Why is the suction pressure to be zero for the submerge pit pump? 1

[silebi66]

I would like to ask a very trivial question.
In fact, this is my boss always said to me, "If these answers could be found in the Google, don't ask me."
So, please bear with me if you think like my boss.

I do a pit pump calculation to deliver this pump design sheet to mechanical staff.
This axial-flow pump to be installed in the pit used to pump untreated waste water ,and the normal liquid level is around 2 m in the pit.

I wonder why the suction pressure to be 0 for this axial-flow pump case in the pit.
If it is true, the shut-off pressure to be 0 causes there is no suction pressure.

Could anyone please help me out in this?

Thank you so much.

 

[TD2K]

I would guess that they are sizing the pump for minimum level in the sump. The normal level may be 2 m but there's likely a min and max level also.

The pressure can't really be zero since you need some liquid head to cover the pump inlet but it's going to be pretty small so to argue whether it's 0 or 0.05 barg is splitting hairs. The shut-off head isn't zero just because you have 0 inlet pressure. At shut-off the pump still delivers its shut-off head.

I'm ignoring the case where the liquid level is low enough that the pump cavitates because you don't want to operate there.

 

[bimr]

The minimum fluid level in your pit will be determined by the need to prevent a vortex from forming at the inlet to the pump.

That elevaton would be anywhere from .3m to .6m and will be a function of the pump and flowrate. That minimum elevation (head) would also be the same as to the suction pressure. The suction pressure will not be zero. It will be the fluid head at the minimum fluid level.

 

[thorium1]

This might be a nice read for you

http://85.185.231.196/mekanik/CD_3/pump handbook/intaks.pdf

 

[bimr]

From thorium1's link:

Centrifugal pumps in intake pumps must be submerged deeply enough to provide
• A pressure sufficient to prevent cavitation in pump first-stage impellers, referred to as
NPSH (it is assumed that the proper pump has been selected to perform satisfactorily
with available NPSH)
• Prevention of vortexing and associated pit flow problems detrimental to pump operation

 

[silebi66]

TD2K's point is reasonable to me.

Also, some process engineer suggests me that the other conservative approach to estimate the pit pump's shutoff head.


1). Shutoff head = Max water level above pump + 1.2 * differential pressure between suction and discharge.

or

2). Shutoff head = 1.25 times the normal differential pressure.

Also, he point out in either calculation method, we generally get the shutoff head is under 150LB

,but this approach seems safe and conservative to me.

I am not sure my thinking is acceptance or not in process engineer’s view.

Please give me comments if I am wrong.

I really appreciate for your help.



Thank you very much.

 

[bimr]

There is no reason to guess at the pump shut off head. The shut off head is determined from the actual pump curve. Look at the left access of the pump curve.

Rather than using the actual suction pressure, the process engineer is probably rounding off the suction pressure to zero.