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New Member


Wednesday, April 28th 2010, 3:10pm

PVB sizes

When seaching for PVB devices, what determines that size? The opening for the incoming H2O or the opening for outgoing H20 or are they the same for both?

Thanks :thumbup:


Advanced Member

Posts: 66

Location: east coast


Wednesday, April 28th 2010, 6:37pm

Size selection is based on your allowable pressure loss through the device as well as the velocity,usually around 7-8 ft./sec. Both inlet and outlet are the same size.


Supreme Member

Posts: 5,382

Location: Metro NYC


Wednesday, April 28th 2010, 8:59pm

Backflow preventers are flow-rated the same as water meters are, which means they can pass much more flow than you would normally have in a pipe of the same size. For a PVB, you can match the supply size of the pipe coming into the water meter, and not be killing system performance. The reason you see a lot of one-inch PVBs on systems fed from 3/4-inch plumbing, is that the zone valves will be one inch, and the plumbing is simplified by using all one-inch.


Supreme Member


Thursday, April 29th 2010, 9:20am

If you are having issues with water pressure and you're trying to minimize pressure loss, you would want to compare pressure loss charts for different sizes to see which has the least pressure loss for your designed flow rate. Counter to what you might think, there are times where you will get less pressure loss at a given flow rate in a back flow preventer that is SMALLER that one that is bigger, especially if you are looking at RPZ devices.

If marginal water pressure isn't an issue for you, then you fall back on Wet_Boots advice and use the size that is most conveniant.

BTW, the same advice applies to sizing valves.


New Member


Friday, April 30th 2010, 11:48am

Thanks all for your advice and information.

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