Always-Running Domestic Pumps OK with Fire?

11/30/2022

I am doing some work on a combined fire-fighting/domestic pump house for a small community.

A couple things look odd to me.

They have a 12-inch flanged basket strainer after the pump discharge just before the mains. I was under the impression that any devices or fittings between pump discharge and need to be listed for fire protection use.
Instead of a jockey pump, they have three potable water pumps which run all the time pushing potable water to the mains. The primary fire pump automatically operates when the pressure drops to 4.5 bar and the backup pump at 2.3 bar.

There seem to be two deviations from NFPA standards here:

First, the jockey pump should be small , so that in case of fire it is the fire pump taking the load, and second, the starting sequence requires the jockey pump to start above the fire pump churn, the first fire pump a little lower and so on.

Is there any exception for such combined domestic/fire protection systems in the NFPA line of standards?

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3 Comments

Dan Wilder

11/30/2022 08:07:34 am

I had a similar setup with a house on a hill with the feed main serving both a hydrant (long road up the hill and the FD did not want to run supply hoses up/down the hill during winter and possible snow/ice), fire protection residential system, and domestic.

The fire pump house at the base of the hill included 2 domestic pumps (primary and backup) that acted like the jockey pump. We did need to adjust the FP/JP on/off pressures to be lower than the working domestic pump to avoid the JP running full time and false starts from the FP. The domestic pumps were strong enough to run the FP system on their own.

Our solution (with FD/FM approvals) was to still provide the jockey pump in case the domestic pumps were not operational (still knowing that the fire pump would likely kick on about the same time the JP would, and later confirmed during one of the commissioning scenarios...only about 5PSI difference with a delay). The domestic pumps did most of the work and there was discussion about an interlock to shut down the domestic pumps when the fire pump kicked on, but the FD/FM wanted to keep all available resources (water supply was not an issue due to the lower overall demand for the single hydrant, domestic demand, and FP system)

As to the strainer AFTER the pump....seems a bit backwards as this does nothing to protect the actual device providing the pressure and flow from damage. There are listed strainers available however this wouldn't be a big sticking point to me as long as a ITM schedule was followed.

Panos

12/1/2022 04:08:57 pm

Thank you Dan and Glenn! Valuable information!

After reading your replies, I spent some time on NFPA 20, and found some interesting things:

"Fire pumps that are installed in parallel with domestic pumps on a combined fire/domestic water distribution system require special consideration.
The domestic pumps should operate at or close to the fire pump pressures.

For systems that use fire system piping for both domestic and fire protection purposes, the domestic pumps typically maintain the system pressure. (4.27)
The operating pressure should be maintained close to the fire pump churn discharge pressure to minimize pressure surges.

The pressure maintenance pump shall be sized to replenish the fire protection system pressure due to allowable leakage and normal drops in pressure. (4.27.2.1)

Pressure maintenance pumps shall have rated capacities not less than any normal leakage rate. (4.27.3)

Pressure maintenance (jockey or make-up) pumps are generally low-flow, high-pressure pumps. For sprinkler systems, jockey pumps are usually sized to flow a quantity of water less than or equal to that required by a single sprinkler.

The pressure differential between the pressure maintenance pump and the fire pump should be a minimum of 10 psi (0.68 bar) to avoid false starting of the fire pump due to pressure fluctuations in the system piping.

A rule of thumb for sizing jockey pumps supplying underground piping is to use 1 percent of the fire pump rated capacity and add 10 psi (0.68 bar) to the pressure rating of the fire pump.
The fire pump system, when started by pressure drop, should be arranged as follows: (A1.4.2.6)

(a) The jockey pump stop point should equal the pump churn pressure plus the minimum static supply pressure.
(b) The jockey pump start point should be at least 10 psi (0.68 bar) less than the jockey pump stop point.
(c) The fire pump start point should be 5 psi (0.34 bar) less than the jockey pump start point. Use 10 psi (0.68 bar) increments for each additional pump."

So, the existing setup definitely defies NFPA 20 in terms of pump sequence pressure settings.
Regarding the strainer, NFPA 20 does not specifically address strainers after pump discharge, but there are quite a few indirect recommendations:

"The discharge components shall consist of pipe, valves, and fittings extending from the pump discharge flange to the system side of the discharge valve. (4.17.1)"
(nothing else I may add)

"Listed sprinkler system strainers are installed within a sprinkler system to help prevent the flow of foreign debris, such as grit, stones, leaves, etc., beyond it. It is typically required in sprinkler systems equipped with small K-factor sprinklers and the strainer must be cleaned after every operation, including flow tests. "

So the intent of the Standard is not strainers with flowing water continuously.

Glenn Berger

11/30/2022 08:24:18 am

In my read of the comment, you have a pump that provides additional pressure when a fire flow event occurs. I do not see conflicts with NFPA standards.

Always-Running Domestic Pumps OK with Fire?

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