15 replies to this topic

[anshumanag]

Hi all,

I am looking for the meaning for the statement "Pump is design for end of curve operation". It is mentioned for intermittent pump.

Thanks in advance.

[fallah]

Hi all,

I am looking for the meaning for the statement "Pump is design for end of curve operation". It is mentioned for intermittent pump.

Thanks in advance.

It means the motor would be designed such that the pump can handle the flowrate corresponding to H=0 on Q-H curve,regardless of intermittent or continuous operation.

[process_engg]

fallah can you explain... what do you mean by H =0? Can you explain this a bit and give a real time example of this statement?

[fallah]

fallah can you explain... what do you mean by H =0? Can you explain this a bit and give a real time example of this statement?

That is very clear.

H=0 means while differential head of the pump would be equal to zero and the system curve converts to a line just on Q axis.Therefore system curve and pump performance curve meet each other on Q axis where H=0.This point would indicates the pump operation at maximum flowrate.

The real time example is when a breakdown occured in a point of discharge line such that discharge head would be very close to suction head.

Hope this could clear the matter.

[Jiten_process]

Please find attached sample curve where i have marked the point which is called as end of the curve.

@fallah,
as shown in curve you can see that end of the curve is not at the point where H=0, the end of the curve depends on the model of the pump which is a vendor item. H=0 is the point where flow becomes infinite, which is not possible. isnt it?

generally, motor is sized taking 20-25% marzin on BKW for the operating point as per curve and taken next size higher standard motor rating. In some cases motor is sized keeping next higher standard rating on the point where curves end, which is the case you have.

Please correct me if i am wrong.

Attached Files

•  pump_curve.xls   199KB   284 downloads

[fallah]

Please find attached sample curve where i have marked the point which is called as end of the curve.

@fallah,
as shown in curve you can see that end of the curve is not at the point where H=0, the end of the curve depends on the model of the pump which is a vendor item. H=0 is the point where flow becomes infinite, which is not possible. isnt it?

generally, motor is sized taking 20-25% marzin on BKW for the operating point as per curve and taken next size higher standard motor rating. In some cases motor is sized keeping next higher standard rating on the point where curves end, which is the case you have.

Please correct me if i am wrong.

In the cases similar to that you presented as a sample,you are right.

But we may faced with steep performance curve having intersection with Q axis and relevant pump is going to be started with open valve.In this case just at start-up moment considering no resistance to flow, we may expect to have a head near zero with maximum flowrate."No resistance to flow" conditions also may occured when we have a sudden breakdown in discharge line of mentioned pump (steep curve) close to its discharge nozzle during normal operation.This was the logic behind of my previous conclusion.

Of course,now i think generalizing the issue for all cases is wrong,and also as you mentioned, with high probability there is no case in pump operation leading to H=0.

Thanks a lot for your precise attention.

[DRS]

I would like to present a small clarification here. Though end of the curve operation is what exactly Jiten mentioned, this will not entirely take trouble out of the operation. Flows corresponding to zero head (more specifically called as Run Out condition) may still happen, as in the cases suggested by Fallah. There may even be chances that pump may just run beyond the end of curve (not necessarily upto run out condition). We should note that pump curve doesn't end exactly at the point where the manufacturer ended it. There can be many reasons as to why the manufacturer ends the curve where as pump can still run beyond this.

The first case is that manufacturer may be suggesting us to chose another pump (with a bigger impeller or another model) beyond this point, so that the efficiency is better and pump operation is optimum.

Secondly, flows beyond the end of curve may warrant higher NPSH otherwise the pump may cavitate.

Third reason is that the pump may be non overloading type beyond a certain point.

Another clarification is that the maximum discharge of a pump is a function of the impeller diameter and the rotational speed and there is a limit for that.

[fallah]

Secondly, flows beyond the end of curve may warrant higher NPSH otherwise the pump may cavitate.

You are kindly requested to clarify the above matter.

[rsk]

In case of centrifugal pumps, these are started with discharge MV in closed condition as otherwise these will start & follow the performance curve from the end of curve. This may require sizing of motor to be corrersponding to end of curve thereby increasing cost. above is true for pumps with radial characteristic where power required increases with flow. But with mixed ie radial and axial characteristic power decreases with flow, so why these pumps are not started with MV in full open position, instead started with MV in crack open position.
regards

[Qalander (Chem)]

I jump in with my little bit;During my stay with my previous employer we use to slightly crack open MOV or Hand operated Block valves.

Since otherwise a soft start of these Electric Motor driven multistage centrifugal pumps was not possible as

either high Amperes observed for longer period(corresponding to initial starting torque) and tripping occur.
or
the pump tripped on overload initially(if valve opened excessively)
Hope this helps
Qalander

[DRS]

NPSHr published by pump manufacturers is a function of impeller geometry. This increases with increase in flow rate. Though NPSHr can be calculated, pump manufacturers do practical testing to arrive at NPSHr data.

Maintaining NPSHa equal to that of NPSHr doesn't mean trouble free operation as NPSHr is slightly a misnomer. NPSHr actually corresponds to NPSH where the performance(differential head) of a pump reduces by 3% (NPSH 3 is more likely to replace NPSHr in future). This is the reason why Hydraulic Institute recommends a margin of 1.1 to 2.5. Your pump manufacturer can tell you what margin you have to maintain. As the performance curves are generic, I opine that manufacturer may built in the safety margin and cut off the curve at certain flowrate beyond which it is difficult for us to maintain this margin.

Hope I made my view clear.

[djack77494]

This may be a very simplistic approach to the situation, but "bad things" may happen if you attempt to operate beyond the curve as presented by the pump manufacturer. There are various parts to these "bad things", including mechanical and hydraulic aspects which may change rapidly with small changes in flowrate near the end of curve. Best to avoid that area.

[std]

The real time example is when a breakdown occured in a point of discharge line such that discharge head would be very close to suction head.

Can you explain this that how breakdown make close discharge and suction pressure?
Can you give some other example and when end of curve operation is used?

[djack77494]

The real time example is when a breakdown occured in a point of discharge line such that discharge head would be very close to suction head.

Can you explain this that how breakdown make close discharge and suction pressure?
Can you give some other example and when end of curve operation is used?

???std, I am puzzled by the quote which is not mine despite being attributed to me. Furthermore, I cannot answer your question since I don't know anything about a supposed "breakdown". High flowrates equate to low head, but I've never heard of concerns about suction and discharge heads being close. I also don't like that terminology. There is suction pressure, discharge pressure, and pump head. So far as I know, pump operation to the right of the BEP (best efficiency point) is generally discouraged.

[std]

[/quote]

The real time example is when a breakdown occured in a point of discharge line such that discharge head would be very close to suction head.

[/quote]

I am sorry djack.... The question is not intended for you.

fallah Can you explain this that how breakdown make close discharge and suction pressure?
Can you give some other example and when end of curve operation is used?

[fallah]

I am sorry djack.... The question is not intended for you.

fallah Can you explain this that how breakdown make close discharge and suction pressure?
Can you give some other example and when end of curve operation is used?

-Mechanical breakdown due to any reason
-Pump start-up with open discharge valve