High Temperature - High Pressure Compressed Air 1

[PEDARRIN2]

I am assisting another engineer who is trying to design a piping system for a research client that requires compressed air at 150 psig at 800 F. They require stainless steel to minimize any impurities from the piping entering the air stream.

This is outside the standard compressed air system design parameters - so standard design approaches are not going to work.

The design pressure/pressure exceeds the limits of ASME B31.9 Building Services Piping so I am assuming B31.1 applies.

Looking at a table based on B31.1 (found on Engineering Toolbox) indicates 10" schedule 40 pipe at 750 F is rated for 578 psig, schedule 80 is rated for 956 psig.

Since the design temperature is fairly close to the upper limit, I am thinking either schedule 40 or 80 would be sufficient.

But I do not want to just depend on a table that I am unfamiliar with.

So I am going to determine the MAOP, based on ASME BPV, Section 1, which is referenced in ASME B31.1. If I get values that correspond to what the table indicates, then I will trust those results.

Does this sound reasonable or am I off base?

 

[moltenmetal]

Unless the hot air is used for power generation, I would not assume B31.1 applies- it is likely B31.3 piping if it is feeding a chemical process.

Do not design piping in accordance with a boilers and pressure vessels code- use the stress values and other rules spelled out in the applicable piping code.

 

[PEDARRIN2]

The reason I went to B31.1 is it states, "100.1.2 Power piping systems as covered by this Code apply to all piping and their component parts except as excluded in para. 100.1.3. They include but are not limited to steam, water, oil, gas, and air services." Nothing in 110.1.3 indicates compressed air piping is not allowed.

The only other section that might be applicable is B31.3, but I haven't been able to get access to the document to look at yet.

I do not know of any other code that might apply.

 

[XL83NL]

The best approach would be, I think, to get a gold of B31.3 and do the calcs for the required thickness yourself. After having found the minimum required thickness, select the first higher commercial available schedule and use that.

 

[curtis2004]

PEDARRIN2,

Can you describe more in detail what hot temperature compressed air does? What is actual temperature ranges? Is pipe insulated?
Do you know other piping systems at this site designed to? It is important because it is owners prerogative to tell what design standard their piping should be designed to.

Regards,

Curtis

 

[racookpe1978]

OK - AQ me on this:

The original poster has an unusual condition, one not inside the standard "code rubber stamp" of steam and power plants (31.1) nor API (high pressure, high temperature flammable/explosive petroleum mixtures) right?

BUT - He DOES have very high temperature air under relatively high pressures -> A dangerous situation he needs to protect against. But, are not the API spec's more appropriate in this case because, while he is well inside the high-pressure and temperature limits of the power plant steam and feedwater, his extremely hot air will not be condensing if leaked or blown out of failed fitting/valve/flange/weld.

Thus, while not flammable, it appears his hot air will be better contained within a more conservative code (although at a slightly higher cost) using the API spec's.

 

[PEDARRIN2]

I am not certain the exact application of the process, but as I understand it, it is a university research lab which requires this type of heated compressed air. The air is compressed to ~150 psig. The user wants it at that pressure because the process of filtering it and then heating it will drop the pressure down to where they want it. But 800 F is not typical and falls outside of B31.9 parameters.

Guidance from the user is they wanted it to be stainless steel and to be flanged. Other than that, they have no standards and they are leaving it up to the engineers to determine the applicable codes/standards. The pipe will be insulated due to the high temperatures.

The flanges will have to be specified as well as the gaskets for the pressure/temperature expected.

This system is the only one we are designing that would be outside of B31.9.

And since the pressures/temperatures fall within parameters similar to high pressure steam, I was assuming specifying pipe thickness with B31.1 would be applicable. Pipe thickness is a function of the pipe material, pressure, and temperature. And since the air will be dry, I do not see much of any potential for corrosion.

I was originally asked only to determine the pressure drop (and sizing the pipe) for the system. Since it was higher pressure/temperature, I was assuming schedule 80, although schedule 40 would likely work, based upon the tables I found which are based on B31.1. While I will use tables to serve as a basis for design, I do not necessarily trust tables that I do not know their derivation. I want to have actual calculations that can be referenced, especially since another engineer found a table from Atlas Steels (

http://www.atlassteels.com.au)

which indicates that 10" Schedule 5 or Schedule 316SS pipe would work at these pressures.

I am not familiar with API, other than knowing what the acronym stands for. Are they similar to ASME B31.8 or are they more stringent?

 

[curtis2004]

What kind of air flows we are talking here?
How they are going to heat compressed air? Fired Heater?

