Do Orifice Plate Flow Meters account for varying densities?

[n8cole]

I am currently working on a process that uses inert nitrogen (N₂) to purge organics from a tank. The organics are then fed through a process line into an oxidizer. Initially, the process line contains primarily organics, but as the tank empties, the process line becomes more nitrogen-rich. This results in a variation in the bulk density of the process line.

I need a method to measure flow that accounts for this change in density. I initially thought an orifice plate would be ideal since the orifice plate formula includes density. See reference equation below:

From the FE Handbook​

However, my supervisor informed me that orifice plates do not account for density variations. As a recent graduate, I am aware that I may lack some industrial common knowledge. Despite my research, I have not found definitive sources confirming or denying that orifice plates account for varying densities.

Could you please advise on this matter?

 

[georgeverghese]

Your supervisor is correct, the flow element just measures DP, and some other device is required to get an estimate of density in this application in order to derive density compensated flow. There are 2 choices:
a)Use an online gas chromatograph to obtain gas composition, then feed this to an algorithm to derive density
b)Install an online gas densitometer

Of the 2 methods, (a) is the least reliable. Densitometers are not reliable either. I would try to find a way out of having to measure flow accurately in this case, given these limitations, if I were you. Can you tell us why / what you plan to use this density corrected flow for at the oxidiser?

 

[pierreick]

Hi,
Yes, your supervisor is correct. As you can see in your formula if the density changes, Gamma will change. Gamma =(Ro*g).
Better to use a mass flow meter (to get access to mass flow, density, temperature of stream).
Good luck
Pierre

 

[katmar]

You need to take a step back and look at the bigger picture.

Why do you want to measure the flowrate? Does the flow to the oxidizer need to be varied according to the organic content of the off-gas? If yes, how will you measure the organic content? If you do measure the organic content then you can calculate the gas density on the fly and adjust the orifice calculation.

If the oxidizer flowrate does not need to be varied then I would simply move the orifice to the nitrogen inlet line to the tank so that you can control the volumetric flow into the tank. If the tank remains at a fairly steady pressure then the volumetric flow out of the tank will be the same as the volumetric flow of nitrogen into the tank.

What is the variation in the organic content? Does the average molecular weight of the organics vary with time? How different is this molecular weight from the MW of nitrogen? Calculate the variation in MW of the off-gas over the full batch and work out what error it will introduce. Is this error acceptable in terms of what you need to know and/or control?

I have used coriolis meters very successfully for measuring mass flow rates of liquids of varying composition. These meters give both the mass flow and the density and this has allowed me to determine the composition of the liquid in real time. I don't know if the coriolis is good for gases - call your supplier for information. But note that a coriolis meter is very much more expensive than an orifice meter.

 

[n8cole]

You need to take a step back and look at the bigger picture.

Why do you want to measure the flowrate? Does the flow to the oxidizer need to be varied according to the organic content of the off-gas? If yes, how will you measure the organic content? If you do measure the organic content then you can calculate the gas density on the fly and adjust the orifice calculation.

If the oxidizer flowrate does not need to be varied then I would simply move the orifice to the nitrogen inlet line to the tank so that you can control the volumetric flow into the tank. If the tank remains at a fairly steady pressure then the volumetric flow out of the tank will be the same as the volumetric flow of nitrogen into the tank.

What is the variation in the organic content? Does the average molecular weight of the organics vary with time? How different is this molecular weight from the MW of nitrogen? Calculate the variation in MW of the off-gas over the full batch and work out what error it will introduce. Is this error acceptable in terms of what you need to know and/or control?

I have used coriolis meters very successfully for measuring mass flow rates of liquids of varying composition. These meters give both the mass flow and the density and this has allowed me to determine the composition of the liquid in real time. I don't know if the coriolis is good for gases - call your supplier for information. But note that a coriolis meter is very much more expensive than an orifice meter.

Thank you for your detailed response. Unfortunately, Coriolis meters are not good for gases. Additonally, in order to obtain an adequate destruction efficiency of organics in the oxidizer, there must be a minimum residence time of 1 second. This is why the flow rate needs to be measured and accounted for.

