main and reserve pump turn on sequence 3

[Erfan z]

hi fellows;
as per client demand, in one of our projects, we have 3 line pump assembly
1 jockey pump and 2 main pumps
we arranged the pump turn-on sequence as explained below:
1) after a minor pressure drop, the jockey pump came online and started to raise the pressure
2) if the pressure drop continues, the main pump no.1 starts.
3) if pump no. 1 failed to provide the demanded pressure, pump no.2 started, and 3 sets of pumps worked together Simultaneously.
so I have these questions about this type of pump work sequence:
1: After pump no.1 failed to provide pressure, is it wrong that pump no.2 started Simultaneously with pump no.1?
2: during the fire, if we want the main pumps to work as a single unit and not work Simultaneously in case of excessive pressure drop, how could we synchronize two controllers of pumps to each other?

 

[skdesigner]

I assume that your pumps are arranged in parallel, so that neither pump relies on the other for its water supply.

I will further assume for question #2, that the word 'simultaneously' should be replaced with 'sequentially' (i.e one after the other).

If you're using diesel engines as drivers, there's no need for a time delay from one to the next. They can both crank at the same time without issue. Starting two large electric motors at the same time is a massive load on the electrical system, and NFPA 20 requires a sequential start to avoid that (see 10.5.2.5 in the 2016 edition).

So to answer your questions:

1 - No, it is not wrong that the second pump started when the first pump could not maintain system pressure. That's what it's there for.

2 - If you want both engines to crank simultaneously on a pressure drop, you need to set both controllers to start the engines at the same pressure. Note that this only applies to diesel-driven pumps. For electric motor-driven pumps, NFPA 20 prohibits simultaneous starting of multiple units.

 

[RRaghunath]

The description suggests that the pumps are art of Fire water system.
Jockey pump is smaller in rating and is meant to cater to small leakages during normal service (without a fire incident). If the pressure drop is excessive under normal conditions, it means there are heavy leakages in the fire water piping which need to be addressed on urgent basis.
Discussion about starting of Main pumps is not important here.
In case of real fire, the pressure drop is genuinely high as the water from the Fire water piping is being used for fire fighting and the main pump is expected to start.
The second main pump is installed to act as a stand by to the Pump-1 and thus is not required unless there is genuine problem with Pump-1 or if there is some other exigency.

R Raghunath

 

[LittleInch]

It's not that pump one "failed to provide pressure" necessarily, but that this is a very common way to avoid running pumps when you don't need to.

As a fire starts, any hydrant or sprinkler system or monitor opens the pressure falls below what the jockey pump can provide as its flow is typically 5% or less of a main FW pump. As more hydrants etc are opened up then the flow increases from pump 1, but as it goes along the pump curve then the pressure falls to the point where the second pump kicks in to add to the flow and increase pressure ( there is usually a dead band or timer on this pressure switch so that it doesn't start and stop too many times).

Sometimes systems are set up with 2 x 100% units, sometimes both are required for a "full scale" fire.

So e.g. you would find pressure switches set at something like
Start / stop Jockey pump - 8 / 10 bar
Start / stop Pump 1 - 7 / 9
Start / stop pump 2 - 6 / 8

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