Material selection downstream of 'cold' PSVs 1

[tenac]

I've seen in a couple of old cryogenic gas processing plants that sometimes stainless steel (or 3.5% Ni steel) is used in PSV discharge lines for a short length (e.g.: 10 to 15 m), before the discharge line ties-in to a carbon steel flare header.

This is applied in PSVs that relieve gases at low temperatures (e.g.: -100ºC from a de-ethanizer safety valve).

But, considering that the coincident pressure downstream the safety valve is low (= back pressure, up to 1 barg for example) carbon steel could be used (e.g.: API 5L) in spite of the low temperature.

Then,
- why is stainless steel used for a short length? is it really necessary?
- which criteria would be used to determine the required length of stainless steel piping?

The gases relieved really don't heat up much in 10 m, sometimes only from -100ºC to -90ºC considering they take heat from the air surrounding the non-insulated discharge pipe.

Thanks.

 

[sshep]

tenac,
This seems like a questionable practice that was rationalized as a cost savings measure, but which might not stand-up to serious scrutiny in todays world- this is my opinion anyway.

Besides the sensible heat cold considerations (serious enough by itself), you should consider that the vapor being relieved may contain droplets of crygenic liquid either entrained or resulting from the expansion. The cooling where liquid contacts carbon steel pipe can result in localized thermal stresses which when combined with loss of carbon steel toughness can provide the shock needed to precipitate brittle fracture. One ethylene plant I work in previously (2-3 years ago) experienced some crygenic liquid getting past the flare drum vaporizer and backing into a carbon steel flare line near the flare stack base at the point where the cryogenic flare header joined the carbon steel header. This situation resulted in a crack in the flare line which required a shutdown to repair.

best wishes,
sshep

 

[tenac]

Thanks. I can understand the cost saving... in some plants, instead of this solution (short length of SS downstream cold PSVs), I've seen a different approach: two separate flare systems (flare header, KOD & flare):
- one for 'hot' discharges, made of carbon steel
- one for 'cold' discharges, made of stainless steel

Based on item 323.2.2 (d) of ASME B31.3, carbon steel piping could be used down to those temperatures if the pressure is low enough. But the possible carry over of liquid droplets should be analysed (I could find no 'rule of thumb' regarding the required length of SS pipe to be installed before joining the CS header).

 

[SmartEngineer]

I have seen similar to your approach with two separate headers once SS and once CS header in an ethylene plant.
For the cold flare system, it is usually a supeheater HE is provided for the cold vapor coming off the cold KO drum. The outlet piping from the HE is CS piping all the way to the intersection point with the hot vapor.
Sometimes, a jacket piping is provided in replacement with HE superheater.

Hope this help