Download - 01 General Control Valves Training.

Engineering Centre of ExcellenceECoE - Doha, Qatar

Instrument training – 16th July 2011General Control valves.

Prepared by: Gavhane D.V.

General Control valves.

2General Control Valves.

BY:-DNYANESHWAR GAVHANE




INTRODUCTION :-

Control valves also called final control element are a most important element in the control system of a process plant.

It is the last device in control loop. DEFINITION :-

A control valve is a variable orifice used to regulate the flow of a process fluid in accordance with requirement of process.

FUNCTION :-

It absorbs the proper amount of pressure drop to maintain system balance under all operating conditions.



Features of Control valves.

THERE ARE FOUR FEATURES OF CONTROL VALVES :

CapacityRangeability

CharacteristicsPressure drop

CAPACITY : The capacity is normally measured as Cv (Flow Coefficient) which is defined as :

“The number of US gallons per minute of water at 60oF that will flow through a valves with 1psi pressure drop .

RANGEABILITY : Rangeability of a Control Valve is the ratio of the maximum to the minimum controllable flow.



Control valves and Flow Characteristics.

The relationship between control valve capacity and valve stem travel is known as the Flow Characteristic of the control valve.

The Curve shown are based on constant Pressure drop across the valve and called Inherent flow characteristics.



Linear Characteristics. Flow Capacity increases linearly with valve (stem) travel. In

which the valve differential pressure drop is Constant over the travel range.Linear valve plug shall for liquid level control and control application requiring constant gain.

LINEAR VALVE TRAVEL IS DIRECTLY PROPORTIONAL TO THE VALVE STROKE

FOR SLOW FLUID TRANSFER PROCESS

MORE THAN 40%OF SYTEM PRESSUREDROP ACROSS VALVE



Equal Percentage Characteristics. Flow Capacity increases exponentially with valve trim travel .Equal

increment of valve travel produce equal percentage change in existing Cv.Equal Percentage valve plug shall be used where only small percentage of system drop is available in Control valve.

EQUAL INCREMENT OF VALVE TRAVEL PRODUCE AN EQUAL PERCENT FLOW CHANGE

FOR FAST FLIUD TRANSFER PROCESS

HIGH RANGEABILITY



Quick Opening Characteristics. Provides large changes in flow for very small changes in lift or in which a maximum Cv is achieved with minimal closure member travel. It has too high valve gain.

LARGE INCREASE IN FLOW WITH A SMALL CHANGE IN VALVE STROKE

Quick opening valve are usually specified for “ON-OFF” Service such as sequential operation in either batch or semi continuous process and in self actuated control valve . When maximum valve capacity must be obtain quickly.

Cage- A part of a valve that surrounds the closure member & can provide Quick opening flow characterization.



Cavitation. Cavitation is condition that occurs in liquid flow where the internal pressure of liquid at set point falls below vapour pressure and vapour bubbles form and at downstream rises above vapour pressure again.As this pressure recovers so that bubbles collapse and cavitation take place.It is possible to predict where cavitation will occur by looking pressure condition and valve recovery factor.( sigma Method )Cavitation sounds like stone passing through the valve.

Bubble form Bubble collapse

Vena Contracta

P

P

Pv

PRESSURE PROFILE ACROSS SINGLE SEATED CONTROL VALVE



Effect of Cavitation.

Erosion in rotary plug valve body induced by cavitation



Flashing.Flashing is condition that occurs with liquid flow where the Pressure fails below it . There are then two phases flowing ( i.e. Liquid and vapour ) down stream.

Several damage can occur inside a valve due to erosion caused by the impact of liquid droplet traveling at high speed.

Bubble form

Vena Contracta

Bubble continue down stream

P1

Pv

P2




Effect of Flashing.

Typical trim erosion damage due to flashing



Velocity Profile across Single seated Valve.

P1

P2

V 1(Liquid)V 2 (Liquid)


Pressure Recovery:- At vena contracta,the velocity is greatest & substantially decrease in pressure .Further down stream, as the fluid stream expands into a large area, Velocity decreases and pressure increases. This phenomena is called Pressure recovery.As a general ,globe valves have Lower pressure recovery than Ball and Butterfly Valves.

