Pressure gauges are an essential indicator of whether a system or component is operating correctly. Gauges generally require little or no maintenance other than periodic calibration. However, if gauges require minimal maintenance, why do some seem to wear out continually?
It is usual for gauges to wear out over time. If gauges are failing prematurely, or the perception is they are being replaced too often, some common factors should be examined. Too often, indicator failures are the result of incorrect application or abuse. The following are some crucial factors to help ensure the instrument used is appropriate for the application.
First, before any instrument is removed from a system, it is vital to verify the system has been de-pressurized, the meter is at a safe temperature to handle, and is free of any toxic or hazardous process media. Pressure gauges must not be reused for applications other than the original process fluids due to the risk of potential cross-contamination between incompatible process fluids.
NIST traceable calibration is recommended against a test meter with at least four times the instrument’s accuracy under test. Ideally, the pressure range of the test instrument should be the same as the device tried. However, a digital test instrument with a range within an acceptable error rate may also be used. The user should determine the frequency of calibration according to the criticality of the application and consider the many factors that contribute to the life of an instrument.
Pressure Range Selection
One of the most common forms of incorrect application is improper pressure range selection. Pressure range selection is critical and should consider operating and maximum system pressure. The maximum pressure, in this case, is the absolute maximum pressure the gauge could be exposed to on the system in question.
The working pressure is the average pressure the pressure gauge will continuously expose. Therefore, operating pressure should fall within the middle half of the pressure gauge dial, as indicated in ASME B40.1 standards and the pressure range selected should be at least two times the operating pressure but not less than the maximum system pressure (i.e., working pressure of 160-psi and the maximum pressure of 300-psi, should use 0- to 300-psi pressure range).
Pressure more significant than the full-scale reading should never be applied to a gauge. Using pressure greater than full-scale reading can cause the instrument to become inaccurate, cause system leaks, and severely shorten the gauge’s life. If overpressure is suspected in a gauge, the maximum and working pressure should be confirmed, then the correct range selection. Suppose maximum system pressure is high enough to make it unfeasible for the working pressure to be easily readable on a dial. In that case, a pressure relief device may be installed to prevent exposure of the instrument to overpressure.
Temperatures below or over manufacturers’ recommendations can cause significant damage to a pressure gauge. Therefore, the system’s operating and maximum temperature ratings should be verified. In addition, manufacturers’ specifications for maximum ambient and maximum media temperature should be consulted to ensure the pressure gauge is not exposed to a temperature beyond maximum ratings. Temperature reduction can be achieved using devices such as a siphon, capillary, cooling tower, or other devices.
Wetted Parts Compatibility
Another common misapplication is wet room compatibility. For example, a corrosive process media may not harm thicker walled components of a system (such as piping and valves). However, the inside of a pressure gauge must be pressure sensitive. Therefore, the wall thickness of pressure gauge internals is thinner than many system components and more susceptible to eventual leakage due to exposure to even mildly corrosive process media.
A chemical compatibility chart should be used. First, refer to the wetted parts of the pressure gauge and process media; if the compatibility charts show a score lower than the maximum possible (usually “A”), select the wetted part with the most significant resistance to the process fluid. Typically, gauges are produced with only a choice of three wetted parts. These options are usually a copper alloy, 316 stainless steel, and Monel. If different wetted components (such as exotic metals or polymers) are required, this can be accomplished through a diaphragm or chemical seal. A diaphragm or chemical seal is a device that isolates the interior of the meter from the process fluid.
If a pressure gauge is installed in the steam service, regardless of its wetted parts, a siphon should permanently be installed before the gauge to prevent damage to the gauge internals. In addition, all indicators intended for use on oxygen service must be cleaned for oxygen service to appropriate standards and clearly and permanently labeled.
Remote mounting via capillary may be advised if the vibration is present in the application. If this is not possible or undesirable, a liquid-filled case can be used to dampen the needle’s vibration and lubricate the internals. Always verify gauge fill is compatible with process media at the point of accidental contact.
Pulses, Pressure Peaks, and Pressure Cycles
If regular pressure pulsation or cycling is present in the application, the correct pressure range selection should first be checked. Pressure spikes can be detected using a gauge with a maximum pressure pointer. This is a pressure gauge with an additional pointer pulled by the central pointer. When left on a system over some time will indicate the maximum pressure displayed during that time. Alternatively, a digital meter with data logging capability can be used.
Many types of dampening devices are available to help slow the effects of pulsation and cycling. While these devices cannot prevent pulsation or pressure spikes altogether, they can mitigate these conditions’ effects and significantly extend gauge life. Installation Damage as a result of the installation is common. Pressure gauges should permanently be installed using a wrench on the socket. Never installing a pressure gauge by hand or turning the case is essential.
In troubleshooting or installing a brand-new pressure gauge, the above factors are among the first to be examined. The majority of these factors are often overlooked. When troubleshooting and increasing pressure gauge longevity, these primary factors are an excellent place to start. Note, however, that this isn’t a comprehensive guide and is intended only for your reference. It is always a bright idea to consult ASME B40.1 standards before maintaining or installing a pressure gauge.