Difference btwn NPSH and NPSHA 3

[tbarkerjr]

I came across these two definitions for Net Positive Suction Head and Net Positive Suction Head Available:

NPSH = p(s)/y + v(s)^2/2g - p(v)/y

and

NPSHa = p(atm)/y - h(e) - h(l) - p(v)/y

where:

p(s) = static pressure in fluid
y = specific weight of fluid
v(s) = velocity of fluid
g = acceleration of gravity
p(v) = vapor pressure
h(e) = elevation from surface to pump
h(l) = head loss

Correct me if I am wrong, but these two formulas should be representative of the same quantity, theoretically. This is, of course, the vessel from which the fluid is being pumped is at atmospheric pressure. Could someone confirm or explain this?

p(s)/y + v(s)^2/2g = p(atm)/y - h(e) - h(l)

I appreciate all the help.

 

[BigInch]

I (finally think) I see the problem. The formula I wrote 12 Jan is referenced to a point at the liquid surface in a tank or reservoir ... where the velocity and resulting velocity head is zero, so you can remove the v^2/(2g) if you like.

CJK, It would have been a lot easier on me, if you just said that. Sorry it took me so long to figure out what you were after. That green dust on the floor under the bed was obviously kryptonite.

**********************
"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/

 

[BigInch]

Here's a diagram I posted to an NPSH problem a couple of years ago. Perhaps more clear...or maybe not.

**********************
"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/

 

[akshehata]

BigInch,

you were right V2/2g != 0 (not equal) since V=Q/A where Q is the flow rate and A is the Cross sectional area at pump inlet. Your reference point should always be the pump inlet where you measure the static & dynamic heads (i.e. the total head) as well as the vapour head.
In most cases V can be ignored. If you prefer you can still calculate it using V=Q/A.

Regards,

 

[micalbrch]

I agree that you can ignore the velocity head, but only if the suction pipe is rather short and properly sized. If this is not the case, the friction caused by the velocity will be significant. And if additionally your static head is low, the fluid is warm or the atmopsheric pressure is low, the velocity head can make the difference between cavitation and a trouble free pump operation.