, or at a

Retainer

Clamped seat with sealing

Defined pressure

Valves withclamped seat

Advantages

Quick exchange


Influencing factors on the overall economics 2524 Significance for economics and plant safety

pass leakages. The tightening torques are quiteoften well above 1000 Nm and are alsostrongly dependent on the material pairing andthe condition of the thread. In contrast, in avalve with a clamped seat, the seating sealingforce is unequivocally defined by the tolerancebetween housing, seat retainer and seat ring as well as by the design of the seating seal.Reproduction of the necessary tolerances canbe ensured by means of CNC production tech-nology. The replacement of the seat ring isconfined to insertion of a new seating seal and a new seat ring, and a simple tightening of the bonnet bolting “onto the block”.

Stem sealOne of the central requirements for anyvalve is permanently reliable sealing to theexternal environment. Just as significant isthe choice of an economical sealing solution,in particular with regard to good operabilityand ease of replacement. Here, triple-stemsealing systems (Figs. 11a, b, c) have be-come established in the market. For the se-

Fig. 11:a) V-ring packing,

self-adjusting

b) Graphite or PTFE packing,adjustable

c) Bellows seal withsafety stuffing box

Fig. 10:Control valve withscrewed-in seat

Anti-rotationdevice

Defined seatingsealing force

Triple-stem sealing systems



• characteristics• setpoint direction• split-range operation• actuator range limitations• tight sealing function• malfunction detection• actuating times and• dead zone

is done during initial commissioning, or isautomatically determined by means of a testrun. Alternatively, these parameters can be op-timised on site or via digital communicationwith the control system. Thus even the re-placement of a positioner while the plant isoperating is very easily possible.Microprocessor-controlled positioners with adigital output (via piezo-controlled binarypneumatics) have significant advantagescompared with conventional positioners, andalso compared with microprocessor-con-trolled positioners with an analogue pneu-matic output, especially with regard to theconsumption of compressed air. While a con-ventional positioner consumes on average6000 Nm3 of compressed air in one year(costing approx. € 400 per year), a piezo-controlled positioner requires just 150 Nm3

of compressed air per year, with correspond-ingly lower costs. The amortisation period fora unit of this kind is a maximum of twoyears.If it is not possible to mount the positionerdirectly on the actuation unit for reasons ofhigh temperatures or extreme levels of vibra-tion, a positioner with an external positionencoder is used. The very robust positionencoder (ambient temperature up to 120°C) isfitted to the control valve, whereas the actualpositioner is then placed at a suitable locationthat can be at a distance of up to 20 m.


lection of the suitable stuffing box system,the key parameters are pressure, temperature,process fluid and type of movement (rotaryor linear).An important component of the stuffing boxsystem is the valve stem and its surface. Toavoid wear, the surface should be as hard aspossible and should exhibit a low level ofroughness. As a rule, valve stems are fine-turned or ground and then roller-burnished.In this way, it is possible to achieve a surfacequality of Rz < 4 to 5 µm. Too fine a surface,however, has the disadvantage, particularlyin the case of graphite packing, that theundesirable stick-slip effect can occur as aresult of adhesion forces. Moreover, withgraphite packing in particular, care has to betaken in the selection of the materials for thestem and the stuffing box chamber in orderto avoid galvanic effects that encourage cor-rosion. Thus it is possible, for example, tointegrate good protection against galvaniccorrosion with a bonnet completely made ofstainless steel, or a stainless steel stuffingbox chamber in the form of a screw-insleeve. Further design features that con-tribute to a permanent stuffing box seal areeffective scrapers and a distance betweenscraper ring and the stuffing box area that inall cases is greater than the valve stroke.These protect the stuffing box chamber fromdirt particles and any damaged areas of thevalve stem.

Cost savings by means of intelligentpositionersModern microprocessor-controlled positionersprovide the following operating modes: auto-matic operation, manual operation, initialisa-tion, parameterisation and diagnostics. Theparameterisation of:

Risk of stick-slip

Optimisation ofthe parameters

Lower airconsumptionsaves costs

Option of exter-nal positionencoder


Plant safety and SIL classification 29

ity of people being present in the hazardousarea. The basic standard for this purpose isEN 61508 (Parts 0-7) “Functional safety ofelectrical, electronic and programmable elec-tronic safety-related systems”. Moreover, EN 61511, EN 62061, EN 954-1 and alsoISO 13849-1 must be observed as applicationstandards, in particular for mechanical com-ponents.The regulations cited above are extremelycomplex in parts, and often lead to a very freeinterpretation of design rules, and as a result toambiguities between plant operators, plannersand valve manufacturers. For this reason, the Verband Deutscher Maschinen- undAnlagenbau (VDMA) [= Association ofGerman Machinery and Plant Manufacturers]has produced a set of “SIL Guidelines” forvalves and valve actuators (www.vdma.org),the contents of which will be described herebriefly.The essential theme of these guidelines is thatthe SIL classification is established only for acomplete safety system comprising sensors,control system and actuators. For the individu-al components (e.g. control valves), only keyparameters (e.g. mean time between failures,MTBF, safe failure fraction, SFF, or diagnos-tic coverage, DC) can be defined. The keyparameters in question for the individual com-ponents are quantitatively evaluated, and thesethen lead to an SIL classification for the over-all system. It should be noted here that in theevaluation of the overall system, the controlvalve is weighted with up to 50% of the totalfailure probability. Based on this quantitativeevaluation, it is up to the plant planners toachieve a prescribed SIL either by the selec-tion of high-quality equipment or by the intro-duction of system redundancies into the sen-sors, control system and/or the valves.


Control valve diagnosticsDigital positioners use parameters derivedfrom normal operation, for example residualcontrol deviation, variation of adaptive controlparameters, variation of mechanical param-eters such as stroke and fixed stops, as well asservice life values such as stroke and directionchanges, to aid in monitoring the controlvalve. All values cited are registered, evalu-ated and stored in the positioner, even in theevent of auxiliary power failure. With appro-priate software or an expert system, thesevalues can be displayed and evaluations can be generated that accurately describe the con-dition of the control valve. This continuousmonitoring helps to avoid unforeseen failuresand enables the timely scheduling of spareparts or spare units. Digital positioners will infuture replace the regular control valve main-tenance cycles (which will then in most casesbe completely superfluous) and will thus con-tribute very decisively to the reduction of life-cycle costs.

Plant safety and SIL classificationIn line with European standardisation, plantsafety is these days linked with the term SIL.SIL stands for “Safety Integrity Level” andspecifies the requirements for the reliability ofthe safety functions of electrical, electronicand programmable electronic systems at fourstages of safety. The task of the safety func-tions consists in removing process risks thatcan represent a hazard for people, the en-vironment and objects. The SIL required for asafety system is established in the context ofsafety considerations which include, amongstother factors, the evaluation of possible dam-age to people and the environment in theevent of plant malfunctions and the probabil-

Exact descrip-tion of condition

SIL

Based on EN 61508

SIL guidelines

SIL classifica-tion only for thewhole system


Plant safety and SIL classification 31

protection. It was not until the introduction ofextremely low-power piezo-controlled valvesin 1992 that it was possible to reduce thepower consumption appropriately.In order to design intelligent positioners ac-cording to the “Intrinsic Safety” type of igni-tion protection, a significantly higher level ofcomplexity is required than in the case of con-ventional units. On the one hand the capaci-tances are not negligible, and on the otherhand the power consumption of processor,memory and actuator are today still at least50% higher than for conventional positioners,so that the self-heating effect in the event offailure assumes a greater significance.

