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Situation when not enough liquid to be pumped

Situation when not enough liquid to be pumped

(OP)
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.

RE: Situation when not enough liquid to be pumped

More student questions?

It doesn't make sense as how is the inlet flow restricted to 5m3/hr?

Throttling on the inlet to pump to reduce flow is a bad idea as it usually leads to cavitation.

Draw your system and describe what is happening better and convince us you're not a student.

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

RE: Situation when not enough liquid to be pumped

Simple answer is the pump is too big.
If only a short term requirement - maybe you could impose more head on the pump and move the operating point to the left (5m3h), however - to sanction this you would need to supply the pump curve and installation data etc. for clarification.

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.)

RE: Situation when not enough liquid to be pumped

(OP)
Thank you for spending your time to response to my thread.
I appreciate if you can treat me as a student. I don't have much experience as I have less than 1 year working experience in Engineering field. I don't know what is the "more detail" that I need to share. I apologize if I am asking stupid questions.
If the following situation happens:
Pump A pumps the water from source to Tank A. Pump B pumps the water from Tank A to Tank B. Let's assume the capacity of both pump is the same. One day, there is a leak located somewhere from Pump A to Pump B (if nobody discover this). Until a time that the volume of liquid is not enough to be pumped by Pump B (I don't know if this situation will happen). Cavitation will happen at Pump B (but it is not due to insufficient NPSHa)? Pump B will still run at same speed (not using variable speed drive), the capacity will decrease (due to insufficient water)?
I apologize again if you still cannot understand what I want to describe. I apologize if these description is not professional. I apologize if these questions are easy to answer or no solution as I don't provide sufficient details.
However, thank you for viewing my response.

RE: Situation when not enough liquid to be pumped

Still unclear what your problem is and what you are trying to do.
Forget about NPSH and speed, treat it as 2 separate functions. If pump A has a leak in the discharge the pump could overload due to increased flow. If pump A doesn't supply enough water to tank A, pump B will eventually run out of water as incoming supply can not meet the demand of pump B.

So what is/are your question/s?

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.)

RE: Situation when not enough liquid to be pumped

(OP)
"Cavitation will happen at Pump B (but it is not due to insufficient NPSHa)? Pump B will still run at same speed (not using variable speed drive), the capacity will decrease (due to insufficient water)?"
These are the questions and I thought it is obviously the questions?
Ok, let me explain more. As the incoming supply cannot meet the demand of pump B, it means the capacity of the water being pumped is reducing. In this case, I don't know if pump curve is still applicable. As the capacity (flow rate) is reduced, the head will be increased, NPSHr will be decrease. But NPSHa might not changing much, as tank A is still located at the same height and pressure drop along pipeline is small. If NPSHa is still higher than NPSHr, there will be no cavitation. Is my understanding correct?
I am not sure that if incoming supply is not enough mean capacity (flow rate created by pump) is reduced. I am not sure if pump curve is still applicable in this situation. I am not sure if cavitation will happen.

RE: Situation when not enough liquid to be pumped

(OP)
I am not sure if these relation of parameters (flow rate, head, NPSHr) is still application when the incoming supply cannot meet the demand of pump. I am not sure if pump curve is still valid in this situation.
I apologize as I do not know much. I haven't see thing much and I am lacking of experience. Am I asking in a wrong way? Can you understand what I want to deliver? If yes, how can I deliver it clearly? Any example or advise to me?
Thank you again for your reply.

RE: Situation when not enough liquid to be pumped

Why will pump B cavitate, yes flow will vary slightly if static head in tank A is reducing, but only until the tank volume and inflow doesn't meet pump output, pump B will then go off prime and cease pumping.

As I said earlier, treat the installation as 2 separate systems because that is what you have, pump A to tank A, and pump B pumping from tank A. You should sketch out the installation and post the pump curves etc.

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.)

RE: Situation when not enough liquid to be pumped

(OP)
"pump B will then go off prime and cease pumping", is this mean all pump have this feature (although I am not installing any instrument or control system)?
Or what is mean by this sentence (maybe I get it wrong)?
Thank you.

