What pipe schedules are required?

[I LUV NH3]

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. 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]

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]

Thanks I would like to hear what your boss has to say.

[US Iceman]

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]

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]

...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]

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]

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.

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]

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.

[NH3LVR]

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]

What state are ya in? I won't name contractors on this forum but these people are notorius for doing this.

[Ponca Dave]

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