“EEx d – Pressure-Resistant Encapsulation”type of ignition protectionThe principle of pressure-resistant encapsula-tion is that any ignition occurring in the interiorof the unit cannot propagate to the externalenvironment. Means for limiting energy levelsare therefore not necessary. Equipment enclos-ures and operating controls must be designedwith so-called “ignition-propagation-proof gaps”which are subject to very tight tolerances andtherefore require complex machining of thehousings. Since as a matter of principle, thehousings of pressure-resistant encapsulated unitsmay only be opened under special safety pre-cautions, a modern positioner must also allowon-site operation without the need to open thepressure-resistant housing. One possibility foroperation from the outside, for example, is bymeans of a sight glass and control buttons thatare designed for external actuation.

Explosion protection for non-electricalequipmentWith introduction of the 94/9 EC Directive(ATEX), all components that are introduced


Plant safety by means of explosion protectionAs a result of the development of electricaland electronic control systems and the increas-ing complexity of chemical plant installations,the requirement has ensued that electric sig-nals are to be used for the control of pneu-matic actuation equipment. Research demon-strated that the electropneumatic converterproviding the conversion of the electric into apneumatic signal needs to be integrated intothe positioner and designed so as to be pro-tected against explosions. The principle ofpressure-resistant encapsulation has beenknown for many years from the mining indus-try. However, the German chemical industry inparticular developed its own type of ignitionprotection “EEx i – Intrinsic Safety”, which inrecent years has also become internationallyestablished. Devices that are designed accord-ing to this principle prevent ignition by limit-ing the energy supplied into the hazardousarea, are typically smaller, and may also bemaintained while in the operational state. Theyare moreover less expensive overall.

“EEx i – Intrinsic Safety” type of ignitionprotectionFor conventional positioners which achieve anI/p conversion by means of a fixed or plungercoil and a mechanical-pneumatic control ac-cording to the force equalisation principle, the“Intrinsic Safety” type of ignition protectionpresents few problems. When the first intelli-gent positioners were developed, their powerconsumption was still relatively high. Andbecause the power of intrinsically safe powercircuits was limited to no more than approx.150 mW, these positioners were unable at first tofulfil the “Intrinsic Safety” type of ignition

Limitation of theenergy supply

More efforts required

Ignition-propagation-proof gaps


33

Specification andselection criteriafor control valvesThe selection of a control valve that is suit-able for the application in question is an inter-active process that takes place between theoperator or planner and the supplier. Here, thebasis is always the data sheet for the controlvalves.

Data sheet for control valvesThe data required for the selection is normallysummarised in a data sheet for control valvesin accordance with IEC 60534-7 or ISA.Mandatory basic data elements that must beprescribed by the planner or operator include:

• nominal size and nominal pressure (of thepipe system), type of connection

• design pressure and design temperature• data concerning the available auxiliary

power and other requirements for processintegration

• process fluid• data for one, preferably three operating

points (mass or volume flow, upstream pres-sure p1, downstream pressure p2, and fluidtemperature T1); at steam conditioning sta-tions data for T2 is also required.

Moreover, any properties of the process fluidthat have a significant influence on the selec-tion of the control valve should be defined.For liquids these are density and vapour pres-sure, for gases and vapours they are standarddensity, isentropic exponent, and also the realgas factor.


into potentially explosive environment mustbe accompanied by a risk analysis with regardto ignition safety. Only “simple components”such as pipe elements, tanks or valves that,apart from a build-up of static charge, do notexhibit any further ignition sources, are expli-citly excluded from this directive. For all com-plex components, in particular for all actuatorsand also for non-electrical installation parts, arisk assessment must be performed by themanufacturer. The result of this risk assess-ment is either a designation of the unit accord-ing to ATEX, or a declaration by the manufac-turer that states that the unit in question is notsubject to the ATEX requirements.

Execution of arisk analysis

Basic data


Calculation of flow coefficients, selection of valve trim … 35

• predictions concerning the flow conditionsin the valve

and in most cases:

• the required nominal valve size or• the flow velocity for a prescribed valve size.

The calculation programs of the control valvemanufacturers also usually enable the opti-misation of valve types and trims with regardto their noise levels and cavitation charac-teristics.

Selection of valve trim for liquidsIn the first calculation in which, as a rule, asingle-stage parabolic plug is assumed, one ofthe following flow states is predicted:

• subcritical (laminar or turbulent) flow• cavitation• vaporisation (flashing).

If laminar or turbulent flow is present, thesingle-stage parabolic plug is the correctchoice.For the case in which the first calculation pre-dicts cavitation, it must be clarified whetherthe cavitation, which does not necessarily sig-nify destruction of the valve, can be tolerated.The destruction of the valve as a result of cavi-tation is a complex process that is essentially afunction of the energy conversion of the valve,the downstream pressure p2 and the materialand/or wear protection of the valve seat andvalve plug. If destruction by means of cavita-tion must be assumed, usually accompanied byan intolerable sound level >85 dB(A), the onlyeffective option for reducing the level of cavi-tation or reducing the damage caused by thecavitation is the distribution of the pressuredrop, or more precisely the pressure ratio, overa number of control stages. In most cases the

34 Specification and selection criteria for control valves

For a sensible choice of valves from the eco-nomic viewpoint, the particular task of thecontrol valve within the overall plant alsoplays an important role. It has to be decidedwhether the control valve during intendedoperation of the plant is in constant use, orwhether it is used only in special circum-stances (during start-up/shut-down, or forsafety functions). This information is normallynot included in the control valve data sheets. Itis therefore sensible in such cases if the plan-ner or operator allows the supplier to see theR&I diagram in which the media flows and, inparticular, in which the control circuits can beseen. Selection of an optimised control valvenormally takes place in three stages:

• calculation of the required flow rate coeffi-cient and the flow conditions present in thevalve – selection of suitable valve trim styleand the nominal valve size

• selection of the valve type and design• selection of materials for the valve housing

and valve trim as well as selection of mater-ials for the sealing elements.

Calculation of flow coefficients, selection of valve trim and nominal valve size To determine the required flow coefficients,calculation programs from the valve manufac-turers (such as ARCAVENA) or independentengineering tools (Conval® amongst others)are available. These programs basically fea-ture:

• a calculation of the flow coefficient in ac-cordance with IEC 60534-2

• an estimation of the sound level (under stand-ardised conditions) in accordance with IEC 60534-8-3 or 60534-8-4, as well as

Constant use oruse in specialcircumstances

Three selectionstages

Calculationprograms

Flow conditions

Distribution ofpressure drop toseveral controlstages



housing can be minimised by the use of anglevalves, since the flow of the process mediumbehind the throttling point is not deflected anyfurther. When using parabolic plugs, in particular withliquids with a high pressure difference ratio(not necessarily linked with cavitation or va-porisation), vibration of the valve plug some-times occurs which can often even lead tofracture of the valve stem. The cause for thesevibrations is that the parabolic plug, sub-merged in the flow, is always in unstableequilibrium, as dictated by the Bernoullieffect. Thus if the plug deflects to one particularside, a higher flow velocity occurs at exactlythis side and there is therefore further loweringof the pressure. Although this physical effect cannot beavoided in the case of a parabolic plug, it ispossible to ensure the stability of the plugsimply by means of a robust guide (usuallydouble-guided plugs with one guide eachabove and under the plug). In conventional


optimal solution remains the use of a parabolicplug with up to five matched control stages inseries (Fig. 12).In the event of vaporisation, also called flash-ing, a mixture of fluid and vapour remains pre-sent behind the throttling point. On account of

