Suction Lift Capability of Centrifugal Pumps

[Tilas]

Dear All,

I have a requirement to replace a sanitary water pump(centrifugal - submerged pump and motor)to an above ground centrifugal pump. The pump flow rate is 4 m3/hr and the head is 21 meters. The suction pressure available is - 0.4 kg/cm2g (negative).The new pump suction will be in a pit below ground.

My question would be:

1. Is it common for a centrifugal pump to handle this negative suction pressure?
2. Do I need to opt for a self priming pump or just the normal centrifugal pump?

Thanks in advance.

Tilas

 

[TenPenny]

Suction lift is quite possible with a standard centrifugal. The lift possible is simply the difference between NPSHa and NPSHr, although you want to keep a bit of margin in there.

You do not need a self priming pump IF you have a way to maintain the pump prime when the pump is shut down (generally, a foot valve), and a way to prime the pump on initial installation.

 

[Artisi]

Your first problem will be finding a centrifugal pump for such a low flow at the head required.
Are there any solids in the pumped liquor?
Why replace the submersible?


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.)

 

[bimr]

Agree with the previous comments. Whether you need a self priming pump will depend on if the pump shuts down, as the pump will have to be primed again each time the pump restarts.

Check with Gorman Rupp to determine if there is a pump available in the flow and pressure range that you request.

 

[Compositepro]

Google "shallow well" and "deep well" pumps.

 

[LittleInch]

Tilas,

1. Is it common for a centrifugal pump to handle this negative suction pressure?
Yes, a 4m lift should be within the range required, but you're eating into your NPSH number so be careful with other losses and vapour pressure

2. Do I need to opt for a self priming pump or just the normal centrifugal pump?
If you can
A - somehow get a first fill of water and
B - ensure that the pump doesn't backflow or run dry then a normal centrifugal will do

If not then get a self priming pump or use a PD pump of some sort.

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

 

[Tilas]

TenPenny , Artisi , bimr , Compositepro , LittleInch,

Thanks for your valuable responses.

I am having difficulty in getting selection from Vendors (Self Priming Pumps) since the flow/head is too low.We might go with a bigger capacity pump self priming pump and just add a continuous re-circulation line in this case.

There are some solids in the stream about but we don't know the particle size. The analysis does not mention it. TSS is around 48 mg/l.

We are considering a suction strainer and a foot valve for the application also as it is to be operated intermittently several times a day.

Thanks,
Tilas

 

[Artisi]

Foot valve and strainer on sewage effluent is asking for big trouble. Why are you not replacing the submersible with a submersible?

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.)

 

[NickJ67]

I would completely agree about the foot valve and strainer. The latter will require maintenance that won't get done and thus clog up and the former won't like the debris in the water and fail to seal, becoming a useless partial obstruction.

In my opinion this is a PD pump application (progressive cavity pumps get used alot for this) due to combination of low flow and relatively high head, though the low flow means a small (and thus easily blocked) machine, so in-line macerator up front would be the usual way to go. Many small rural sewage pumping stations in the UK are done like this.

Nick

 

[bimr]

Considering the issue of the limited number of pumps that are available that will meet the performance requirements (very low flow and high discharge head), you may want to consider another type of pump. For example, the E-One pump station is capable of meeting the performance requirements.

E-One Link

 

[TenPenny]

I agree that a PC pump might be a really good choice, they check all the boxes.

 

[Tilas]

H Artisi,

They are changing the submersible to a self priming due to maintenance difficulty.

To add some more information , the liquid is not totally sewage water . TSS is around 48 mg/l and particle size of is about 3mm max in size.

Just another question, if we install a foot valve and a strainer , is there a possibility of making an arrangement that the strainer and foot valve could be pulled out from the pit when doing maintenance so that we do not need to go inside the pit/drain the pit every time the strainer gets choked?

We are also thinking of putting another spare suction point complete with strainer and foot valve in the pit so if the operating strainer gets choked and we still need to run the pump , we can just move the suction of the pump to the other spare suction point. We can then complete the job and do the maintenance later for the choked strainer.

NickJ67 , bimr , TenPenny ,

Thanks for the response, i will definitely look on the PC pump option and try to convince end user (if i find a process compatible PC pump).

Thanks,
Tilas

 

[Artisi]

There are strainer/ foot valves available that are arranged with an external connection to the foot valve so you can lift the valve from its seat to allow the liquid above the f/t to flush the strainer.
If you had maintenance problem with the submersible - expect trouble with the proposed fix-up.

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.)