Pump curve doesn't cross system curve
Pump curve doesn't cross system curve
(OP)
Hi everybody,
Process team have informed me a rated head of 41m, then I have selected a pump.
Now I have been informed that the new head is only 15m.
The problem is that the pump has already been bought! Neither with minimum diameter impeller the pump curve crosses new system curve!
What kind of problem will occur in operation? Will these pumps operate?
See attached drawing.
Tks!
Rodrigo Alves
Process team have informed me a rated head of 41m, then I have selected a pump.
Now I have been informed that the new head is only 15m.
The problem is that the pump has already been bought! Neither with minimum diameter impeller the pump curve crosses new system curve!
What kind of problem will occur in operation? Will these pumps operate?
See attached drawing.
Tks!
Rodrigo Alves





RE: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
Your option is to instal some sort of flow control valve and accept the loss of energy or buy a different pump. See the "system curve" post below for a similar discussion on alternatives. You could replace the motor with a lower speed unit.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
Assume it is 4 pole 60Hz, check with the pump supplier or manufacturer and discuss 6 pole motor or post a pump performance curve of the pump showing H.Q,P,NSPHr - and it can be reviewed here by our intelligent, helpful and always ready to unravel the confusion members.
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: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump curve doesn't cross system curve
Of course if the OP has it fixed in his mind that the existing pump must be used then the effort is wasted in trying to be logical about the problem - so stick on a valve of some configuration and throttle it until you get the required flow.
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: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
http://www.slideshare.net/bin95/143-don-truncentri... ... for awhile. Let the OP evaluate whether he can afford the maintenance and early retirement, the pump's and maybe his own too.
What he didn't say was what flowrate he wants to run. Since he does say his new head requirement is 15 m, that would imply at 130 m3/h. SO, if he can slow the pump's speed down, he can certainly run at 130 m3/h with head reduced to 15 m.
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
Did the process team give you that 41m requirement in writing or an e-mail? At my company, they'd be getting their paychecks dinged to pay for a new pump.
Best to you,
Goober Dave
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RE: Pump curve doesn't cross system curve
"Some pumps don't lose so much efficiency when moving to the right"
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
The other alternative is to see if a slower motor speed can do the job. Standard motor speeds are 3600 rpm, 1800 rpm, 1200 rpm, 900 rpm 600 rpm. The motors get bigger in physcial size the slower the rpm.
You might be able to install a vfd to decrease the speed. But you have to change the motor to be inverter duty rated.
RE: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
First off, looking at that curve, I doubt the pump will do what you want by slowing it down, by the time you get it slow enough (even with a trimmed impeller), your flow rate will be beyond runout.
So, the quick and easy solution is, as suggested above, throttle that baby to add in enough head to bring it onto the pump curve.
The 'proper' solution is to get the right pump. Depending on your pump supplier and what model and size pump, you may be able to convert it to a smaller pump relatively easily, or if your pump supplier is a good partner with your plant, you might be able to work a good deal to trade for the correct size pump. How much piping, foundation work, etc is done?
RE: Pump curve doesn't cross system curve
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
EngMecBra should now go to the pump supplier to get pump curves developed for different speeds. He will now see with accuracy where the system head curve will intersect with the pump curve. His alternatives to slowing the speed down is to put on a sheave and pulley or install a vfd. VFD's do have the advantage of unlimited number of starts and stops on the motor
RE: Pump curve doesn't cross system curve
I don't think the problem is that the pump has already been bought, that can usually be addressed when the difference is reasonably small (trim the impeller a little bit.) The problem is some major change or oversight that was discovered very late. I'm curious what that could have been.
This looks way different than the usual "safety factor on top of safety factor" pump oversizing. Based on that system curve sketch, it looks like elevation is the main component that they changed on you (or, I am putting too much faith in the sketch, which I think is very possible.)
What are some of the system details? Did somebody put parts of the system at a different elevation than originally planned? Did someone forget to consider suction pressure at the pump, and size it for discharge pressure instead of differential pressure? I know I'm not the only one who is curious.
RE: Pump curve doesn't cross system curve
New flow coordinate is Old_Flowrate * new_rpm / old_rpm
New head coordinate is Old_Flowrate * new_rpm2/ old_rpm2
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
RE: Pump curve doesn't cross system curve
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: Pump curve doesn't cross system curve
We don't stop until the questions stop.. sometimes not even then.
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
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: Pump curve doesn't cross system curve
Problem solved.
RE: Pump curve doesn't cross system curve
Independent events are seldomly independent.
RE: Pump curve doesn't cross system curve
I have read all posts, but I don't have any news from pump manufacturer, e.g. new curves considering different speeds and a smaller impeller.
Soon I will bring information to discuss more with you.
Best ragards