Fig. 12:Three-stage parabolic plug

the residual gas fraction, flashing is not linkedwith very high sound pressure levels; on ac-count of the high velocity of the liquid/vapourmixture in the area of the throttling point,damage to the valve inner parts and in ex-treme cases even damage to the housing is pos-sible as a result of droplet impact. Under thesekinds of operating conditions perforated plugs(Fig. 13) are preferably used, in the case ofvery small Kvs values parabolic plugs are also used, the preferred “medium closes” flowdirection preventing damage to a large extent of the functional parts of the valve such as the stem and the guide bushing as a result ofdroplet impact. Under extreme operating conditions, the risk of erosion on the valve

Fig. 13:Single-stage perforated plug

Controlstages

Perforated plug

Use of perforated plug

Bernoulli effect



required, and the lower guide bushing is more-over open to the bottom and is thus insensitiveto contamination. A further advantage consistsin the fact that these trims can be simply andcost-effectively retrofitted as required.

Selection of valve trim for gases and vapoursVarious flow conditions are also predicted byvalve calculations in the case of gases andvapours. A differentiation is made betweensubcritical and supercritical expansion(choked flow). The latter is distinguished bythe fact that the sonic velocity of the gas isexceeded within the throttling point and theloss of energy no longer takes place throughturbulence but in the form of compressionshocks (similar to the supersonic boom of anaircraft). These act on the inner parts of thevalve and can, for example, lead to fracture ofthe valve stem by vibration. In an analogousmanner to cavitation, a supercritical expan-sion can only be avoided by distributing thepressure ratio over a number of expansionstages.In the case of gases, the supercritical expan-sion is almost always accompanied by a veryhigh sound level. In the case of subcritical ex-pansion, a sound level also very often occursthat is far above the values that can be tol-erated, which as a rule lie between 80 and 85 dB(A). For gases and vapours, there aretwo possibilities of reducing the sound level:

• multi-stage expansion• distribution of the total flow into as many

flow passages as possible (see section on“Sound emissions”, p. 56 ff.).

Sound-reduced valves for gases work mainlywith a combination of both options. There are,for example, designs with one- or multi-stage


valves this is achieved with the aid of a specialbottom flange; this, however, requires an add-itional static seal and is moreover very sensi-tive to contamination (Fig. 14). In the case of a quick-exchange system with adouble guide (Fig. 15), these limitations donot occur. Here, no additional sealing point is

Fig. 14:Double-guided parabolic plug (conventional system)

Fig. 15:Double-guided para-bolic plug (quick-exchange system)

Quick-exchangesystem

Subcritical andsupercritical expansion

Sound reduction


Selection of the valve design 41

fluid mechanics point of view – and indeedfor all probable operating conditions – theperforated discs must always be considered inconjunction with the control stages installedin the valve. Control valves in which fluids are expandednear their boiling point and in which a signifi-cant level of vaporisation occurs must likewisebe considered critically with regard to theiroutlet nominal size, since the vapour fractionhas about 800 times the volume of the corres-ponding liquid fraction. The vapour fractiondownstream of the valve, and with it also theflow velocity at the valve outlet and in thedownstream pipe, can be determined with theaid of thermodynamic calculation methods.Perforated discs cannot be used in such ap-plications because of the droplet impact that is always present. Experience shows that thenominal pipe size in such cases is often under-dimensioned by the plant planners. This can-not be compensated by the control valvealone.

Selection of the valve designThe selection of a valve design suitable for theapplication in question is dependent on the de-sign temperature, the design pressure and theproperties of the process fluid. With regard toits universal applicability, the single-seat con-trol valve in globe style (entry and exit on op-posing sides) always represents the firstchoice. By means of appropriate bonnets andvalve trims it can be matched to almost anyapplication.In the selection of the bonnet, the permissibleor optimal temperature in the stuffing boxarea, the possibilities for thermal insulationand – in the case of valves for cryogenic ap-plications – also the introduction of heat, are


perforated plugs in combination with fixed-flow dividers (perforated discs and cylinders,wire meshes or labyrinth inserts). These de-signs should, however, only be used with“clean” process media. For gases (and ofcourse also for liquids), which, for example,contain solids or contaminants, perforatedplugs and labyrinth inserts can only be used toa limited extent. Compromises must be madehere. In such applications, only valve trimsshould be used that cannot be affected bysolids or by polymerisation.

Determination of the suitable nominal valve sizeIn most cases, the minimum nominal valvesize is already prescribed when the Kvs valuehas been determined and the choice of thevalve trim made. It must merely be checkedwhether the nominal valve size matches thespecified nominal pipe size. This is the case ifit is not larger than the nominal pipe size, and,for mechanical reasons, is also not smallerthan half the nominal pipe size.If control valves are being used for the expan-sion of gases at high pressure ratios, it is es-sential when selecting the valve size to beaware of the gas velocity with reference to thenominal size of the valve outlet. The outletvelocity should in no case be more than half,preferably no more than a third of the sonicvelocity. If the outlet velocity is higher thanthe sonic velocity, there is a risk that the valvewill be damaged by shock waves. A moderatemeans for reducing the outlet velocity con-sists in raising the downstream pressure bymeans of perforated discs arranged down-stream. This assumes, however, that the nom-inal valve size is smaller than the nominal pipesize and that the perforated discs are installeddownstream of the pipe expander. From a

Matching tonominal pipesize

Attention to theoutlet velocity

Large vapourvolume

Single-seat control valve foruniversal use


Selection of the valve design 43

the better capability for insulation of thevalves. If the operating temperature liesabove 450°C , it must be ensured by an ap-propriate design of the cooling fins and bymeans of heat insulation of the valve that thetemperature in the stuffing box area, even inthe most unfavourable case, does not exceed450°C.At cryogenic temperatures an insulation col-umn (Fig. 16c) is used. This serves to ensurean energy exchange between the cryogenicprocess medium and the environment that isas low as possible, and also serves to protectthe stuffing box from icing. According torequirements, the range of design stretchesfrom a simple extension tube with a massivevalve stem up to an insulation tube with a hollow stem filled with mineral foam thatreduces convection both within and also outside the valve stem to the greatest possibleextent.In the case of severely “contaminated” processmedia, the control valves should be selectedappropriately, i.e. any designs featuring fineopen structures should be avoided as much aspossible. For large nominal diameters or highpressure differences, double-seated valves pro-vide a suitable alternative to pressure balan-cing systems. With abrasive media, angle-style valves comeinto use on account of their free outflowconditions; in combination with suitable ma-terials they achieve a high lifetime even underextreme conditions. If the process fluid has atendency towards polymerisation, both thevalves and also the pipe system should beheated (Fig. 17). For this purpose, heatingjackets that use thermal oil or steam are suit-able. Care must be taken that the stuffing boxarea and/or the bellows are also heated ifnecessary.


key factors. The standard bonnet, is the typ-ical bonnet for the temperature range between–10°C and +250°C. In the range up to+200°C and up to a nominal pressure (PN) of40 bar, a self-adjusting spring-loaded stuffingbox (Fig. 16a) is state of the art. At operatingtemperatures above 250°C, cooling fins (Fig.16b) usually come into use, in conjunctionwith an adjustable graphite stuffing box. Al-though graphite stuffing boxes can actuallybe used up to a temperature of 450°C, fortemperatures above 250°C, cooling fins areused as a matter of principle on account of

Fig. 16:a) Spring-loaded

stuffing box withstandard bonnet

b) Bonnet with cool-ing fins for use athigh operatingtemperatures

c) Insulation column

a)

b)

c)

Springs

Cooling fins

Bonnet withcooling fins

Insulation column at cryogenic temperatures

Measures specific to theprocess fluid


Selection of materials for valve housing and trim 45

• wear resistance (abrasion, erosion) and the• regulations (DIN or ANSI, TRB 801).