 

[n8cole]

Hi,
Yes, your supervisor is correct. As you can see in your formula if the density changes, Gamma will change. Gamma =(Ro*g).
Better to use a mass flow meter (to get access to mass flow, density, temperature of stream).
Good luck
Pierre

Your supervisor is correct, the flow element just measures DP, and some other device is required to get an estimate of density in this application in order to derive density compensated flow. There are 2 choices:
a)Use an online gas chromatograph to obtain gas composition, then feed this to an algorithm to derive density
b)Install an online gas densitometer

Of the 2 methods, (a) is the least reliable. Densitometers are not reliable either. I would try to find a way out of having to measure flow accurately in this case, given these limitations, if I were you. Can you tell us why / what you plan to use this density corrected flow for at the oxidiser?

I like method 2. However, density is intrinsically related to pressure, as described by the equation ( P = \rho g h ). Therefore, if the density of the gas changes, the pressure should also change. I am confused as to why this logic does not hold up.

 

[TiCl4]

Unfortunately, Coriolis meters are not good for gases.

Please explain your reasoning. Coriolis meters are good for gas flow, and are perfect for your application, as they can be used to directly measure volumetric flow rate. E&H E300, a middle-of-the-line model, has a +/- 0.5% error on gas mass flow, and a decent range on allowable gas velocity. The limits are clearly defined in the manual - if gas velocity is too high, simply go a size up.

 

[TiCl4]

I like method 2. However, density is intrinsically related to pressure, as described by the equation ( P = \rho g h ). Therefore, if the density of the gas changes, the pressure should also change. I am confused as to why this logic does not hold up.

Because density is a function of molecular weight of the gas flowing through the meter as well, which is the crux of your issue. Density is a function of both pressure and molecular weight. Ideal gas: rho = P*(M.W.)/ (R*T).

 

[n8cole]

My vendor told me they had nothing in their catalog that would work. Thank you for bringing this to my attention. However, I do not understand the maximum full-scale value for liquid? Where does this number come from? Also, the gas density at operating conditions will change, so why does the formula have it has a constant? I am assuming it is the max density of the gas.

 

[LittleInch]

I like method 2. However, density is intrinsically related to pressure, as described by the equation ( P = \rho g h ). Therefore, if the density of the gas changes, the pressure should also change. I am confused as to why this logic does not hold up.

That equation is the pressure difference at the base of a column of fluid h metres high caused by gravity.

Density is related / proportional to the density of a gas at an absolute pressure of 1 bara if you know density at standard conditions.

 

[n8cole]

Because density is a function of molecular weight of the gas flowing through the meter as well, which is the crux of your issue. Density is a function of both pressure and molecular weight. Ideal gas: rho = P*(M.W.)/ (R*T).

I understand, thank you!

That equation is the pressure difference at the base of a column of fluid h metres high caused by gravity.

Density is related / proportional to the density of a gas at an absolute pressure of 1 bara if you know density at standard conditions.

You found me again LittleInch.

 

[katmar]

in order to obtain an adequate destruction efficiency of organics in the oxidizer, there must be a minimum residence time of 1 second. This is why the flow rate needs to be measured and accounted for.

The residence time is a function of only the volumetric flowrate and the volume of the oxidizer, so you have no need to measure the density.

All you want to measure is the volumetric flowrate - regardless of what the density is. This makes a laminar flow meter the obvious choice. It is a differential pressure device like an orifice but by forcing the flow into the laminar regime the volumetric flow (or velocity) is linearly related to the differential pressure and changes in density are irrelevant provided that the flow remains laminar. This seems to be the behavior you were hoping to get with the orifice.

 

[georgeverghese]

If you go through the GDM video from Emerson on densitometers, it will become apparent that this device can only be used in clean gas service - no particulates or waxy components, anything that can plug up the delicate measuring internals.

Its still not clear what you are deliberating on - if you have a fixed reactor (combustion chamber) volume, residence time is directly related to the displacement flow of N2 sweep gas ? Some temperature correction may be required to this volumetric flowrate if displaced gas in the organics tank is warmer then feed N2 gas.

 

[pierreick]

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Pierre

 

[n8cole]

If you go through the GDM video from Emerson on densitometers, it will become apparent that this device can only be used in clean gas service - no particulates or waxy components, anything that can plug up the delicate measuring internals.

Its still not clear what you are deliberating on - if you have a fixed reactor (combustion chamber) volume, residence time is directly related to the displacement flow of N2 sweep gas ? Some temperature correction may be required to this volumetric flowrate if displaced gas in the organics tank is warmer then feed N2 gas.

There is a forced draft process blower between the tank and the oxidizer. The blower is what is controlling the flow, not the nitrogen.