pressure tmin for thick piping 1

[NovaStark]

Hi all,

I have to calculate the tmin for a 1/2 inch XS pipe at 150 psi and 100F. I'm using ASME b31.3 however my t is greater than D/6 indicating that I need to use an equation for thick wall I believe.

Now in the code there is a provision to calculate the coefficient Y for t > D/6. Does that imply that I am to use the same equation for thin wall piping or use Lame's equations ?

The line is just a branch connection going to an analyzer so there aren't any thermal stresses or fatigue to generally heavy loads on it.


Using the thin wall equation, I get t would be 0.003 inches + corrosion allowance of 0.05 inches = 0.053 inch.

 

[racookpe1978]

If you have "chosen" a 1/2 dia XS pipe, your t_minimum IS specified. It cannot get "minimimmer" nor "maximummer". (Mill tolerances excepted of course.)

Now, invert the problem, you have a low temperature system 100F, right?
You have a specified pressure (150 psig), right?

What is the calculated wall thickness?
Does a 1/2 dia XS carbon steel pipe meet or exceed that wall thickness you calculated?

 

[LittleInch]

Same as racookpe,

All you have to do is make sure that your t min is < t of a 1/2" XS pipe.

1/2" XS is 0.147" thick. You've calculated 0.053". You're good to go. The thin wall equation normally gives a bigger, but seeing as this is 0.003", then I think you're OK.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NovaStark]

Thanks for the response guys.

I was mainly asking as I was trying to figure out the plant designer's reasoning for using schedule XS for a 1/2 inch piping of such low pressure and temperature. As XS is around 0.15 inches i.e. 50 x tmin.

As this line is just a branch going to an instrument, there wouldn't be any excessive bending or cantilever sections, so I was wondering if a different thickness equation was required.

 

[racookpe1978]

Sometimes it becomes a "default" or "I specified that wall thickness so I won't screw up when it does matter" type of protectionism. Other times, it is "We know people are going to grab the instrument lines or step across them" ...

And sometimes, it is because they didn't think.

 

[LittleInch]

At small pipe sizes, practicality and experience come into play.

Hence many designers will simply use XS or sched 80 (same thing at 1/2") as a minimum for strength purposes.

Sure, in theory you only need a paper thin piece of pie, but it would collapse as soon as you tried to lift it, clamp it, attach anything or blow on it.

The thickness is pretty low ( 3.7mm). there's simply no long term benefit to reducing it any further.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KevinNZ]

Do you need a corrosion allowance?

 

[NovaStark]

Thus it is not always necessary to produce a structural calculation showing the minimum thickness for this being greater than the thickness for pressure then.

Do you need a corrosion allowance?

There is a corrosion allowance of 0.05 inches specified by the plant designer based on the service.

 

[LittleInch]

Not sure if I follow that exactly, but at those sizes, you just need to do as you've done and make sure that the required thickness for pressure and corrosion allowance is less than your chosen standard wall and for that size XS/schd 80 is a perfectly reasonably standard to choose which has stood the test of time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NovaStark]

Not sure if I follow that exactly

What I meant is that normally when doing something like calculating the remaining life based on some corrosion rate, if you calculate a structural thickness requirement as more than the thickness due to pressure, the structural one becomes the tmin.

But I am guessing that for these things, the pressure t-min is sufficient.


Also ultra-pipe produces a structural tmin which just subtracts the corrosion allowance and mill tolerance from the nominal thickness of the piping that you input.