If the material of the pipe system is defined inthe specification, this material or a corres-ponding casting material should be used forthe valve housing. If the process medium doesnot demand any particular measures and se-lection of the material is based only on oper-ating pressure and temperature, the materialselection reduces to a few material groups(Table 1).

The limits of use for the materials for pres-sure-bearing parts are usually represented as afunction of the nominal pressure in pressure-temperature diagrams. If, in addition, there isalso the risk of corrosion attack arising fromthe process medium, one must resort to the useof special alloys. Specialised casting foundriesoffer many alloys that have been developedpurely for particular applications, as well astheir corresponding corrosion resistance tables. Very detailed information can be found in the Dechema Materials Handbook(www.dechema.de). For certain process fluids,tight guidelines with regard to material se-lection and design requirements have beenproduced by the user organisations. Ex-amples of these are the Eurochlor Guidelines


Selection of materials forvalve housing and trimThe key factors for the selection of valvehousing materials are:

• operating pressure• temperature range• chemical resistance and interaction with the

process fluid• mechanical strength

Heating connections

Process connections

Heating connections

Fig. 17:Control valve withsteam jacket forvalve body and coverflange (ARCAECOTROL®)

Table 1:Material selectionfor valve housing

European Standard For ASTM For temperatures temperatures

1.0619 GP240GH –10 to +400°C A 216 WCB –28 to +400°C

1.4408 GX5CrNiMo 19-11-2 –196 to +300°C A 351 CF8M –196 to +400°C

1.4581 GX5CrNiMoNb 19-11-2 –10 to +400°C – –

1.6220 G20Mn5 –40 to +400°C A 352 LCB –50 to +400°C

1.6982 GX3CrNi13-4 –120 to +400°C – –

1.7357 G17CrMo5-5 –10 to +530°C A 217 WC6 –28 to +530°C

Selection criteria

Special alloys


Selection of materials for valve housing and trim 47

count of the different thermal expansions ofthe materials used – is a “ceramics-compat-ible” design of the control valve as well as theexperience of the valve supplier in selectionof the suitable compound. Ideally, the sup-plier should make use of a modular systemwith which such solutions can be imple-mented in a flexible, replaceable and eco-nomical manner. Figure 18 shows a controlvalve from a modular system with a tungsten-carbide valve plug and a dual-sided tungsten-carbide valve seat; the modular system sig-nificantly reduces the costs associated withthe use of the normally very expensiveceramic inner parts.


GEST 98/245 (www.eurochlor.org) or theOxygen Guidelines of the European IndustrialGases Association (www.eiga.org).With severely corrosive process media, it isnot sensible to make a valve housing out of ahomogeneous corrosion-resistant material forreasons of cost. Here, lining the valve housingwith corrosion-resistant metallic, ceramic orpolymer materials provides a solution. In thiscase, it is absolutely essential that the pres-sure-containing housing does not under anycircumstances come into contact with theprocess medium. Particular attention must begiven to the bearings, the inner parts and allsealing points. For throttling elements and valve seats, eitherchromium steels (e.g. 1.4021/A473), austeniticchromium-nickel steels, chromium-vanadiumor chromium-molybdenum steels are used.The material of the control valve functionalparts should be of at least the same quality asthe housing material, and if possible of evenhigher quality. The materials used for thevalve trims and for the particularly highlyloaded areas in the control valve are sum-marised in Table 2.By means of surface treatments such as ni-triding or the Tenifer® treatment it is possibleto produce hardness values that are specif-ically different in the friction pairs, for ex-ample in the perforated plugs and in the guidebushings of the valve. In particularly highlyloaded areas of the valve trim and in areas of very high flow velocities, additional hard-platings and ceramic materials such asStellite®, tungsten carbide, aluminium oxide,zirconium oxide or silicon nitride are used, inparticular when abrasive process media andhigh pressure differences are present. Anessential prerequisite for valve trims made ofcarbides or ceramics – in particular on ac-

Table 2:Material selectionfor valve trims

Material For temperatures Typical application

1.4021 X20Cr13 –10 to 400°C Standard applications withwater, steam andnon-corrosive media

1.4571 X6CrNiMoTi17-12-2 –196 to 400°C Applications with increasedrequirements for corrosion resistance

Stellit® alloy 6 –196 to 400°C Increased mechanical(often as weld-on strength with good corrosionhard facing) resistance

1.4112 X90CrMoV18 –10 to 400°C Steam and water with high(hardened) differential pressure

1.4922 X20CrMoV11-1 –10 to 580°C Especially for use above480°C

Tungsten carbide –10 to 400°C In case of risk of erosion by Special ceramics solids in the flow medium

Lining withcorrosion-resist-ant material

Different hardness valuesby surface treatments “Ceramics-com-

patible” design


Selection of materials for sealing elements 49

ical resistance is indispensable. The tables of the well-known seal manufacturers provideappropriate information.


Selection of materials forsealing elementsFor static sealing applications, high-qualitygaskets made of graphite with a spiral inserthave established themselves. Here, the spiralinsert provides a defined pre-load that ismainly generated in the force bypass. Ifgraphite is not permissible for process rea-sons, seals made of a PTFE compound canalso be used. For the stuffing box area, pack-ings made with a PTFE or graphite basematerial are mainly used (Table 3). PTFEpackings are configured either as self-adjust-ing V-ring packings or as manually adjust-able packings. Pure graphite packings arereserved for higher temperatures on accountof their much less favourable friction prop-erties.Other seal materials used in control valves arelocated in the seat seal (in the case of soft seal“bubble-tight” valves), or in sealing rings forpressure balancing systems, or in O-rings forspecial sealing tasks. In the selection of mater-ials for the sealing elements, knowledge of the limits of use of the material in questionwith regard to pressure, temperature and chem-

Fig. 18:Control valve withtungsten-carbideplug and dual-sidedtungsten-carbide seat

Table 3:Static seals and theirareas of application

Max. Max. Gasket form Static Packing Typicaltemp. press. Process flange valve type application(°C) (bar) gaskets

250 100 Simple flatgasket

Flat gasket ofspecially treatedPTFE

Spiral-woundgasket with sealing filler (graphite orPTFE)

General engineering foruncritical media

Standard specification inchemistry, petrochem-istry, apparatus engineer-ing and power industry

Braided PTFE packing

PTFE V-ringpacking

Thin gasket ofpure graphite

Spiral-woundgasket withgraphite sealingfiller

480 160 Spiral-woundgasket with sealing filler(graphite and/orPTFE)

Standard specification inchemistry, petrochem-istry, apparatus engineer-ing and power industry