RE: Situation when not enough liquid to be pumped

(OP)
The above situation is what I image. What I want to know is the situation might happen when not enough liquid to be pump. Maybe I should write the incoming supply cannot meet the demand of pump (as I don't know what is the better term to be used). In this case, should I draw the installation system too as it is straight forward question (if you understand what I want to ask)?
As I mention, centrifugal pump is used. I thought performance curve wouldn't vary much (I mean the curve on the graph, not the size of the parameter) and this is what I imagined, so I can simply sketch the performance curve (like x axis for flow, y for head or efficiency, and etc), but not giving any value?

RE: Situation when not enough liquid to be pumped

What I want to know is the situation might happen when not enough liquid to be pump

It does not function in this way. The pump cannot be dissociated from the system.
It is where the system and the operating curve exactly intersect that things will start to function.
If your system curve is far off your pump´s curve, what have you done?

If what you mean is that the system (curve) has changed, then you should reformulate the original question.

RE: Situation when not enough liquid to be pumped

(OP)
System curve represents the friction loss along the pipeline and it is constant if the system does not change. Am I right?
If the incoming supply cannot meet the demand of the pump, can i still refer to the pump curve and evaluate all the parameter at the point where the capacity of the incoming supply locates (let's say the incoming supply is reducing to half of the capacity)?

RE: Situation when not enough liquid to be pumped

NO, re-read my post where I said that the pump will continue to operate until such times as the liquid in the tank and the inflow are less than the pump output capability, 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.)

RE: Situation when not enough liquid to be pumped

You should stop pump B if the water level in the tank drops to a minimum level. You can have automatic controls to start/stop the pump at level 1 and above level 2 respectively.

RE: Situation when not enough liquid to be pumped

Loo Kian Sing

- Say system curve is constant.
-> Put no pump, you get no flow.
-> Put a small pump, you get little flow.
-> Put a big pump, you get big flow.

As long as you have big pump running, you will get that big flow. Ok?
And big flow will remain until the pump will suck the last drop of liquid available as long as its controls (provided it exist) do not trigger a stop (min. liquid level, etc.).

PS: If you restrict the supply, for example by mean of throttling a valve upstream, you effect the system curve. There is no such thing as short supply, its due to an intervention.

RE: Situation when not enough liquid to be pumped

(OP)
"the pump will continue to operate until such times as the liquid in the tank and the inflow are less than the pump output capability, the pump will STOP pumping. " But if no control system to the pump, it will stop automatic when the inflow is less than the pump capability?

So, if incoming liquid is not enough until it is less than the pump output capability, there will be cavitation inside the pump, is my understanding correct?

RE: Situation when not enough liquid to be pumped

Pump can spin without any fluid flowing if not protected by a control switch or whatever protective measure.

I suppose a tank at the suction can deplete without necessarily inducing cavitation. If the NPSH available vs. required is favorable and control switch does not trigger stop, ultimately pump will spin without any fluid flowing. Guess it will first deplete the suction tank, the feeding line and then it will start pushing air or whatever gas is there - very ineffectively.

RE: Situation when not enough liquid to be pumped

OP, do you understand what cavitation is? Suggest you obtain a few books and do some reading on what a pump is and what they can and can't do.
You are currently going round in circles going nowhere.

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.)

RE: Situation when not enough liquid to be pumped

(OP)
When incoming supply is not enough, will pressure drop also?
If NPSHa drop to a certain level (let's say lower than NPSHr of pump), then cavitation will happen. Is my understanding right?
Sorry, I know I am lacking in knowledge. Sorry.

RE: Situation when not enough liquid to be pumped

Think you should sketch your installation so we can see what is going on, at the moment it's all guess work.
NPSHa / NPSHr.
Under some inlet configurations reduced flow can cause a pressure reduction while others it won't.
If you want meaningful help - supply the necessary information.

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.)

RE: Situation when not enough liquid to be pumped

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.

RE: Situation when not enough liquid to be pumped


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

RE: Situation when not enough liquid to be pumped

(OP)
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.

RE: Situation when not enough liquid to be pumped

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.)

RE: Situation when not enough liquid to be pumped

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.

RE: Situation when not enough liquid to be pumped

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.

RE: Situation when not enough liquid to be pumped

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-I1BKg0...

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.)

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