Third Pump - Significant decrease in flows 8

[catplank]

Hello All,

I’m a little stumped the following… any able to offer any insight?

Normal Operating conditions 2x pumps in parallel. Third pump kicks in on level control. When third pump kicks in its flow/current in the middle pump (pump 2) is significantly lower.

Pump 1 & 3 = 40L/s @120A
Pump 2 = 30L/s @100A

If pump 2 is operating under duty conditions it is inline with the other pump (maybe 1-2L/s lower)

I've attached a section view of the pump station, my thoughts is that it needs an inspections to wear ect but I've been told that its always happenened (even other pump i.e. 1 or 3 were doing the same).

 

[moideen]

The impeller might slightly be more trimmed on 2nd pump from beginning, and when running alone it will pump more due to the less friction loss in the circuit than working pair with 1&2. Check the shut of head of each pump whether it is identical.

Thanks
Moideen-DUBAI

 

[LittleInch]

Running only one unit looks to be a bad idea as you are at or off the end of the pump curve.

A simple dead head check will tell you something more, but it's looking like pump 2 isn't the same as pumps 1 & 3.

Incidentally it looks like your normal flow of 49l/sec is way too close to the end of the curve. Poor efficiency and potential for long term damage.

But without shut in head and differential head at the flows you quote we're all just guessing.

Also see why pump 2 looks newer than the others.

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

 

[Artisi]

Starting to sound like the kids song, "The wheels on the bus go round and round all day long". Slimply flow test 1 and 2, 2 and 3 and 1 and 3 and 123 to get some decent data to work with, plus is 2 the same as 1 and 3.

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.)

 

[catplank]

Hi All,

Apologies for such a delayed update (I was struck down with C19...). Two important pieces of information has now come to light.

1. Pump 2 - whilst specified as an exact duty match has a slightly different curve due to model changes in the product line (the third pump was added about 10 years)
2. The original duty point selected wasn't the best in reality there is between 25-30m less head in the system (namely due to a mine change that changed the pump station elevation and this wasnt picked up)

I have since throttles the pumps to push them to a better operating point their respective curves - Now in 3x pump operation the flows are much closer (although P2 will never match P1 & P3 - due to the difference in the curves)

Thanks all for your help with this and apologies for the long delay in closing it all out

ps. If you can make sure you get vaccinated!

 

[TugboatEng]

Thank you for the follow-up.

Get 3 doses if you can, I've had mine and im doing fine.

 

[Artisi]

Good result and great that you got back with the answer, most don't, they just disappear into thin air.
Also, this was a good exercise in pointing out the need for good accurate numbers, without flow and head it was all guess work or in your case finding that the operating system had been changed.

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]

catplank,

Thanks for letting us know and being able to close this out and seems we did finally hone in on pump 2 being not the same as the others.

Glad you got it sorted and also that you've got over C-19.

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