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Process Gauge Isolation - Onyx Valve

1 Process Gauge IsolationDiaphragm Seals and Isolator RingsBy David Gardellin, President Onyx Valve CoPressure measurement on a compressed air or cooling water line is fairly straightforeword: simply tap a Gauge at some point in the line. Fluids such as air and water haveno adverse effects on the internal components of the Gauge . Other fluids, however, maynot be so benign. Many fluids subject to pressure measurement are corrosive, or theycontain solids, or they crystallize or fluids include strong acids and bases, adhesives, slurries, sewerage,sludge, pigments, food products such as cheese or ground meat, molten plastic and mineore these fluids, we have to isolate the Gauge from the Process fluid with a barrier thattransmits pressure while isolating the Gauge from direct contact with the fluid beingmeasured. The typical response is to apply a " chemical seal " like the one illustrated infigure 1 Typical chemical seal with GaugeThe pressure Gauge is separated from the processfluid by a flexible diaphragm.

4 There is more surface area in an Isolator Ring than there is in a traditional chemical seal. Some pressure instruments have relatively large control volumes.

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Transcription of Process Gauge Isolation - Onyx Valve

1 1 Process Gauge IsolationDiaphragm Seals and Isolator RingsBy David Gardellin, President Onyx Valve CoPressure measurement on a compressed air or cooling water line is fairly straightforeword: simply tap a Gauge at some point in the line. Fluids such as air and water haveno adverse effects on the internal components of the Gauge . Other fluids, however, maynot be so benign. Many fluids subject to pressure measurement are corrosive, or theycontain solids, or they crystallize or fluids include strong acids and bases, adhesives, slurries, sewerage,sludge, pigments, food products such as cheese or ground meat, molten plastic and mineore these fluids, we have to isolate the Gauge from the Process fluid with a barrier thattransmits pressure while isolating the Gauge from direct contact with the fluid beingmeasured. The typical response is to apply a " chemical seal " like the one illustrated infigure 1 Typical chemical seal with GaugeThe pressure Gauge is separated from the processfluid by a flexible diaphragm.

2 The interior spacebetween the diaphragm and pressure element isfilled with an intermediate fluid that transmits thepressure applied to the diaphragm to the is critical that all the air is evacuated from thesystem prior to injecting the fill fluid. Any airbubbles entrained in the system will collapseunder pressure , forcing the elastic membrane toover stretch. Air pockets introduce unacceptableerrors in the Gauge reading, or will cause thegauge to stop functioning airbubble as small as a grain of rice can cause thegauge to malfunction, so it is imperative that avacuum pump be used to evacuate the systemprior to filing. Once the system is evacuated andfiled, if it is disassembled for any reason theevacuation and filling operation must be repeatedfor it to function diaphragm and flange connection areexposed to the Process fluid.

3 The materials ofdiaphragm and flange must be suitable towithstand the temperature and chemistry of theprocess , capillary tubing provides the connection between the diaphragm seal and thepressure-measuring instrument to bridge the distance or protect the Gauge from hotprocess tubing influences the response time of the are two ways to isolate the Process fluid from a pressure -measuring instrument: A conventional diaphragm seal . An Isolator use of a chemical seal or isolator ring is not restricted to a pressure Gauge as shownin the illustration, but works with transmitters and pressure switches as either type of isolating device into a pressure measuring system protects theinstrument from the Process fluid but introduces errors in the factors determine the increased magnitude of the error in the Gauge Volume of the Gauge : To drive the gaugemechanism and rotate the pointer around the dial, you have to inject a small butfinite amount of fluid into the Gauge Bourdon tube.

4 The volume of fluid requiredto drive the pointer from zero to full scale is called the required displacementvolume or control Volume of the diaphragm seal or isolator ring: Theamount of fluid which can be squeezed out of the diaphragm seal is the MaximumAvailable Displacement area of the isolating flexibility of the isolating order to function properly, the maximum available control volume in the seal must begreater than the required control volume of the Gauge . Once the maximum availablecontrol volume has been extracted from the seal , the diaphragm hits the backstop and thegauge stops in its tracks. Isolator rings, even in small sizes, typically have more than 10times the available control volume compared to any of the standard size diaphragm SealsNominal sizeAvailableControl VolumeIsolator RingsAvailableControl in3 in3 in31 2 in33 in34 in33 Surface Area of the Isolating Device.

