Piping Spec Question

[CharlieH]

Can a piping spec be used if the process conditions do not fall into the specified range?

For instance, a spec states:
285 PSIG at min temp through 95 PSIG at 750 F

Can this piping spec be used if I want to pipe gas vapors at 0 PSIG (exhaust pipe open to atm) at 1200 F?

Can anyone direct me to a "down to earth" explanation of piping specs and how to use them? I've never officially been trained on this topic and find it a little confusing when I try to apply intuitive reasoning.

Thanks for the help!!!

 

[BigInch]

You must define the design code before you can determine if the material and thickness is appropriate for your pressure and temperature.

Your spec isn't too terrific, since it apparently makes you follow some allowable stress line between temperatures and pressures given for two points. Most specs developed from a particular piping design code will give you an allowable stress that will be valid between two temperatures. With the allowable stress you can determine the allowable pressure, calculated from the appropriate formula given in your design code. For example in B31.4 Liquid Pipelines, allowable stress values for various piping materials are given for -20F to 250F. When used at temperatures outside the -20F to 250F range, the allowable stresses must be reduced. (Keeping in mind that cold steel is subject to embrittlement and hot steel will have a reduced yield stress).

The reduction at 1200F coul be significant, making for a very low pressure allowable, but you will also probably have very little pressure in your piping because, since exhaust gas pressure must be very low leaving the cylinders, there will not be very much backpressure building back up from your 0 psig atmospheric outlet into the cylinders. So, maybe it will be OK, but we really can't say because you don't give us the design code, the material or the wall thickness. But that's OK too, 'cause we don't want to take your job away from you either.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[dtn6770]

Make sure to consult with whomever's piping spec you're using. Have a technical basis (design code) for knowing the spec deviation (variance) is acceptable, assuming it is. With that technical basis in hand, make a recommendation to the spec owner (client?) that you be approve the deviate from the spec, as it's writen, for your particular application. Document that approval. Or, you could notify your client that you have an application that doesn't match any of their available specs and request they provide you with one that does.

Specification deviations should trigger some sort of management of change protocol that forces a technical review.

 

[dcasto]

1200 F, can you say phase change. The exhaust pipe doesn't require a specification for hoop stress. You are looking for some structural strenght and most of all a material that won't oxidize to a pile of rust at 1200 F. You'll need some CrMo or SS.

 

[NozzleTwister]

CharlieH,

At zero pressure, you probably won't have any code requirements. You are outside the B31.3 scope.

You're right, your piping falls outside the Pipe Spec. range but the Pipe Spec is written to meet a piping code such as ASME B31.3 and your pipe does not need to meet B31.3.

I've had similar experience with regeneration vent piping. The client wanted to use carbon steel at 1200 deg. and zero pressure. The lines were to be used for a short period every 2 years or so and the client didn't care if it warped and stayed that way. He only wanted it to not affect other lines.

The bottom line was he felt it was more economical to repair or replace the line when required than to invest in an alloy line.

If you have this situation, design for the thermal expansion, minimize the support loads and keep the bare pipe off the steel.

Good luck,

NozzleTwister
Houston, Texas

 

[CharlieH]

I'd like to give some specifics that may shed light on the project. We have an existing natural gas fired waste gas compressor (12 cyl Ingersoll Rand) and we're replacing the existing exhaust silencer with a combined silencer / catalytic converter. We're also replacing the upstream and downstream piping (total about 25 feet). The existing exhaust pipe is 14" Std Wt (3/8" wall thk) and according to the spec it is plain carbon steel.

I was provided pipe surface temperatures in the 1100 - 1300 deg F range. The problem is that the existing spec appears to max out at 750 deg F at 100 PSIG. My application is essentially at 0 PSIG since the cat converter generates about 5 in H2O backpressure and the exhaust is open to atmosphere. So, my dilemma is whether this spec can be extended to a higher temperature since the pressure is greatly reduced.

I appreciate all the feedback and I am gathering info to present to the facility as a request for direction since their specifications do not cover this situation.

On the published spec, I see a Press-Temp Rating Table that goes from 100-750 deg F and the corresponding pressures are 275-100 MAX PSIG. This is where I deduced the 750 deg F at 100 PSIG ceiling. #1) Am I correct with this determination?

#2) Do these values come form a graph of some sort that is published somewhere?

#3) Can someone verify if I am reading table 331.1.1 in ASME B31.3 (Reqs for Heat Trmt) correctly?
***Carbon Steel*** <= 3/4" wall thk does not require heat treatment.

Thanks,
Charles

 

[dtn6770]

Hi Charles,

#1) Your 750F at 100psig correlation is reasonable.
#2) As previously mentioned, you need to determine which, if any, design code the spec meets and if the owner/developer included preferences. The temp/pres ratings come close to those of B16.5, 150 class, carbon steel flanges but that doesn't tell us much.
#3) Your interpretation of 331.1.1 is correct. Besides, the exhaust temperatures you were given are of heat treating magnitude.

The spec you have is most likely more than adequate for the intended service but because you’re applying it to conditions beyond its published limits you should get approval from your client or from whomever you’re accountable to

 

[BigInch]

Some things that may help.

From indicated sections of B31.3,

300.1.3 Exclusions. This Code excludes the following:

(a) piping systems designed for internal gage pressures at or above zero but lss than 105 kPa (15 psi), provided the fluid handled is nonflammable, nontoxic, and not damaging to human tissue as defined in 300.2, and its design temperature is from -29ºC (-20F) through 186ºC (366F).

302.3.1

The stress values in Table A-1B, and A-2 are grouped by aterials and product forms, and are for stated teperatures up to the limited provided in para. 323.2.1(1). Straight line interpolation between temperatures is permissible.

So, in para. 300, it seems that you could argue that the pressure is so low, the code doesn't intend to cover that pipe, although a counter argument could be established based on the high temperature.

In 302, higher temperature allowable stress is reduced along a straight line, and at 1200F, most steels will obviously have enough stress allowable remaining to hold that pressure, but the problem is that very few materials have a listed allowable stress at that temperature. It could be argued that the unlisted values can reasonably extended to the 1200 F temp and thereby result as sufficient.

Given the number of possible arguments, all with answers that would either exclude the pipe from coverage or yield an acceptable "stretched" result, if it were covered, I'd be tempted to exclude it from code coverage, then pass it based on the same engineering principles contained in the code.


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Arg! Forgot,

No heat treatment is necessary, but its gonna' get it anyway.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com