measuring/installing pipe unions

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josh123

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Hello all

I was removing the backflow preventer in preparation for winter. The unit is a series of components and union joints. Whenever I turned one piece to loosen, the other side would tighten. I was able to remove the device by tightening one of the union pipes a good amount, allowing me to remove one side. This got me curious from a general plumbing perspective. If you have to fit a pipe into a fixed space with a connector, it seems like you would have to tighten the union all the way on one side, insert the otherside and thread back the otherway. So if you had six threads on one side, when you loosen that side and connect to the otherside you would have 3 threads on each.

What's the guideline/rule of thumb when doing something like this?
 

Jimbo

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If you are talking about threaded couplings....you can't do it that way!!! First off, if you 'overtighten' a coupling, then loosen it, it will likely leak.

Instead, pipe systems are assembled by starting at one end and working through to the other end. An actual UNION can make a joint in the middle if necessary.

To do what you seem to describe, there is a "left/right" coupling which uses a nipple and coupling which have left hand threads on one end. As you twist the nipple, BOTH ends get tight. Those have somewhat fallen out of favor.
 

hj

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union

IF that was the way you had to remove the "union", then it was NOT a union, but rather a coupling. You damaged SOMETHING, whether it was the pipe or the fitting or both we cannot tell you, by stressing it that much.

quote; So if you had six threads on one side, when you loosen that side and connect to the otherside you would have 3 threads on each.

Once the fitting is screwed on to the pipe, you CANNOT unscrew it without causing a leak, which is why you CANNOT put a conventional coupling into a line the way you did. Doing it your way you would have 3 tight threads and 3 loose ones which would leak.
 
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