I need to confirm if my results are correct about time to depressurize a pipe

[Vivaldi M. Smith]

I am trying to verify if my time to reduce pressure in a line pack scenario is correct:
Conditions :
Natural Gas
Initial Pressure: 1400 PSIG
Initial Temp: 128 F
Final pressure: 750 PSIG
Final Temperature: 65 PSIG
Pipe Length: 108945 Ft
Pipe Size: 24 inch
If I take out of this pipe 55 MMSCFD, I am getting about 5.5 hours to reach 750 PSIG as the final pressure
My total volume is 13MMSCF.
Can someone please help me see if my values are correct?

 

[1503-44]

How is the gas being removed?

If you are depressuring by drawing down the volume during normal operation, you must know how much is being delivered to customers, or, if there is a compressor sta downstream, the inlet flow rate to the station. You must know how, or be able to determine the flow rate of the gas leaving the pipe segment. If you do not know the flow rate, you must know enough about the blowdown process to be able to determine the flow rate, such as, if venting to atmosphere, what size is the vent pipe and if it passes through a valve, what type and size of valve, etc.

How do you know the outlet temperature? How is the gas being cooled from 128 to 65°F (not psig BTW). How is the gas being cooled? Presume the pipeline is underground (and not insulated), so conductive gas cooling rate will usually not be all that high. Do you allow the gas to cool to 65°F before starting blowdown? Do you need to know how long that will take to cool? What is the soil temperature, type of soil, water content, or its thermal conductivity, burial depth and air temperature?

Without knowing flow rate, I can only check your volumes. Time could be anything. And the thermal process could be equally important. Are you blowing down while the gas is cooling? It may take more than 5.5h to cool the gas. If you blow down quickly, the gas may do some of its own cooling as it depressures.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[katmar]

Assuming that you can keep the discharge rate constant at 55 MMSCFD I come to about 4.5 hours with a total of about 11 MMSCF discharged. But you have not specified what your standard conditions are so me working at 60F and 14.696 psia may be wrong.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

They look about right, (5.5 to 4.5 hrs) but you need to include compressibility factor.

How accurate do you need to be?
There will also be some pressure drop from one end to the other during flow so when you get to 750 psi at the outlet then turn it off the pressure will stabilise over a few minute to something higher.

Use a transient simulator and it will give you the JT cooling as well.

You can't calculate time without specifying a constant flow rate

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Vivaldi M. Smith]

Katmar and LittleInch thanks for your responses. Yeah my standard conditions are 60 F and 14.7 PSIA. I have a compressibility factor of 0.95 and a volume correction of 1.3

 

[Vivaldi M. Smith]

Constant flow rate is estimated to be 55 MMSCFD

 

[Vivaldi M. Smith]

1503-44 the pipe runs under seabed for about 20 miles

 

[1503-44]

So how does this gas get to 128°F for its entire length? Little Mermaid says its not so hot down there. OK the 55 is the withdrawal rate.

It is not at all clear how you go from 128F, apparently in the entire line pack to 65F. Anyway OK, fine, but if you start or finish assuming the wrong volumes at the wrong temperatures, you get the wrong answers.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

Inventory released = 12.8mmscf, so at 55mmscfd constant depressure rate, time = (12.8/55) *24 = 5.6hrs.