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Effect of Compressor on Orifice Flow Measurement
Effect of Screw Compressor on the upstream Orifice Flow Measurement.

Hi Guys

We have an orifice DP Flow measurement mounted on a Natural Gas Line to measure its flow. At the downstream of the orifice, there is a screw compressor to compress the gas to a higher pressure. After the compression, there is another orifice flow meter installed.

This screw compressor, recycles the flow back to the inlet through its recycle valve. What is important to know that the orifice flow measurement installed at the upstream of the compressor is actually present in the upstream of this recycle line connection to the main inlet line.

I would like to know that due to this installation, is it going to affect the upstream orifice flow measurement? This is because, some flow is always recycling back to the inlet at the downstream of this orifice. What do you think?


Check your piping layout drawings. It will answer you query...

My friend d

Thanks for your response but its not helpful at all. Please respond if you can help anyway. For sure i have checked the piping layout, that is why i have tried to explain piping configuration in my question.


Got it, misread the sentence describing the recycle tie-in.

As long as you have adequate run length up and down stream of your meter, and know the properties (pressure, temperature, sp. grav.) of the gas the meter calibration can be checked.

If you are using transmitter with square root extraction, that has to be coordinated with the monitoring equipment (DCS,PLC,display). Double square-roots can be a common issue.

If you flow is pulsating that to can create metering error.

By W.L. Mostia on 2 March, 2018 - 5:33 pm

I am not exactly sure what you are asking but I try to give you an answer. If your inlet orifice meter is upstream of the recycle connection, it is reading the compressor natural gas throughput. One concern for such arrangement is how close the recycle tie-in is to the orifice as such a connection may require a longer orifice downstream straight run piping for accurate measurement. At steady state operation (constant flow & recycle), this meter should agree (within reasonable accuracy) with the downstream orifice meter (assuming you have got all the orifice factors correct, compensated for pressure and temperature, and the downstream orifice is downstream of the recycle connection). At steady state the recycle flow does not matter as the recycle flow is in a loop around the compressor (e.g. the same cubic feet of natural gas is being compressed through the compressor). The recycle connection can effect the amount of upstream straight run the downstream orifice needs.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.

Hi Guys

Thanks for your help.

As the typical recycle flow arrangement, the orifice for the inlet flow of compressor should be at the downstream of the recycle tie in. But in this case, the orifice for the inlet flow of compressor is at the upstream. So, my concern, is that it should bring some inaccuracies in the flow measurement.

I have observed one thing here, though the inlet and outlet runs as per the orifice are enough long but still there is a huge fluctuation in flow observed. Do you think it is because of this recycle line?


Sounds like a poorly designed system.

The quick answer to your question is yes, in many ways.

beyond that you need to release your p&ids, piping layout and instrument specs along with the compressor data from the vendor, not usual in a public forum .....

Assuming by your username this is an oil injected screw used in a GT feed application trying to maintain a set flow into the GT?

Firstly, I'm confused by where you say your flow meters are placed and I think you have mixed up upstream and downstream.

Your package will have a suction line going into the screw and a discharge line going from the screw into the process and a recycle line going from the discharge line back to the suction line. The recycle line will have a modulating control valve to (if this is indeed a GT feed application) open when the discharge pressure gets too high and close if the discharge pressure is too low. It will probably do this in conjunction with the screw's slide valve if it has one.

You'll probably have a flow meter at the package discharge (at the end of the discharge line AFTER the recycle take off - as the gas leaves the screw package into the process) used to measure the flow getting pushed forward into the GT. This is by far the most important one for this application. The confusion I got from reading your post is where is this other flow meter? Its unusual to have two flow meters in the gas line as your normally only concerned with what your pushing forward in this application. So is this other flow meter in the suction line before the recycle line ties back in? In this case your measuring what comes in and what comes out. Is it in the suction line after the recycle tie in, in which case your measuring what's coming and your recycled gas. Or is it in the recycle line itself so your simply measuring the amount recycled?

Either way, so long as its a gas turbine feed application and the discharge flow meter is located after the recycle take off you're fine as this what the recycle control valve and slide valve will be controlling on.


Thanks for your responses. Let me explain my question again.

Yes you identified it correctly that this is basically a gas booster compressor whose function is to compress the gas to high pressure and that high pressure gas is fed to the gas turbine.

We have an inlet orifice flow meter for the compressor but this is installed before the compressor recycle line tie in. Then we have the Gas turbine inlet flowmeter which is of course just before the gas turbine. It is of course assumed that this flowmeter will show what is the output from the compressor after recycle line.

The problem, which I'm observing is the inlet orifice flow meter (before recycle line tie in) as this faces a huge fluctuation issue. As far as the straight runs of pipes are concerned, we have enough lengths available at the upstream and the downstream. But do you think that because this flow meter is installed before the recycle line tie in point, that is why it is facing fluctuations.

This is because, the recycle flow could only be the varying factor which may be generating some back pressure on orifice downstream. This may be resulting in the flow fluctuation. What do you think?

By W.L. Mostia on 8 March, 2018 - 3:49 pm

Have you looked at the control output to or position of the recycle control valve to see if it corresponds to the flow fluctuations? Have you considered connecting a pressure gauge or transmitter on the upstream orifice connection of the compressor inlet orifice meter to see if you are getting pressure fluctuations?

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.

It sounds like the booster is a constant displacement type device, meaning that its' inlet flow would be constant. The turbine's demand would not be constant, therefore you need the recycle line to get rid of the excess. The upstream flow meter would therefore fluctuate with the demand of the turbine.

Is this just a generic inquiry low and high pressure flow measurement?


Are you specifically having fluctuating flows or with stable meter readings, differences betweel low and high pressure?