Modest Cryogen Compressed Air Process Pipeline - Materials Selection Question

[jmedclay]

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

 

[pmover]

remove the water from the air after compression. there are readily available desiccant (for dewpoint temps to -40F) air dryer systems and refrigerated air dryer systems to 39F dewpoint.
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.

 

[SJones]

Carbon steel will also rust from the outside, and 40 years will be at least two repaints! Stainless might also 'rust' if it is not fabricated correctly.

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.

 

[BigInch]

Air is not cryogenic at -20F. What are you talking about, cold air?

Do you not have an instrument air package/ compressor/filter/dryer?

Independent events are seldomly independent.

 

[ElSid1]

Compressed air has several options. You may want to look at Aluminum prepacked systems. If not, what about sch 80 CS? With air being dry (assuming a low dew point), it should not be a problem. I've worked on SCH 80 CS steam systems. Old ISA instrument air required -40d dew point ... so very doable but $$$. Newer ISA has instrument air as -19F below ambient ... if memory serves.

 

[jmedclay]

I meant to say near cryogenic temps, which on reflection is an artifact of previous thoughts that the process air might be -50F or so. In any event this is nothing but cold air - process air, not IA and no serious attempt will be made to dry it. Once it's in the pipes it could get as cold as -20 F. The question confronting me is: Assuming modest internal condensate, will CS piping rust to the point of generating enough scale and garbage to damage the globe style metallic valve trim or vortex shedders, or compromise the pipe structurally as it relates to span between racks, all over a 40 year operational lifetime. It's not a salty environment.

 

[Duwe6]

In my experience, you will not get flakey, exfoliating rust. After normal drying, your air will [briefly] cause a little powder rust, and then it will stop. Partly due to the oxidized, passive layer, but mostly to a thin, thin, thin layer of air-compressor lube oil that will slowly be 'plated out' onto the interior of the lines.

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

 

[ElSid1]

After normal drying

no serious attempt will be made to dry it

I recommend you follow the ISA IA standard for drying air. It is set at 19F below ambient for the dew point. You have not defined the conditions of the air, even if it's plant. If it is not dry enough and there are not enough drip legs with blow downs. I have seen SCH 40 piping in mining applications where it was constantly used and created little to surface rust at 16F. Same location, different system ... piping was corroded due to sitting water and not blowdowns or frequesnt use. I concure with Duwe6

price of painted Sch40 4" carbon steel pipe is very similar to the price of Sch10 304 stainless.

. Especially with so many unknowns.