Flange Connections on Suction Side of Fire Pump?

12/6/2018

Many fire pump arrangements I've seen include flanged fittings and pipe connections on the suction side of the fire pump. Is there a requirement for this?

Are grooved connections allowed for pipes and fittings on the suction side of the pump assembly?

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4 Comments

RichP

12/6/2018 10:24:31 am

I've always been taught that grooved fittings should not be used on suction pipes because of potential for negative pressures for which they are not designed or approved. Viking Johnson style couplings should be used to allow any flexibility.

12/6/2018 12:21:11 pm

As far as I know, there is no code requirement for flanged fittings in pump suction. NFPA 20 section 5.13.2 says grooved joints are acceptable. Section 5.14 does not limit this. (It says the suction piping is subject to NFPA 24, which allows grooved joints when used per their listing.)

I don't think negative pressures at the pump suction should be a big concern, but check with your manufacturer or supplier. I know that Victaulic does offer some vacuum-rated grooved couplings.

NEW PE

12/6/2018 02:34:10 pm

I have also heard that it should be avoided, putting grooved fittings/couplings on the suction side of a pump, but I have not seen evidence that their use poses any threat. Not only, as it is mentioned above, is there no code prohibiting their use, but if the suction side of a pump were to become low enough that it risked pulling the gasket out of the coupling, the pump would be seriously over-sized for the available water source such that it would be dangerously close to causing cavitation. I am not saying that gaskets haven't pulled out of couplings, but in those cases the couplings probably weren't installed properly. I mean, what pressure is that? The vapor pressure of water @ 25C is about 24 torr, or ~ 4.6 psia, which is roughly -10psi gauge. We size a pump based on the maximum demand for the FP systems, then based on the feasibility of the water supply in both terms of suitability and reliability we either use that supply or specify a suction tank. So, included in the supposition, "potential for negative pressure", that potential should not exist. Negative compared to what? Using the 20 psig benchmark imposed by some municipal purveyors on the suction side, even if there was some problem and the supply pressure approached the vaporization pressure, you have to take into account that pressure is only acting on the 1/16 " thickness of the gasket on the inside diameter pulling it in the radial direction. How much force could it possibly translate to given the limited area it is acting on? So, I just don't see it being a real problem. If someone is calling for them, yes, you should use a vacuum rated coupling, but you should also question the logic.

Franck

12/7/2018 10:22:10 am

Just one comment to ZS, it is now in section 4 and not 5.
Regarding the use of flanged fittings, it is clear in NFPA 20 that this is the "preferred" method (A.4.13.2), but is not the only one.
If you are looking to 4.14.6.5, strain relief is required when the pump and suction supply are on separate foundations. Normally, strain relief is achieved by using an expansion joint. But there are many installations where a series of grooved couplings achieve the same purpose.
You can have additional information if you have a look on NFPA 20 handbook (more pictures, more explanations than just NFPA 20 standard) at 4.16.6.5 (Strain Relief).

So, yes, flanged fittings are preferred (more robust), but grooved couplings are fully acceptable.

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