Situation when not enough liquid to be pumped 1

[Loo Kian Sing]

Let's say a centrifugal pump is designed to operate at a flow rate of 10 m3/hr with head of 50 m.

If the liquid from the suction is not enough to be pumped (only 5 m3/hr liquid is available):

1) Will the pump still function at the designed rpm?
2) Will the pump capacity still remain as 10 m3/hr or change to 5 m3/hr?
3) Will the NPSHr of the pump changes (that might caused by the capacity of pump)?
4) Will the suction pressure and NPSHa decrease?
5) Will the cavitation happen (due to the insufficient liquid to be pumped) although NPSHa (after decreases of NPSHa that might caused by insufficient liquid available) is still higher than NPSHr?

Thank you for anyone who pay attention to the question i ask.

 

[LittleInch]

Loo Kuan,

You're making this more complex than it is. If the water level in tank A falk below the inlet line to pump B but some water is still flowing in, then pump B will start to pump a mixture of air and water.

Centrifugal pumps don't like doing this (pumping two phase fluid) so working out what the head is well be difficult, but the fliw will equal the inflow into the tank. Operating like this for a long time will damage the pump.

Think of it like a washing machine pump. When you first turn it on water flows out fast until the drum empties and then the flow starts to reduce and pulse. Those pumps are designed to run dry or partly water filled, most others are not.

Cavitation is something very different.

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

 

[hantrongtai1]

1)Yes
2) Will the pump capacity still remain as 10 m3/hr but runing 30min
3) no
4) no
5) no

 

[Loo Kian Sing]

Dear Artisi,
For your information, for this thread i really dont have any installation sketch or much information. I am just curious of what might happen if incoming supply cannot meet the pump demand (suddenly).
Thank you for your willingness to help. I will try to input more detail, sketch, drawing and etc. if I ask something in the future. Thank you again for your advise. Appreciate it.

Dear LittleInch,
Noted of your explanation and thank you. For my understanding, vertical pump, submersible pump or self priming pump can be used if tank fall below the centerline of the pump (depend on application also). How about PD pump to be used in this similar situation if application (system, fluid and etc.) is suitable?

Dear hantrongtai1,
Thank you for your answer.

 

[Artisi]

To sum up, if supply reduces below pump requirement, tank will eventually empty and the pump will stop pumping.

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.)

 

[LittleInch]

Yes you can use a PD pump as many are self priming and more accepting of a mixture of air and water. However they need pressure relief systems and are often bigger and more expensive. See my reply in your other post (next time keep it all in one post please).

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

 

[pumpingtips]

Loo Kim,
When the tank runs low the pump will pump a mixture of air and water. Therefore the pump curve does not apply exactly. Many pumps, like sump pumps run on low water levels and end up pumping air and water. It's not cavitation as there are no high suction pressures. It's just what we call "snoring". Horizontal Slurry pumps also pump a lot of air in situations where there is a lot of froth. maybr this helps you, maybe not if I'm guessing wrong.

 

[Artisi]

pumpingtips: correct - especially NPSHa/r comment, however - the OP has, we assume fixed installed centrifugal units and air entrainment will kill their operation. For froth pumping , depending on the froth factor - the pumps may need to be sized 2 - 5 times larger than the actual liquor flowrate and would be "froth pumps" ie, vertically mounted tank pumps.
Posted for interest of the OP to save more confusion.

https://tse2.mm.bing.net/th?id=OIP.KKRQOAS7-I1BKg0CyF5xEAEsEO&w=214&h=193&c=7&qlt=90&o=4&pid=1.7

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.)