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Maximum velocity and pressure drop limits for pumps pipingHelpful Member! 

PKEngineer (Chemical)
28 Nov 09 21:40
Could I get guidance to find out the capacity of light hydrocarbon lines (butane) both for centrifugal pump suction and discharge piping?
What is the industry standard for maximum velocity and pressure drop limits. I know API RP 14E recommends 3 ft/sec and 9 ft/sec max for velocities for suction and discharge lines. Hydraulics Institute is less conservative to choose the numbers.
We have two pumps in parallel taking butane product form storage sphere. We need to find out how much more liquid can be pushed through this line?
Your comments will be highly appreciated for a newbie in the industry and this forum as well.
Helpful Member!  BigInch (Petroleum)
29 Nov 09 0:57
Butane as a liquid or as a gas?

Probably around 20 fps as a liquid, not applying to pump suction lines.  It may depend more on what pressure drop you can allow in the piping.  Do not drop the pressure below butane's vapor pressure anywhere in the piping, especially suction lines.  That varies from around 1 Barg at 5C, to 4.5 Barg at 50C.  

60 fps as a gas.

You may find some helpful info here too.
http://www.corken.com/publications/training.html

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

PKEngineer (Chemical)
29 Nov 09 3:12
Thanks for providing your valuable comments and link which is quite useful especially as being new in the LPG industry.

Butane is stored as LPG so in liquid form and is always be in the boiling range. We have 25-30 ft suction head available all the time through the storage sphere liquid level so have some room for pressure drop in the suction line.

Don't you think 20 ft/sec limit is on the higher end for most on-plot lines (not pump suction).

Regards
BigInch (Petroleum)
29 Nov 09 6:44
Its on the high side for the usual 3 to 10 fps in liquid lines, but butane being relatively light should not develop the same waterhammer pressures as heavier fluids would and ... you did ask for the maximum.  That is the max I'd try, followed by a transient pressure check to be sure, as "high" transient pressures are always relative to how close your actual operating pressure is to the maximum allowed operating pressure.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

PKEngineer (Chemical)
1 Dec 09 0:20
BigInch
Thanks for your valued comments.

I have one other issue.

We have a second pump which is getting Isobutane charge from the pump as discussed.The second pump is about 250 feet away from the first pump and is Vertical Turbine type.

Not sure whether suction pressure of this pump needs to be above vapor pressure all the time or will need to have only the static head required in order to avoid two phase flow/ pump cavitation.

Also as both pumps are on the same grade level so may not have static head available. What would be the NPSHA (equation) for this pump. Line loss in this pumps' suction line is about to 2 psi.

Regards
BigInch (Petroleum)
1 Dec 09 4:09
In the butane vapor space above the liquid level, assuming its all butane vapor, pressure = vapor pressure.  Pressure below the liquid level increases with the density * depth.  If you provide at least the vapor pressure in the space above and the recommended liquid liquid level above the intake, I think you'll be OK.  Convert the vapor_space_pressure + liquid_level_head over the intake to all head units.  That's the NPSHa which should be > NPSHr, if the pump mfgr gives you a NPSHr value for that turbine pump.  

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

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