Bent pipe in lieu of fittings

[STex]

We are having problems locating the proper fittings, especially in larger diameters, for use in hydrofluoric acid service. A106 would seem to indicate that the use of bent pipe is acceptable as long as it meets the material requirements set forth for the pipe specs. My internal inquiries bring about concerns for SCC as a result of the bending process as well as a few other less serious concerns. Thus far I have not been able to find anyone who has used bent piping in HF service. I am not thrilled with the prospect of using seamed fittings. Are any of you aware of bent piping being used in HF acid service, H2S service or other services subject to SCC, SOHIC and such?

 

[metengr]

Bent pipe for use in service where residual tensile stresses from post forming operations can contribute to environmental assisted cracking should be removed/reduced by mandating a post bend thermal heat treatment. By the way, ASTM A 106 does not address use of service in specific corrosive environments.

What is the concentration and temperature of the hydrofluoric acid?

 

[STex]

Metengr,

Concentrations of HF are around 90% while temperatures range from ambient to ~150F. We are dealing with an HF Alkylation unit. I haven't been able to find any specific guidance condoning the use but on the other hand I haven't found anything prohibiting its use either. API-751 doesn't address the issue. The current mindset is that if there is nothing stating it is okay then we are going to assume it is unacceptable.

Would using properly manufactured bent pipe be an worse or provide more potential for problems than using a seamed fitting?

 

[EdStainless]

I have seen many systems built by bending the piping. In some cases a stress relief after forming was used.
This approach can greatly reduce the number of fittings, and fitting welds in a system and improve overall reliability.
The most common use is for long radius elbows, not much strain and it assures smooth interior surfaces.

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Plymouth Tube

 

[STex]

EdStainless,

I have seen bent piping in lieu of fittings in catalyst transfer service which for all intents and purposes is not process piping. Failed piping in these locations can typically be replaced without impacting unit throughput.

I see the value of using bent pipe in place of fittings I am just having a hard time finding support to do so.

 

[EdStainless]

What is the spec for the fittings that you would use?
Can they be made from pipe?
Then follow those rules.


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Plymouth Tube

 

[metengr]

Post bend thermal treatment should be required for any carbon or low alloy steel bent pipe and most specifications do not address this issue.

 

[EdStainless]

But the fitting specs do require post forming anneal, or at least stress relief. Whatever the requirements for fitting are what should be used for formed pipe sections.

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Plymouth Tube

 

[STex]

That's the quandary I've run into. Bent pipe is recognized in the various codes put forth by ASME and ASTM as pipe not as a fitting. But because it is used in the manner of a fitting some want to apply the pertinent fitting specs.

Is bent pipe that has been properly fabricated in a manufacturer's shop in accordance with the pertinent pipe codes and in keeping with the customer's material specs any more susceptible to SCC, SOHIC, etc than corresponding forged fitting? A forged fittings is extruded and then pounded into shape. Bent pipe is extruded and then cold or induction bent into shape. If both meet the same carbon content, residual element, carbon equivalent, tensile strength, hardness specs and have undergone heat treatment and the requisite testing such as RT or UT are they for all intents and purposes the same?

 

[metengr]

There is nothing in the ASME piping codes to prohibit the use of bent pipe versus fittings. The use of formed pipe versus a fitting boils down to cost, piping configuration and stress analysis (stress intensification) of the piping system. As I said before, post thermal treatment for bends should be mandated if not already addressed by ASME B31 piping codes.

 

[SJones]

API RP 571 details the degradation mechanisms that will apply and the materials restrictions that can serve to minimise risk of failure.

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.

 

[STex]

metengr,

That is also my take on the matter. Unfortunately, my take is not the final say. Part of my search is to find code cases or documented instances of the use of bent pipe in hydrocarbon service with particular emphasis on H2S or HF service. We are currently of the mindset that if we haven't seen it we won't use it, even though it is fully accepted within the governing codes, because there maybe something not covered in the codes that will cause a problem. Organizationally, the best way to overcome this reservation is to see proof of use.

 

[metengr]

Take a look at the reference below.

http://ballots.api.org/ecs/sc5/ballots/docs/5LIB_ballot.pdf

 

[STex]

metengr,

Excellent!! I'll have to bring this to the attention of our materials folks. Much appreciated.

 

[metengr]

STex;
From the Nickel Development Institute publication 10-074 regarding materials in hydrofluoric acid, hydrogen fluoride, and fluorine... for carbon steel to resist HIC or other hydrogen-induced phenomena, carbon steel should be low strength, low sulfur grade with a maximum hardness of 22 HRC.....

http://www.nickelinstitute.org/~/Me...cAcid_HydrogenFluoride_andFluorine_10074_.pdf

 

[SJones]

I thought all the fuss was over cold bending. Since induction bending is now on the table, ASME 16.49 may have been a good starting point.

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.