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RO Calculation for Flow Measurement
RO calculation for variable area flowmeter before it.

In primary seal gas of ethylene compressor of our plant there are two variable area flow transmitter which are over range i.e 100 kg/h (10..10 kg/h). With a RO before these flow transmitters with bore diameter of 2.5 mm. First of all what could be the problem for this over range situation? What is calculation for flow range in regard to this RO? And what data we need to calculate flow? is it same as dp orifice flow or not?

I appreciate any help in this regard.


The RO's are likely for safety to limit the max flow. The variable area meters typically have an adjustable valve upstream for your recommended seal flow.

Some seal gas installations also use a pressure regulator upstream of the variable area meter.

By W.L. Mostia on 18 April, 2019 - 4:26 pm

We need a bit of more information.

What do you mean when you said this:

"First of all what could be the problem for this over range situation" What over range situation?

And what exactly is the problem you are asking about?

The RO is there probably to restrict the seal gas flow to an acceptable level or to reduce the orifice downstream pressure, as was mentioned in a previous post. As far as measuring the flow through the RO, you maybe able to get a rough estimate. A RO is not typically a sharp edge orifice that is used for flow measurement so reading the differential pressure would only give a rough estimate. You can find the orifice calculation in on-line or in a flow measurement book on flow measurement like Flow Measurement Engineering Handbook by Richard Miller and Principles And Practice Of Flow Meter Engineering by LK Spinks. If the down stream pressure is less than one half the upstream pressure, the orifice is choked (at sonic velocity) and only the upstream pressure needs to be read. There are equations for this also. Also, you may not have enough straight run to get a good measurement. To use the TO as a measurement orifice, you will need to have more information about the orifice plate and the process conditions.

You need to find a competent instrument engineer and see if he can figure out how to do what you want to do.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Engineer (TUV Rheinland)
Winner of the 2018 ISA Raymond D. Molloy Award
Sr. Safety Consultant
SIS SILverstone, LLC