Backflow Allowed Before Fire Pump Suction?

2/1/2021

We have a building with a double check backflow assembly on the suction side of a fire pump. Normally there is a minimum of 10 pipe diameters of vertical plane pipe just before the suction flange.

Is the double check backflow allowed within that distance?

Are there requirements for the backflow to have flanges vs. grooved connections?

A colleague said the control valve should be OS&Y rather than butterfly on the suction side only. Why would this be the case? I would assume the OS&Y would reduce turbulence better than a butterfly, but wouldn't the checks also cause turbulence?

Thanks in advance.

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8 Comments

Brian Gerdwagen FPE

2/1/2021 09:20:50 am

Control valves on the suction side of the pump should always be OS&Y per NFPA 20.

There is no requirement for grooved versus flanged connections as long as the pressure ratings are acceptable for the application. Regular grooved fittings are fine for the suction side.

Backflows can be on the suction side, but you are correct, it is good practice to keep them farther away from the pump than directly connected to the suction flange.

Jesse

2/1/2021 09:28:42 am

Backflow preventers can be on the suction side, keeping in mind OS&Y Valve requirements, etc.

Some AHJs require them on the suction side, as they consider the pump to be a contaminate source. NFPA 20 nd testing does a pretty good job dispelling this.

The only concern I'd have is ensuring the hydraulic hit we take in friction loss through the BFP doesn't create a problem with available pressure at the suction inlet. If your latent water supply is sketchy with low pressures, that may be a problem.

Dan Wilder

2/1/2021 09:25:03 am

Well, couple of Q's and some A's for ya....

Yes, you can have a backflow on the suction side of the pump and as long as it's allowing the 10 pipe diameters requirement, it is acceptable*. Grooved or Flanged makes no difference, the internal flow disruption from the vane is the issue as you pointed out.

*I say acceptable because the backflow would require OS&Y's, if you were to have butterfly valves the distance (overall) increases to 50' (20-19' - Section 4.16.5.4).

One issue with backflow on the suction side occurs when the pump is tested to 150% as many backflows are not listed at the higher GPM (Rated Flow) and can cause failure of the device.

CJ Bonczyk

2/1/2021 09:37:26 am

NFPA 20 prefers BFP's/RPZ's on the discharge side of the booster pump. However many jurisdictions such as ours require the BFP on the suction side. If the device is on the suction side the following rules apply:

1. Devices must be installed per NFPA 20 4.29
2. The device (BFP) must be a minimum of 10 pipe diameters to the suction flange of the pump (with the installation of a OS&Y type BFP).
3. If the BFP has butterfly control valves the BFP is required to be at least 50 feet from the pump suction flange measured along the route of the pipe in accordance with 4.16.5.4.

*Both types of BFP's will have check assemblies, however, OS&Y's are the preferred type of valve as they cause less turbulence than a butterfly style valve.*

5. Pay attention ti the increased suction pressure loss from the BFP when sizing the pump based off the system demand.
6. When a BFP is installed the final arrangement shall provide effective pump performance at the lowest permissible suction pressure.
7. ***The discharge flow rate shall meet or exceed the fire protection system design flow.***
8. The discharge flow rate shall meet or exceed 100% of the fire pump rated flow rate.
9. Determination of effective pump performance is to be documented by engineering calculations and tests.

Pay attention to any retroactive BFP installations on the suction side of a booster pump as outlined in 4.29.4.1.6 & 4.29.4.1.7

Lastly pay close attention to the technical performance & installation data for the BFP itself, and ensure you have enough space for clearance and removal. I would personally recommend to add a bypass meter on the BFP even if not required by the local waterworks authority.

Mike

2/1/2021 09:51:31 am

Devices allowed in the suction piping and installation requirements are shown in section 4.15 Suction Pipe and Fittings of NFPA 20, 2016 if you have anymore questions.

Stephen L Aikey

2/1/2021 11:39:38 am

As usual there are great comments. I would like to highlight the BFP technical performance & installation data portion of this discussion.

Most who do this for a living are aware of the friction loss requirements we input into our hydraulic programs for this application to ensure the design is acceptable.

However, I want to point out the BFP check valve spring tension requirements. Have not seen anyone bring this up.

The minimum upstream water supply pressure required to open the internal check valve springs which are under spring tension.

Some BFP Manufactures have high flow / low pressure drop BFP's that are great for hydraulic calculated designs. That being said, you should be aware it may take at least 16 psi. to open the internal check valves.

Also note some Manufactures post that data on their data sheets. And some do not!

The BFP spring tension information is very important when working with water supply tanks and or low pressure city mains.

Dale

2/8/2021 09:27:06 am

I was always instructed by my fellow well experienced Engineers to install the backflow on the suction side, as the backflow may not be listed for the now increased system pressure that the Fire Pump is providing.

Gear Pumps link

2/13/2021 07:03:53 am

Thank you very much for sharing about backflow allowed before fire pump suction here… I really hope I can work on your tips and it works for me too, I am happy to come across your article. Great post I must admit, keep sharing more…

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