Field Welds & Fillet Welds 1

[paulbrown]

H.P. Steam 90bar 490C (1300psi 914F)

When we have to repair pipework during shutdown if we cannot Hyd. Press. test the pipework we 100% Radiograph all Final butt welds (butt welds are the only weld we allow as final welds even down to sizes we would normally use Socket weld fittings on).

While I agree with this on pipework that is going to be stressed
Is it acceptable to use a Socket weld fitting on small short lines (vents & drains with no stress)without having done anything other than MPI ?

I just need some advice on this as I cannot find any literature to back up my gut feeling.

 

[DSB123]

paulbrown,
On our site we would not allow socket welds on HP steam if they were not subject to a pressure test. Recently we have had a socket weld (which incidently passed the pressure test)fail after only 18 hrs under steam blow conditions. Socket welds are very susceptable to fatigue failure. With the steam blow conditions the "piping" was humming but with the weight of the blanked flange joint this was sufficient to cause failure. Only yesterday an adjacent plant reported a socket weld failure on an existing system. So my attitude is to use butt welds whereever possible. MPI will not tell you if there is a crack at the root of the weld so the weld could be almost ready to fail and the MPI would say it's O.K.

 

[paulbrown]

Thank's dsb123
The reason I asked this is that quite often vents for Hyd. press. testing are fillet welded after testing is finished and I wondered whether this logic could be used for other low stress branches.

 

[bvi]

The hydro test plugs are presumably threaded and then seal welded, so it is a different circumstance.

If you are following API 570, Piping Inspection Code, you would be required to use butt welds with 100% inspection. Socket welds without pressure test are not an option.

 

[Cockroach]

I have used Socket Weld Flanges and fittings on orifice meter runs for years, up to and including ANSI Class 600. Typically these are 1440 psig MOP and 2160 psig MTP with temperature ratings (-20,+100)F.

In my experience, I have had no problems up to NPS 6 Schedule 40 (ST) and Schedule 80 (XH) pipeline. There is a code in the Canadian jurisdiction which challenges greater pipe sizes, these are under CSA Z245.15 for steel valves. To my knowledge, socket weld equipment is used primarily on NPS 2 and 3, sometimes on NPS 4 but only as fittings such as threadalets, etc on NPS 6 and above.

I typically call out for 100% inspection on all circular welds on flanging, magnetic particle or equivalent on fittings. The industry norm is typically 10% inspection.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[paulbrown]

bvi
On a lot of plant's you are not allowed to seal weld threaded joints, so the threads are cut off after Hyd. test & a socketweld fitting is used.

We do not use API570 but I will have to check it out

Regards
Paul