NFPA 20 Recycle Lines

[dadof89]

We have a firewater system, including a firewater tank, in a rather large industrial plant designed to NFPA 20. As such, there are no valves on the recycle/relief lines back to the tank off of two electric pumps plus one diesel pump except for a common valve at the tank nozzle. My question is how did NFPA 20 expect a user to maintain their relief valves without taking the complete system down since there are no individual discharge block valves on the flow through reliefs, just a discharge cone with sight glass. We are contemplating cutting in isolation valves but wanting to understand the intent of NFPA 20, reference this question. Also, what exactly is this closed discharge cone?

 

[SprinklerDesigner2]

NFPA #20
2-13 Relief Valve.
2-13.1*
Where a diesel engine-driven fire pump is installed and 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated, a relief valve shall be provided.

2-13.3
The relief valve shall be located between the pump and the pump discharge check valve and shall be so attached that it can be readily removed for repairs without disturbing the piping.

First question that comes to mind is the relief valve absolutely required?

2-13.3 does not allow for an isolation valve to be installed probably the reason being if the relief valve is down the intent is to have the pump out of service as well. Having a valve closed does two things; 1)takes away the urgency for repairs and 2)if very high pressures are possible, say you have a 150 psi pump with 140% churn pressure taking suction from a source with 40 psi suction pressure, damage could be done to the system should the pump come on.

 

[LCREP]

How about installing a check valve on the discharge side of the relief valve going into the tank? The tank water pressure would keep it closed and the relief valve can open and relief pressure if required. You can take the valve apart to repair service as needed.

 

[DavidCR]

NFPA 20 does not tell you where to discharge the flow of the relief valve.

If you direct the discharge of the relief valve to an open discharge point of the same level or lower, the funtion of the relief valve will be much simpler (loose valuable water of course).

For Diesel units if your static suction pressure + churn pressure + 121%, will not damage your system, NFPA 20 2003 does not require relief valve. For electric units relief valves are not a code requirement, but could be suggested or neccesary.

The discharge of the relief valves back to an aboveground storage tank (of different pumps) shall not be dischargin to a common pipe. (See 5.18).

The idea of the code reffering the discharge cone deals with the possibility of observing the opening of the relief valve (to adjust pressure, to be aware of the lost water that could be used for fire fighting, etc.)

So a little observing window is provided for closed cones in order to allow to observe the openiong of the valve.

In case you discharge to a river and you can see and even ear the splash, you need nothing, but in case you discharge back to an aboveground water suction tank, you need to observe the flow (with a sight glass in the cone or other type a flow detector), you need independent pipes, and you need to be sure that the back pressure does not affect the capacity of the relief valve. And even more if you need to facilitate the maintenance of the valve you need more requirements.
In general it is recomended to discharge in the top of the tank, so in case of maintenance you only need to drain a portion of the discharge pipe.

 

[dadof89]

DavidCR
Thanks for the input. Unfortunately, while our 3 pumps have independent relief/recycle lines, they all tie in to a common tank nozzle with one common block valve, at the bottom of the tank.So not 100% independent. Seems like we may have a bit of a design bust.

Is there any internals on these discharge cones? Ours is about an 8-10' long spool and could be easily modified to cut in a full bore locked open valve, unless of course there is some kind of internals to them.

 

[DavidCR]

I´m not sure if I get the right idea about the cones you are referring. I only know the closed cone wich is a steel reduction both ends ANSI flanged and sight glass on opposite sides. So it has no internals.