backflow device question

[cdafd]

fire sprinkler system wet with single riser.

1. only check valve installed in building is the backflow device. Legal under 2002 nfpa 13

2. The fdc connection is after the backflow device

3. so if you pressure the fdc you pressure against the backflow device.

anyone have heart ache with the set up??

are backflows designed to handle pressure 150 to 200 psi against them??? maybe 175 psi max.

 

[TravisMack]

This is basically the only way I have seen a system installed. I can't say I have seen an FDC installed upstream of a backflow preventer, unless it was a field error when a site contractor would install it incorrectly.

Not to sound sarcastic, but has there been a lot of cross contamination issues lately with backflows failing when the FDC is pressurized against the backflow preventer?



Travis Mack
MFP Design, LLC

 

[cdafd]

I can't say I have seen an FDC installed upstream of a backflow preventer

1. the underground comes into the building
2. first thing you hit is the osy backflow osy than feeds into the building system

3. being a wet system and the fdc on the building, the fdc pipe comes in and ties into the system after the backflow


Not to sound sarcastic, but has there been a lot of cross contamination issues lately with backflows failing when the FDC is pressurized against the backflow preventer?

no problem yet but have not had an occassion for the fdc to be pressuriezed.


just have not seen the set up before with out an alarm valve after the backflow, but meets nfpa 13

 

[cdafd]

after rebooting the brain, I have seen the set up, just not much in this area. Just did not look right and wanted to question it. normal main pressure around here is 80, and if the fd has to pump into it there are bigger problems going on.

 

[PEDARRIN2]

We always design for the FDC to be downstream of the backflow. The intent of the backflow is to protect the main from contamination. The main may also feed domestic water and there is the concern that the water in the tank on the fire truck or the hoses or something else on/in the fire department equipment could introduce something into the domestic if the FDC is upstream of the backflow device.

Now it is usually given that the feed to the fire service of the building is the same or very close to that of the domestic and that if the pumper has enough pressure boost to cause backflow into the main there are more problems - but putting the FDC downstream makes all these issues moot.

 

[SprinklerDesigner2]

For what it is worth Toledo, Ohio requires all sprinkler systems to have an additional check valve downstream the backflow preventor assembly.

Additionally all fittings from the underground flange and spigot to the second check valve must be cement lined. Oh, and Uni-Flanges are not permitted.... only flange and spigot pieces.

IMHO I agree with the way they do it. Not bad if everyone is forced to do it the same way (and they are).

 

[Cidona]

IMO, the working pressure of these is usually 175 psi. The same minimum (and usual) working pressure as all components of the system are required to be (unless you're dealing with a high pressure system). If the FD overpressurise the system enough to cause the backflow damage leading to contamination, there would be lots of other problems with all the other 175psi components. It is my understanding that the fire pumper truck operator is meant to be aware of the system's working pressure (signage on FDC is other than 175 system) and should stay within those parameters while pumping.
In my neck of the woods we typically don't require Alarm valves. So it is very usual to have the incoming water come in, split for domestic and FP, backflow on the FP line, a connection from the FDC into the FP pipe between the backflow and a riser manifold. In this scenerio the backflow would be subject to the same pressure (during FDC operation) as the system described in cdafd's post. Have not heard of any particular issues with this arrangement. When performing the system pressure test the valve is subject to 225 psi test pressure if the working pressure is @ 175. This is allowed for in the 175 psi rating the manufacturers provide.
Interesting to hear about Toledo. We (Chicago area) can go galv from the incoming flange up to the backflow, then sch 10/40 whatever from there on.