Closed tank level measurement – DP transmitter

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Closed tank level measurement – DP transmitter

Level measurement is the hot topic on Visaya for May. However, we can connect this with the pressure topic of last month, right? With that in mind, let’s talk about level measurement in a closed tank with a differential pressure (DP) transmitter.

Now most technicians and engineers know exactly how to install, run, and maintain a DP transmitter in a closed tank for level measurement. However, questions constantly pop up on Linkedin’s groups and forums, mostly about how to use one and calculate its level range.

In fact, the basic equation is easy, but it gets more complex once you start layering in factors like installation, type of transmitter, and more.

Let’s dig in and see how many layers we can peel on this onion today.

Impulse lines

When you have a closed tank, the product may create gas, vapor, or you can have a pressured vessel. This situation requires compensation, using the low-pressure side of the DP transmitter. That brings us to impulse lines.

We call the two different impulse lines wet leg and dry leg. These terms crack me up in Portuguese, but I’ll hold off on the jokes for now. Depending on your product, you have to use one of them. For example, if the product may create condensate, then you’ll use the wet leg. Otherwise, you can go with the dry leg.

Courtesy of Emerson Experts

So what do these weird terms mean? First of all, a mechanical structure connects both sides of the DP transmitter to the vessel. Wet legs have impulse lines filled with liquid, and you can have different liquids in them. Dry legs have columns filled with vapor, gas, or whatever may come from the process without condensate.

You can certainly have issues with both installations, like condensate in dry legs and leaks or clogs in wet legs. You can avoid these issues with the solutions discussed in this article, but you just need to understand the concept for the math part here.

Capillary systems

As I said, impulse lines have disadvantages, so somebody created the capillary system to solve this problem. Frankly, I hated working with capillary transmitters, but I’ll admit they solve those impulse line problems such as evaporation, condensation, leaks, clogs, and so on.

In a capillary setup you mount a remote seal system and a sensing diaphragm in the process with oil-filled capillaries. When you have a force deflecting from the diaphragm of the remote seal, it sends the pressure through the oil. Then the transmitter captures that to show the process measurement.

When we talk about a capillary system, usually we mean a balanced system. What does that mean? Basically, you have the same remote seal and an equal length of capillary on each side of the transmitter. In theory, you can avoid problems with temperature shifts and other issues. Of course, in the real world – well, you know.

Fancy options

You can find still more solutions to address the above problems with impulse lines and capillary systems. For example, one solution has a new way to compensate the capillary, reducing issues with the balanced system.

You also have electronic remote sensors, which different companies call by different names. Rather than a mechanical structure or remote seal system, you have a sensor that communicates digitally. This option fixes all the problems we have with impulse lines and capillary systems! At least that’s what the sales reps say.

So how do I calculate the level range of the transmitter?

Ready? Because we have a hard way to do this and an easy way. Which do you prefer? Hard way, got it! Just kidding. We’ll start with the basics and build you up, okay?

The basics

First, let’s find the level range using an impulse line with a dry leg. Below we have an example where we have only the standard gravity of the process liquid, with the transmitter installed at the same level as the zero level measurement. With me so far? Because right after the graphic comes the math!

Dry leg and the SGf is filled with vapor -Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGp * H) – LP (SGf * H)

Maximum = level at 100% =  HP (SGp * H) – LP (SGf * H)

  • HP = high pressure
  • LP = low pressure
  • SGp = Specific gravity of process
  • SGf = Specific gravity of fluid
  • H = height

Saved by zero

This example has a wet leg and the transmitter installed below the zero point, which we call suppressed zero.

Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGs * h1)

Maximum = level at 100% =  HP [(SGs * h1) + (SGp * H)]

  • HP = high pressure
  • LP = low pressure
  • SGp =Specific gravity of process
  • SGs =Specific gravity of seal
  • H = height
  • h1 = height of suppressed zero
  • H2 = height of full tank

Raising the bar

In this third example, we elevated the minimum level in a wet leg system.

Courtesy of Visaya Solutions

Minimum = level at 0% = HP (SGp * h0) – LP(SGs * H2)

Maximum = level at 100% = HP (SGp * H+h0) -LP(SGs * H2)

  • HP = high pressure
  • LP = low pressure
  • SGp =Specific gravity of process
  • SGs =Specific gravity of seal
  • H = height
  • h1 = height of suppressed zero
  • h0 = height of minimum level
  • H2 = height of full tank

Creeping into capillaries

Did you get all that? Take your time. Now let’s check a capillary system. The math stays the same, but the data going into it changes.

Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGp * h0) – LP (SGs * h)

Maximum = level at 100% =  HP (SGp * H) – LP (SGs * h)

  • HP = high pressure
  • LP = low pressure
  • SGp =Specific gravity of process
  • SGs =Specificgravity of seal
  • H = max. level height
  • h0 = min. level height
  • h = height of full tank

The one with the electronic sensor

Last but not least, when you have an electronic sensor, you go back to something like a dry leg equation. The diaphragm connects directly to the tank.

Courtesy of Visaya Solutions

Minimum = level at 0% =  HP (SGp * H0)

Maximum = level at 100% =  HP (SGp * H)

  • HP = high pressure
  • LP = low pressure
  • SGp =Specific gravity of process
  • H = height
  • H0 = minimum level

Conclusion

You can use DP transmitters in closed tanks with various setups. We listed the most common, any of which you can use as a base to understand the concept and calculate even more complex processes. Good luck!

This article is part of our Book of Instrumentation. To learn more about Process Instrumentation check out the whole book here.

Related tags: closed tank DP transmitter level level measurement Pressure tank
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Closed tank level measurement - DP transmitter
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