A106 Carbon Steel piping - heated above 1000degF ?

[bfkirk]

I'm currently using 6inch A106 300lb. rated seamless piping in a pressure vessel that operates at about 150 psi and 750degF. During a mishap, the vessel was heated in excess of 1300 degF for a period of several hours, forming a small bulge in the pipe (nothing drastic). According to the ASME Boiler and Pressure Vessel code, the temperature limitation of A106 is 1000 degF. Has heating the pipe to 1300 severely damaged the carbon steel itself? Should I replace the pipe, or continue to operate at 150psi and 750 degF? I can't find any information on this issue.

 

[Rich2001]

Basically, you heat treated the steel, Depending on how fast it was cooled down will influence the mechanical properties.

Have a NDE companty take some hardness reading (with a portable hardness gauge) this will tell you if the steel was affect by the temperature excursion.

Poratble Hardness tester -

http://martinmetsupply.com/hardnesstest_portable.htm

 

[PAN]

bfkirk,

Pressure rating of carbon steel flange Class 300 at 1000 F is only 50 psig. You should also consider about operating pressure during select temperature limitation.

 

[hacksaw]

if you have visibly bulged the pipe, then it has failed, unless of course you work for nasa. in that case....

In all seriousness, there are established engineering procedures for evaluating your pipe and adjacent equipment. It won't be cheap.

One question: how was the equipment protected from overpressure and was a Hazop performed?

 

[poppeye]

BFKIRK,
As Rich2001 stated, you have heat treated the metal causing the molecular structure to change or crystalize in nature. By hardening the metal, you have changed its elastic properties and the ability to thermally expand and contract safely. The Navy found out about this the hard way during the 60's. Carbon steel was used primairly in the construction of all steam piping withing the engineering plants of vessels. But the newer 1200psi 975deg designs of the early and middle 60's showed that operating above 775deg was definately a no no. Quite a few mishaps were reported where after several heating and cooling cycles, piping would catastrophicly fail. Not a seam or simple parting of the piping, but the piping would fracture and explode. That is why during the late 60's and 70's the Navy instituted levels of assurance in all construction materials. Level-1 any temps above 775deg, liquid oxygen and pressures above 3000psi, Sub-safe, level-3 etc.
Having a bulge in this piping shows that the metal has under-gone some type of structural change. The only way to know for sure if the metal properties have changed to an unsafe condition is to perform a microscopic examination to see if the grain of the metal has crystalized.
If you have any doubt what so ever about the safety of the piping replace it. It's better to be safe at any cost than having to, as we use to call it in the Navy, "Sit and the end of the table that has no water glass." OSHA investigations are extremly thorough. If your records show that overheating caused a structural change in the piping, and was not corrected. You will be on the hook.

 

[DLANDISSR]

BFKIRK:
The temperature limit of A106 pipe is 800Deg F Per B31.1 Power Piping Code. This is due to graphitization of the carbon in the steel. The metalurgy changes above 800Deg F with prolonged exposure. The stress value of A106 Gr B at 800 deg is only 10,800 PSI.

Since there is a bulge in the pipe you have obviously over stressed the pipe. The wall thickness at the bulge is less than the remainder of the unbulged pipe and will be a weak point.

ASME B16.5 tables for flange rating go up to 1000Deg F for ASTM A105 material which is what would normally be used with A109 pipe. At 750 Deg F a 300lb flange is rated at 505 PSIG and at 1000Deg F the rating drops to 50PSIG.

It is permitted to use A105 and A 106 above 800Deg F but it is not recommended because of the carbide phase of the steel may be converted to graphite. Since the pipe was not exposed to the high temperature over a porlonged period the steel is probably not aprecibly changed in metalurgy but may have lost any tempering and therefore be softer than before. Depening on how it was cooled and how long it took to cool it may have been hardened. This is an unknown without destructive testing.

In any case the condition of the pipe is unknown at this time and will reqiure destructive testing to find out its condition. The pipe should, in my opinion be replaced.

 

[bcarp01]

You also may have a code issue (and could be a state regulatory issue since many of the government regulations are carbon copies of the ASME codes)with overtemperature/overpressure of a system depending on which code is the governing code for your system. For example, B31.1 section 102.2.3 and 102.2.4 give pretty clear guidelines on how long a system can exceed design limits. I agree with the last comment. I would replace this pipe.