Pv



Piping Influence/Reynolds Number/Choked Flow. Piping Influenence:- Capacity reduction due to the presence of Reducer Up & Down stream of the valve. Correction factor will be available from manufacturer .Reynolds Number :- For Reynolds number application such as viscous or small flow , the basic sizing equation can not be used without correction, since the equation is based on turbulence flow . Correction factor will be available from manufacturer .(The ratio of dynamic forces to viscous forces) Usually the correction can be ignored when Reynolds number is greater than 4000,the correction will be 10% or less .Choked Flow:- When the pressure at vena contracta drop below the vapor pressure of the liquids, bubbles will form in the stream .The formation of bubbles causes a crowding condition at vena contracta which tends to restrict flow, and further increase in pressure drop will not produce increase in flow. In case of gas , choked flow means outlet velocity reaches sonic velocity.The limiting Pressure difference ΔPt is called Maximum allowable differential pressure for sizing.



Contents of Control Valve:– Data Sheet PROJECT:- GPS DOC NO:- -

CONTRACT NO:- SUB CONTRACTOR DOC NO:-

SHEET NO:- 1 OF 1

DOCUMENT NAME:-

SPECIFICATION AND DATASHEETS FOR CONTROL VALVES CLIENT DOC NO:-

DATE:-

REV NO:-

DATASHEET - CONTROL VALVES

1GENERAL

DATA

TAG

2 SERVICE BLANKET FUEL GAS FROM OIL STORAGE TANK 0-T-3603

3 LINE NO. / PID NO. 0-14"-FG-43 0017-AA3-H(E) / S2-01-YS207019-PID-P-5004 Sht 1 of 2

4 FLUID TANK VENT/FUEL GAS CRITICAL PRESS. Pc

5

SERVICE

CONDITION

UNIT MAX FLOW NORM FLOW MIN FLOW SHUT-OFF

6 FLOW RATE m3/h 5166 1847 525.3 -

7 INLET PRESSURE psi(g) 0.0568 0.093 0.0975 15 bar(g)

8 OUTLET PRESSURE psi(g) 0.037 0.0087 0.00234

9 INLET TEMPERATURE Degree C 50 50 50 100(Note 1)

10 SPEC WT/DENSITY/ MOL WT - /kg/m3 / kg/kmole /-/1.378/40.30 /-/1.378 /40.30 /-/ 1.381/40.30 -

11 VISCOSITY /SPEC HEAT RATIO Cp / 0.00924/1.133 0.00924/1.133 0.00924/1.133 -

12 VAPOR PRESSURE inH2O NA NA NA -

13 REQUIRED CV -

14 TRAVEL % 0

15 ALLOWABLE /PREDICTED SPL dBA < 85/ < 85/ < 85/ -

16 SET POINT inH2O(g) 2.75 2.75 2.75



Contents of Control Valve:– Data Sheet. 17

LINE

PIPE LINE SIZE & SCHEDULE

IN 14" & SCH20 56

ACTUATOR

(PNEUMATIC)

