Hi,

I am currently on a new project where the client has intended using a DP transmitter to measure Interface level on a vessel. I know it is possible, but it is not the best instrument to use. But now I want to get some facts to put on the table and explain exactly why its not the best option.

In this specific application, the two mediums will be water and oil/petrol. The vessel is approx 30m high with the bottom 10m would have water and the oil on top of it.

Does anybody know the exact engineering reasons and proven facts why this is wrong, and if so, would you mind letting me know so that I can tell the client.

Thanks,
Brett

 

Hello brett,

The first and foremost thing is that the DPT can be used to measure level of liquid in a tank where the liquid is not a mixture of 2 or more liquids because the level is measured by multiplying the pressure generated on the dp cell with the density of the liquid.

Level=specific gravity or density (*) pressure.

Thus in your case oil has different s.g and water has different s.g.,which makes the use of dp cell inappropriate for your app.
(Note:different liquid posses different pressure in a tank of same level and same volume due to the fluids s.g).

For further conceptual explanation have a look at my blog,
Www.raj7eie.blogspot.com

 

I am afraid that I have to disagree with the below statements regarding using a differential pressure transmitter to make an interface measurement: The first and foremost thing is that the DPT can be used to measure level of liquid in a tank where the liquid is not a mixture of 2 or more liquids because the level is measured by multiplying the pressure generated on the dp cell with the density of the liquid.

> Level=specific gravity or density (*) pressure.
>
> Thus in your case oil has different s.g and water has different s.g.,which makes the use of dp cell inappropriate for your app.
> (Note:different liquid posses different pressure in a tank of same level and same volume due to the fluids s.g).

D/P transmitters have commonly been used for many years to detect interface between two liquids. Guided wave radar is also commonly used these days for interface measurement. See below for an explanation and discussion of using D/P transmitter for interface measurements.

http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00816-0100-3207.pdf

For an explanation of GWR interface measurement see: http://www.servinstrumentation.fr/commun/pdf/en/E_Rosemount_3300_OK.pdf

By the way, I have no connection with Emerson.

William (Bill) L. Mostia, Jr. PE
Fellow

SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

Hello W.L. motsia,

I am new to instrumentation and thank you for your valuable information, which made me to gather new concepts.