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. Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
13 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.
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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.
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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.
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Alex
12/8/2022 08:19:46 am
Showed up late this morning and everyone hit the nail on the head.
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Anthony
12/8/2022 08:54:04 am
QUESTION:
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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.
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Nimal Tissa Wijetunga
12/8/2022 07:58:36 pm
This comment by Mr. Glen Berger really hit the nail on the head.
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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.
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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.
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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.
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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
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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.
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D
8/15/2024 09:22:59 am
Following up on this topic from a few years ago - what are the ITM requirements for this type of valve?
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