Dry pit submersible 3

[Lan123]

Could a dry pit submersible pump operate under negative suction provided that NPSH and min. submergence of the suction bell conditions are met at the design flows?
I have a situation where I am trying to replace an existing dry pit with a new dry pit submersible. The minimum water level in the wet well is about 16" below the C/L of pump impeller.
I thought it is possible until the new pump manufacturer said that the pump has to be flooded above the volute. This is a 50 hp dry pit submersible pump supplied with cooling jacket.

Thanks

 

[ashtree]

Lan123

The fact that it is a dry pit submersible does not make any difference as such, it still just works like a pump. If the old pump worked than another pump can work as well, provided it is the same style and the ratings are correct.

I suspect however that your pump does not have a footvalve assembly so that to start pumping the suction has to be flooded and the pump primed. If the pump does not have a footvalve the suction should always be flooded to a level above the pump volute and the control levels should be adjusted accordingly.
Pumping below the pump volute will be possible but on shut down you risk the pump loosing its prime.

The other issue may be that to meet the rating required the particular pump must have a flooded suction. This may be a particular issue if the pump rates have been upgraded in the past but the suction pipework has remained the same.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

The key aspect here like agree says is what is the situation at pump start? If the water level can be guaranteed to be above suction level then you shouldn't have a problem at that sort of lift.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

As a dry pit operating pump, we can assume that the start is activated with a level in the wet pit considerable higher than the pump centreline, you should consider the pump as flooded suction and therefore self-priming, once primed and operating it should be able to pump down to the low level in the wet sump. If you think there might be a problem, simply raise the minimum level in the wet sump to suit.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Lan123]

Thanks all for the replies.. Correct; the suction line has no foot valve. The pump start level is much higher than the volute level. The current pump stop level is 16" below the pump volute c/L. I can always raise this minimum float level. However during an emergency I want to have the ability to pump all the way to the wet well min. level which is below the pump center line.

 

[Artisi]

See no problem in pumping down to the low level point provided the pump was started at a higher level.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)