5 replies to this topic

[luuquocdai]

Dear friends,

I'm having a problem what I want to consult with you.

Assume a natural gas offshore or onshore pipeline (only gas phase), that has inlet and outlet shutdown valve.

• The length of the offshore pipeline is 298 km; the onshore pipeline is 29 km long. The Nominal Pipeline size is 18".
• Average pipeline pressure is 90 Barg; average temperature is 25 ^oC
• Assuming an accident, a hole (for example: a hole 1") can occur in the pipeline.

A question arises: how long (in minutes) from occurance of the hole, will it take the pressure in the pipeline to decrease to atm. pressure?

Looking forward to information from you.

Thanks,
luuquocdai@gmail.com

[Zauberberg]

You can create a Blowdown case in any of the flowsheeters available to you, and simulate depressurization of the pipeline through 1" orifice. That will give you an idea how much approximate time you need for complete blowdown.

[Art Montemayor]

Luuquocdai:

I believe the specific answer to your specific, stated problem can be found the following paper that Milton Beychok has made available at his website:

Calculating Accidental Release Flow Rates From Pressurized Gas Systems

The specific webpage where you can read and download this paper (and sample calculations) is:
http://www.air-dispe...m/feature2.html

I don’t know of a more authoritative professional engineer or author on this subject than Milton Beychok. You will find his writings and explanations concise, simple, and totally understandable. And on top of that, Milton is a great individual and human being.

One word of caution that I should add to your post is that you should be more detailed and specific in your basic data. You mention two sections of the same pipeline – one offshore and another onshore. However, you fail to state WHERE the leak is assumed - offshore or onshore? If the leak occurs offshore, it is impossible for the pipeline to achieve atmospheric pressure. You obviously need to be more specific.

[rathodms]

Luuquocdai:
,You can create a Blowdown case in any of the transient simulators availble with you, and simulate the depressurization of the pipeline through 1" loutlet pipe(leak). OLGA 5.3 can be useful in this case.


regards,

Madan

[Texas14]

In reference to Milton Beychok's equations, I don't believe they can be applied here since they are for vessels and not pipelines. The best resource I have found is this paper: http://www.qub.ac.uk...ts/chapter5.pdf
A ways down, there is a pipeline blowdown through an orifice equation, but it is in terms of mass flow instead of pressure. But you can calculate the mass-flow over small time increments to decrease the pressure a certain set amount, say 10 psia, and then add how much time total to get to atmospheric.

[paulhorth]

This sounds exactly like a typical exam question. When I was at university many years ago we would be faced with answering questions like this, in 35 minutes, without a computer.

Here is my answer.

Mass of gas in pipeline m = (density).volume = p.M/(z.R T).V where V is pipeline volume, p is pressure
or, abbreviating, m = p.(K1)

Differentiate: dm/dt = dp/dt. (K1).........................................(1)

Critical flow through orifice is given by flowrate dm/dt = K2. p...................................(2)

From equations (1) and (2), dp/dt. (K1) = p.(K2)

Integrate: ln (p1/p2) = (K2/K1).t

where t is time for pressure to fall from p1 to p2

Just evaluate the constants K1 and K2 for your pipeline and your hole size and you have the solution.
This does assume constant temperature, which is a simplification, but is not far wrong if most of the pipeline is under the sea where it will tend to keep warm. It is at least a simple and reasonable approximation.

Paul

Edited by paulhorth, 03 June 2011 - 05:54 PM.