Orifice Chart recorder problems

[gastrouble]

Friends,

Got problems. It may be easy for you guys to figure out but i am struggling.

I have several wells plumbed into a main gas gathering line that is 8 to 10 miles long. Each well has its own check meter (Orific Chart recorder). There is also ONE end point chart recorder that sums up all gas going through that gathering line before selling via a custody transfer meter. The ONE end point chart recorder is within 2% of the custody transfer meter.

The problem is the following. The ONE end point check meter and the summation of gas from the individual check meters do not add up. They are over 40 to 50% off. Why is that?

Could it be because of the following?

1. Individual chart recorders are at a high pressure (15 to 25 psi) compared to the down stream ONE end point check meter (4 psi). (I wouldnt think so since all measurement is in MScf/D

2. Gas temperatures at the individual wells could be as high as 100'F compared to 50'F at ONE end point check meter? (I dont think so either since it is in MSCF/D)

3. Could it be liquids dropping off in the line? I have no idea how much liquids have dropped off in the line and i know that was probably "accounted for" in the individual orifice check meters.

Template Gas Analysis attached.

Thanks for all your help!!

 

[zdas04]

You didn't say if the sum of the well meters was higher or lower than the check meter and the custody transfer meter. It matters.

If the wells were lower (most common), then I would look at (1) do the individual meters have the actual gas analysis or a default (or air)? (2) What elevation is in the calculations (i.e. what is the local atmospheric pressure)? (3) are the right pipe ID and orifice OD entered into the RTU? and (4) are any plates in backwards? I've done that exercise and it is just work. You have to look at every RTU, every parameter, and every plate. When I started my exercise, the sales meter was 30% higher than the sum of the wells. When I finished the sum of the wells was 1.5% higher than sales. It can be fixed, it just takes focused effort.

If the wells are 40% higher than sales then I would be really nervous about a major leak. You can still check all the things above, but most of them will find under measurement; not over measurement so much.

Since you have such a range of temperatures I'd also check that all of the meters had thermowells with RTD's in them, and the RTD being used in the calculation. It is usually a pretty small error, but your temp range is nearly 10% and worth investigating.

David

 

[gastrouble]

David - thanks for your response. We are registering 40 - 50% higher on summation of individual well meters.
Other thank leaks - can you think of anything else? What about Liquids?

 

[zdas04]

In that case I'd look at the meter parameters really close. That kind of an error is not condensation in an impulse line, a backwards or nicked plate. It is something big. I've seen 2-inch tubes entered into the calculation as 4.026 inches. Results in a really high flow rate.

David

 

[gastrouble]

David - wont liquids dropping out cause shrinkage to gas? Could it in any way be the 40% number we are looking for?

 

[zdas04]

Not a chance in hell. Condensation could be a couple of percentage points, but nothing like 20% let alone 40-50%.

Boiling point of normal pentane is 82F at atmospheric pressure (higher at elevated pressure). So if you are dropping from 100F to 50F, you could be condensing normal pentane and heavier, but most of your wells that would be a couple of percent (one was over 10%) if you condensed it all (which is unlikely).

David