5 The surface area and the modulus of elasticity (stiffness) of the flexible element affect theaccuracy of the SealsNominal sizeAreaIsolator in2 in2 in21 2 in23 in24 in2 The elastic membrane in an Isolator Ring has a lower Modulus of Elasticity (it's moreflexible) than the material in the traditional chemical seal , which is usually stainless RingLdAIso 2rAdiaph , psiReq'd Control Vol, in3 Error vs diaphragm1/2" Iso Ring3" Iso Ring4 There is more surface area in an Isolator Ringthan there is in a traditional chemical pressure instruments have relativelylarge control volumes. For example, a turretcase Gauge designed to measure low pressure inthe range of 5 psi or less, uses a convolutedbellows as the pressure measuring element.(See figure-4 at left.)This type of instrument has a control volumemore than ten times the volume of a typicalbourdon tube.

6 This type of Gauge can not beused with a diaphragm seal . However, isolatorrings 6 and larger have enough displacementvolume to operate this kind of instrument andstill provide accurate Bellows type measuring instrument5 Elevation effects of Caused by Capillary Tubing:As you change the elevation of the Gauge with respect to the isolator ring, you introducean elevation error. This error is due to the static pressure of the liquid in the change in Gauge reading caused by elevationchanges of the pressure sensing instrument can becalculated in advance (and compensated for by re-calibrating the instrument) using the followingequation: The Onyx standard fill fluid is silicone oil with aspecific gravity = at 77 polarity: If the Gauge is below the isolatorring, then the elevation term in the above equationis positive; if the Gauge is above the isolator ring,the elevation term is the Gauge or transmitter has a zero adjustcapability, the elevation error can be eliminatedcompletely by re-setting the zero adjust tocompensate for the elevation Time with Capillary TubingCapillary tubing introduces a response time lag in the instrument reading.

7 This delayedreaction time is influenced by: Length of the capillary tube Internal diameter of the capillary tube Control volume of the pressure -sensing instrument Viscosity of the fill fluid, including temperature effects fill fluid viscosityResponse time is defined as the time,in seconds, for the pressure -sensinginstrument to register of astep change in reason the definition is not basedon a 100% change is because,theoretically, it takes an infiniteamount of time for any pressure -sensing instrument to respond to astep change in pressure . That sPressureTime, ofpres sure changeTime in s ecStep cha ngeindicate dpres sure6because the Gauge pointer moves slower and slower as it gets closer to the control volume of a pressure -sensing instrument such as a Gauge or transmitter isdefined as the change in volume required to deflect the bourdon tube or sensingdiaphragm from zero to 100% smaller the control volume, the better the performance.

8 Instruments with smallercontrol volumes exhibit less temperature error and time lag than instruments with greatervolume. As a general rule, higher range instruments have a smaller control volume thanlower range instruments; for example a 100-psi Gauge has a much smaller control volumethan a 15-psi general rule is that electronically amplified devices have a smaller controlvolume than mechanical devices; again, to use an example, an electronic transmitter hasabout 1/100ththe control volume of a bourdon tube Gauge of the same isolator rings are supplied with capillary tubes with ID = inches, and the fillfluid is Silicone with a viscosity = 100 centistokes. Typical time lag with 5 feet ofcapillary and a 4 turret case Gauge with a 60 psi range is about 2 from the geometrical configuration of the measuring system, the temperature ratingof the viscosity of the fill liquid also influences the response time.

9 A fill liquid of lowviscosity reduces the response Effects on Accuracy with Capillary Tubing:The complete isolating assembly, consisting of a chemical seal (with or without capillary)and pressure measuring instrument is filled with a certain amount of filling liquid at aspecified temperature (in general 20 2 C), referred to as the filling temperature. Temperature changes cause the liquid inside the capillary tube to expand and contract,changing the volume of the fill fluid. The resulting error is a function of the total volumeof the tubing, pressure instrument, and isolator the rubber sleeve in theisolator ring has a much lower modulus of elasticity compared to a diaphragm seal , it canabsorb most of the volumetric change resulting from temperature differences throughoutthe usable temperature range for isolator rings. A typical error in Gauge reading througha temperature swing from 0 F to 120 F is about psi depending on isolator ring size andgauge type.

10 This is roughly a quarter the error expected with a standard 60 mm stainlesssteel diaphragm sealUsers who want a more accurate prediction of performance for a measuring systemconsisting of an isolator ring, capillary tube and pressure sensing element can bring thefull capability of mathematics to bear on the problem. For calculating the volume insidethe capillary tube: Onyx isolator rings are supplied with capillary tubes with ID = SgageambcapilambringisoprocessREVolTVolT VolTErr Where:Err = Error in reading expressed in inches H2O water =Expansion factor. If you use degrees F and inches for all the terms in the aboveequation, then E for Silicone fluid = =Spring rate. For isolator rings, Rsis related to nominal size such that:insizeNomRS,100 The error as a percent of reading is therefore:1002% OHinchesinSpanMeasuredErrErrorVolume of Onyx Isolator Rings:sizeVol, CCVol, in3sizeVol, CCVol.


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