PROCESS AUTOMATION

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MANAGEMENT OPERATION MAINTENANCEDESIGN

OIL/GAS

WATER/WASTEWATER

CHEMICAL FOOD/BEVERAGE

PHARMA/BIOTECH

Diaphragm pressure gaugesare suitable for gauge, ab-solute and differential pres-

sure. Their core, the diaphragmelement, is a circular, corrugateddiaphragm which is clamped orwelded between two flanges. Thiselement is generally manufacturedfrom resilient steels such as stain-less steel or Inconel. On pressureloading, the deflection of the pres-sure element, proportional to the

SPECIALISTSPictu

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ika[M]Anetzberger

S I L V I A W E B E R *About the use and the advantages of diaphragmpressure gauges — When compared with Bourdon tubegauges, these will always take second place—thoughwhere the “top seller” in mechanical pressuremeasurement technology is reaching its physical limits,diaphragm pressure gauges come into the frame. Theyare the specialists in the process industry when it comesto critical measuring tasks such as with highly corrosiveor viscous media or when it comes to low pressure andalso overpressure.

FROM THE SECOND TIER

Pressure entry

Lower flange

Diaphragm element

Push rod

Movement

Diaphragm pressuregauge model 432.50

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incident pressure, is transferred toa movement via a push rod.

The usable diaphragm travel isas short as approximately one mil-limetre, which offers a high repeat-ability. However, this characteristicis related to corresponding stand-ards for quality and tolerances ofthe diaphragm materials. In addi-tion to the material itself, the ma-terial strength, wave form and di-ameter determine the metrologicalcharacteristics of the diaphragm,and with this, the quality of themeasuring result for the appropri-ate measuring range. Diaphragmelements basically possess a cor-rugated profile. A smooth metaldiaphragmwould deform plastical-ly under loading and would there-fore make a precise pressure meas-urement impossible.In the past, developing a dia-

phragm element required numer-ous tests. These days, the designof the pressure element has beendetermined with the finite ele-ments method (FEM), saving timeand money. The graphical rep-resentation of this calculationshowswhere the pressure-relatedstresses are the highest. From thismodel, it can be derived how themetrological characteristics—i.e.diaphragm travel, linearity andhysteresis—and the service life ofthe diaphragm can be optimised.Set against the Bourdon tube

gauge, this “colleague” with a di-aphragm measuring element hasunique advantages.

Low pressure measurement

Diaphragm pressure gaugesfromWika measure pressures from16mbar, with the maximum valuebeing, on average, about 25bar.For measuring instruments withBourdon tubes, in contrast, thelowest measured value achieved isaround 600mbar. It is easy to seewhether the instrument is speci-fied for “bar” or “mbar” measuringranges: The larger the diaphragmdiameter, the lower the measura-ble pressure, in accordance withthe formula Pressure = Force / Ar-ea.

At pressures below 16mbarhowever, diaphragm elements arepushed to their limits: They wouldhave to be extremely thin toachieve the requisite elasticity, andwould thus no longer be reliablystable. This problem can be solved,though, with a special form of thediaphragm gauge: the capsulepressure gauge. Capsule elementsconsist of two diaphragmsweldedtogether, which are generally load-ed with pressure from the inside.This results in a double tube travel.This means that lower pressurescan be measured without a reduc-tion in the wall thickness. Capsulepressure gauges, however, havethe disadvantage of more limitedfields of application. Since theirpressure chamber is not self-drain-ing, they are not suitable for appli-cations with liquid media.

High overpressure safety

Measuring systems with dia-phragm elements, on the groundsof their design alone, offer goodprotection from overpressure,since the diaphragm can supportitself against the upper flange.Using a contour with flattened

waves, the protection factor is fur-ther increased as a result of thegreater contact area. Diaphragmpressure gauges from Wika, forexample, thus have a standardoverpressure safety of five timesthe full scale value—with Bourdontube pressure gauges, this is onlyaround 1.3 times as standard.The overpressure safety with

diaphragm element instrumentscan still, however, be increased upto one thousand times, up to400bar is possible. This holds true,even for measuring ranges up to16mbar. In this case, the upperflange has a specially turned, me-tallic diaphragm bed, into whichthe diaphragm element seats,should the full scale value be ex-ceeded.Through this full-surface con-

tact, any plastic deformation isexcluded and the long-term stabil-ity is increased. From the sameresistant-to-deformation approach,safety against underpressure canbe manufactured: with a corre-sponding diaphragm bed on thelower flange.

* The author is Product Manager Process Gaug-es at Wika; Contact: Phone +49-9372-132-0

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Use with critical media

Here, diaphragm pressure gaug-es offer the user considerable lee-way. In order, for example, to maketheir diaphragms resistant againstaggressive substances, a wholemultitude of special materials canbe exploited—from PTFE, via tan-talum, Hastelloy or titanium,through to gold. For Bourdontubes, only stainless steel andMo-nel are typically suitable.

Depending on the requirements,the diaphragm element has a foilfrom the special material glued toit, for gold-plating it can also begalvanised.Provided that these materials

have the required resilience prop-erties, the pressure element canalso be manufactured partially orcompletely from them. With par-ticularly aggressive media, thelower flange of the diaphragm canalso be lined, thus the area in con-tact with the product is fully pro-tected.

Diaphragm pressure gauges alsoprove their worth with measuringtasks where viscous, contaminatedor crystallising media are involved.These can clog up the narrow pres-sure ports of the threaded connec-tors or, with Bourdon tube instru-ments, also the internals of thetube.In order to avoid such an inci-

dent, diaphragm pressure gaugesare connected to the process withan open flange. This generates alarge pressure chamber, in whichno media of any sort can settle.

For operation in sanitary appli-cations, variants with flush dia-phragms are available. The pres-sure element is welded directly tothe aseptic process connection, inorder to ensure a dead-space freeconnection. Since diaphragm pres-sure gauges operate mechanically,the possibility of contaminationthrough a transmission fluid is ex-cluded.

Conclusion

Low measuring ranges, highoverpressure safety, special mate-rials for aggressive media, openflanges for viscous media: The ver-

satility of these gauge models inthe process industry, demonstrat-ed here, is strongly underlined.Diaphragm pressure gauges arealmost predestined for the highestdemands with the stability againstaggressive materials, such as onecomes across in the chemical andpetrochemical industries. Theyhave also been designed for harshenvironments in the oil and gasindustries: Wika diaphragm pres-sure gauges in a standard design(stainless steel or Inconel) are, forexample, qualified in accordancewith the requirements of NACE0175 standard, up to a temperatureof 120°C, and thus are suitable forsour gas applications. This capa-bility is confirmed through testsfrom an independent institute.Mechanical pressure gauges

such as diaphragm pressure gaug-es play an important role, also inthe digital age. Your advantage:Measurement and display reliablyand without the need for an exter-nal power supply. Also, with these,one can bridge to electronic pres-sure measurement: through de-signs with various switch contactsor analogue output signals. Elec-tronic data processing of the meas-ured value with an additional on-site display, based on the increas-ing safety requirements in plantdesign and construction, is further-more, an interesting perspective.

• Meet the experts fromWika at the Powtech2016 in Nurembergfrom April 19. – 21. ,Hall 4, Stand 531

• Learn more andwatchwebcasts from thecompany: www.youtube.com/wikagroup

PROCESS-Tip

The diaphragm element, the core component of a diaphragm pressure gauge and itsmultitude of special materials.

Pictu

res:Wika

Material stress ofa diaphragmelement as anFEM illustration

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