Pumps & Systems, January 2009
Mechanical pressure gauges, which require no external power, provide an affordable and reliable source of accurate pressure measurement. Using the criteria below can help maximize gauge life.
Step 1: Gauge Process
The wetted parts of the pressure gauge, the Bourdon tube and socket, must be compatible with the process media. If the media are not compatible with the wetted parts of the gauge, corrosion will occur. Corrosion of gauge wetted parts will eventually cause gauge failure and possibly safety issues.
Bourdon tubes are most often manufactured from phosphor bronze, 316 stainless steel or Monel. 304 stainless steel, Inconel or steel are used less frequently. If the process media is not compatible with the wetted parts of the gauge, a diaphragm seal/gauge isolator can be used to prevent the process media from contacting the Bourdon tube. The diaphragm seal, when supplied with wetted parts compatible with the process, not only protects the gauge from corrosion attack, but also serves to prevent viscous or extremely dirty media from clogging the Bourdon tube. When attached to a gauge, a diaphragm seal will add an additional minimum error of 0.5 percent to the gauge/seal assembly.
Process temperatures should also be considered as they will affect the accuracy of the reading, and could also cause damage to the gauge. Extreme process temperatures can be reduced before the media contacts the gauge by inserting a (pigtail) siphon inline below the gauge. The siphon will act as a heat sink to reduce the media temperature before it enters the gauge. This is especially prevalent in steam pressure applications.
Both pulsation and vibration will decrease gauge life if not properly dampened. Gauges subject to pulsation and vibration should be liquid filled or internally dampened.
Available accessories that minimize stresses on the Bourdon tube/movement and extend the life of the gauge include pulsation dampeners, pressure snubbers, gauge savers, diaphragm seals and (pigtail) siphons.
Step 2: Pressure Gauge Range
ASME B40.100-1998 recommends that normal operating pressure be confined to 25 to 75 percent of the scale. If pulsation is present in the process, maximum operating gauge pressure should not exceed 50 percent of the full-scale range.
Step 3: The Environment
Environmental considerations include ambient temperature, air-borne particulate, condensation, humidity, water and chemicals, all of which can affect gauge performance.
Ambient temperature may affect the accuracy and integrity of the gauge. Gauges are available either temperature compensated or non-temperature compensated. Ambient conditions may require that the gauge be isolated from temperature extremes. The gauge should be operated in an isolated environment from temperature extremes by extending the length of the connection to the process with flexible capillary.
When ambient conditions are corrosive, contain a large number of particulate or are wet or humid environments including wash-downs or rain, specify a gauge that is weatherproof/hermetically sealed or liquid filled.
Step 4: Accuracy
For a mechanical pressure gauge, accuracy is defined as a percentage of the full-scale range. While requirements differ from one industry to another, the following are general guidelines:
- Test Gauges and Standards: 0.25 through 0.10 percent full scale
- Critical Processes: 0.5 percent full scale
- General Industrial Processes: 1.0 percent
- Less Critical Commercial Uses: 2.0 percent
Refer to ASME B40.100-1998 or the DIN specifications for more information on accuracy.
Step 5: Dial Size
Pressure gauge dial sizes range from 1.5- to 16-in diameters. Generally, readability requirements, space limitations and required gauge accuracy determine dial size. Accuracies of 0.25 percent or 0.10 percent generally have dial sizes of 4.5-in or larger since more dial graduations are required.
Step 6: Gauge Connection
Gauges are available with a variety of connections including NPT, DIN, JIS, BSP and SAE. Process pressure gauges with 4.5-in dial sizes or larger are most often supplied with a 0.5-in NPT connection to best support the gauge.
Factors to consider when selecting a pressure gauge connection include process pressures, gauge size and weight, space limitations, leak integrity and past experience.
Step 7: Mounting Requirements
Consider the following mounting options when selecting a pressure gauge:
- Direct stem mount lower connect
- Remote wall/surface mount lower connect
- Panel surface mount back connect
- Panel hole U clamp flush mount back connect
- Panel hole front flange flush mount back connect
Conclusion
To properly select a pressure gauge, consider the gauge process, range, environment, accuracy, dial size, connection and mounting requirements. When selected using these seven steps, the gauge performance and reliability will be greatly enhanced.