TYPE SINGLE-ACTING FAIL SAFE SPRING RETURN –DIAPHRAGM TYPE

18 OUT 14" & SCH20 57 MFR & MODEL VTS

19 PIPE LINE INSULATION HEAT CONSERVATION (ELECTRICAL TRACING) 58 SIZE / EFF.AREA VTS

20

SPEC.& CODES

VALVE SIZING ISA S75.01 59 ON/OFF MODULATING YES

21 VALVE SEAT LEAKAGE CLASS IV AS PER FCI 70-2 60 SPRING ACTION ON FAIL CLOSE

22 61 MAX ALLOWABLE PRESS. VTS

23 62 MIN REQUIRED PRESS. VTS

24 63 AVAILABLE AIR SUPPLY PRESSURE kPag

MIN NOR MAX DESIGN

25

VALVE BODY

TYPE BUTTERFLY 64 400 850 900 1000

26 SIZE VTS 65 BENCH RANGE VTS

27 MAX PRESS./TEMP. VTS 66 ACTUATOR ORIENTATION VTS

28 BODY/BONNET MATERIAL A105 or A216 WCB 67 HANDWHEEL TYPE YES VTS

29 LINER MATERIAL / ID 68 AIR FAILURE LOCK RELAY REQD. SET AT Note 5

30 69 AIR BOTTLE REQD. WITH ACCESSORIES

31 END CONNECTION IN/ OUT FLANGED 70 FIRE SAFE YES

32 RATING & FLANGE FACE FINI. 150# RF ASME B16.5 71

POSITIONER

INPUT SIGNAL 4-20 mA

33 END EXTENSION/MATERIAL ASTM A105 carbon steel 72 TYPE ELECTRO-PNEUMATIC (HART)

34 FLOW DIRECTION YES 73 EXPLOSION PROTECTION EExd

35 TYPE OF BONNET BOLTED 74 ON INCR SIGNAL OUTPUT INCR/DECR FIELD REVERSIBLE

36 LUB & ISO VALVE / LUBE 75 GAUGES Yes BYPASS Yes

37 PACKING MATERIAL PTFE 76 CAM CHARACTERISTIC

38 PACKING TYPE VTS 77 MFR & MODEL VTS

39 BONNET GASKET VTS 78 SWITCHE

S

TYPE - QTY -

40 MFR & MODEL VTS 79 MFR & MODEL -



Contents of Control Valve:– Data Sheet . 41

TRIM

TYPE VTS 80

CONTACTS/RATING -

42 SIZE RATED TRAVEL VTS VTS 81 ACTUATION POINTS -

43 CHARACTERISTIC VTS 82

44 BALANCED/UNBALANCED VTS 83

AIR FILTE

R

MATERIAL DIE CAST ALUMINIUM

45 RATED CV VTS FL VTS XT VTS 84 FILTER SIZE < or = 25 Micron

46 PLUG/BALL/DISC MATERIAL F6 85 SET PRESSURE

47 SEAT MATERIAL F6 SEAT HARD FACED WITH STELLITE6. 86 INTEGRAL PRESS.

GAUGE 50mm DIAL

48 CAGE/GUIDE MATERIAL VTS 87 CONNECTION 1/2"NPT

49 STEM MATERIAL F6 88 MFR & MODEL VTS

50 89

51 SPECIALS /

ACCESSORIES

NEC CLASS GR. DIV. 90

TESTS

HYDRO PRESS. YES

52 IEC ZONE GR. TEMP. CLASS CLASS I, ZONE 2 , GROUP IIA & T3 91 SEAT LEAKAGE TEST YES

53 92 CAPACITY TEST YES

54 93

55 94

NOTES : VTS-Vendor To Specify



Globe Control Valve Construction. .



General Control Valve Classification.

CONTROL VALVE

LINEARMOTION

ROTARYMOTION

Diaphragm valveGate valve

GLOBE VALVEGlobeAngle

Three way

Eccentric plug Butterfly

Ball



General Types of Control Valve. CONTROL VALVE TYPES

VALVE TYPES BASED ON PLUG CONSTRUCTION ARE

GLOBE VALVE BALL VALVE BUTTERFLY VALVE

Control:- Linear & equal %Use:- Throttling Service,Flow Regulation, FrequentOperationApplication:- Liquid, Vapour,Gases, Corrosive SubstancesSlurries.Advantage: Efficient throttlingAvailable in multiple ports.Accurate Flow control.Disadvantage: High pressureDrop, more expensive.

Control:- Quick openingAnd linear.Uses:- Fully open / closedlimited-throttlingApplication:- Most liquidsHigh temperature, slurriesAdvantage:- High capacityLow leakage and Maint.Tight sealing with low torqueDisadvantage:- poor Throttling characteristicsProne to Cavitation

Control:- Linear & equal %Uses:- Fully open / closed orThrottling services, FrequentOperation, Minimal fluid Trapping in line.Application:-Liquids, gases,Slurries, Liquids withsuspended Solids.Advantage :- High CapacityGood flow control, Low Press.Drop, Low cost and Maint.Disadvantage:- High torqueRequired for controlProne to cavitation at lowerFlows.ECENTRIC PLUG / CAMFLEX DIAPHRAGM KNIFE GATE



Eccentric Plug / Camflex Valve Construction.

Side view of an eccentric plug valveShown in partially open…..


