Thursday, June 25, 2015

Some lessons about pipe dope

Mostly a lab note to myself: I've been having a lot of gas leaks, and have been experimenting with ways to ensure that they don't recur.  Ideally I should seal something once, and it's sealed, and not have to test, re-test, etc.  So I've tried using pipe dope instead of teflon, and have had somewhat improved results.  But ordering matters; the solenoid valves from Alibaba leak around the joints if you don't do it right, probably because they're cheap.  So, first I apply dope to the nipple going into the inlet, then I apply dope to the female threads of the valve.  I thread the nipple and tighten it down with a vice grip, then I can hand tighten the valve into the rest of the assembly.  Above the valve everything will be at atmospheric pressure (because that's where the gas comes out) so the seals are less important.

A problem I've had for a long time is ensuring that multiple pipes line up appropriately, and I still don't have a great answer for this.  Roughly speaking, imagine you have two pipe tees connected by a pipe.  I'd like to have them both facing the same way, but that means that they need to screw in tight into exactly the same position, and they often don't.  Sometimes hand-tight is at 90 or 180 degrees; in this case, wrenching it down to the same position either takes a lot of force, or leaves them slightly loose.

The answer here and here seem to suggest that a lot of pipe dope and/or teflon should help, plus there should be at least +/- 1 turn slack on a joint that's tight enough.  For water I'm sure it's good.  For 30 PSI gas, not sure. 

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