Allowable Stress for Steam Blow Piping

[ak2004]

Hi all,
How can I get (and maybe raise) the carbon steel’s allowable stress for the purpose of steam blow piping stress analysis? The temperature is in excess of 1000 degF.

 

[jte]

Simple enough. Reduce the design life to a few minutes so that creep is not an issue.

More seriously... Using the right materials will reduce the likelyhood of people getting hurt and killed. Steam at over 1000 deg is not something to play around with. I guess the best way to deal with the issue is to melt down the carbon steel and add somewhere around 5 to 9 percent chrome and a dash of molybdenum to the mixture, then freeze it and reform it to pipe.

jt

 

[ak2004]

jt
I have been asked to work on the temporary steam blow piping for venting steam to the atmosphere during the cleaning of the main & reheat steam lines.

The temporary piping material will be A106B but the permanent piping will be all made of P22, for the reason that you've already mentioned. The allowables shown in table A1 of B31.1 stops at 800degF but the steam to be used for cleaning will be over 1000degF.

Taking out the creep factor might be the best approach since these temporary piping will only have a few hours of life and will probably be junked forever. But how do you get the allowable values using this approach?

Thanks in advance!

 

[metengr]

You don't. No Code or Standard is in the business of looking at temporary anything because of the risk factor. You can use carbon steel, but be warned, this is at your own risk.

By the way, if this is temporary vent piping with no isolation points to atmosphere, you still might be able to get by at your own risk with using SA 106B carbon steel, assuming the maximum 1000 deg F for allowable stress per ASME Section II, Part D (I hope my esteemed ASME B31 colleagues don't figure out my alias).

 

[RossABQ]

Power plants typically use CS piping for discharge piping away from the release valves because the piping is not at the design conditions of the system. Also, blows are not typically done with the boiler at the design conditions. Ak2004, what does your boiler mfr's blowing procedure call for?

 

[ak2004]

Ross,
The cleaning steam condition varies as per the individual line or common header to be cleaned. But it appears that a section of the temporary carbon steel piping (between the CS/P22 interface up to the first temporary desuperheater) will be subjected to a condition up to 1022degF & 80 psi. It is this section of the pipe that I am needing stress allowables. The highest design condition of the permanent piping, which is the main steam line, is 1065F & 690psig.

 

[RossABQ]

I would have to assume that whoever spec'd the CS pipe has some experience justifying its use; Is it the boiler manufacturer? The pressure is quite low.

I would hope they prohibit re-use or re-sale of the piping!

 

[ak2004]

We are cleaning the steam piping that runs from the boiler up to the turbine which is several hundred feet long.
I have asked the system engineer who spec'd the temporary piping and it looks like he could provide me the data I needed based from previous projects although he could not explain how those were obtained.
My understanding is that the CS piping will be junked after the steam blowing.

Thanks for all your help!

 

[insult2injury]

Will the pipe be chemically cleaned? If not, the system may require extensive steam blows.

I2I

 

[CW3]

I used to do a lot of steamblowing. Back in the day, there was not as much concern as to what grade of pipe was used, however wrong that may be. More recently, much more consideration is given to proper engineering in this endeavor. In sections of the temporary steam blow piping that are upstream of temporary desuperheaters (quenchers), P11 pipe and fittings are whatwe used. B31 shows P11 material at up to 1100F as having a maximum allowable stress of 2.8 ksi. If you were looking at temps substantiallyhigher, you would look to P5, P9, P91 material. Tes, it's more expense, but ourclients usually appreciated that fact when we explained the risk factors involved. Although rare, there have been issues involving steam blows that proved unsavory.

I'm not in that business now, but I thought I'd toss in what I know from what we used to do....in that exact business.

Thanks,

CW3

 

[bvi]

If you are looking for design rules for A106 Gr B at over 1000F, ASME B31.3 provides allowable stresses up to 1100F, but they will be very conservative for a temporary steam blow operation, as they are based on long term creep properties. The allowable stress is 1 ksi.

 

[RossABQ]

The last blow I witnessed was a 500 MW 2400psi/1005/1005 utility boiler. They gutted the turbine stop valves and attached temporary piping to their "lids"; is that what you're doing, AK?

FWIW, I was in the field engineering building, at least 300 yards away, when a 12" crescent wrench, some odd pieces of pipe, and some 3/4" bolts/nuts sprayed the side of the building on the 2nd blow -- actually the wrench went thru the wall and onto a co-worker's desk. Needless to say the direction of the discharge pipe was changed the next day...

 

[ak2004]

bvi,
I was given the value of 4ksi @ 1050degF which is what was used from the copied job. I have no idea how it was obtained. I'm still trying to find out.

Ross,
The project we are doing is a 470MW 2085psi/1050. The main line just before the turbine stop valve was designed with an end pipe that is blind flanged during the normal operation. It is at this end pipe that our temporary blow piping is attached to. No chemical cleaning. Only continous blowing.