Prevent Fire Pump Start with Low-Water Levels?

9/2/2022

Is there any NFPA provision for forbidding a fire pump to start in the event a water storage tank has a water level that is too low?

I'm wondering what might happen if the water storage gets down to ~25% of the design level, or lower, and the potential need to shutdown the pump before it runs dry.

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11 Comments

James Phifer

9/2/2022 08:14:40 am

If the water supply can't meet 150% of the pump's rated pressure, then it must meet the greater of either 100% of its rated pressure or the maximum flow demand of the fire protection system.
NFPA 20: 14.2.6.2.3.1 (2016 ed).

if you can't meet that demand you won't be able to do the test.

Jesse

9/2/2022 08:21:22 am

Interesting question. The ramifications of a pump interlock to low water can't be understated.

If the tank were low for maintenance, I've seen the pump taken off line and impaired - then fire watch established.

Autofill switches on tanks are super important and in their absence I've recommended daily visual inspections of water level

Glenn Berger

9/2/2022 08:30:27 am

See NFPA 20 Section 4.16.9 and Section 4.17.10 and others. These references are from the 2019 version of NFPA 20.

Greg

9/2/2022 08:38:07 am

When viewing the fire pump as emergency equipment during and emergency, its purpose can be likened to a baseball analogy, where a batter has a runner on first and hits a pop fly ball, a sacrifice fly… the runner will advance. In the world of fire, sprinklers and fire pumps, when it’s all on the line, the fire pump is the batter. Sacrifice is a distinct possibility.

But to the facts within the question, standards and the like. When the tank is 25% is this level within a refill period and does the refill rate match NFPA for tanks supplying a fire water pump? How did it get to 25% ? Is this actually an impairment to the fire protection system, where compensatory measures are needed because the fire pump must be turned off, or someone stationed at the pump while a refill is occurring?

In my experience, the only time a fire pump is off is under impairment due to a problem with the pump or what the pump could do (e.g. pipe work downstream and the pipe is open with no capacity to isolate ahead of the opening ).

If the pump is called to run, in the early days, it ran until a button was pressed in the pump room to shut down the pump. Advances in technology provide for run timers however, these are often discouraged from use if the premises is insured by a major carrier who sees the insured as Highly Protected Risk and warns against the use of timers that stop the pump for any reason other than a manual stop.

Hopefully this explanation provides a framework for considering why the pump might require isolation under impairment but that fire pumps, although an expensive player on the team, may be called upon to sacrifice for the sake of the firefight.

Alex

9/2/2022 08:42:40 am

When are you expecting the tank level to drop below 25%?

- If its during testing or maintenance, I would require a fire watch.

- If you are saying it may occur at any time, I would add a float switch to monitor the water level with an indicator to alert someone that the water has dropped below the required level. You can then use a level control valve to then add more water if connected to the municipal supply or have someone come top off the tank.

Alex

Tim

9/2/2022 08:45:11 am

The system would not be compliant to standards if automatic start prevention due to low level is introduced. I will suggest to install low water level alarms to trigger maintenance intervention.

Diesel driven water cooled pumps from experience will run less than 10 minutes dry before damage occur. Destruction beyond repair follow not much later. Electrical pumps will not get motor damage very quickly but pump will most certainly sustain damage very quickly as well if it runs dry.

The low level alarm will only help to mitigate some risk preventing accidental dry running of equipment.

schulman

9/2/2022 08:47:00 am

if youre not using the "handbook" versions of the codes with commentary you are doing yourself a disservice in my opinion

NFPA 20 Commentary on this subject is spot on

Chris

9/2/2022 08:48:21 am

There should be a low-water alarm that goes off at 4" below minimum tank capacity to meet system demand. And its always been my understanding that a pump is "sacrificial" and will either run until pressure is restored and the timer shuts it down, or the pump burns up.

Casey Milhorn

9/2/2022 08:54:22 am

https://www.zurn.com/products/fire-protection/pilot-operated-valves/zw215fp

You can install a fire pump suction control valve like this one, but these aren't designed for use with tanks as far as I know. Their adjustable pressure range is 10 psi to 30 psi. At 25% full, there is a good chance your tank is already close to, or below the minimum 10 psi. All this valve would do is to slow down the output of the pump as the tank level lowered.

As Greg said, a pump is designed to run until it can't run anymore. When you are solely on tank supply, there is a very real possibility that it's going to run through that water.

The real question is why is the tank being allowed to reach that low of a level.

Franck

9/2/2022 01:05:15 pm

The only alarm that is allowed to stop the pump is the overspeed with a diesel engine.
No stop with high temperature or even with no lubricating oil.
So no stop either with a low water level.
And not even with a fire in the fire pump room !
The main reason has been indicated above : the pump must start during a fire event and shall not stop until someone decided the fire is controlled.
So why stop on overspeed ? It is not to protect the engine nor the pump, but the sprinkler system. Systems are designed for 225 psi max (even 175 psi for most sprinklers). With overspeed, the delivered pressure might go higher and damage the fire protection system.
In all other cases, the fire pump shall start and run until the fire is out or the pump is dead.
Even with 25% of the water in the tank you want to take the chance to control the fire (all the more since 95% of fires are controlled with 5 sprinklers or less, which means that 25% of water is sufficient for several hours).
In addition you have a low water level with a technical alarm.
So there should be no reason to have 25% of water and not knowing it.
Good maintenance and regular test of technical alarms and regular test if the fire pump is the only way.

sean

9/2/2022 06:09:07 pm

no the code would rather it run dry and get what water it can the the fire the protect the pump

Prevent Fire Pump Start with Low-Water Levels?

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