Just to think out loud – I don’t have a perfect understanding of how very low flow works through backflows… but:
It seems like the system static that is locked in is usually pretty close to the flow test static. I wonder if the noticeable pressure loss to initially open the BFP, that goes down once flow starts, if that pressure drop might approach zero as the flow goes from positive back to zero. That is – the pressure drop seen at a flow that is just barely over zero depends on whether you are approaching that flow from zero, or from [more than zero]. For example: if you slide a block across a table – the force it takes to move the block at a very low speed will depend a lot on whether you start from a standstill, or a faster speed. (that illustrates a different principle, but shows how the approach could change) Thoughts? Matt From: Sprinklerforum [mailto:[email protected]] On Behalf Of Roland Huggins Sent: Wednesday, January 11, 2017 6:07 PM To: SprinklerFORUM Subject: Re: Static pressure after backflow preventer I'm flying by the seat of my pants (with no lights) but until I prove to myself otherwise (or am beat into submission by the technical committee), I don’t see why we should beat ourselves up and default to what ever seems to be the most conservative. Our supply curve is a straight line relationship from static to some lower residual pressure. We now incorporate a device that has a crazy high initial pressure loss that then drops back into a more normal curve. In looking at SYSTEM DEMAND are we well served by ignoring the actual available pressure that was otherwise lost due to accounting for a wiggle at the front end. For now, I’ll stick with my mantra: Lead the way, Mr. Custer. I’m right behind you. Roland Huggins, PE - Senior VP Engineering American Fire Sprinkler Assn. Dallas, TX http://www.firesprinkler.org<http://www.firesprinkler.org/> Fire Sprinklers Saves Lives On Jan 11, 2017, at 12:27 PM, Dewayne Martinez <[email protected]<mailto:[email protected]>> wrote: I have used the static without the backflow loss in the past but I noticed with the SprinkCad software I am running it uses the mfg curves and it was showing less static than I had figured. If I take into account the BFP loss I can eliminate 6 pressure reducing hose valves…..Looks like I will err on the side of caution and design them in. Thanks, Dewayne
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