High-temperature application especiallywith steam

Carbon-fibre/graphite braidedpacking

Pure graphitefoil packing(rolled)

Serrated gasket250 160to480

Spiral-woundgasket with sealing filler(graphite and/orPTFE)

Highly stressed valvesin chemistry and petro-chemistry

Very high tightness requirements (TA Luft)and very corrosive media

Carbon-fibre/graphite braidedpacking

Bellow sealing

Metallic lensgasket

Metallic ring-joint-gasket

>500 400

420

Metallic lensgasket

Metallic ring-joint-gasket

High-pressure and high-temperature application,also vacuum technology

Chemistry and petro-chemistry, mostly inAnglo-American countries

Bellow sealing

Carbon-fibre/graphite braided packing

Tungsten-carbide seat Tungsten-carbide plug

Graphite or …

… PTFE compound


External tightness 51

ing elements are present, the movement of thevalve stem is accommodated by the deform-ation of a component especially designed forthis purpose. The classic variants of a hermeticstem seal are the diaphragm seal (Fig. 19a)and the bellows seal (Fig. 19b).The diaphragm seal is used for pressures inthe range up to approx. 10 bar. Amongst itsimportant advantages are low price, compactconfiguration and also a potential sterilisationcapability. The bellows seal has established it-

50

EmissionsThe worldwide increase in awareness of en-vironmental issues and the resultant emissions-related obligations for the operation of processtechnology plants in the form of legislative re-quirements, guidelines or standards affect allindividual components of the plant, and in par-ticular the control valves. For control valves,the following three features play key roleswith regard to environmental compatibility:the external tightness, the internal tightness,and the sound emissions.

External tightnessThe external tightness is an essential criterionfrom the ecological viewpoint and is clearlyspecified in numerous acts and standards (e.g.in ISO 15848) as a function of the mediumthat is being sealed. It is essentially deter-mined by the pressure boundary parts such ashousing, bonnet, bonnet bolting, and the seal-ing between valve housing and bonnet.Here, the central role is played by the valvestem seal. The traditional valve stem seal is aself-adjusting or adjustable stuffing box.With a high-quality self-adjusting stuffingbox, it is possible to achieve leakage valuesthat meet the requirements of ISO 15848Class B or the TA Luft (the legislative re-quirements in Germany for media that are in-jurious to health). These requirements are sostrict that for a car tyre filled with helium,for example, they would correspond to apressure drop of 0.1 bar over a period of 200 years.If the sealing requirements on valve stem sealsare even higher, so-called hermetic seals areused. In these sealing systems, only static seal-

Diaphragm

Fig. 19:a) Diaphragm seal

b) Bellows seal withintegrated anti-rotation device

Bellows

Safety stuffing box

Anti-rotation device

Three essentialfeatures

Standard: self-adjustingstuffing box

Diaphragm seal

Bellows seal


External tightness 53

tion. This means that any fracture of the dia-phragm on the primary side (the medium side) or on the secondary side (the sealingfluid side) is detected. In either case the her-metic seal remains intact, however, and nocontamination of the process medium by thesealing fluid can occur. In this way, the valveremains fully functional up to the planned re-moval of the defect. Moreover, as a result ofthe hydraulic assistance to the diaphragm, thissealing design minimises the necessary actu-ator force and the size of actuator. Thus itcombines:

• low overall height• gap-free design• absolute hermetic external sealing function• a large pressure and temperature range as

well as • redundancy by means of the double dia-

phragm with diaphragm fracture monitor-ing

52 Emissions

self as a universal stem seal for control valves.While it requires more space and is also moreexpensive than a diaphragm seal, if designedto meet high quality standards it provides, be-sides the guaranteed sealing function, a longservice life (far in excess of one millionstrokes) and covers the complete range of op-erating pressures up to 400 bar and tempera-tures up to 500°C.An essential feature of a well-designed bel-lows seal is adequate protection of the bellowsagainst torsion, which can significantly reducethe lifetime of the bellows. Torsion can ariseas a result of incorrect installation (for ex-ample, when coupling valve and actuator), butalso as a result of torques which, arising fromthe valve or the process, act on the valve trimand thus on the bellows. These torques shouldbe eliminated by means of an anti-rotationdevice integrated into the bellows housing.A forward-looking development which com-bines the advantages of a bellows seal (pres-sure range and operational reliability) withthose of a diaphragm seal (compact configur-ation and a sterilisation capability) is repre-sented by the hydraulically-assisted dia-phragm seal. In this type of seal, the dia-phragm continues to function as a separatingelement between the process medium and thesealing fluid, but now no longer has to with-stand the static pressure of the processmedium (Fig. 20). The alteration in volume ofthe sealing fluid during the movement of thevalve stem is compensated for by the corres-ponding deformation of the diaphragm. This is configured as a double diaphragm. This de-sign ensures that it meets the most stringentrequirements regarding plant safety and avail-ability. Between the two diaphragms is locateda non-pressurised intermediate space that canbe monitored by means of a testing connec-

Fig. 20:Hydraulically assisted diaphragmseal (ARCAOPTISEAL®)

Test connections

Doublediaphragm

Sealing fluid

Adequate protectionagainst torsion

Hydraulicallyassisted dia-phragm seal

Remote monitoring


Internal tightness 55

• DIN IEC 60534-4 Class IV corresponds toapprox. 27.5 Nm3/h

• DIN IEC 60534-4 Class V corresponds toapprox. 0.038 Nm3/h

• DIN IEC 60534-4 Class VI corresponds toapprox. 0.009 Nm3/h.

The internal tightness is tested individually aspart of the final testing for control valves andis recorded in a test report. In the case ofscrewed valve seats, the internal tightness canalter so strongly in operation under alternatingload conditions, even after a short period oftime, that the reality no longer corresponds tothe measured leakage value recorded in thetest report. Clamped valve seats do not showthese deviations with regard to the internaltightness. In the soft sealing shown in Figure21, an additional metallic sealing between thevalve seat and the valve plug ensures that thepermissible surface load of the PTFE soft sealelement is not exceeded, even for over-dimen-sioned actuators, and therefore no so-called“cold flow” of the PTFE material occurs. Thedefined pre-load of the sealing element is per-manently guaranteed by means of an O-ringspring system. This soft seal configuration hasproved itself under the most difficult condi-

54 Emissions

and is thus an economical, and in many casesalso a technically better, alternative to the con-ventional bellows seal.

Internal tightnessThe internal tightness describes the possibleleakage of a valve between fluid entry andvalve outlet when in the closed position. It isof decisive importance for the process and isdetermined by:

• the design configuration of valve seat andvalve plug

• the form of attachment between valve seatand valve housing, and

• the dimensioning of the actuator.

