Minimum submergence level for positive displacement (PD) pumps

[Ktracid]

Hi all

1. Is formation of vortices (and air being sucked into) a general concern for positive displacement (PD) pumps?

2. If yes, how can the minimum submergence level be calculated? Can the equation available in HI 9.8-1998 Par.9.8.7.3 be used??

Thanks.

 

[micalbrch]

Ktracid,

1) Not really. Especially oscillating PD pumps like piston (not plunger!) and diaphragm pumps can handle entranced air but also peristaltic pumps are not sensitive to that. Rotating PD pumps like progressive cavity (eccentric screw) pumps are more sensitive but triple screw pumps for example are very suitable to work with gas/fluid mixture. It all depends upon the amount of gas and its distribution.

2) HI = Hydraulic Institute? I do not know the corresponding chapter but as long as the suction entry is flooded all PD pumps work well - not considering NPSH, but that is a different topic.

 

[LittleInch]

1), Yes it is a general concern because your system won't work as you intended and getting air into systems on a continuous basis leads to two phase flow, potential for air locks, surges and unstable flow. However for PD pumps per se, they can generally handle some entrained air without major problem as micalbrch notes. Its generally not too difficult to put in an anti vortex inlet header or even just expand the inlet to reduce inlet velocity.

2) Not come across this before, but looked it up and seems to be the business for pump inlets (I learnt something new...). As said there are other easy things you can do apart from submergence depth to reduce or eliminate vortices such as bigger inlet, orientation of inlet, inlet guide vanes, end plate and side entry slots.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way