Process engineers always want more instruments. However, project managers invariably balk at the cost, especially for devices not needed when an operation is running right. The usual compromise leads to a mix of instruments (some tied into the control system and others purely for local indication) as well as connection points for future instruments.
When a process has problems, the local instruments and connection points may play a critical role in finding the culprit. Local pressure readings often are essential.
These pressure readings usually rely on either a mechanical or electronic gauge. The most common conventional gauges use a Bourdon tube to measure the pressure. This coiled tube uncoils when the pressure differential between the inside and the outside of the tube increases. A mechanical linkage converts the Bourdon tube movement into a dial reading.
ASME Standard B40.100, “Pressure Gauges and Gauge Attachments,” covers mechanical, analog, dial-type gauges that use elastic elements (including Bourdon tubes). The standard specifies several accuracy grades. Grades 1A (±1% of scale) and 2A (±0.5% of scale) suit many uses. Grades 3A (±0.25% of scale) and 4A (±0.1% of scale) are expensive enough that plants rarely have them, and usually require special effort to get. Other, less-accurate grades are available. Whenever you use a pressure gauge, check its grade and look up the accuracy rating.
The accuracy ratings assume good mechanical handling, specific operating conditions, and no vibration or other mechanical damage. I’ve found that even gauges in “good” nominal condition in the field frequently have two-to-three times the error of the manufacturer’s stated values. This reflects age, handling practices (dropping, etc.) and how the plant’s instrument shop reconditions the device. Gauges in semi-permanent installations for long periods of time often have larger errors.
When using a pressure gauge attached to a rarely turned valve, the key question is what happens if you can’t shut the isolation valve again? Recommended practice is to put a new valve and bleeder between the pressure gauge and the process. This way, if the current valve doesn’t seal completely, you can resort to the new valve. Then, you can remove the old gauge and install a plug if needed.
Other key questions in valve selection include materials compatibility, process pressure and temperature, ambient conditions, service specific requirements, size, mechanical conditions of the service, and mounting.
To check materials compatibility, review the gauge materials, the piping materials and the process fluid involved. Materials must be safe to use and not contaminate the process. Additionally, if the gauge is to remain in place for an extended period, the materials shouldn’t create a corrosion weak point or promote corrosion elsewhere.
You must select a gauge with the right pressure range for useful readings. It also must offer safe mechanical characteristics and adequately tolerate the maximum possible pressure and expected temperature ranges for the process. In addition, it must suit ambient conditions, including the possibility of dust and harsh weather as well as locally corrosive atmospheres.
Specific services may impose special requirements. These may stem from industry standards, regulatory requirements, corporate guidelines or prudent judgment. Common services requiring specific gauge choices include acetylene, ammonia, oxygen, severe chemical applications and lethal services.
Larger gauges are easier to read but usually are more expensive and harder to find a location for mounting. At a minimum, opting for a gauge with a nominal 4-in. (100-m) face is good practice for taking accurate readings.
Vibration poses the most significant mechanical issue for a gauge. It can make the gauge difficult to use and also lead to mechanical fatigue. When the needle is oscillating, how do you take a pressure reading? Mechanical vibration of the pipe and equipment or pressure fluctuations in the process can cause such shuddering. The most common tactic to dampen vibration is to use either a liquid-filled gauge or a pulsation dampener; however, both tend to reduce gauge accuracy.
Liquid-filled gauges have larger possible errors due to temperature effects but improve gauge life, dampen needle vibration and decrease corrosion problems inside the gauge. Pulsation dampeners tend to reduce gauge sensitivity and plug more easily than conventional gauges.
Consider these factors when selecting a pressure gauge. However, often a troubleshooter must use whatever instrument the plant has available. In that case, understand the gauge, safety requirements and the likely accuracy or error in a reading.