Eccentric Disk/Ball/Knife Valve Construction

22

V-Notch Ball Valve Knife Gate ValveEccentric Disk Valve

Prepared by: Gavhane D.V.General Control Valves.

Butterfly/Angle/Three Way Valve Construction

23General Control Valves. Prepared by: Gavhane D.V.


Diaphrgm Type Valve Construction.

Seal over a seat ( Straight type)

Weir type ( Saddle type)



General Types of Actuators . Actuator is the part of the valve that responds to the applied signal and causes modification of the fluid flow through stem & plug. TYPES OF ACTUATORS:1) Diaphragm Actuator Direct Acting Reverse Acting Size depends on output thrust required and supply air pressure available.2) Piston Actuator Use of high pressure air to 150 psig., eliminating the need of pressure regulator. Used where High & Fast Stroking Speed is required.3) Electro hydraulic Actuator: Electro-hydraulic Actuator is single unit contains Hydraulic Pump and Reservoir & Electric Motor. Ideal for isolated locations4) Manual Actuator Used where no auto control is required



Direct – Acting Actuators .



Actuators Fail-Safe Requirements. Fail Safe is the action of the actuator by which the valve CLOSES (FC) or OPENS (FO) fully in case of air supply failure.



General Types of Bonnet & Accessories . Bonnet - Bonnet is a part of the valve body assembly through which the valve plug stem or rotary shaft moves.

Types Of BonnetsPlain or Std Bonnet :-Operating Temp below 2000 c.Finned Bonnet :- Operating Temp above 2000 c or Equal.Extension Bonnet :- Operating Temp below 00 c or Equal.Extension Bonnet :- Operating Temp above 4500 c.Bellow Seal Bonnet :- where no stem leakage is tolerated For Toxic, Expensive Fluids.

• Positioners• Limit Switches• Solenoid Valves• Position

Transmitters• Volume Boosters

General Control Valve - Accessories



General Construction of Bonnet . Forged –Valve

BonnetFinned Bonnet

Bellow Seal Bonnet ( Enviro Seal)

Std./ plain Bonnet

Extension Bonnet



Control valves Characteristics. PROCESS APPLICATION INHERANT

CHARACTERISTICS

PRESSURE LIQUIDSGAS (LOW FLOWS )GAS (LARGE FLOWS)ΔPmax/Δpmin < 5ΔPmax/Δpmin > 5

EQUAL %EQUAL %

LINEAREQUAL %

Temperature Liquids & Gases EQUAL %

Flow Load ChangesSet point change

EQUAL %LINEAR

Level ΔPmax/Δpmin < 5ΔPmax/Δpmin > 5

LINEAREQUAL %



Control valves :- Interface

PROCESS INTERFACEProcess to provide process data to Instrument for specifying the valveInstrument to provide slected valve size to Process for incorporating in P&IDsInstrument to provide instrument air consumption figures for the valves to Process to decide air line & header sizes.

PIPING INTERFACEInstrument to provide Face to Face Dimensions, End Connection Size & Rating, Actuator Height & Orientation, Envelope Dimensions (for 3-D Model), Weight, etc.Piping to provide location of control valves to Instrument for deciding instrument air manifolds.Piping to provide Pipe Material Specification to Instrument for specifying the valves.



Control valves Body Material.

Frequently Used Valve Body Cast Materials Are:CARBON STEEL (ASTM A216 GR WCC) up to 4270 CCHROME MOLY STEEL (ASTM A217 GR WC9) up to 5930 CCARBON MOLY STEEL (ASTM A217 GR C5) up to 5930 C304L STAINLESS STEEL (ASTM A351 GR CF3) up to 8160 C316 STAINLESS STEEL (ASTM A351 GR CF8M) up to 8160 CCAST IRON (ASTM A126) up to 2320 C

Special Materials Used Depending On The Process RequirementsHASTEALLOY BHASTEALLOY CMONEL ALLOY



Control valves Trim Material.

