When is Low-Suction Valve Req'd for Fire Pump?

12/8/2022

I am trying to understand how low suctions control valves in a fire pump work.

At what point would you need to have one? Could anyone help me understand that?

Thanks in advance.

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12 Comments

Brian Gerdwagen

12/8/2022 08:05:42 am

When the pump could cause cavitation on the main by pulling too much water out of it and reducing the residual pressure below 20 psi, you need low suction control. Or some jurisdictions, like Arkansas, require low suction control on every fire pump.

It works by throttling closed the discharge side of the fire pump to maintain a net positive pressure on the suction side.

Travis Mack

12/8/2022 08:07:55 am

We've had a couple jobs where these are required. It is usually the water purveyor that requires them. Basically the valve throttles the water being discharged so that you never go below a set pressure in the suction lines. Typically that set pressure is 20 psi.

Eric R

12/8/2022 08:08:47 am

A low suction valve is used to protect the underground supply piping from the public waterworks connection from falling below the minimum required residual pressure set by the water purveyor. This is done to prevent back-siphoning of contaminated water from other connections on the line (which is also why water companies require backflows) The generally accepted value for this is 20psi.

I would say that you really don't want to put one in until you are sure the fire pump is putting the water supply at risk, since the valve will reduce the performance and reliability of the fire protection systems supplied by the fire pump.

A suction pressure below 20psi at the fire pump inlet does not necessarily mean the underground line on the public way is also at or below 20psi, and you would want to do some calculations for pressure losses in the supply piping and also probably perform a fire pump flow test while recording residual pressure from a hydrant on the public system before installing one.

Alex

12/8/2022 08:19:46 am

Showed up late this morning and everyone hit the nail on the head.

Anthony

12/8/2022 08:54:04 am

QUESTION:

If you have enough water for your sprinkler system and hose demand but the hose demand is 'close' would you recommend this? How much hose demand is 'actually' going to be used. Think very sharp curve on your water supply.

Thoughts?

Glenn Berger

12/8/2022 08:57:37 am

Summary - Low Pressure Suction Valves are required where the water supply is not capable of properly supplying water to the pump at 20 psi, which is basically where you need to provide an on-site water storage system.

Nimal Tissa Wijetunga

12/8/2022 07:58:36 pm

This comment by Mr. Glen Berger really hit the nail on the head.
Water supply must be by private mains through a water storage tanks directly connected to the fire system.
NFPA must look in to allowing water getting thru public mains and in my view shall discard that option.

Casey Milhorn

12/8/2022 10:12:49 am

Everyone pretty much covered it. What's interesting is how many opinions there are on why 20 psi is the floor. Cavitation, back siphoning, etc... from my understanding the main concern are the gaskets on push on pipe and fittings. If you are at 20 psi at the suction of the pump, I would imagine there is a chance that you might have close to 0, or even some situations where you might even see a vacuum (negative pressure) on the pipe. Especially in situations where your pump is in a basement, and you may have close by areas where the city main could be higher in elevation due to terrain. Remember, just because you see 20 psi at suction of pump, doesn't mean the system isn't experiencing less farther upstream.

Mark Harris

12/8/2022 05:13:20 pm

Not sure if in the clean water act or just water departments but not uncommon to see a boil order requirement if supply pressures drop below 20 PSI.

Eric R

12/9/2022 08:52:54 am

I don't believe it's specific to the CWA, but yes, low line pressure requiring boil water orders is standard practice, and IMO the primary reasoning for the minimum suction pressure requirement. I'm pretty sure that physical pipe failure/collapse is more urban legend that has just been repeated often enough to become the standard reasoning.

Water supply reliability is an increasing concern for municipalities and boil water notices are becoming more and more common in the news so its real issue. However, Brian's comment about municipalities requiring these valves on every system feels like a very "cover the land in leather instead of wearing shoes" response to the problem.

It's probably pretty evident that I'm not a fan of these things. Any device designed to actively reduce fire flow water should only be used in specific situations where other design alternatives can't solve the problem.

When water supplies are strong, it's no big deal to slap together a design that over discharges and needs high pressures. The challenge is when designers get into a habit of high pressure layouts and stop thinking critically when faced with less adequate supplies.

Just for an example, not that long ago I reviewed a drawing for an Ordinary 2 mercantile design (A legit one for a small toy store with nice flat 10' ceilings) with a calculated discharge of 550gpm(yes, before including hose streams), and a 750gpm pump specified. Simply by switching the heads from 5.6K to 8.0K and balancing some pipe sizes the design dropped down to 340gpm and eliminated the fire pump all together. Now the water utility doesn't need to be stressed by yearly 1125gpm flow tests for no good reason.

I'll get off my soap-box now.

Rich Baxendale

12/9/2022 09:53:24 am

Make sure to check your applicable building & fire codes. In Ohio, our state EPA requires a low pressure cut-off, low suction throttling valve, or VFD controller to maintain a minimum of 10 psi in the suction line and this requirement shows up in the building code but is originating from the Backflow Prevention and Cross-Connection Control chapter of the Ohio Administrative Code

Josh

12/9/2022 02:44:16 pm

Just to add to all this info - I use the pump curve and look at the NPSH to help make decisions based on my available flow from the city. If you select the right pump with the NPSH curve in mind, you can really cut down on the need for pump low pressure suction valves.

When is Low-Suction Valve Req'd for Fire Pump?

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