Backflow preventors - maximum rated flow

[SprinklerDesigner2]

Per NFPA #20 Table 2-20 Summary of Fire Pump Data the minimu suction and discharge for a 1,500 gpm booster pump is 8".

Of course we all understand that is the minimum allowable size and it often happens site conditions dictate a larger, usually on the suction side, size be used.

Backflow preventors I believe an are excellent example.

Take a look at a snapshot taken from Ames for the 3000ss double check.

http://img21.imageshack.us/img21/7019/amesqg5.jpg

For an 8" 3000ss the "Rated Flow" is 1,600 gpm but beside that is the point where UL tested and the test appears to run out to about 2,350 gpm.

I've always worked under the assumption I can use this only where my required flows do not exceed the maximum rated flow or 1,650 gpm. To me this is not unlike fire pump test curves which exceed 150% of rated capacity. The factory may very well test a 1,500 gpm pump to 2,500 gpm and supply the certified curve but that doesn't mean I can use that curve to design a system requiring 2,300 gpm.

In the case of this particular 8" Ames am I correct in saying this particular backflow preventor would not be acceptable on a system requiring more than 1,650 gpm?

Then there is the fire pump itself.

Here in Georgia the state requires all fire pumps to be able to operate to 150% without drawing any portion of the suction below 20 psi and they are starting to require it be demonstrated on fire pump tests.

I'm not sure but I think this is a requirement everywhere throught the EPA... something to do with the clean water act of a long time ago but not so sure of that.

Anyway, am I right in my way of thinking?

Thanks.

 

[FFP1]

I am not sure, but I think UL tests the backflow prevention units at 150% of the rated flow (similar to fire pumps). I am not aware of any requirement in NFPA which limits the design of sprinkler systems to the rated flows for the applicable backflow prevention unit. I am curious to see if anyone posts a link which provides this requirement.

In the absence of any specific requirement outlined in NFPA, I would assume there is no limitation as long as the hydraulic calculations work out right. If there is a specific requirement, I would assume NFPA would require the system demand (combined sprinkler system & hose streams) be less than 150% of the rated flow (rather than limiting to the 100% rated flow). We all can make a strong case for limiting the system design to 150% of the fire pump rating, but I am not sure there is an obvious reason for limiting the system design to 150% of the backflow prevention unit's rated flow. Friction loss will increase at a higher rate as the flow rate increases, but the available volume and pressure will not drop as drastically as a fire pump from 150% to say 200% as with a fire pump.

Very interesting post!

 

[SprinklerDesigner2]

I am thinking it is much like water meters that have a "maximum flow rate" and then a "maximum continuous" which is always less than the "maximum flow rate". By meters I am speaking about the type you would encounter in a residential 13D or even 13R type system.

So, speaking of meters, what is "maximum continuous" and what does it mean to me?

Same thing with the backflow preventors and the "Rated Flow". It's there, it's on the published literature but what is it and what does it mean to me?