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I LUV NH3
01-11-2006, 02:15 AM
Howdy,

Can anyone tell me what is required for NH3 pipe schedules. I know that if I run anything less than 2" it needs to be schedule 80 seamless. Pipes from 2" to 6" schedule 40 seamless or ERW. But what about piping above 6" in diameter? It really gets confusing on these pipe sizes.:confused: Where can I find this information? ANSI/IIAR 2 1999 is blowing me away as it is calling out pipes that are schedule 20 and even schedule 10. I have been taught that pipes 12" and above should be standard wall. I cant find any code to back this up though.

Thanks in advance.

NH3LVR
01-11-2006, 03:29 AM
I do not know the codes well. But then I do not do much heavy fitting anymore.
However we only use Sch80 for 2 inch and under, and Scd40 for the bigger stuff.
The State has really clamped down hard on NH3 Piping the last few years. We have to have test results on each lot of pipe we bring to the job site.
If you wish I can check with the Boss on requirements. He deals with it every day.

Welcome to the Forum, I LUV NH3

I LUV NH3
01-11-2006, 01:19 PM
Thanks I would like to hear what your boss has to say.

US Iceman
01-11-2006, 02:19 PM
ANSI/IIAR 2 1999 is blowing me away as it is calling out pipes that are schedule 20 and even schedule 10.


First, let me welcome you to the RE forums. I hope you will be able to participate and contribute to the material, and maybe even have some fun too.

What ANSI/IIAR 2 is trying to tell you is, various wall thicknesses can be used, but you have to be able to prove it by engineering calculations.

There are several requirements in selecting a suitable pipe wall thickness. Some of these are:

Pressure containment (same as design pressure)
Corrosion allowance (if provided or used)
Temperature (below -20F you need to use a different material [A-333], or derate the A-53/A-106 pipe material, or use stainless)
Temperature and Pressure cycles (an attempt to compensate for pressure and temperature cycles that occur)
Actual stresses (unsupported spans, expansion/contraction, and other stresses than develop in piping.ASME B31.5 is the refrigerant piping code here in the US. This sets the prescribed method for determining the above. You have to do calculations to prove the pipe wall thickness is thick enough to absorb all of the stresses without failure.

The smaller diameter pipe needs to have a thicker wall, than the larger pipe.

Someone can correct me if I'm wrong on this next point, but I wanted to add it into the conversation.

The use of Sch. 80 wall thickness is interesting. In the past before systems started to get so big, almost all of the liquid lines were under 2". The use of Sch. 80 was prescribed due to the high potential for failure due to corroded pipes and the strange phenomena that can occur (water hammer) when solenoid valves open and close fast.

NH3LVR
01-11-2006, 02:41 PM
Talked to the boss this morning. He was not sure of the codes over 6 inches, although he thought it might well be allowed in theory. But he said he did not believe that the local inspectors would pass it.

US Iceman;
I do not believe that you are wrong. But in a refrigeration system people end up crawling over the pipes etc. and mechanical strength is important. Especially when you have vibrations issues.

The other consideration is when you have threaded connections on flanges etc. Threaded Scd 40 has very Little wall thickness left after threading.

US Iceman
01-11-2006, 02:55 PM
...But in a refrigeration system people end up crawling over the pipes etc. and mechanical strength is important. Especially when you have vibrations issues.

The other consideration is when you have threaded connections on flanges etc. Threaded Scd 40 has very Little wall thickness left after threading.


Quite right. This is similar to other things I have seen such as using much heavier wall thicknesses for 1/4" or 3/8" pressure piping back to control boards too. This was threaded also.

I LUV NH3
01-11-2006, 03:41 PM
Ok, I got a copy of ASME B31.5-2001.

They give a formula for straight pipe under pressure.

P = maximum anticipated pressure
DO = Outside diameter of pipe
S = allowable stress for the material
y = coefecient for materials

t= pressure design thickness

tm= t + allowance for corrosion + threaded groove thickness etc..

t = (P*Do)/(2*(S+P*Y))

So for a large diameter pipe say 18"

Do = 18 in.
S = 12,000 psi for temps (-20F - 100F)
Y = 0.4 for A-53 grade B


So

t = (250 psig * 18 in.)/(2*(12,000 psi + 250 psi *0.4))

t= .1859 in


Now

tm= t + C
Corrosion allowance is 50% so that means C = also equals .1859 in.

Tm = .1859 + .1859

= .3718 in

This means that the 18" pipe needs to be Standard wall .375 in.



I guess I answered my own question. Maybe this excersize can help someone else.

