This is not necessarily a CII question. However, since this forum is an excellent way to contact other members of the piping engineering community, I would like to submit it here in the interest of broadening piping knowledge. Rich - if you feel this particular message is an inappropriate use of this forum, please feel free to delete the message.

A client of mine wants to use a 2" API-type screwed hammer union to attach a turbine flowmeter into the line. They want to use a union because the meter must be removed from the line periodically for service. The operating pressure is 4700 psig. The system is an oilfield waterflood measurement and distribution manifold.

They have asked me if it is "okay" to seal weld, or back-weld as it is sometimes called, the threads that attach the union to the nipple. They are afraid that the threads will back off during subsequent hammering events of the union when the meter is removed; that is why they want to backweld the threads.

In my former life at Mobil, we never allowed the general seal welding of threads. Seal welding was done only by specific permission in specific instances. However, I never could get anyone to give me the science or the experiences behind this. What do you think? Entrapment of welding gases? Weld cracking? Heat deformation of the thinned pipe x-section that exists at the thread root? Anyone?!?!?

Thanks for any help/ideas/referrals.

Pete


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Thanks,
Pete
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Thought for the day:
Good judgment comes from experience;
Experience comes from bad judgment.