For control valves the leakage classes accord-ing to IEC 60534-4 apply, and thus other test-ing methods than those for shut-off valves.This difference is based on the fact that the re-quirements in control operation (stability ofthe throttling element) and absolute sealingfunction represent somewhat contradictory re-quirements. Today, the usual sealing class forcontrol valves is class IV, i.e. the leakage flowrate is one ten-thousandth (0.01%) of the flowrate at fully opened position (presupposing aconstant pressure difference).The higher sealing classes V (“metallic-lapped”) and VI (“bubble-tight”) are usedmainly only when there are particular require-ments, for example, in the case of valveswhich expand either directly into the atmos-phere or (in the case of combustible media)via a flare, and whose leakage rate thus signi-fies a direct energy or product loss. As an ex-ample of such cases one can consider a flarevalve with a nominal size DN 100 for hydro-gen at a plant pressure of 40 bar. The follow-ing applies:

Fig. 21:Soft sealing with additionalmetallic sealing(ARCA patent)

Sealing class IV …

… or highersealing classes

Additionalmetallic sealing

Metallic stop

Soft sealing


Sound emissions 57

within tolerable limits, cavitation sound caneasily exceed these limits, even in the case ofsmall valves. In addition, cavitation mustalways be seen as a possible cause for thedestruction of the valve inner parts.For gases or vapours the main cause of soundemission is, for subcritical expansion, the par-tial conversion of the energy into sound en-ergy. Because of the significantly higher flowvelocities compared with liquids, the soundpressure level increases sharply with risingpressure difference. Even for relatively smallvalves, it can already lie above permissiblelimits and cause impairments to health. If thepressure ratio across the control valve exceedsthe XT value, shock waves are the main causeof sound emission in the expansion zone.

Primary sound reduction measuresIn gaseous flows, a reduction of the level ofsound generation is achieved by distributingthe throttling area into many smaller individ-ual flow passages (perforated cage, perforatedplug, multi-slot plug). In this manner, thesound-generating source is divided into manyindividual sound sources. On account of thelower extent of the turbulence zone and thehigher frequency range, these generate in totala lower level in the A-weighted sound spec-trum relevant to human hearing. The secondeffective measure is the distribution of thethrottling process into a number of stages. Inthis manner, a lowering of the flow velocities,which are causally responsible for the soundgeneration, is achieved in the individual throt-tling stages. Here too, the sum of the indi-vidual sound levels adds up to a significantlylower overall level in comparison with thesingle-stage throttling process. Included in themulti-stage throttling design concept are alsouncontrolled (static) throttling stages such as

56 Emissions

tions over more than one million operatingcycles (see section on “Control valves forpressure swing adsorption plants”, p. 62 ff.).

Sound emissionsDuring the pressure reduction in a valve, partof the energy of the process medium is con-verted into sound energy and radiated bothfrom the valve itself, but also primarily fromthe pipe system. Guidelines as well as healthand safety at work legislation are pushing to-wards “quiet” valve solutions; sound level re-quirements of 70 to 75 dB(A) are not unusual.The increasing demand for lower sound emis-sions from process plants often come upagainst not only economic boundaries but alsotechnical limitations. Low-noise valves requirenot only more complex inner parts, but oftenalso a larger housing. This is reflected in sig-nificantly higher costs.Extreme levels of sound emission are alwaysalso an expression of mechanical stress.Whenever considering sound emission it mustalways be borne in mind that while the soundis in fact generated in the valve, the soundradiation actually emanates from the down-stream pipe system. With reference to soundgeneration, a differentiation must be madehere between incompressible and compres-sible media.

Sound generationFor liquids, the sound sources are in the tur-bulent flow conditions at the throttling pointsand downstream. Here, the energy of the fluidis converted into not only heat but also soundenergy. If the pressure ratio across the valveexceeds the critical pressure ratio, cavitationsound also occurs. While the sound level re-sulting from turbulent flows generally lies

“Quiet” valvesolutions

Sound radiationinto the pipesystem

Turbulent flowas the reason

Cavitationsound

Rise of soundpressure level

Distribution intomany individualflow passages

Multi-stagethrottling


Sound emissions 59

Secondary sound reduction measuresSecondary sound reduction measures are con-cerned not with sound generation but rathersound radiation. The components used for thispurpose are mainly insulators, downstreamsound dampers or acoustic enclosures. Sincethe sound radiation of the acoustic energy gen-erated in a control valve occurs over a verylong length of pipe, extending sometimesmore than one hundred metres, the introduc-tion of secondary sound reduction means isresource-intensive and should therefore alwaysbe considered as an additional measure only.Sound-amplifying interactions can occur be-tween the valves and any pipe bends that arelocated too close downstream, and also anyother items installed in the pipe system. Forthis reason, consideration must be given at anearly planning stage of the pipe system to asound-optimised installation of the valve. Thesound predictions given by the valve manufac-turers, however, are related to “idealised” up-stream and downstream conditions. The pipesystem itself must also always be consideredas a potential sound source whenever the flowvelocity exceeds Mach 0.25.

58 Emissions

perforated cages and perforated discs. In par-ticular, if cavitation and supercritical expan-sion are present, distribution of the throttlingprocess is always to be considered as a pri-mary measure. In spite of their “open” flowareas, perforated discs, perforated cages andperforated plugs also have an encapsulating ef-fect on the sound generated by the upstreamstages and thus act further to reduce soundlevels. Figure 22 shows a three-stage valve with

Fig. 22:Three-stage controlvalve with controlledperforated plug/cagecombination, pres-sure balancing sys-tem and downstreamperforated disc

Perforated cage

Perforated plug

Perforated disc

Additional encapsulating effect

Reduction ofsound radiation

Sound-optimisedinstallation location

a controlled perforated cage/perforated plugcombination and a downstream set of per-forated discs that is integrated into an enlarge-ment of the housing. With combinations ofthis kind, it is possible to achieve sound levelreductions of 10 to 30 dB(A), relative to asingle-stage parabolic plug. Even a simple per-forated plug can result in a sound level reduc-tion of up to 15 dB(A), depending on the pres-sure ratio.


Control valves in power generation 61

valve. Only special control valves with a verywide rangeability are suitable for this pur-pose. The steam is then fed into the super-heater. Here, the temperature is adjusted bymeans of injection coolers, or also by meansof 3-way control valves, before the steamenters the turbine. A whole series of controlvalves operates in the turbine environment(e.g. sealing steam control valves, drainagecontrol valves) through to the extraction andby-pass stations, which in each individualcase is represented by a pressure control valvewith integrated cooling water injection tomaintain a prescribed temperature. Thesevalves maintain correct boiler operation if theturbine itself is not in operation. Thus variouslow- and medium-pressure systems must alsobe supplied, or the live steam must be com-pletely condensed and fed to the water-steamcircuit. Steam is also required for the heating of vari-ous heat exchangers and supply systems up tothe heating of the feed water tank, and is pre-cisely controlled by many control valves ac-cording to the required process and the per-ipheral conditions. Wherever thermal energy isextracted from the steam and converted tocondensate, control valves are used for thecontrol of pressure and temperature.Here, easy-to-maintain valves with clampedvalve seats that can be used on both sides(quick-exchange systems) have proved to beparticularly advantageous, especially duringcommissioning, since in spite of taking thegreatest care, it is not possible to flush the pipesystem so that it is really clean. Thus contam-inants remaining in the system inevitably leadto damage of the valve seating surfaces duringthe start-up of a plant. In this case, the valvecan be opened without special tools and theclamped seat turned over. This particular ad-

60

Modern solutionsfor special areas ofapplicationThe range of control valves is extraordinarilylarge and the number of applications is cor-respondingly numerous. In the followingsections some applications are described asexamples.

