The Calculation for Piping Pressure Drop

[leel]

Hi All,

May I knw how to calculate pressure drop in the gas piping which is installed as a loop?

Besides, for the piping with a lot of branches, how to calculate its pressure drop from the supplier untill the last point of use?

actually, I have raise the question before, But I really don't know why I can't find the forum from Eng Tips.

Thanks
Lee

 

[CRG]

Take a guess where the low pressure node is on the loop. Look at flow in both directions from the high pressure node to the low pressure node. Iterate until you find the low pressure node’s location and the mass flow from each side of the loop. You will know when you have it right when the calculations from both directions yield the same pressure at the low pressure node and the mass balance for the system is correct.

Use computer software to do this for you. You can easily check the computer program’s output with hand calculations, and I recommend you do this until you have a high degree on confidence in the results. I have found errors in a commercially available program that could not calculate a system with several nested loops. The program gave the wrong results that were a apparent when a mass balance of the system was done with its output. I evaluated other computer programs for the project and purchased one that worked properly and was easy to use. Note: that this was an unusually large network (300+ nodes) with several main loops that had nested loops. The software that failed worked well on less complex analysis.

 

[leel]

Hi CRG,

Thanks for your reply, As you know, while calculation the pressure drop we will use the equation which related to the initial P, Flowarate, gas density and so on...
But, for this case should we use the Hardy Cross Network Analysis to calculate?

Thanks
Lee

 

[CRG]

leel,
I have not used the Hardy-Cross Network method for some time. If I recall the Hardy-Cross method uses the Hazzen-Williams formula to determine friction losses. I believe that this is usually used in liquid service by the Civil Engineering guys and is not a good choice for gas flow. I am sure someone will correct me if I am wrong.

You have not given enough information for me to give much more advice than I have. The GSPA Engineering Data Book has a discussion of various formulas for calculating pressure drops in gas piping systems. It really depends on the inlet pressure and the pressure drop through the system to be able to pick the best calculation method. Typically I would use the Darcy-Weisbach equation to determine the pressure drop. So, what is the system pressure, what type of pressure drop is anticipated, and how accurate do you want your calculations?

 

[steveen]

If this is a single loop, calculation can be estimated well by hand. See Crane Handbook "Flow of Fluids" for calculation of pressure losses via nomograph.

Assume x flow through loop and Y direct. Calculate losses and adjust flow until you get a match.... Same as CDG above.

Actually setting up a simple spreadsheet, you can have the program solve for you if you are computer savvy. If not just do an interative process. If you need additional help, call me 713-374-7974. Steve

Hardy Cross works well with liquids and slow moving gases (low pressure losses relative to starting pressures)
Be careful to understand the formula the nomographs are based on and that they apply to your case....

 

[leel]

HI All,

Thanks for your reply, how about I come out a senario for you all, please try to solve it for me.

Type of Gas - Nitrogen
No of Points of Use - 11 points
Tool's Working Pressure / flowrate - 100 psig / 32cfm
Main Line - 2"
Droppers - 3/4"
Supply Pressure - 120psig
Pipe length for main line - 150 m
Dropper from main line to tool - 3m

Just Imagine the doppers are having the same distance from each other.

So, what is the DP for looping main line and straight main line to the last droppers?

Thanks
Best Regards
Lee

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