Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
(OP)
I need advice from folks who know more about this than I do; sorry I have to be so vague.
Question: What material should be used for this service, CS or Stainless are the only options for various reasons including -20F operational temps and plant prohibitions. It’s a 4” compressed air process line and my concern is scale/garbage accumulation in the line over time, causing problems for the control valves and/or flow transmitters.
Situation: Nominal 4” line, 1000’ feet long carrying compressed process air at 125psig, roughly 500 scfm. There will be internal condensate, a large number of flow control valves will receive this process stream and the expected life of the process is 40 years. The branch connections can be from the top of the 4” header and the header can be equipped with accumulation points and drains, but the branch process lines may not have strainers – just the control valve and vortex flow transmitters:
• CS will rust internally, and must therefore generate scale – but will it be enough to damage the vortex unit, or accelerate the wear of the control valve and trim, to an unacceptable degree? Unacceptable...say, 1.2X the normal wear rate, for some attempt at scale.
• Will internal corrosion be a problem structurally given the process lifetime? Target rack spans are 18 to 20 feet.
SS makes the corrosion & scale problems go away (though I need to check span limits) but of course nobody wants to pay for it.
Thoughts, recommendations on the question as posed?
If alternate materials were permitted, irrespective of rack spacing, what might they be (aside from copper)?
Thanks much,
John
Question: What material should be used for this service, CS or Stainless are the only options for various reasons including -20F operational temps and plant prohibitions. It’s a 4” compressed air process line and my concern is scale/garbage accumulation in the line over time, causing problems for the control valves and/or flow transmitters.
Situation: Nominal 4” line, 1000’ feet long carrying compressed process air at 125psig, roughly 500 scfm. There will be internal condensate, a large number of flow control valves will receive this process stream and the expected life of the process is 40 years. The branch connections can be from the top of the 4” header and the header can be equipped with accumulation points and drains, but the branch process lines may not have strainers – just the control valve and vortex flow transmitters:
• CS will rust internally, and must therefore generate scale – but will it be enough to damage the vortex unit, or accelerate the wear of the control valve and trim, to an unacceptable degree? Unacceptable...say, 1.2X the normal wear rate, for some attempt at scale.
• Will internal corrosion be a problem structurally given the process lifetime? Target rack spans are 18 to 20 feet.
SS makes the corrosion & scale problems go away (though I need to check span limits) but of course nobody wants to pay for it.
Thoughts, recommendations on the question as posed?
If alternate materials were permitted, irrespective of rack spacing, what might they be (aside from copper)?
Thanks much,
John





RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
with -20F operational temps (assuming inlet air), there is very little moisture in the air, unless the air compressor is placed in a location in which moisture is added to the air.
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
All answers are personal opinions only and are in no way connected with any employer.
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
Do you not have an instrument air package/ compressor/filter/dryer?
Independent events are seldomly independent.
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question
Also, the price of painted Sch40 4" carbon steel pipe is very similar to the price of Sch10 304 stainless. It is quite expensive to paint pipe to a reasonable outdoor industrial specification. It takes a good blasting [think 3 or 4-sided object], then an immediate [prior to more than rust 'measles'] primer coat with at least one turn to get all around, touchup of the rack-marks, cure of primer, then an intermediate coat + turn + touchup, then top-coat + turn + touchup. Then field fabrication fitting and welding. Then field priming of the weld areas, then inter-coat, then topcoat.
Properly applied industrial paint is time consuming, thus expensive. And a 'slapped-on' coat of enamel over the un-blasted, unprimed pipe is not really any more life-prolonging than just leaving the pipe 'bare' and letting the weather 'passivate' it. Both give a corrosion rate of .05 - .2 mils/year. don't try 'bare' or 'slapped-on paint' anywhere near salt water
RE: Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question