Control valves in power generationIn power stations, control valves are used inall areas. These extend from regulation of thefuel through to control of flue gas cleaningplants. However, one of the most interestingareas of application for control valves is thefeed water/steam circuit.Under normal operating conditions, controlvalves in the feed water circuit do not causeany problems whatsoever. An interesting chal-lenge arises, however, if control valves areused not only to supplement the feed waterthat has vaporised, but also to fill-up or start-up boilers. In this event, the high-pressure feedwater under start-up conditions is firstly re-duced to atmospheric pressure. Here, the con-trol valve passes through all stages of flashingand cavitation and is extraordinarily highlystressed. Only with the aid of special designmeasures can these operating conditions becontrolled with “normal” feed water controlvalves without the need for an additional fill-ing and start-up valve.During the start-up of the boiler, saturatedsteam vapour firstly enters into the steam sys-tem and is then exhausted via the start-up

Use for fill-up or start-up procedures

Temperaturecontrol

Valves withquick-exchangesystems


Anti-surge control valves on turbocompressors 6362 Modern solutions for special areas of application

vantage of the quick-exchange system ensuresthat commissioning can be continued withoutfurther delay.

Control valves forpressure swing adsorption plantsPSA plants (PSA = Pressure Swing Adsorp-tion) are used to separate and clean gasessuch as hydrogen, helium, nitrogen. In thecase of hydrogen, it is possible to achievecleanliness levels of 99.9999% by means ofthe PSA method. The method is very energy-efficient since it takes place at ambient tem-perature. At the heart of this method are 4 to 12 pressure tanks that are filled with anadsorbent. This adsorbent has the property ofadsorbing certain gases at a particular pres-sure and of releasing them again at a lowertemperature. By means of a systematic trans-fer of the gas between the individual ad-sorbers, the gas is cleaned in a staged manner.The typical cycle time of a PSA plant liesbetween 20 seconds and several minutes andthus places high requirements on all plantcomponents – particularly on the controlvalves. These must work reliably for morethan one million operating cycles per yearand moreover must guarantee a bubble-tightshut-off. Control valves optimised for alter-nating pressure plants:

• are made of superior quality cast steel with areduced carbon content to minimise the riskof hydrogen embrittlement

• have a reliable soft seal function and• at nominal valve size of 80 (3") and higher

provide a reliable pressure balancing sys-tem, since the travel times required are pos-sible only with relatively small pneumaticactuators.

The trend to ever larger actuator forces withinever shorter switching times leads to the pre-ferred use of double-acting piston actuators(Fig. 23). In recent years, this type of actuatorhas proved itself in a large number of instal-lations.

Anti-surge control valves onturbocompressorsIn turbocompressors, a control valve is in-stalled in the bypass that blows the com-pressed volumetric flow of gas back into theintake of the compressor, or (if the processmedium is air) blows it off into the atmos-phere. These valves are used in the firstinstance during the start-up and shut-down ofthe compressor. During critical running condi-tions, or with fluctuating consumption in spiteof a constant rotational speed, they are alsoused in order to blow off excess gas quantities.

Fig. 23:Double-acting piston actuator

Low-friction guide and sealing

More than one million operating cycles

Double-actingpiston actuator


Anti-surge control valves on turbocompressors 65

This requires reliable control of the valves bypositioners, boosters and solenoid valves, andcareful selection and matching of the pneu-matic components and pipework (Fig. 24).

64 Modern solutions for special areas of application

Their most important task, however, is to act as a safety valve in surge limit controlfunctions.Anti-surge control devices have the task ofpreventing so-called “pumping” in turbocom-pressors under all circumstances – this worddescribes the stalling that occurs at the com-pressor blades if a minimum flow through thecompressor is not maintained. Stalling cancause vibration of the compressor rotor andthus damage to the bearings, to the rotor itselfor to the blades. In the compressor characteris-tics, the blow-off line is therefore establishedat a certain safety distance below the surgelimit. For economic reasons this distanceshould be as small as possible. Reduction ofthe safety distance inevitably raises the re-quirements on the anti-surge control valve.The control valve must on the one hand besensitive, but on the other hand must also openwithin the shortest possible time without over-shooting, i.e. without opening fully in theblow-off case.Single-seat control valves with a linear ormodified characteristic have proved them-selves as anti-surge control valves in turbo-compressors. Instead of parabolic plugs, per-forated throttling elements are used today evermore frequently, since these guarantee soundreduction, even for valves that blow off intothe atmosphere. Since turbocompressors are becoming evermore powerful, the nominal diameter of by-pass valves has in recent years extended up toDN 1200 with correspondingly large actu-ators. Here, the following travel times areachieved:

• opening via solenoid valve < 1 to 2 seconds• opening via positioner < 3 to 5 seconds• closing via positioner 6 to 20 seconds.

Fig. 24:Pneumatic schematicof an anti-surge con-trol valve with volu-metric flow rate asmeasured parameter

3

1

2

P

AR

3 1

2

3

A

Venturi tubeTurbocompressor

Surge limit

TsFC

∆ps

Z

1

out sub

sigout in

2

Prevention ofstalling

Single-seat control valves

Stroke times

Function as asafety device


Process valves terminology 67

when compared with a piston actuator) a frictionless stroke movement andthus a very uniform actuation movement.

Downstream pressure increase For the purposes of sound reduction, thedownstream pressure of the valve is increased by the use of fixed throttlingstages (perforated discs and perforated cages) downstream of the valve. Lowerpressure differences thus occur within the valve and, in the case of gases,reduced outlet velocities.

Flow direction In globe valves, this is predominantly against the closing dir-ection of the plug. In the closing direction, sealing is assisted, but involves therisk of unstable (pneumatic) actuators and, in particular in the case of liquids,of pressure shocks.

Flow rate limitation Beyond a certain pressure difference ratio x =(p1–p2)/p1, the mass flow through a valve can no longer rise by means offurther reduction of the downstream pressure p2. Sonic velocity then occurs atthe narrowest throttling cross-section.

Flushing trim Place holder (mainly only for welded-in valves) mounted as asubstitute for the inner parts to protect the valve during pipe flushing and pick-ling processes.

Heating jacket Pressure-tight enclosure of the valve body and the bonnetthrough which steam or heat transfer oil flows. Heating jacket valves are oftenused in smelting and for polymerising media.

Hygienic valve A valve for applications in the food and pharmaceutical in-dustries with a special form of housing that counteracts the accumulation ofdeposits and enables simple cleaning.

Hysteresis In a control valve, the difference between the stroke positionsthat ensues with the same required stroke signal but with opposing directionsof movement.

Installation position An installation position with a vertical stem (actuatoron top) is preferred, because valve actuators cannot then exert any lateralforces or bending moments on the axisymmetrically aligned valve trim andtheir seals.

Instrument air Dry and oil-free air is indispensable for the fault-free oper-ation of positioners and other pneumatic components (IEC 770).

Insulation column Extension of the valve stem used with cryogenic tempera-ture media in order to protect the packing, actuator and positioner from icingcaused by the cold medium, as well as to enable effective insulation.

Kv value Coefficient for the flow capacity of a control valve. 1 Kv correspondsto a flow rate of 1 m3/h of water (20°C) at a pressure difference of 1 bar (see Figure 26 on page 68 for empirical formulae for calculation purposes).

66

Process valves terminology Acceptance testing The testing procedures defined in the regulations andstandards to guarantee safety, function and quality of the valve.

Acoustic conversion factor Coefficient for the ratio of the power loss gener-ated by the control valve that is converted into sound. In subcritical flow, theacoustic conversion factor is of the order of 10–6 to 10–4, depending on trimand valve shape.

ATEX European directive for the correct use of equipment in potentiallyexplosive environments.

Bellows seal Hermetic sealing on the valve stem against hazardous media bymeans of a metal or polymer bellows.

