Pump curve

[WilliamTz]

Hello

If I suspect my pump is generating low Differential Pressure and off the pump curve (see attached in red area) would it be worth closing off the discharge valve to increase the system pressure drop and force the system curve back onto the pump curve. I suspect my pumps are currently not delivering the required flow rate.

Regards
W

 

[Artisi]

Yes.

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]

William,

Your post doesn't seem to make sense unfortunately as you provide no numbers.

You say the pump is producing "low" differential head. How low? This is an engineering forum, please be precise and use numbers.

But then you say you're not getting the required flow?? This would indicate that you are at the left hand side of the curve? What is the flow you're getting?? In numbers.

Maximum efficiency is at a diff head of around 16 to 18 m at a flow of between 150 to 200 m3/hr.

So artisis "Yes" is the short answer, but it doesn't appear to be as simple as you're making out.

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

 

[Artisi]

Yes, because if it's so far out to the right, it could be cavitating to the point flow has virtually stopped.
But without full detail it's all guess work (as USUAL)

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.)

 

[rotw]

What is not delivering the required flowrate, too low or too high flowrate?

If actual flow is too low compared to what you require, your options are:

1/- Increase pumps speed in combination with discharge throttling to restore the operating point into the curve operating range.
The question is, would you have that speed variation capability in place or that can be fitted.

2/- Change the pump for a new one that is bigger, designed for higher flow duty point (and efficiency by the way).

If its too high, throttling the discharge should do the trick.

 

[MJCronin]

What has changed in your system ? .... Is there a story here ????

Let me guess, was this pump used for another service and is now exhibiting these problems in the NEW service ?

Was this relocated pump evaluated for the new service by evaluating the new operating point and the pump curves ???

Be aware that running a pump under continuous valve throttling comes with it's own set of problems. not the least of which is massive energy waste ....

MJCronin
Sr. Process Engineer

 

[LittleInch]

Is this post connected to your two previous posts??

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

 

[WilliamTz]

Hi LittleInch Artisi, MJ & Rotw....
Linked to the pumps question on vibration last week....same pumps...
Getting the pump DPs measured on Friday so Ill be back with definitive results
Gauges on the pumps are faulty and unreliable...all showing different readings...and Im suspicious with respect to total flow from each. They pumps into a combined header that goes on to serve 3 x large boilers 5,4,2MW ea.
Ill be back
Thanks for the feedback

W

 

[LittleInch]

Were those pumps fixed speed or variable speed?

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

 

[WilliamTz]

Fixed speed these ones and the ones that vibrate...start on a soft starter...

 

[georgeverghese]

Check your flow meters - do you have gas bubbles in the flow stream?. Bubbles can give you erroneous high readings, and can also result in low dp at the pump. Look at the source vessel for these pumps - is the operating level adequate to ensure proper degassing of the exit liquid stream?

 

[TugboatEng]

The melting solder type overload heaters (Square D as an example) don't typically return to their original rating after tripping and should be replaced. I don't know that you're using these but have had erroneous motor trips in my past and that was a cause.

Keep in mind that for centrifugal pumps everything working properly is the heaviest load on the motor. Gas entrainment, throttled discharge, cavitation, etc... all reduce the motor load. I would be very suspicious of an electrical side issue if you can confirm the motor is properly sized for the pump.