NPSH Calculation

Benfa · February 14, 2013, 13:59

Hi,

I am calculating a flow of diesel trough a valve. Therefore the outlet is defined by a mass flow while the inlet is specified as static pressure with pressure 0. The pressure of the diesel in the pipe is about 250bar. So I put the ambient pressure in domain settings to that value. I calculated the flow in the valve and now I am trying to get the NSPH-value for this specific case. The definition of NSPH=(Totall Pressure inlet-Pvap)/(g*Density). I have to problems:
1. Does anybody know the vapor pressure of Diesel at abot 30-40°C and a global pressure of 250 bar?
2. If I use the total pressure at inlet to calculate the NSPH value I get unreasonable values. The flow at inlet is about 3 cm/s and so the dynamic pressure is about some tenth of Pa. Do I need to use the absolue pressure for that specific case. Otherwise If I had a velocity equal to zero it would mean that I get cavitation already at the inlet with zero velocity (makes no sense).

I appreciate any help on the questions! Thanks in advance.

ghorrocks · February 14, 2013, 16:09

You will have to look up the vapour pressure for Diesel. And by the way, the ambient pressure has nothing to do with the vapour pressure - make sure you understand the concept of vapour pressure before doing simulations on it.

Yes, you need to use the absolute pressure in the NPSH calculation - again, make sure you understand the concept of vapour pressure before doing simulations on it.

Benfa · February 15, 2013, 12:46

Thanks for your fast answer. Yes we will defitnily dig into cavitaition theory. Vapor pressure is dependent ond material and temperature.

February 14, 2013, 13:59	NPSH Calculation	#1
Benfa Member Benny Join Date: Apr 2012 Posts: 40 Rep Power: 14	Hi, I am calculating a flow of diesel trough a valve. Therefore the outlet is defined by a mass flow while the inlet is specified as static pressure with pressure 0. The pressure of the diesel in the pipe is about 250bar. So I put the ambient pressure in domain settings to that value. I calculated the flow in the valve and now I am trying to get the NSPH-value for this specific case. The definition of NSPH=(Totall Pressure inlet-Pvap)/(g*Density). I have to problems: 1. Does anybody know the vapor pressure of Diesel at abot 30-40°C and a global pressure of 250 bar? 2. If I use the total pressure at inlet to calculate the NSPH value I get unreasonable values. The flow at inlet is about 3 cm/s and so the dynamic pressure is about some tenth of Pa. Do I need to use the absolue pressure for that specific case. Otherwise If I had a velocity equal to zero it would mean that I get cavitation already at the inlet with zero velocity (makes no sense). I appreciate any help on the questions! Thanks in advance.

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February 14, 2013, 16:09		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You will have to look up the vapour pressure for Diesel. And by the way, the ambient pressure has nothing to do with the vapour pressure - make sure you understand the concept of vapour pressure before doing simulations on it. Yes, you need to use the absolute pressure in the NPSH calculation - again, make sure you understand the concept of vapour pressure before doing simulations on it.

February 15, 2013, 12:46		#3
Benfa Member Benny Join Date: Apr 2012 Posts: 40 Rep Power: 14	Thanks for your fast answer. Yes we will defitnily dig into cavitaition theory. Vapor pressure is dependent ond material and temperature.