Cavitation Occurs when there is a temporary pressure drop in liquids belowthe vapour pressure. The collapse of the vapour bubbles generated at the throt-tling point leads to shock waves that can cause erosion damage when they im-pinge on valve bodies and internal parts of the valve. Cavitation is reduced bythe use of multi-stage throttling bodies. To increase durability, hardened materials are used (see Fig. 25).

Fig. 25: Pressure distribution in the valve at flashing and cavitation

Vaporisation (flashing)p1>pV>p2

Cavitationp1>p2>pV>pVC

p1

pVC pVC

pVp2

p1

p2pV

Characteristics These describe the ratio between valve position and openingcross-section prescribed by the shape of the throttling element. Linear orequal-percentage characteristics are customary.

Choked flow See flow rate limitation.

Critical pressure ratio See flow rate limitation.

Cryogenic temperature valve Special valve for liquid gases at temperaturesbetween –196°C and absolute zero (–273°C).

Diaphragm actuator Pneumatic control actuator in which the pressurechamber is sealed off by a diaphragm. The diaphragm allows (particularly


68 Process valves terminology Process valves terminology 69

Plant characteristic The characteristic that ensues from the characteristic ofthe control valve, taking into account the pump characteristic as well as allpipe components. The valve authority is calculated from the plant character-istic.

Pressure balancing Elimination of the hydrostatic valve plug forces bymeans of pressure equalisation between the upper and lower sides of the valveplug. Requires complex design of the control plug into a piston-cylinder sys-tem with a sliding radial seal.

Pressure shock (Joukowski Shock) Sudden pressure rise evoked by therapid deceleration of a flowing fluid in the pipe system caused as a result ofclosing a valve. The pulse of the flowing mass has a powerful potential fordestruction.

Rangeability Generally the ratio of the largest to the smallest controllableflow rate. The inherent rangeability corresponds to the ratio of the largest tothe smallest flow coefficient.

Reversing For pneumatic actuators, the change of the safety position (springforce) in the event of air supply failure.

Safety position Valve position prescribed in the event of failure of the actu-ator energy source: closed, open or locked.

SIL Abbreviation for Safety Integrity Level. Provides a classification to pro-tect the health and well-being of people, the environment and of goods. Con-trol valves are essential components in the assessment of process technologyin this respect.

Silencer Static downstream throttling stages for sound reduction purposes(see downstream pressure increase).

Split-range operation Distribution of the flow between a large main controlvalve and a small fine control valve if the rangeability of the large valve is notsufficient to cover all working points

Start-up valve Additionally installed valve whose prime function is to startup the process. It is characterised by extreme operating conditions, but is notsubject to constant load.

Steam conditioning station A valve for the combined pressure and tempera-ture reduction of superheated steam by means of simultaneous throttling andinjection of cooling water.

Stick-slip effect The difference in frictional forces during sliding and releas-ing the radial seals can, in conjunction with weakly (i.e. purely statically) de-signed pneumatic actuators, lead to undesirable jerky movements that preventan exact positioning of the valve stem and plug.

Fig. 26: Empirical formulae for the calculation of flow coefficients

Pressure loss

Kv

Kv

For fluid For gas with temperature correction

For vapours

p2p12

>

p12

<

Q31.6

ρ1Δp

=QN514

ρNΔp

T1p2

=G

31.6v

Δp=

Kv

p2p12

<

Δpp12

>

Q31.6

ρ1Δp

=2QN

514 p1ρN T1=

G31.6

2vp1

=

Δp

Q Volumetric flow rate m3/hG Mass flow rate kg/hp1 Upstream pressure bar abs ρ1 Density at the operating condition kg/m3

QN Standard volumetric flow rate of gases Nm3/hp2 Downstream pressure bar absρN Density at the standard condition kg/Nm3

T1 Absolute upstream temperature K v Specific vapour volume at T1 m3/kg

Laminar flow Characterised by the uniform flow of flow particles alongneighbouring paths without turbulence. Laminar flow only occurs in controlvalves at extremely low flow rates or with high-viscosity fluids.

Leakage class In a closed valve, there is always some leak flow betweenseat and plug. The requirement for the seat tightness and test methods is estab-lished in international standards (EN 60534-4).

Lining The pressure-containing metallic housing of the valve is clad on theside exposed to the medium with a chemically-resistant material.

Packing Dynamic sealing of the valve stem against the external environ-ment.

Parabolic plug Simplest shape of a control plug that in conjunction with acircular opening (valve seat) forms an annular throttling point.

Perforated plug Control plug, configured as a perforated cylinder, thatslides in a seat ring and according to its position opens up more or less holesand thus alters the throttling area.

Piston actuator Pneumatic actuator that possesses a dynamic piston sealinstead of a diaphragm. Is often double-acting and is used for long strokes.


70 Process valves terminology

Stuffing box Module for sealing of the valve stem with the packing as anessential component.

Subcritical pressure ratio The pressure difference across the throttlingelement is relatively small, so no flow rate limitation or cavitation occurs.

Supercritical pressure ratio The pressure difference across the throttlingelement is relatively high and causes flow rate limitation or cavitation.

TA Luft Abbreviation for “Technische Anleitung zur Reinhaltung der Luft”(Technical Instructions for Maintaining Clean Air). German regulation thatplaces increased requirements on the stem sealing function in particular.

Turbine bypass station Steam conditioning station which in the event offailure of the turbine takes over its throttling and cooling functions.

Turbulent flow Irregular non-parallel flow. Flow-conditioned pressure dif-ferences are generated within the fluid and thus turbulence is generated.

Two-phase flow A mixed gaseous and liquid (e.g. wet steam/condensate)flow or a fluid flow with solid components (e.g. cellulose).

Valve authority A key figure that describes the influence of the valve on theprocess parameter to be controlled.

Valve characteristics These describe the ratio between valve stroke andopening area prescribed by the shape of the control plug. Linear or equal-per-centage valve characteristics are customary.

Vaporisation Partial conversion of a liquid medium into the gaseous stateduring the throttling process to a pressure that lies below the vaporisation pres-sure of the liquid (see Fig. 25).

The company behind this book

ARCA Regler GmbHKempener Str. 18D-47918 Tönisvorst, GermanyPhone: +49 2156 7709-0Fax: +49 2156 7709-55E-mail: [email protected]: www.arca-valve.com

Valves, pumps and cryogenics worldwide

As one of the leading companies in flow control technology,ARCA Regler GmbH has concentrated for more than 90years on the development, manufacture, marketing, and notleast on the service of pneumatically actuated control valves.

The company is the parent company of the internationalARCA Flow Group, with branch offices in India, Korea,China, Mexico, Switzerland and the Netherlands, and a mar-keting network that operates worldwide. In addition to con-trol valves, pumps for slurries and other critical fluids, levelmeasuring equipment, and injection coolers and control ballvalves are included in the Group’s product portfolio.

ARCA offers innovative technology of the highest qualityand reliability and is a proficient partner for plant construct-ors and end users in all industrial sectors – from power sta-tion technology, refineries, chemical plants, iron and steel-works through to pharmaceutical and food production.

The ongoing development of ARCA products is evidencedalso by many patents and awards received as one of the mostinnovative TOP 100 companies and as a TOP JOB employerin the German medium-business sector. Furthermore, ARCAis featured in the “German Standards – World Market Lead-ers of German Industrial Companies” Encyclopaedia.
