Backflow Preventer on Discharge Side of Pump?

2/19/2019

Is there any benefit to having the backflow preventer on the discharge side of the fire pump? We've seen several RFPs recently that are requiring the backflow to be on the discharge side of the fire pump.

To me it seems as though this arrangement unnecessarily leaves more pipe and mechanical components exposed to the public water supply, but I'd be interested in others' opinions.

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10 Comments

Brian Gerdwagen FPE

2/19/2019 10:08:35 am

This seems to be a hold-over from an older version of UFC 3-600-01. The newer versions do not require this and defer to NFPA 20.

JOE MEYER

2/19/2019 10:42:02 am

Where I've seen it - it has been military requirements for the backflow to be on the discharge side, but only required on a specific project. As you've mentioned UFC 3-600-01 doesn't require this arrangement.

James

2/19/2019 10:11:59 am

From my experience, almost every water utility company will require a backflow device at the incoming service location after the house control valve. I would imagine not having one until after the fire pump be a big "no-no".

James

2/19/2019 10:14:24 am

Ah I might have misread what you asked. Is this a BFP that is additional to the one at the incoming service? I personally have never installed a BFP after a fire pump, just single check valves as per NFPA 20 diagrams.

Pierre Lemieux link

2/19/2019 10:14:46 am

There is only one probable answer to this that I can think of.
As the backflow preventer springs create a pressure drop across checks #1 & #2, this may cause vortexing or cavitation and lower the availablbe npsh.

PETE

2/19/2019 11:35:17 am

PIERRE- The backflow pressure drops are consistent with their listings. Any instantaneous cavitation that may occur on pump-run, is addressed in NFPA 20 with a 10 x suction nominal diameter distance upstream of the pump suction flange to the backflow discharge.

Franck Orset

2/19/2019 10:36:07 am

As it has been said, a check valve is necessary on the discharge side of a pump.
A backflow preventer (large device), is normaly required at the connection from the city water supply to your sprinkler system, if you have a booster pump. This is mostly to avoid a "pollution" of the city system from the sprinkler system.
In that case, you might have the "backflow preventer" at the suction side and there is no need to add another one on the discharge side (but you still need the check valve).

C.T. Carbonari link

2/19/2019 12:40:17 pm

Backflow Prevention Assemblies (BFPA), by definition, protect the potable water supply from contaminants. Ref: Manual of Cross-Connection Control developed by the Foundation for Cross-Connection Control & Hydraulic Research.

BFPAs are categorized into (2) Classifications. Those that protect against Health Hazards (RP, RPDA, PVB) and those that protect against Non-Health Hazards (DC, DCDA).

(Typically, Firelines use Dector Assemblies (BFPAs with a 3/4" By-Pass Assembly) to monitor any and all flow below 3 gpm.)

Installing a BFPA on the discharge side of the pump (without an additional BFPA upstream of the system) would defeat this purpose.

To answer the original question: The only "Benefit" I see to downstream installation would be the psi savings you would get on the suction side of the pump due to not having to account for the headloss across the BFPA on the supply side of the flow curve. Or, possibly, installing downstream due to difficulty in adhering to 10 Pipe Diameters of straight pipe on Pump Suction.

This has been one of the more interesting questions I've read on MeyerFire and I enjoy everyone's insights. I learn a lot from others through these posts.

On a side note: The City that I do a lot of work in has recently ammended their code requirements to mandate installation of an RPDA on Fire Protection Systems for High Rise Buildings. How the Fire Protection System for a High Rise Building introduces a potential Health Hazard to the City Water Supply (whereas a Low Rise does not) is a complete mystery to me. Any insights would be appreciated. There is no such thing as a "greater" or "lesser" degree of protection with BFPAs.

J. Diaz

2/19/2019 03:40:00 pm

This is one of the "suggestions" from Factory Mutual. Depending on the reviewer, it could be seen as the only approved installation method. Generally, they are open and could accept the conventional way.

See data sheet 3-07 Section 2.3.1.7 and 2.3.1.8

2.3.1.7 Do not install a device or assembly (including, but not limited to backflow prevention devices or
assemblies) that will stop, restrict starting, or restrict the discharge of a fire pump or pump driver in the suction
piping.
2.3.1.8 Locate backflow preventers on the discharge side of the fire pump whenever possible, due to the
increased friction loss and the potential negative effect on pump performance.

As per your specific question, I agree with others. There's no benefit; it is indeed detrimental since in that arrangement, the pump itself (mechanical equipment requiring maintenance or possible replacement) may be the culprit of contamination of a municipal systems. Most cities prefer to have the BFP as close as possible of the city mains.

Ed Glynn

2/19/2019 05:48:00 pm

There is an issue where the sole source of potable water is strictly a tank that has head pressure only.

It would not be a "traditional" potable source but a potable water source nonetheless.

As a result of head pressure only, the checks would not have enough pressure to open therefore a positive displacement pump would be required and a backflow on the discharge side of the fire pump.

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