valve for water powered sump pump?

About a year ago a friend asked me about installing a water powered sump pump for him. I checked with the town and they require an expensive valve to insure there is no backflow between the pump and the water supply; if I remember correctly, there are two kinds that could be used, and they require the expensive one. Ultimately the friend decided not to do it, so I forgot about it.
Today he called wanting it to go in before he goes on vacation Monday! It is a simple enough job, but I can't remember what the valve the town required was. An ideas?
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toller wrote:

http://www.irrigationtutorials.com/links/backflowmanf.htm
Jim
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backflow valve.

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toller wrote:

this is a bit OT but...Why would anyone install a water powered one?? Wouldn't a standard electrical one do the job? This is a serious question, not trying to be funny or anything. I can only figure one possible reason - power outage with city water still available.
Harry K
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Harry K wrote:

You're right! During the Great Blackout of 2003, it would have worked ... until the water towers emptied out (which happened here). To avoid a replay, city spent $$$$ on several huge diesel-electric units to supply emergency power to the muni pumps.
Roby
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We are 40 miles north of the lake that provides our drinking water, and about 1000' lower elevation. The whole thing works on gravity forever. Despite this I recommended a battery powered pump, but he likes water power.
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toller wrote:

Thanks. Does make sense in that situation.
Harry K
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They probably require a reduced-pressure principle backflow preventer- The cheaper one is probably a double check valve with an atmospheric break-
Dan
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wrote:

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