Frequently Used Valve Trim Materials Are:• 302 SS : ASTM A 276 TYPE 302• 304 SS : ASTM A 276 TYPE 304• 316 SS : ASTM A 276 TYPE 316• 316 L SS : ASTM A 276 TYPE 316L• 410 SS : ASTM A 276 TYPE 410• 17-4 pH SS : ASTM A 461 GR 630 [High Tensile Strength, High Yield point, Hardened Material]• HASTEALLOY ``B’’ : ASTM A 335• HASTEALLOY ``C’’ : ASTM A 336



Control valves Codes and Standards. ANSI B 16.34 : • Covers Pressure, Temperature rating, Dimensions, Materials, NDT RequirementsANSI B 16.5 : • Covers design of flanges and flanged fittingsMSS SP 67 : • Covers design and test performance requirements of Butterfly valves

and divides in to three leak classes.ANSI B 16.10: • Face to Face dimensionsANSI B 16.37: Hydrostatic testingLEAKAGE RATES AS PER ANSI/FCI 70-2 Formerly (ANSI B 16.104) for seat leak classes and testing procedures- FCI



Vendor Design Documents- Sizing & Selection

Step # 1:- Define the systemExample :- The system is pumping water from one tank to another through a piping system total pressure drop is 150psi.The fluid is water at 700F.Design (maximum) flow of 150 gpm,operating flow rate of 110 gpm,and minimum flow rate of 25gpm. Pipe diameter is 3 inches. At 700F, water has has a specific gravity of1.0

Key Variables:- Total pressure drop , design flow ,operating flow, minimum flow diameter, specific gravity.

Step # 2:- Define Max. Allowable Pressure Drop For The Valve When defining the allowable pressure drop across the valve, you should first investigate the pump.What is its maximum available head? Remember that the system pressure drop is limited by the pump. Essentially the Net Positive Suction Head Available (NPSHA) minus the Net Positive Suction Head Required (NPSHR) is the maximum available pressure drop for the valve to use and this must not be exceeded or another pump will be needed. The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater. For above example step#1 system, 10% of the total pressure drop is 15 psi which is what we'll use as our allowable pressure drop when the valve is wide open (the pump is our system is easily capable of the additional pressure drop).



STEP #3: Calculate the valve characteristic



Step#4:-Preliminary Valve Selection

The Cv value should be used as a guide in the valve selection, not a hard and fast rule. Some other considerations are: a. Never use a valve that is less than half the pipe size b. Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10-80% stroke range.

Before a valve can be selected, we have to decide what type of valve will be used . For our case, we'll assume we're using an equal percentage, globe valve . The valve chart for this type of valve is shown below. This is a typical chart that will be supplied by the manufacturer.

For our case, it appears the 2 inch valve will work well for our Cv value at about 80-85% of the stroke range. Notice that we're not trying to squeeze our Cv into the 1 1/2 valve which would need to be at 100% stroke to handle our maximum flow.



Typical Chart Supplied by Manufacturer.



Step# 5:-Check Cv & Stroke % at Min. Flow

If the stroke percentage falls below 10% at our minimum flow, a smaller valve may have to be used in some cases. Judgments plays role in many cases. For example, is your system more likely to operate closer to the maximum flowrates more often than the minimum flowrates? Or is it more likely to operate near the minimum flow rate for extended periods of time. It's difficult to find the perfect valve, but you should find one that operates well most of the time. Let's check the valve we've selected for our system:



Step#6:Check Gain across Applicable Flow Rates• Gain is defined as:

• Now, at our three flowrates:Qmin = 25 gpmQop = 110 gpmQdes = 150 gpm

we have corresponding Cv values of 6.5, 28, and 39. The corresponding stroke percentages are 35%, 73%, and 85% respectively. Now we construct the following table:Flow (gpm) Stroke (%) Change in (gpm) Change in Stroke (%)

25 35 110-25=85 73-35=38110 73150 85 150-110=40 85-73=12

Gain #1 = 85/38 = 2.2Gain #2 = 40/12 = 3.3



General Control valve Gain. The difference between these values should be less than 50% of the higher value.0.5 (3.3) = 1.65and 3.3 - 1.65 = 1.65. Since 2.2 is closer than 50%, there should be problem with controlling the valve. Also note that the gain should never be less than 0.50. Turndown :- It is ratio of the calculated Cv at maximum condition to the calculated Cv minimum.Rangeability :- It is ratio of Cv of the valve fully open to the minimum Cv at which it can control.The rangebility of the selected valve must exceed the turndown requirements of the application.




Any Question ?