I think, I would first determine overall system components, their operational conditions. Looks like compressed air is easiest part of overall system.

Regards,

Curtis

 

[PEDARRIN2]

The air flow is in the range of 15,000 scfm and the air is heated after it is compressed. I am not sure of the mode of heating it.

I was tasked with determining the appropriate pipe size and when I used S80 piping as my basis, the senior designer stated he wanted to use S40, although he thought S10 or even S5 would work.

This started the discussion of what code/standard applied so we could determine the correct method to determine the pipe wall thickness. I am thinking B31.3 is more applicable over B31.1.

 

[kacarrol]

PEDARRIN2:

Are you in Canada?

 

[PEDARRIN2]

No we are in the USA.

Since this design is obviously outside of B31.9 and outside the realm where the Building Code Official would inspect the design/construction, we are going to bring on board a consultant who has more experience with this type of piping design.

But I am going to make sure I get to see the design process and calculations for my own professional development.

Thanks for all the help.

 

[curtis2004]

Pedarrin2,

You can use either B31.1 or B31.3. Please do not get fixated to pipe wall thickness calculations. Piping code is much more than that.

I would suggest at this stage just state that piping system will be designed to B31 code. Which one will be decided later on. It is because at point you have two halves of the system (compressed air and process) hooked into "black box - heater" you do not have any idea what it will look like.

At this stage just calculate wall thicknesses by both standard formulas and get thicker one (it even could be the same schedule in both cases).

You could also contact insurance underwriters on the research facility which may have their own rules and requirements depending on what kind of process equipment been used.

When you have whole picture of overall project with detailed equipment specification, get information from insurance company, get local jurisdictions requirements for system being built, after that you can make a call what standard will be appropriate in the circumstances.

Regards,
Curtis

 

[PEDARRIN2]

Curtis,

I did the calculations and come up with Schedule 10 pipe, but since I am not as familiar with the code, I do not have the confidence that I am inputing the correct numbers (stress values, weld adjustments, etc.) in the equations. I do not have the experience to have the confidence in my results. And nobody else in our office have it either. That is why we are going to get another consultant to perform the calcs.

In Ohio, where this is being built, the local jurisdiction basically "punts" the responsibility for design and inspection and puts it on the owner.

 

[racookpe1978]

At that temperature for any length of time, I do not believe Sch 10 wall thickness - especially of stainless - will be sufficiently strong enough against long-term sag and distortion from the gravity and heat loads. But, certainly 800 F is easier to resist than 1200 or 1500 degree F.

You have the advantages of no corrosion margin (very hot, very dry air is nice!) but that high a flow flow rate WILL cause physical erosion problem that will cause a thin-walled pipe to erode through at elbows, bends, and fittings. Basically, where ever the air flow is going to be "bumped" against the wall of the pipe and away from the neat, pristine centerline turbulent trajectory of a perfect pipe. A research facility has an advantage of (perhaps) a short lifetime - please factor that in. On the other hand, once built, it will sit there in place "forever" because there will be no funds to take it down, and some future university/lab/project will most certainly "resurrect it" for some other application/test/project/research.

 

[Compositepro]

You could also consider placing insulation inside the pipe. High temperature autoclaves have insulation on the inside. You would still have thin walled steel pipe or sheet metal inside but it would not have to contain the pressure.

 

[moltenmetal]

Get the code and do the calculation. If the code calc indicates that sch10S welded seam 316/L piping has more than sufficient wall thickness at your RELIEF pressure (which will be ABOVE your 150 psig operating pressure- find out what the pressure source is, how it is relieved, and what its setting is and design for THAT) and 800 F, then you should not hesitate to use it. Do not go up to sch40 out of worries over erosion etc. unless your velocities are much greater than typical for flows of gases in pipe (100 ft/s or so) or the stream contains particulates or other corrodents that you haven't mentioned. From what I've heard, it's dry hot air, and stainless will last FOREVER.

Any joints in that piping, especially branches, even for small instruments etc., need to be designed and installed per the piping code, or per ASME VIII-1. Welding must be per ASME IX, requiring RT examination etc. etc. etc. At 800 F it is no longer a Category D (nonhazardous) fluid service just because it's air- it is general fluid service.

Do not forget that this pipe will experience a LOT of thermal growth between 70 and 800 F- thermal flexibility analysis and proper support design is a MUST. But if this is done, the pipe will not sag or otherwise suffer from the choice of sch10S- in fact sch10S will make your flexibility design easier than using sch40S in a lot of cases.