US Iceman
01-11-2006, 04:43 PM
Bingo, that's it.

Just make sure you use all of the stress reduction factors and compensate for the pressure/temperature cycles. These are listed in a chart. Hot gas defrost headers are a big concern with this due to the many defrost cycles this pipe can see.

You also need to verify the stresses/moments at joints or other "supported loads" connected to the pipe in question.

I really like the use of a corrosion allowance, since this is really the only protection you are adding when the vapor barrier system fails.

Notice I said When, not If.

As you pointed out, the generous corrosion allowance doubles the wall thickness. Therefore, if someone less discerning did not use a corrosion allowance they could make a case for using thinner wall thicknesses and still be CODE LEGAL.

It could be legit, but not necassarily the right thing to do for the customer. The thinner walls would of course be less expensive though.:D

Look at stainless steel too. No corrosion allowance required and it works very well for low temp.;)

PS. What part of the country are you in?

I LUV NH3
01-11-2006, 06:54 PM
US Iceman,


I live in the Midwest. I have stumbled across installations that have schedule 10 pipe installed! The installer simply quoted IIAR Refrigeration Piping guidlines. 9 times out of 10 IIAR is not a code! ASME B31.5 and ASHRAE section 15 are codes! Be carefull of the piping that is installed in your facility. Know what is code and what is a suggestion.

:D

NH3LVR
01-11-2006, 07:29 PM
I went and looked that up. (Not working today and cold and windy outside, playing with my new Linux machine and watching the Forum)
We are talking one HALF the wall thickness! Leaves a lot of room for bad welds. And no corrosion allowance at all.
We would not get away with that in my State. (Or maybe these jobs are not inspected in the Midwest)
Glad I do not live next to one of these Plants!

I LUV NH3
01-11-2006, 07:42 PM
What state are ya in? I won't name contractors on this forum but these people are notorius for doing this.

US Iceman
01-11-2006, 08:18 PM
Know what is code and what is a suggestion.


Very true. Unfortunately, a lot of people have a hard time understanding ammonia systems, let alone the codes.

The owners are concerned with what cooling does for them. Not too much of how the cold is produced.

This business can appear to be a lot of "smoke & mirrors" to the untrained person, so eventually some one gets "bitten".

If the contractor can provide a set of calculations that shows Sch. 10 is "adequate" for the project, then an inspector could formally accept that. The downside is, while the calc's may be legitimate they are in all likelihood not properly addressing the requirements.

If we tried to make a case of: the pipe has to be thick enough to contain pressure only; then the wall thickness can be very thin.

Doing it as you did in the example, shows more insight into the real world environment and is much closer to what I would call acceptable.

The vapor barriers are so dependent on a good installation to be effective. If the assumption is the pipe will never rust, someone is living in a fantasy world. Of course, there are some pipes that won't rust by virtue of being very cold all of the time.

Taking a job with Sch. 10 pipe and an insulation job that is poorly done (leaks) is just asking for trouble. But then again, it would provide job security. At least until the owner starts to figure it out.:D

I'm in the upper Midwest.

PS. I would not mention names either. At least publicly. I am not a contractor either.:p

Andy
01-11-2006, 10:35 PM
The vapor barriers are so dependent on a good installation to be effective. .:p


Generally Sch 40 is used in everything up and including 8" there would be an option for 20 erw above this.
I would say if the piping is properly insulated and the insulation is maintained sch 40 is fine.

Kind regards Andy:)

I LUV NH3
01-11-2006, 10:51 PM
Essentially it boils down to this.

1-1/2" and smaller = Sch80 seamless
2" through 12 = Sch 40
14" and up is standard wall.

Always check your local codes. I know California and Minnesota are pretty strict. Infact Minnesota does not allow anything less than schedule 40.

US Iceman
01-11-2006, 11:36 PM
Ohio is strict too from what I have heard. I think most of the states are beginning to take this pretty seriously.

The info I wrote about earlier is NOT an attempt to condone or recommend using thinner wall schedules. It was an example of how someone might try to argue this practice could be acceptable.

Ponca Dave
04-02-2008, 08:36 AM
Codes vary state to state. Minnesota has probably one of the strictest codes.
the schedule depends on the state and pressure of the fluid.
Liquid and hot gas usually are 2" and below and should always be 80.
I am an overdesigner and have a built in safety factor of 2 for everything.That slight "overkill"
is cheap compared to litigation costs.
Suction lines can be sch.40 (no less) because t
(the thickness increases with size.)
Insulation with a good vapor seal is the best prevention of disaster especially with pipes that
get cold and then defrost.
Common sense should always prevail over a few $'s