Pump NPSH - water or liquid ?
Pump NPSH - water or liquid ?
(OP)
Hope that you can help me here.
In the pump data sheet ( for pumping gasoil ), stated the NPSH is 12 m. Is that 12 m of water or 12 m of gasoil ?
Background to the above question is my attempt to justify lowering the low alarm subject to the NPSH.
Thanks.
In the pump data sheet ( for pumping gasoil ), stated the NPSH is 12 m. Is that 12 m of water or 12 m of gasoil ?
Background to the above question is my attempt to justify lowering the low alarm subject to the NPSH.
Thanks.





RE: Pump NPSH - water or liquid ?
For the long answer - see thread378-175054: NPSHa Units
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RE: Pump NPSH - water or liquid ?
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RE: Pump NPSH - water or liquid ?
NPSHr may wary due to different properties of the liquid=different dP and different vapour pressure.
Best regards
Morten
RE: Pump NPSH - water or liquid ?
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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: Pump NPSH - water or liquid ?
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pump NPSH - water or liquid ?
Best regards
Morten
RE: Pump NPSH - water or liquid ?
RE: Pump NPSH - water or liquid ?
"With so much literature available, one might assume that NPSH and its relationship to cavitation is well understood. Nothing could be further from the truth. To this day, NPSH is still misunderstood, misused, and misapplied, resulting in either costly over-design of new systems or unreliable operation of existing pump installations."
RE: Pump NPSH - water or liquid ?
My two cents worth:
Maximum water density occurs at 4 degree C or 39.2 degree F. At 4 degrre C fresh water has a specific gravity of 1.00. In almost all cases NPSHr is plotted in terms of fresh water at a specific gravity of 1.00. You can convert the feet of fresh water to the specific gravity of liquid you have by dividing the catalog NPSHr by the specific gravity. As an example if the gas/liquid mixture has an average specific gravity of 0.85 and the NPSHr is 10 feet the adjusted NPSHr would be:
10 feet / 0.85 = 11.76 NPSHr Required
NPSHa as defined by the Hydraulics Institute is:
NPSHA or the "NPSH available" is Atmospheric pressure + static head + pressure head - the vapor pressure of the pumped product - loss in the piping, valves and fittings. Therefore, NPSHA becomes the amount of pressure available to prevent liquid vaporization or pump cavitation.
In this case the two very real keys to the definition are the vapor pressure and intake piping losses. Crude oil has a very high vapor pressure whereas gas typically has a very low vapor pressure. Long intake piping with a lot of fittings reduces the available intake pressure.
I would suggest a laboratory test for the vapor pressure, limit the amount of friction between the source and pump intake and adjust the NPSHa liquid level accordingly.
The simple thing would be to ask your pump manufacture.
D23
RE: Pump NPSH - water or liquid ?
If your pump curve specifies a requirement of 10 ft of NPSH and you are pumping water you could say that the NPSHr is either 10 ft of water or 4.335 psi. On the other hand, if you are pumping a hydrocarbon fluid with an SG of 0.85 you could say that the NPSHr is either 10 ft of fluid or 3.68 psi. This was generally agreed in the other thread.
Katmar Software
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http://katmarsoftware.com
RE: Pump NPSH - water or liquid ?
I apologize, but I am very short on time right now. I will do a little looking as time allows for support documentation to this post.
I know the published information from HI, Goulds, Flowserve etc. states that NPSHr is in feet or meters so no conversions will be required. I have doubts about that. In general terms NPSHr is the required head in feet or meters that is required to prevent the impeller eye pressures from going below vapor pressure. This value (NPSHr) must account for all intake losses. With liquid other than water the vapor pressure, viscosity etc all tend to change which will affect the NPSHr. I don't believe there is a blank check for any liquid without knowing the liquid properties. In this specific case there is a gas-oil mixture to be considered. Not knowing the specific application I have to assume the oil is crude at "X" viscosity and the gas is some hydrocarbon base like methane. The gas will have a lower vapor point than water and the oil will have a higher viscosity than water. This means there will be higher intake losses and lower vapor pressures to overcome.
Typically with cases that two liquids are involved they are not mixed very well. In the case of oil and gas the oil is heavier, so it separates from the gas. The oil is on bottom and the gas is on top. During pump operation you will get some mixed flow and some slugs of one liquid. In this type operation I would suggest designing for the worst-case scenario, which would be viscous entrance losses and gas vapor pressures.
The statement that gas bubbles are not the same as "water" cavitation is reasonably accurate. The gas can still compress back into a liquid, which causes implosions. These implosions can still cause damage to the metal they come in contact with.
I will look for some support documentation as time allows however; I believe we will find each application requires review beyond the generic "NPSHr in feet or meters" is the same for all liquids.
D23
RE: Pump NPSH - water or liquid ?
The generally accepted method is to use the NPSHR given by the pump Mfgr for cold water and, when specific problems are encountered for specific products in specific piping configurations for specific applications, ie. gasoline at 44º pumped from a storage tank, is to deal with those based on experience with equivalent systems. In the gasoline case, it is commonly believed NPSHR can be safely reduced, but it still should not be applied as an accross the board solution, due to the risk of increased cavitation damage. Any reduction in NPSHR not sanctioned by the pump mfgr is a very risky business, as many currently believe that the present method of establishing NPSHR based on cold water is not conservative enough for most systems anyway. Recent thought is that the actual beginning point for cavitation may be as high as 20% above that needed for the cold water standard.
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RE: Pump NPSH - water or liquid ?
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