Pumps and Systems, March 2009
Pressure gauges are an important indicator of whether a system or component is operating correctly. Pressure gauges generally require little to no maintenance, other than periodic calibration. However, if gauges require minimal maintenance, why do some seem to continually wear out?
It is normal for gauges to wear out over time. If gauges are failing prematurely, or the perception is they are being replaced too often, there are some common factors that should be examined. Too often, gauge failures are the result of misapplication or abuse. The following are some important factors to consider to help ensure the instrument being used is appropriate for the application.
First, before any instrument is removed from a system, it is important to verify the system has been de-pressurized, the gauge is a safe temperature to handle, and it is free of any toxic or hazardous process media. Pressure gauges should not be re-used for applications other than the original process media, due to the risk of potential cross contamination between incompatible process media.
NIST traceable calibration against a test gauge with accuracy at least four times greater than the instrument being tested is recommended. Ideally, the pressure range of the test instrument should be the same as the instrument tested. A digital test instrument with a range that falls within acceptable error rate may also be used. The frequency of calibration should be determined by the user according to criticality of the application and account for the many factors that contribute to the life of an instrument. These factors include, but are not limited to, vibration, pulsation, pressure spikes, pressure cycles, temperature and other environmental conditions.
Pressure Range Selection
One of the most common forms of misapplication is the improper selection of pressure range. Pressure range selection is critical and should take into account both system operating pressure and maximum system pressure. Maximum pressure, in this case, is the absolute highest pressure to which the gauge could be exposed on the system in question.
Operating pressure is the average pressure the gauge will be exposed to continually. Operating pressure should fall within the middle half of the pressure gauge dial, as indicated in ASME B40.1 standards (see Figure 1), and the pressure range selected should be at least two times the operating pressure, but not less than maximum system pressure (i.e., operating pressure of 160-psi and maximum pressure of 300-psi, should use 0- to 300-psi pressure range).
Pressure greater than the full scale reading should never be applied to a gauge. Applying pressure greater than full scale reading can cause the instrument to become inaccurate, cause system leaks and will severely shorten the gauge's life. If a gauge is suspected of overpressure, maximum and operating pressures should be confirmed and correct range selection should ensue. If maximum system pressure is high enough to make it unfeasible for the operating pressure to be easily readable on a dial, a pressure relief device may be installed to prevent exposure of the instrument to overpressure.
Temperatures below or in excess of manufacturers' recommendations can cause significant damage to a pressure gauge. Operating and maximum temperature ratings of the system should be verified. Manufacturers' specifications for both maximum ambient temperature and maximum media temperature should be consulted to ensure the pressure gauge is not exposed to temperature beyond maximum ratings. Temperature reduction can be achieved by using devices such as a siphon, capillary, cooling tower or other device.
Wetted Parts Compatibility
Wetted parts compatibility is another common misapplication. A corrosive process media may not have a negative effect on thicker walled components of a system (such as piping and valves). However, a pressure gauge's internals must be responsive to pressure. The wall thickness of pressure gauge internals is thinner than many system components and more susceptible to eventual leakage as a result of exposure to even mildly corrosive process media.
To verify no corrosion takes place, a chemical compatibility chart should be used. Refer to the wetted parts of the pressure gauge and process media in question. If compatibility charts indicate less than the highest rating possible (usually "A"), another wetted part with the highest resistance to the process media should be selected. Typically, pressure gauges are only manufactured with a choice of three wetted parts. These options are generally Copper Alloy, 316 Stainless Steel and Monel. If different wetted parts (such as exotic metals or polymers) are required, this can be accomplished through the use of a diaphragm or chemical seal. A diaphragm or chemical seal is a device that isolates the gauge internals from the process media.
If a gauge is installed in steam service, regardless of its wetted parts, a steam siphon should always be installed before the gauge to prevent damage to the gauge internals. In addition, any gauges intended for use on oxygen service should be cleaned for oxygen service according to appropriate standards and clearly and permanently labeled.
If vibration is present in the application, remote mounting via capillary may be advised. If this is not possible or undesirable, a liquid-filled case can be used to dampen vibration of needle and lubricate internals. Always verify gauge fill is compatible with process media in case of accidental contact.