Flow Measurement Pressure-Temperature Compensation

by , | May 25, 2016 | Measurement Instrumentation

Peer-to-peer knowledge sharing provides valuable information beyond what is possible to find in owners’ manuals and other technical documentation from suppliers. You can find it every day in your Google searches about personal technologies that you use and technologies you use on your job.

One of the great places for peer-to-peer knowledge sharing for DeltaV distributed control system users is the DeltaV group in the Emerson Exchange 365 community. I flagged this response by Emerson’s Lou Heavner as a great example of this knowledge sharing in action.

A community member asked a question about how pressure-temperature compensation worked in the DeltaV system.

Lou responded (and I inserted some hyperlinks for additional sources of information):

PT compensation converts a volumetric gas flow into a mass flow. There will be a constant that puts it into the correct units. The underlying assumption is that the gas composition doesn’t change. If you are familiar with the ideal gas law, you know that the moles of gas vary with pressure and temperature for a fixed volume.So if a flow meter is measuring volumes of gas and we want to know mass, we need to correct for changes in temperature and pressure. That means the conversion factor is based on a reference pressure and temperature, typically the reference values used during meter calibration. The calculations are based on absolute pressure and temperature, so if you do the calculations by hand, you would need to correct both from gauge/measured to absolute (e.g. psig to psia and Deg F to Deg K).

With any flowmeter using differential pressure as the flow element, the differential pressure has a nonlinear relationship with flow and the flow will require a square root extraction somewhere. This can be done in the transmitter or DeltaV. It needs to be done in one place or the other. The composite block in DeltaV handles all the math for you, as I understand it. But you should verify in BOL [DeltaV Books Online] if you have any doubts.

For steam, especially saturated steam, you can use the energy pallet and pick the appropriate steam metering calculation. Saturated steam (or any pure component vapor for that matter) is at its boiling point by definition and therefore pressure and temperature are not independent. You only need one or the other. If both are available, pressure is preferable, because the response is faster.

There can be delays in the temperature response to a change in pressure and temperature due to the time it takes to conduct heat through the thermowell to the temperature-sensing element. For high precision work, especially where the gas has variable composition you may want to use the AGA calculation for gas flow compensation. You need the MW [molecular weight] of the gas, which means an analyzer is also required. That is because for a specified volume, pressure, and temperature, the number of moles is fixed, but the mass will depend on the molecular weight.

flow-measurementAGA correction is used in the natural gas processing and transportation industry where the gas is the product and its value varies with composition (and heat content).

Ammonia production is another industry where the feedstock is natural gas and due to the large volumes consumed, AGA gas correction is justified.

Some meters provide an option to provide a mass flow if the meter is a multi-meter that measures volumetric flow, pressure, and temperature.

Then the output would already be converted to mass and would not need to be done in DeltaV. I don’t have the exact formula used available to me to now, but if it isn’t in the DeltaV BOL, you can find general formulas on the Internet or in a flowmetering or chemical engineering handbook.

[From Jim: this Flow Measurement User Manual provides the formulas for your use.]

Liquid flows can also be compensated. But since they are non-compressible, you use a density measurement and correct for changes in liquid density directly by multiplying density and volume flow rate. It will be related to temperature and you could convert from liquid volumetric flow to liquid mass flow with temperature if you knew the relationship between density and temperature for the material being measured. I think the DeltaV composite gives you the option to convert either a liquid flow or a gas flow. You would add the correct inputs accordingly.

They are all required to do the calculation. However, if one is constant, you could use a constant value entered as an input/read parameter instead of an AI to provide the measurement. Also, as I noted in my previous comment, at the boiling point of a pure component pressure and temperature are not independent. But your calculation would need to know the that it is dealing with such a vapor. In practice, I have never seen this done except with steam.

There are many groups within the Emerson Exchange 365 community in which you can join and interact with your peers. Many times, this may be your fastest path to the solution to the challenge in front of you.

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