Gas flow rate at different pressures

[MerlinKnight]

I now the flow of natural gas through an orifice will be the same on both sides with a little pressure differential. But if i have 900 PSIG at 4mmscfd measured upstream and the other side of the valve or orifice is attached to a pipe to atmosphere. Is the flow going to be higher on the pipe to atmosphere due to a lower pressure and gas expansion?
Just want to get this straight.

Thanks

 

[katmar]

It depends on the units that you measure the gas in. If you use mmscfd then the gas flow will be the same on both sides of the orifice because, although the gas expands as the pressure decreases, in both cases (upstream and downstream) it is referred back to a reference temperature and pressure. This is also true if you measure in mass units eg pounds per hour.

If you measure the flow in actual cfd then the volume will be higher downstream becase the gas expands as the pressure decreases.

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[zdas04]

Another way of saying what Katmar said is that volume flow rate at standard conditions is a reasonable surrogate for mass flow rate while volume flow rate at actual conditions is not a reasonable surrogate for mass flow rate. I don't know if that is any clearer, but somehow it is clearer in my mind. Mass flow rate is a measure of how much stuff went from one point to another. Volume flow rate at actual conditions is a measure of how fast a control volume is moving--but it says nothing about the stuff that is in the control volume.

I've found that SCF (or Nm3) is the most important omitted topic in engineering school. We learn a lot about mass flow rate and quite a bit about volume flow rate at actual conditions (although they seldom spell that out, it is just "volume flow rate"). It is symptom of too many instructors having never worked for a living.

We learn about the continuity equation that says that "flow" upstream and downstream of any point must be equal. The examples are all mass flow rate because it doesn't appear to be true with "volume flow rate" and I've seen college professors that couldn't explain why not.

The nice thing about converting volume flow rate to standard conditions is that it is a perfect surrogate for mass flow rate and satisfies the continuity equation exactly.

The bad thing about standard conditions is that you can't get a meaningful velocity from them. Velocity is pressure and temperature dependent and using a made-up pressure and temperature will always give you a nonsense velocity. I see this a lot. In fact the central theme of my Masters Thesis nearly 20 years ago made the mistake of calculating velocity based on standard conditions and generated a nonsense relationship that sailed through my committee review. None of the committee members had ever been responsible for a real flow.

David

 

[BigInch]

Short answer: No! Standard flowrate is the same on both sides.

Why?:

4 MMSCFD is Standard cubic feet, which means it is the volumetric flowrate already referenced to 14.7 psia or 0 psig. Standard flowrates do not care what pressure the pipe has at any point, so the standard flow will be the same whether the pipe is at 900 psig, 800 psig, 200 psig, or 0 psig. Standard flowrates really have little meaning when actually doing hydraulic calculations, but none the less are a very convenient measure for simple communication, because the actual pipe pressures and temperatures can be ignored.

Its easier to explain the difference between standard and actual cubic feet using some quick examples,

At 900 psig ACTUAL flowrate (at pipe conditions) would be,

MMACFD = 4 MMSCFD * 14.7 psia / (900 psig + 14.7) psia
MMACFD = 0.0643 MM_Actual_CFD @ 900 psig

at 500 psig,
MMACFD = 4 MMSCFD * 14.7 psia / (500 psig + 14.7) psia
= 0.114 MMACFD

at 100 psig,
MMACFD = 4 MMSCFD * 14.7 psia / (100 psig + 14.7) psia
= 0.513 MMACFD

at 0 psig (or 14.7 psia)
MMACFD = 4 MMSCFD * 14.7 psia / (0 psig + 14.7) psia
= 4 MMACFD

You can see that actual flowrate = standard flowrate only when the pipe conditions = standard conditions, ie. Pressure = 14.7 psia and temperature = 59 degrees F
(other standard conditions are sometimes used, so if different than the above, they will always be defined in a sales contract or other design document). Actual Flowrates must always have the pressure, and temperature noted alongside.

Small Print:
_{The above calculation neglects the compressibility factor, the measure of how much deviation to expect the natural gas will have from an ideal gas at some given pressure and temperature, which at 900 psig will significantly affect the real answer. It also assumes temperature is constant.}




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