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#### lowvolumejeff

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I'm stumped. I was taught to calculate pressure loss from the most distant head in a zone and work backward to the connection to the main, subtracting that amount from the static PSI. But, what if there is a PRV after the the backflow. Do you take take your static downstream of it, and just work your pressure losses back to it, or carry it all the way to the street?
Example: In the case where first there is Meter then backflow then PRV, do you ignore the loss thru the meter and the backflow and simply subtract losses from PRV to distant head from a Static Pressure taken after the PRV?
Thanks, Jeff

#### Wet_Boots

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I wouldn't bother with calculations. Just measure what's there, and work with it.

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OK. Let's say you're starting with 75 PSI at the source. Working with 10 GPM. We'll give a loss of 2 PSI each through the meter and backflow. So you're down to 71 PSI. But you have the PRV set to 60 (because you actually used a pressure gauge downstream, of course), so none of that matters, because the PRV doesn't care whether it has 75, 71 or 65 coming into it.
In other words, no, losses before the PRV don't matter (provided the PSI is still above what you want to set the PRV to. But if it isn't, you have other problems...).
Hope that helps...

#### Wet_Boots

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the PRV is not a 'passive' device, so you don't have an easy time running numbers for it

#### Waterit

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Taking the last 2 posts and adding them together - you don't need to worry about pressure UPSTREAM of the PRV unless it's ridiculously low, in which case the PRV needs to be removed.

Does my rain-soaked brain have it right, or do I need more caffeine?

#### lowvolumejeff

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Well, that makes itr easy.
Thanks guys.
Jeff

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