Centrifugal Pumps and their Curves (Fire Pumps)

[hgordon]

Hi there,

I am getting my head around fire pumps which start at a constant speed direct on line.
When a pump starts on an empty system there is no back pressure then the pressure builds from friction or lift.
So to me it starts at max flow then the flow reduces as the pressure builds until say the water meets a dead end.
Visualising that on a pump curve - it doesn't follow a system curve because the pipe system is not constant.
It seems the flow and pressure from pump start until churn would follow the pump curve from right to left, is that right?
So for a period the pump is running way off its curve until the backpressure builds to about 100m head? As per attached jpg.
Of course we would never run a system like this due to the massive water hammer that could occur - this is just theoretical.
We use jockey pumps to keep the discharge primed around the churn pressure. However there is another idea of using a pressure relief 'surge' valve also to help water hammer which I am not sure about, the valve is open on start up to release the initial water surge then closes at a set pressure. What if the relief pipeline has not enough backpressure and the surge valve never closes?? And if the surge valve is set to close at only 1-2bar what is the point in having it? If anyone has any info on the benefits on these surge valves would be apprecaited, this is a Cla Val surge/relief valve.

Thanks,

 

[LittleInch]

Fire pumps to NFPA are a law unto themselves.

However you don't have a flat pressure fire system due to the jockey pump.

I don't really understand what this valve is and what it is doing. Do you have a link or details.

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[hgordon]

Thanks for reply,

What is a flat pressure fire system? I thought a jockey pump will help to create a flat pressure fire system?

The valve is here:

https://www.cla-val.com/50-49-pump-startbrpressure-relief-valve-p-149.html

It stays normally open unlike a typical relief valve, to let the initial pump surge be redirected elsewhere.
I think the benefit to this valve is the fact it closes slowly once the surge has past, softening the pressure increase within the pump discharge.

Thanks,

 

[LittleInch]

By "flat" I mean not pressurized.

I think this is a particular operation where by there is an empty seawater fire system which is flooded by the pump.

The valve appears to have two control pilots. One is open at low pressure then closes as the pressure rises, the second acts as standard relief valve.

Clearly the issue is that if operated or set incorrectly, the pressure in the system never rises as all the water is being tipped over the side of the rig ( as in the animation) and hence you don't get to the operating pressure of your firewater system.

It's a new one to me for sure and your initial summation seems about right, i.e. if you try to fill an empty no pressure system your pump on a DOL starter will wizz right off the right hand side and usually trip on excess amps or vibration.

The fire water pumps shown on the cla val video are clearly more robust units as they are somehow sucking water up an unfeasibly high distance and operating on the RHS of the curve during initial operation. Most electric fire pumps I've ever heard of are submersible units in the rig legs and hence have a climbing static head right from the start.

Each system will be different depending on your particular circumstances.

Fire systems, pumps and motors tend to be pretty strong and a bit brutal in operation as you don't want a failure, but this is one reason why jockey pumps exist.

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