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| Industrial pressure gauges (NoShok) |
Because the pressure gage is a purely mechanical device, it is essential to take care of three process circumstances. Temperature, vibration, and pulsation are the three factors that can adversely affect accuracy and performance.
Effects from Temperature
For each 100-degree temperature change from which the gage is calibrated, the user may encounter an extra reading error of up to 2 percent. The cause is the temperature shift in the bourdon tube element's elasticity or spring rate. Although the effects of ambient temperature are difficult to circumvent, we can address the effects of process temperature. In steam service, to dissipate process heat, the common practice is to install coil syphons or pigtail syphons. Another popular practice is to install a capillary diaphragm seal to separate the gage from the source of elevated heat. In the seal and capillary scheme, there are many choices available with fill liquid to withstand temperatures up to 600 degrees F. Many users choose to heat their instrumentation via electric trace or steam trace under severe cold ambient conditions. When choosing and implementing pressure gages, process and ambient temperature is a significant consideration.
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| Pressure snubbers. |
Effects from Vibration
Vibration due to pumps, motors and other rotating equipment may result in excess wear and potential premature failure of the internal working parts of a pressure gage, including the bourdon tube and the movement or gear mechanism. Due to pointer oscillation, vibration also causes difficulties in accurate reading of the gage. Exposure to constant vibration is one of the most prevalent causes of pressure gage failure. Applying and installing a liquid filled pressure gage is the most commonly accepted solution. Either glycerin or silicone is the filling fluid of choice. Liquid-filled case gages not only address pointer oscillation, but also safeguard and lubricate inner geared motion.
Effects from Pulsation
Pulsation of the process can occur around the pump discharge as well as rapid operating
valves. Many consumers believe that pulsation will be completely addressed by fluid filling a pressure gage. While a liquid-filled case gauge helps to dampen the pulsation effects, this process condition is often not fully addressed. Upstream of the gauge socket, pulsation dampers are mounted and can be a piston-like snubber, a sintered metal snubber, or a threaded in-flow restrictor in the gauge socket. Another common practice for addressing pulsation is a needle valve installed upstream of the gage that is "pinched down" or slightly opened. Because the user could inadvertently open the valve and thereby negate the flow restriction, it is not recommended to rely solely on a needle valve to address pulsation. A threaded orifice / flow restrictor or a sintered metal snubber is the least expensive way to deal with pulsation in clean fluids (gases or clean low viscosity liquids). A piston snubber is generally mounted in dirtier and greater viscosity liquids.
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| Diaphragm seals. |
Conclusion
Three process conditions are temperature, vibration and pulsation that adversely affect a pressure gauge. Being aware of and taking the necessary steps to address these three process conditions can help minimize accuracy errors and add to the pressure gauge's service life.
For more information about the proper application of pressure instrumentation, contact Advance Instruments. by visiting https://advanceinstruments.com or by calling (888) 388-6446.
