Pipe Minimum Wall Thickness Calculation

[daviwy]

Hi,

Can someone help me with this? I am trying to calculate minimum wall thickness for pipes according to ASME B31.1 standards,

t (min) = PD/2(SE+PY) + A
t(min)= Minimum wall thickness required (in)
t(corr) = Mechanical, erosion, or corrosion allowance (in), typically 0.05 in. = A
P = Design pressure (psig)
D= Outside diameter of pipe (in)
S = Allowable stress in pipe material (psi)
E = Material and pipe construction quality factors: Table A-1A
Longitudinal joint factor E = 1.0 for seamless pipe, E = 0.85 for ERW pipe
Y = Wall thickmness coefficient Table 304.1.1
for ferritic steels
Y = 0.4 for T<= 900 ºF
Y= 0.5 for 900< T <= 950 ºF
Y= 0.7 for 950 < T <= 1000 ºF


But I keep getting the design wall thickness (on the P&ID) to be almost twice the minimum wall thickness (tm). Although it looks "good" i.e. shows an over conservative design of the pipes. It just doesnt seem right.

Is anyone aware of what might have gone wrong with my calculation? (if any)

 

[DSB123]

daviwy,
The Design (Selected) thickness is always greater than the minimum wall thickness (I would hope). After calculating the min wall thickness the pipe schedule needs to be selected to be greater than the minimum wall thickness. Also you need to consider the pipe's structural strength. It may be, for example that the minimum wall thickness for a certain size pipe is very low due to the design pressure but you would need to also consider the structural strength and select a suitable pipe schedule.

Also it may be that the originator of the P&ID's has a specified minimum pipe schedule from a Company perspective.

 

[StressGuy]

There may not be anything wrong with your calculation. Your design wall thickness on the P&ID may be coming from a standard specification. As such, the wall thickness indicated may not be based on the specific pressure/temperature of the line you are looking at, but may instead be based on a design temperature and pressure for the overall specification. If your line pressure happens to be a good deal lower than the spec limit, that would be a possible explanation.

Especially for smaller lines (12" and less at least), it is often the case that a constant wall thickness is chosen, rather than using "calc wall" to simplify purchasing and design. It's more common to calculate a wall thickness for lines above 24", as they are most often made from rolled plate.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[BigInch]

The minimum wall thickness is for when you have virtually NO Pressure.

What is the design pressure of your system?
Without that, we can only guess if you've got anything right here.

P.S. I hope you've got an experienced engineer somewhere near you that you should be learning from.



**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JohnBreen]

daviwy

Usually we calculate the minimum wall and "round that thickness up" to the next larger standard schedule. Sometimes certain standards for systems (for example condensate return lines) call for additional corrosion allowance which will increase the thickness (and again round up to the next standard thickness). In some cases with small bore piping the thickness might be increased if experience shows that workers stand on the pipe. Some small bore pipe had increased so that it might span the standard spacing of the rack supports.........there might be any of several reasons other than internal pressure.

John

 

[MJCronin]

dave....

Understand that you are talking to a group that has done exactly the same calculations that you have and has made the same observations that you have...

In addition to the sage advice of Mr Breen and Klein above, consider the threading requirements of small bore piping.

This, of course, must be added to the min wall that you calculated.


_mjc

 

[unclesyd]

If you have a pipe in a corrosive service you could use the the calculated minimum wall thickness as the retiring limit of said pipe though there are more conservative calculations to allow for support and operational flexibility. We have a lot of SS steel in very corrosive service where we have constant inspection to determine the current thickness to calculate the remaining life. We would never let a line approach a calculated limit based on your formula.

 

[Zapster]

In addition, there should be a considerations for mill tolerance per the piping specification. Often 12.5%

 

[daviwy]

Thanks all for the responses everything's very useful!
The pipeworks in question are mostly Carbon Steel,
ASTA 106 Gr.B designed for HRSG units on a power generation system.

I've spoken to site personnels and various other information sources. As the powerstation was bought and not built, the transfer of information to an extend have been poor.

Please do drop more information or knowledge you have on this subject they will all be taken in and appreciated.

Thanks