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# Negative Pressure Across A Pipeline

8 replies to this topic

### #1 L6H9B5

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Posted 15 February 2013 - 04:55 PM

Hello, everyone. I have very little experience as a design engineer. I have worked mostly in project management and supervision but in a recent project we were a junior process engineer short and it was decided that I would have to do.

I would rather not go into much detail. Suffice it to say I am required to determine the flowrate at the delivery point of a natural gas pipeline considering that the pressure drop across the pipeline is actually greater than the inlet pressure. From a calculation perspective the calculations would simply not converge. In reality the natural gas would arrive at the delivery point but at an irregular rate and at a much lower velocity than required. These rates are the ones I need to estimate.

I don't need a detailed answer, just a general idea as to how to employ the familiar equations to fit this unfamiliar situation.

HLB

### #2 Art Montemayor

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Posted 15 February 2013 - 05:29 PM

I seriously doubt that what you state can take place in the real world.

In order to have fluid flow through any conduit you must have a driving force available to carry out this Unit Operation.  That, in common language, means you must have a pressure drop (since the pressure is the driving force).  If you have an origin of the gas in question with a pressure of X psig, and a terminal pressure at the end of the pipeline with Y psig pressure, then the pressure drop (driving force) is equal to X - Y.  All this points to is the fact that the pressure drop cannot be greater than the origin.  In order to have what you describe, the terminal pressure must go into a vacuum - and that is limited to approximately 15 psi.  You do not want to have a vacuum - natural gas situation.  The vacuum would draw in atmospheric air through fittings and gaskets and create a natural gas + Oxygen mixture which you want to stay away from.

To calculate the hydraulics involved and the diameter of a pipeline there are various equations involved - Such as the Panhandle ones and others.  This is compressible fluid flow, so it isn't simple nor straight forward.

Why do you state that the natural gas would arrive at a lower velocity?  The velocity increases when the pressure is reduced - because the specific volume is greater within a fixed pipe diameter.  This is one of the problems with compressible flow.

### #3 Bobby Strain

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Posted 15 February 2013 - 05:57 PM

I don't know where you are, but you need to engage the service of an experienced engineer to cover your shortage of people. Seems you're way out of your league.

Bobby

### #4 L6H9B5

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Posted 15 February 2013 - 06:29 PM

Thank you both. I suspected that this would be the answer. Unfortunately, as improbable as this sounds I am being required to do this kind of analysis. Without going into much detail, an existing non-operating pipeline is to be used to transport natural gas temporarily across approximately 230 km using three compressor stations. According to my calculations, this is not feasible because the discharge pressure of the last compressor station is not enough to overcome the pressure drop acros the last section of the pipeline. Increasing the discharge pressure is not an option because then the pipe thickness would not be in accordance with ASME B31.8. Building an additional compressor station is not an option either. However, declaring that the pipeline is unfit for this service is also not something that I'm being encouraged to conclude by my supervisor. He has asked me to determine the flow that would be arriving at the end of the pipeline. Mind you, he does not mean the maximum flow that can be handled by the pipeline with the existing diameter and maximum allowable discharge pressure in the last compressor station, but literally the flow that would be arriving at the end of an incorrectly employed pipeline, which I suspected was not possible, however since I wasn't sure I decided to ask.

As for why I claim the gas would be arriving at a lower flow in real life, it's because that's what I've observed in pipelines where compressor stations aren't working properly and excessive friction losses take place. The gas does not become trapped in the pipeline, it actually does reach the end of the pipeline but at lower and inconsistent conditions of flow and velocity than those the pipeline was designed to deliver.

Either way, thank you again.

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Posted 17 February 2013 - 11:26 PM

1. Do you have the compressor's performance curve? and the minimum flow surge point?

2. If you have you can do the hydraulic simulation to determine the flow rate of gas. There is some flow but not the same as the current design basis.

### #6 Santoshp9

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Posted 19 February 2013 - 03:56 AM

Dear ,

Already you have three compressor station in 230KM span,why dont you try for 4th station after 3rd station to overcome the losses from last station (3rd) to to your destination.

### #7 031

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Posted 19 February 2013 - 01:31 PM

L6H9B5 , You are facing a very interesting problem. For any centrifugal machine, discharge pressure is governed by back pressure of the system. Take into account this issue when you interpret compressor performance curve.

### #8 uxahid

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Posted 07 March 2013 - 02:02 AM

I suggest you to take account of terrain/topography in the simulation model. 230 km is quite reasonable length, so you need to check how pressure drop varies after implementation of terrain changes in simulation model.

### #9 shan

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Posted 07 March 2013 - 08:27 AM

Piping line pressure drop is a dependant variable of fluid velocity.  Tiny fluid flows (tiny fluid velocity) result tiny pressure drops and no fluid flow results no pressure drop.  The problem is if the tiny flow is beyond your compressor surge point, you may be unable to keep the compressor continuously opening.  You may have to close the downstream valve and pressurize your line and then open the valve to depressue the line.  The daily gas volume delivered will be ( (gas contained at high pressure - gas contained at low pressure)*daily pressure swing circle).