1660238324073 Ian

Basic rules for pressure measurement

Sept. 22, 2020
Rules of thumb for this most ubiquitous of process measurements

Though I'm sure the percentage of field measurements based on pressure is decreasing from what I heard at one time was close to 80% of all signals, I'm confident the majority of control inputs continue to be pressure-based, including differential pressure (d/p) for flow and level inference. While many of us take pressure measurement for granted, we must keep in mind several basic rules of thumb, such as placing sensors at the top or side of a line, and using manifolds for isolation and maintenance.

The main reason for placing the sensing line in the correct location is to minimize the chances of something (corrosion, unwanted fluids, etc) in the process affecting the reading. One way in which I have had success in minimizing tap plugging is to use a diaphragm seal instead of a ½- or ¾-in. pipe nipple which, because of its narrow diameter, is more easily “bridged” and hence plugged.

When using a diaphragm, I've learned the hard way to remember the following lessons. First, specify the diaphragm face to be ½-in. to a maximum of 1-in. from the pipe face, especially in slurry or abrasive service. This will prevent the face of the sensing diaphragm from being scoured and damaged by the process fluids. Second, be sure to specify that the inside of the nozzle is ground flush before attempting to insert the diaphragm, which will have a tight tolerance with the pipe wall. You don't want to damage your meter, and then wait until the next outage to complete the project because you “smashed the face” on a piece of slag.

As pressure meter diaphragms are generally quite thin, be very careful when selecting the face materials. In some cases, it may be preferable to select a brittle ceramic rather than one of the metals.

Another source of d/p transmitter problems is the capillary—especially if it extends more than about 10 feet. At this length, response grows sluggish, and accuracy is susceptible to temperature changes if the fill fluid isn't carefully selected. Heat tracing helps, but makes this option quite expensive.

For these reasons and others, several manufacturers now offer “electronic” impulse lines, where two close-coupled (directly connected to the process with a manifold for calibration and maintenance when needed) transmitters are connected electronically rather than via capillary. The electronic-impulse-line approach requires a minimum distance (hence, pressure drop) between the taps to ensure an adequate signal-to-process-noise ratio. Again, depending on the process fluid this distance is coincidentally about 10 feet.

Wireless remote seals?

Electronic impulse lines have been in use for about a decade, and I believe it will only be a matter of time before someone replaces the electronic impulse line with a wireless option. To do so, however, will required continued improvement in wireless power supplies. 

Wireless could also be used to connect an at-grade transmitter with local indication to the sensing head in the pipe rack—again, with a potentially proprietary connection to keep down costs, arguably for security, and of course to keep out competitors.

Fortunately, if you have a reasonable relationship with the supplier of your transmitters, they'll consider many of the above items for you, or at least be asking the questions.

Lastly, though it may be intuitive when you look at the way pressure transmitters are made, practically all pressure transmitters are based on differential pressure sensing, with the one leg open to atmosphere and, hence, measuring gauge pressure.

Pressure measurement may be ubiquitous but that doesn't mean that we can take it for granted. In fact, the argument can be made that because it's so widely used, it should be better understood. Hopefully some of the points made this month will spur that conversation.

About the author: Ian Verhappen

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