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Process Automation

Have you ever wondered what the best way to attach a transmitter to a process is?

I get this question all the time and it’s a fairly simple solution. While you can use remote seals, impulse lines are the most popular way to do this. An impulse line is tubing or pipe between the process and the transmitter.

Why are impulse lines so popular?

Impulse lines are popular for a number of reasons:

It’s important to note that there are some considerations when using impulse lines.

Considerations? What do I need to worry about when I use impulse lines?

Most processes are either liquids or gases, so one of the factors you need to consider is where you’re mounting your pressure transmitter.

For liquid service, you want to mount the transmitter below the tap point so bubbles don’t accumulate in the impulse lines. Remember: air in the lines will cause errors in the transmitter reading because there will be a significant pressure drop across the bubble depending on the size of the bubble due to the energy that will be lost in compressing the bubble.

For gaseous service, you want to mount the transmitter above the tap point so liquid doesn’t accumulate in the impulse lines. Remember: liquid in the lines will cause errors in the transmitter reading because the height of the liquid will add to the process pressure being measured.

What do you do if you have to take the transmitter back to the instrument shop for service or calibration?

In case of maintenance, emergency removal, etc. you should always have an isolation system that allows easy removal of the transmitter. Different systems are applied to different transmitters. For example, if you have a gauge transmitter, you would use a Block & Bleed valve because there’s only one connection to the process. If you have a differential pressure (DP) transmitter, you would use a 3- or 5-valve manifold because you have a high- and low-pressure connection to the process.

How do you know if a remote seal is needed to connect to the process?

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