Relief Valve Inlet Branch Failure in H2S Piping

siva1943

316SS is the recommended alloy for Hydrogen Sulfide gas - I suspect the vibration issue is contributing to the failure (embrittlement). Is it possible to use a higher schedule pipe?

Edmund

thanks for the comment.

a correction,the crack failure is on the elbow upstream and not on the branch weld as reported earliar.

will check the possiblity of using a higher sch pipe for the elbow /bend without restricting the safety valve inlet.

will the higher temp usage 200deg C will lead to cracking(as per the nace mr 0175 dec 2005,clause 4.1.3,'changes that affect only the corrosion resistant alloys"the limit for max service temp is 60deg C for their susceptibility to chloride stress corrosion cracking).we have to confirm the presence of chloride in this case.

will the bend upstream of size 3inches which may be manufactured by cold bending,resulting in cold hardening to approx 22 HRc,lead to the failure?(this also is to be checked)

kindly give your observations

thanks

I wonder if the thermal cycling is part of your problem. If the intercrystaline hydrogen were being increased during the higher temperature parts of the cycle the lower temperature parts of the cycle would become increasingly susceptible to hydrogen embrittlement cracking. Reducing the stress level is about the only way out, but if the stress level is reduced by merely increasing the thickness, the temperature regime toward the inside will become worse. I don't think that it will help much, but you could try 317L and 321 stainless as they might be slightly more crack resistant.

thanks for the new direction.

let me try to suggest a solution in this direction.

Pipe material, acid gas, high operating temperature, corrosion, embitterment are applicable for entire system (discharge branch). While so, what is special (and common, "failure is reported from similar location in other low pressure compressor stages also") at that specific place: elbow upstream of the relief valve?

While the root causes prevail through out the system, the first failure exhibits at the most vulnerable point.

Please check, 'smaller branch off, sudden size reduction, amplifies the amplitude of vibration (increased displacement, having same frequency), could lead to accelerated failure. Allowing or even, restricting the vibrations cause such failures. Clamping arrangement, clamping location or even re-rooting can give better solution.

Caution 1: more surprises are possible at other locations, on days to come, as long as the 'dominant root cause(s)' prevails.

Caution 2: removing/reducing one of the root causes may sober/decelerate failure but may not prevent.

thanks for the comment

yes,the elbow is upstream of the relief valve and shall check the points suggested.

Small bore piping MUST be secured against vibration. All material in H2S service should be post weld stress relieved. If chlorides are present, 316L is required.

we will be stregthening the supports after completing the analysis and site measurement.noted the stress relieving requirement for the welds

thanks