Q: Fiscal meter accuracy change with added tie-ins

[Iradah]

Hello all,

I need your consultation regarding the accuracy of a metering skid.
There is a proposal to connect one stream of gas at the outlet header of a metering skid that has three runs.
So now there will be two streams: one that enters the meter and gets measured and one that joins at the outlet header and does not get measured. I need to know how can this affect the metering accuracy.
The pressure of the line joining downstream is around 3 psi higher than the upstream pressure. However, the end pressure at which both streams are going is lower than both. so, this means that both streams will be flowing towards the low pressure and there will be no back flow from the stream at the outlet header through the metering tubes.
my question is that with knowing that there will be no back flow, can we do this connection for a fiscal meter?
also, is there any effect on the metering accuracy, for example will we have an effect on the flow regime or Reynolds number that can change the metering accuracy?

I look forward to hearing from you. Thanks.

 

[zdas04]

There are really two answers to this depending on how stead the new stream is. If it is reasonably steady (i.e., max flow rate within about 5% of average flow rate) then the new stream is unlikely to impact the fiscal meters.

On the other hand if the new stream is variable (i.e., max flow rate more than 110% of average flow rate, with pertubations happening more than once per hour) then you will get backflow during the surges. I've had similar flow patterns that were significant (i.e., the dP would swing to negative for several minutes several times a day) and the results were measurement that no one trusted. I "fixed" it by putting a check valve on the meter outlet, but check valve slam was nearly as bad as the surges.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Iradah]

Thank you zdas04 for the reply.

the new stream is steady, however, the original stream that will pass through the meter is not. it fluctuates between 380 and 420 psig in seconds. The pressure reaches 380 for less than a minutes and increases again.
Do you see that this will cause back flow throught the tubes from the stream connected at the header?
Also, cannot I consider installing check valve on the metering tubes?

 

[zdas04]

I would definitely put a check valve between the tubes and the new inlet if I had that kind of variability. You want it as far from the plates as you can get it to give you a bit of damping volume to minimize the magnitude of the dP spikes you get on check valve slam.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.