Originally Posted by davegaz
This is an unusual application of a pipe thread. As you know, pipe threads are tapered and as you tighten the joint the threads pull the tapers together forming the seal. The wedging action is what creates the high stress in the female collar. What's wierd is that the fitting tightens down all the way to where the hex "nut" face contacts the flange collar. A typical pipe thread joint will leave one or more exposed male threads. This ensures that the tapers are properly loaded and NOT the nut face. You don't want both.
By adding a washer, but you will lose the seal provided by the taper. You'll be relying on a thick layer of thread sealant. May work ok.
Like someone mentioned above, a straight thread with an o-ring would have been better.
The bigger issue may be the continued material degradation over time. The loads and stress on your flange haven't changed, but the cracks extended. This may be a warning sign, i.e., leak before break.
I agree, and this is why I think a collar around the flange might be the best route to follow. I am not crazy about using sealant to hold back 40+ psi.
Like the one on the right, with little or no lip to get in the way of the sealing edge.
I considered an O-ring, but there is no good way to make it work as is. The washer in installed puts the pressure on the top of the flange, and forces the thread tape to be the seal. It is working, but I do not know for how long as this is not the way it was designed.