available npsh

[prens11us]

hello all,

I am calculating “available npsh” for a drain pump (centrifugal) on a crude oil line. It will drain the line starting from a tie-in point.

Tie in point elevation is 111 meter, suction nozzle elevation is 101 meter. After tie in point the line goes up to elevation of 116 meter.

1- should I use static head value of 111-101=10 meter or 116-101=15 meter.
2- should I include also atmospheric pressure in the calculation if the tie in point is kept open by a vent during draining operation.

Thanks in advance

 

[quark]

|(vertical line)
|
|
|
|
|
|_____________________(pipe line)
|
|
|____(suction line)

Is this the kind of set up you speak about? I would take elevation as 111-101 because once the vertical line is drained, you have to drain the horizontal line and then you may fall short of NPSHa. Yes, pressure is atmospheric.

 

[prens11us]

thanks quark for your help.

set up is similar to mine.

what do you mean by draining horizontal line.



I would like to ask shut-off pressure of this pump.

Shut-off Pressure: Maximum suction pressure (no flow suction at high liquid level) +
120% differential head of normal flow.

is this "differential head of normal flow" selected discahrge pressure of te pump.

thanks

 

[djack77494]

prens11us,
This seems to be a very strange application. Normally, I would think that you'd want to drain your system into some type of vessel or sump. quark has assumed that you want to pump directly out of your line, omiting any type of vessel entirely. From your response, I'm not sure which is the case, but would encourage a more complete description of the process. If you are pumping out the contents of your piping system, then you must ensure that you keep your pump suction flooded. Else you'll form a vortex and suck air into your pump.

In answer to your differential head question, yes, the equation is using the differential head at the normal (design) flow point to estimate the shutoff pressure. I've seen a factor of 125% used here, and even that can be insufficient, depending on your pump's characteristics.
Doug

 

[prens11us]

(114 meter)
__________________
|
(110 meter) |
| |
| |
| |
|_______________[ pump ]_|
(100 meter)


The system as above.

it drains from a line to another line.

Discharged line pressure is known. By back calculation, I have found discharge pressure of pump as 10.7 barg and choose the discharge pressure as 12 barg (adding 10% margin).

I've used differential head at the normal flow as 12 barg in te formulation.

 

[25362]

Not that it matters in regard to the queries, but it seems these probably aren't "above-ground" elevations but taken in reference to a pre-selected level. Right or wrong ?

 

[prens11us]

yes you are right 25362.

All are belowground elevations.

pump is in a pit.

 

[djack77494]

If you maintain the liquid level at or above the 110 m level then you will have an available NPSH of (110-100 = 10m) + atmospheric pressure - the vapor pressure of your fluid, all expressed in the same units. Actually, you must subtract hydraulic and any other losses from the 10m to get the NPSHa.