Troubleshooting Pressure Gauges and Impulse Lines

Remove operational obstacles from legacy pressure instruments. This article comes from the March 2021 Ebook InTech Focus: Temperature and Pressure.

Automation professionals are confronted every day with obstacles caused by outdated instrumentation technologies and practices. When first installed, these solutions were likely state-of-the-art and improved existing installations, but many have now been superseded by even better solutions and may be creating problems ranging from operational annoyances to outright hazards. The causes and effects of the problems are different, but all can be mitigated or eliminated entirely by using advanced instrumentation, with each instrument consisting of a sensor in contact with the process, connected to an electronic transmitter.When it comes to measuring pressure, problems can crop up with mechanical pressure gauges, electronic pressure transmitters, and the connections that carry the pressure to the instruments.

Taking a differential pressure (DP), gauge, or absolute pressure reading from a process involves creating process connections so the pressure can reach the sensor. (We will discuss mechanical gauges later. Here we will concentrate on electronic pressure transmitters.) Frequently this is done via impulse lines that carry the pressure to the transmitter (Figure 1). In some cases, these can be short and very direct, or they may need to be long so the transmitter can be mounted some distance from the process equipment.

● They are part of the process containment.● If they leak, product is lost, with potential safety, economic, and environmental implications.● If process equipment calls for exotic materials, the impulse lines need it too.● They can fill with gas or liquid that compromise their ability to transmit pressure accurately.● They can freeze in cold weather.

Whatever the situation, impulse lines must not impede pressure delivery, so the transmitter can read the sensor value indicating the actual process condition. As an extreme example, if there is an isolation valve on the impulse line and the valve is closed, nothing can reach the transmitter, and its reading will not reflect the process conditions. Such a situation is not always easy to detect because some pressurized fluid may be trapped in the line and reflected by the transmitter. Similarly, inaccurate readings can result when the line is partially plugged, frozen, or there is some other internal obstruction.

Long before there were pressure transmitters, there were mechanical pressure gauges. The concept of a curved Bourdon tube dates back to the mid-19th century, and there are devices available today little removed from that time. Gauges operate using a delicate mechanism with springs and gears, making them vulnerable to shock and damage (Figure 5). Most operators have seen typical failures, including broken glass, bent indicator needles, or needles pointing straight down from broken gearing. In many environments, pressure transmitters are considered disposable due to their low cost and frequent failures.

Electronic gauges, including Emerson’s Rosemount Wireless Pressure Gauge and Smart Pressure Gauge (figure 6), combine the benefits of an electronic transmitter with the usefulness of a traditional mechanical design. These gauges use a solidstate sensor rather than a Bourdon tube, and process the signal electronically rather than mechanically. The needle is driven by a tiny motor, so there is only one moving part, making the mechanism far more resistant to shocks, vibration, and other extreme operating conditions.

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