Diesel Fire Pump Test Loop Overheat

[Oremus]

I know I've seen something online concerning the issue of diesel fire pumps with a flow meter & test loop overheating the water in the loop past the recommended levels. Is this a common occurrence and what can be used to prevent it?

 

[DRWeig]

Hi Oremus,

Please wait for answers from the more experienced fire protection engineers, but here's my take on it:

Any pump (diesel, electric, whatever) can overheat when pumping in a closed loop. Centrifugal pump efficiency is less than 85% typically, the rest of the consumed power goes into heating the water (paddlewheel work).

Good on ya,

Goober Dave

 

[Oremus]

Judging by the lack of responses is this really a non-issue?

 

[786392]

Dear Although clarified above by DRWeig above;I put this slightly different terms for you.

If You have centrifugal pump(Whether driven by diesel engine or some other prime mover)the principal is to change kinetic energy head into pressure head.

If we run centrifugal pump in a closed loop recycling back into suction with little or almost No flow condition

part of the kinetic energy changed into heat energy due to friction etc. is increased(added) continuously as the " flow-out" to take away 'heat energy' is either less than needed or non-existent.

Hope this explains your query with explicit details!

Best Regards
Qalander(Chem)

 

[pipesnpumps]

Well the purpose of the circulation relief valve on fire pumps is so that if the pump is dead-headed (NFPA calls it @ churn conditions), then the amount that flows out of the relief valve is enough to offset the friction heat gain.

However, running in a loop that has little flow restriction, there is going to be a large flowrate in the recirc line. More flow = more current draw and more hydraulic horsepower which turns into friction heat, and the water beings to heat up. The less the flowrate, the less the heat.

It is a different situation than at churn, where there is only a small amount of flow (gpm=the relieved flowrate) and hydraulic work being done.

 

[Oremus]

So the pumps themselves come pre-equipped with the means to deal with any overheating and no extra equipment is necessary?

 

[DRWeig]

No, only if pump is deadheaded (discharge valve closed) will the relief operate.

If you're pumping full flow in a short closed loop, you can still boil the water if you run it long enough. Need to calculate the amount of water in the loop and the rate of heat input (pump inefficiency x horsepower at test conditions) to determine how long it'll take.

Or else you can just monitor water temperature...

That's what I was taught, and I haven't experienced it in a fire pump -- but I have experienced it in chilled water pump applications, and the principle is the same.

Good on ya,

Goober Dave

 

[skdesigner]

pipesnpumps, a quick correction: Diesel pumps don't use a casing relief valve. Discharge water is either siphoned off and run through a heat exchanger, and then used to cool the driver, or a radiator is installed. Don't have my copy of 20 handy, but I think the reference is at the end of 5.11 in the 2007 edition. That being said, a main relief valve is still required downstream of the pump to prevent overpressurization.

Oremus: I've never done a pump installation utilizing a flow meter instead of a test header, but if overheating is a concern, could the test loop be piped through the same heat exchanger as the cooling line?

 

[skdesigner]

NFPA 20 2007 ed.

5.11.1.7 The requirements of 5.11.1 (automatic relief valve) shall not apply to engine driven pumps for which engine cooling water is taken from the pump discharge.