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Three Process Conditions That Affect Accuracy & Performance of Pressure Gauges

 

By: NOSHOK, Inc.

The technology used in today’s pressure gauges has been around since the mid-eighteen hundreds, and the pressure gauge is still one of the most common methods of measuring pressure today.  The majority of pressure gauges today still incorporate the bourdon tube, socket, and geared movement; along with a pointer and dial to indicate process pressure.

Since the pressure gauge is a purely mechanical device, attention to three process conditions is necessary.  The three factors that can adversely affect accuracy and performance are Temperature, Vibration and Pulsation.

Temperature Effects:

For every 100-degree shift in temperature from which the gauge is calibrated, the user can experience up to a 2% additional error in reading.  The cause is the change in the elasticity or spring rate of the bourdon tube element with temperature.  While it is difficult to circumvent the effects of ambient temperature, we can address the effects of process temperature.  In steam service, the common practice is to install coil syphons or pigtail syphons to dissipate process heat.  Another common practice is to install a diaphragm seal with capillary to separate the gauge from the high heat source. There are many options available with fill fluid in the seal and capillary system to withstand temperatures up to 600 degrees F.  In severe cold ambient conditions, many users elect to heat trace their instrumentation via electric trace or steam trace.  Process and ambient temperature is an important consideration when selecting and applying pressure gauges.

Vibration Effects:

Vibration due to pumps, motors, and other rotating equipment can cause excess wear and possible premature failure of internal working parts of a pressure gauge, which include the bourdon tube and the movement or gear mechanism.  Vibration also causes difficulty in accurate reading of the gauge, due to pointer oscillation.  One of the most common causes of pressure gauge failure is exposure to continuous vibration.  The most widely accepted remedy is to apply and install a liquid filled pressure gauge.  The fill fluid of choice is either glycerin or silicone.  Liquid filled case gauges address not only pointer oscillation, but also serve to protect and lubricate the internal geared movement. 

Pulsation Effects:

Process pulsation can occur around the discharge of pumps as well as quick operating valves.  Many users assume that liquid filling a pressure gauge will fully address pulsation.  Although a liquid filled case gauge helps to dampen the effects of pulsation, it often does not fully address this process condition.  Pulsation dampeners are installed upstream of the gauge socket and they can be a piston-type snubber, a sintered metal snubber, or a threaded in-flow restrictor in the socket of the gauge.  A needle valve installed upstream of the gauge that is “pinched down” or slightly opened, is another common practice to address pulsation.  It is not recommended to rely solely on a needle valve to address pulsation, due to the fact that the user could inadvertently open the valve, and thereby negate flow restriction.  In clean fluids (gases or clean low viscosity liquids) a threaded orifice/flow restrictor or a sintered metal snubber is the least costly way to address pulsation.  In dirtier and higher viscosity fluids a piston snubber is usually installed.

Summary:

Temperature, Vibration and Pulsation are three process conditions that adversely affect a pressure gauge.  Being aware of these three process conditions, and taking the necessary steps to address them, can help minimize accuracy errors and add to the service life of the pressure gauge. 

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