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Summary
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. InTech Focus is a bimonthly ebook published by ISA. This March 2021 edition focuses on Temperature and Pressure.
Remove operational obstacles from legacy pressure instruments.
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
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.
Avoiding impulse line problems
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.
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
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
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.
For some applications, the most critical drawback of a traditional gauge is its inability to send information to an automation system. This issue is addressed by the Rosemount Wireless Pressure Gauge because it includes the WirelessHART® communication protocol, which can send the pressure reading and status indications to the host system. This is an optional function and can be used whenever necessary. This communication capability can be deployed in a sophisticated networking environment as the Industrial Internet of Things moves into more manufacturing applications. Even if the wireless capability is not needed today, it may be soon, and the Rosemount Wireless Pressure Gauge future proofs any installation.
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