Failure of Carbon steel piping material

[qualitypro]

We just built 2 HRSG's. During start up,a 2" sch 80 carbon steel (Astm A106 Gr.B)pipe in the IP feedwater to LP drum system failed in the parent material at 2 places with water spurting through the new pipe.The design conditions of this system were 116 psig & 260 F.The section of pipe in the horizontal run that failed was about 5 feet away from an orifice flange with restriction plate. The system has a control valve upstream which regulates the pressure of IP feedwater which has a design of 1044 psig before it goes into the orifice flange. The material used was presumed to be defective and was hence replaced with new material of the same grade and schedule.After 15 hours of operation, much to our surprise the new replaced pipe failed again in the same section of piping.Can somebody explain the cause behind this?

 

[JStephen]

Is there a minimum flow rate at which the control valve is able to maintain the intended pressure? I'm thinking if downstream was shut down completely, you could wind up with that high pressure in the whole system.

You might consider a pressure relief device and a pressure gauge at the point in question to see what is going on.

 

[unclesyd]

You have something very wrong and it's not the pressure as the burst pressure of 2" Sch 80 pipe at your temperature is around 13,000 PSI.
Comeback with a little more information if you can.

Exactly where is the failure in respect to the pipe's geometry?
Is the pipe seamless or welded?
If you have a drop from fabrication grab it and check PT the ends.
Is there any vibration around the point of failure?
Do you have anyone that can possibly look at the failed surface?

Anything else you can think of.

 

[metengr]

This sounds like it could be a flow accelerated corrosion problem (FAC), or internal wastage from fluid erosion, which is common with HRSG systems. As unclesyd states, you need to supply more information.

 

[kenvlach]

Here's my 'shot in the dark:'
Defective batch of pipe, plus a shock wave from rapid closing of a downstream valve to account for failure at the same point (must have been some pretty high flow!).

ASTM A106 pipe of 2” and over is hot finished, so some internal scale (unless otherwise specified, e.g., as in Supplementary Requirement S7 Internal Cleanliness). Hydrostatic testing is required (unless otherwise specified, in which case each length shall be marked 'NH' and the certs. state “Not Hydro statically Tested”). Maybe it was tested with dirty water, not dried and/or stored outside for a couple of years. Galvanic corrosion can occur between the scale and some bare metal. Microbes are always present, too.

The dates of the material certs. (usually from the mill) and of the hydrotesting should allow you to figure out the pipe age and length of storage.
Ken

 

[CRG]

It will be interesting to get additional information from qualitypro.

Is this part of a redundant system? If so, is there any possibility for cool fluid to be isolated by valves and then subject to thermal expansion? If so, is there any possibility that the pipe could have frozen if not in service?

 

[GenB]

you really need to reduce the incoming pressure as the sudden overpressure will act as 'water hammer' creating a hydraulic pressure and burst the pipe, it will only take a few seconds to ruin the pipe, some times we use pressure snubers or fluid relief valves.
genb

 

[Zapster]

I will be surprised if this is a surge problem. However, I have a lot to learn. My thinking that to selectively burst the 2” line without damage to other flanges, gaskets, valves, etc. in not likely.

In regards to a surge problem, thread378-104974 might be of some help.

 

[kenvlach]

Good thread, Zapster[/p]; I especially liked kirsner's website

http://www.kirsner.org

I postulated that the failed pipes were weakened due to pre-existing problem, e.g. corrosion, and that a hyrostatic shock wave created a local pressure point to account for both failures at the same location.
Could also be that pipes sagged a bit in storage, with moisture collected at similar lowpoint.

A examination by metallurgist or corrosion expert is needed. qualitypro, save everything to ensure your firm gets compensation.

 

[qualitypro]

Thanks you guys for your inputs.
I probably think that this could be due to flow accelerated corrosion or a surge problem as brought out by metengr and zapster due to the restriction plate at the orifice flange.I have never experienced anything like this before especially as the boiler was hydrotested at 1.5 times the design pressure at 70 F in accordance with Demin water as per Section I and accepted.The operating pressure of the system was only about 90 psig at time of failure.
I would like to give you more information based on your questions. The first failure was in the bottom and side sections of the pipe. The second failure was in the top and side sections after pipe replacement.The pipe was seamless and there is no question of scales as the whole boiler was chemically cleaned and air blown before start up. The piping system was adequately supported and hence there is no question of vibrations around the point of failure.Visual inspection of the failure pipe revealed pinholes which have opened up to the surface with a small protrusion as if something has been pushed through from inside.The owner has taken the failed pipe and sent it for investigation.I hope this additional info will be useful.

 

[GenB]

pin holes!
before the oriffice you have to get read of entrapped 'air'
or it will kill your pipe
genb