Diesel Driven Fire Pump

[hgordon]

Hi all,

I am troubleshooting an old diesel driven fire pump (sprinklers) which is giving less pressure boost at 5600 l/min than it used to.
In 2009 it produced 5600 l/min at 680 kPa at 1900 rpm, but in 2018 it produces 5600 l/min at 540 kPa at 1900 rpm.
Talking to a diesel guy, he says maybe the diesel has lost the torque it used to have (installed since 1980 Ford Leyland).
Which sounded logical - but would that not mean in 2018 at 5600 l/min torque load the rpms would decrease below 1900rpm because the torque is not there from engine to keep the revs at constant 1900 rpm?
Instead the revs are constant but the boost pressue is not - sounds to me more like to pump imepller is the issue?
Because the pump is still getting the 1900 rpm but not putting out the boost (impeller is always same).
This sounds right?

Thanks,

 

[TugboatEng]

Your diesel guy is flat out wrong. If the engine were losing power it would not come up to speed.


Centrifugal pumps simply add pressure to their suction pressure. If your city main pressure has decreased your discharge pressure will decrease by the same amount.

A reduction in head without a reduction in flow indicates deterioration of the impeller or wear rings in the pump casing.

 

[hgordon]

Thanks for reply,

For the diesel guys defence he didn't have all the details.
The pump is straight off a tank of about 1m of water above pump suction so nothing is really changed there, and the suction pressure is only dropping to -5kPa with the 5600 l/min so a suction restriction shouldn't be the issue.

In the meantime we will be looking at cranking the diesel revs.

 

[ornerynorsk]

I'd check impeller clearances, as TugboatEng mentioned. I've done some of these and a little deterioration and wear can make a big difference.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[NickJ67]

Probably better in the pump engineering section?

Do you have control of the discharge pressure and if so, are you saying that you have to back the pressure off (open snubber valve or whatever) in order to get the flow to come up to 5,600 l/min @ 1,900 rpm? If this is the case then I would completely agree that impeller clearances / condition needs checking as other have previously suggested.

If you don't have control of the down stream condition then it could be that something in the system has changed, reducing the back pressure on the pump.

Nick