Troubleshooting Pressure Gauges and Impulse Lines
Automation professionals are confronted every day with obstacles caused by outdated instrumentation technologies and practices.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installed, these solutions were likely state-of-the-art and improved existing installations, but many have now been superseded by even better solutions and may be creating problems ranging from operational annoyances to outright hazards.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.
Impulse lines and process connection issues
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.
Conventional impulse lines can create a variety of problems:
● 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.
Impulse lines are typically custom efforts and often built in the plant’s maintenance shop, reflecting the skill of local contractors or pipe fitters. A better choice is to use a preassembled instrument, such as the kind available with a DP flow meter like Emerson’s Rosemount™ 3051SFP Integral Orifice Flow Meter (Figure 2), a complete unit built in a factory and fully tested. All fasteners are tightened to the optimal torque level, and the finished assembly can be leak tested. These meters are ready to install right out of the box and even include a calibration report.
Avoiding impulse line problems
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.
Today’s advanced transmitters are able to perform a plugged impulse line diagnostic (figure 3) and detect such situations because they listen to the process noise through the connection. If the noise level decreases or changes character and there is no attributable cause, there is likely an obstruction forming in the lines. Once the change crosses a designated threshold, the transmitter can warn operators and maintenance engineers.
Process intelligence capabilities can also be built into pressure transmitters, allowing them to listen to process noise continuously (Figure 4). Once a baseline of normal noise is retained in the transmitter’s memory, it can perform statistical analysis on what it hears, listening for patterns deviating from normal.
Reasons for such changes can include:
● pump cavitation
● distillation column flooding
● regulator and valve setting changes
● furnace flame instability.
Characterizing and analyzing such noise provides a tool to help operators or engineers identify a likely source. Operators and maintenance engineers can be informed early, so they can correct the situation immediately if necessary or monitor it until a scheduled shutdown.
Process alerts can also indicate upsets and other conditions capable of creating spikes or dips in normal readings. Such alerts can be logged in individual transmitters and accessed during troubleshooting. A status log can look back at the last 10 events, with time stamps to capture extreme readings for later analysis.
Mechanical pressure gauge challenges
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.
So, what is the use case for gauges? They are installed where a reading may be useful for occasional checking, troubleshooting, or maintenance. Any critical output likely already has an instrument installed and connected to the host system. Gauges also serve a critical safety function by verifying the local process pressure when servicing equipment. A gauge must be read by an operator, and given the few manual rounds performed these days, it may not be checked regularly. Functionally, it has to provide a visual, local indication of the pressure. If it could also send the reading to a central location, such as the host system or maintenance shop, it could probably be useful there also.
A modern alternative
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.
Eliminating the Bourdon tube removes a critical failure point. An electronic gauge has multiple barriers of process isolation versus a single process isolation with a Bourdon tube. The overpressure tolerance of this electronic solution is also much higher. The added layers of isolation and overpressure capabilities mean there is far less potential for process fluid escape with an electronic gauge.
Using sophisticated electronics, these new gauges are also able to monitor their own status. There is no way to verify a mechanical gauge is working properly short of removing it from the process and testing, but a glance at an electronic gauge can show its operational status via an LED indicator.