Pipe vs Flange MAWP 1

[eguentsch]

Many times I have heard people make a statement such as "In a spool with pipe and flanges, the flange will always be the weakest link with regard to MAWP".

I am wondering whether this statement is wrong or at least should be worded less general.

From ASME B31.3 I can calculate what pressure a pipe with a wall thickness t can withstand:

P = (2*S*E*t)/(D-2*y*t),

where t = t_nom(1-tol)-ca-threaded-ea

tol: manufacturing tolerance (usually 12.5/100)
ca: corosion allowance
ea: erosion allowance

As an example consider the following:
2" XS CS pipe with #600 CS flanges at 100F, 1/8"ca, no threading, no erosion allowance.
The above formula gives me an MAWP of 1132 Psig, while the flange can withstand 1480 (according to ASME B16.5 table 2, page 22)


So, I think, the above statement should be revised towards:

"In a non-corrosive, non-erosive, unthreaded pipe spool with flanges, the MAWP of the flange is always lower than the MAWP of the associated piping"

This statement is much more restrictive, as it should be.

Your input and discussion is more than welcome.

 

[moltenmetal]

The statement you made is a statement of design intent rather than one of universal truth. It is possible and sometimes sensible to select flanges which have a higher rating than the piping welded or threaded to them. A common example is the use of 300# flanges on low pressure hot oil piping to minimize leakage.

However, some build their pipe specs using a pipe schedule which exceeds the MAWP of the flange class selected throughout its pressure/temperature range, such that the design MAWP is "limited by flanges". Some do the same with ASME vessel design. The idea is that overpressure is more likely to lead to flange leakage than to pipe rupture. But it is acceptable to design piping such that the pipe itself has the lowest MAWP of the assembly.

 

[BigInch]

There are thousands of pipelines in existance everywhere in the world where the flanges and valves have higher allowable pressures.

http://virtualpipeline.spaces.msn.com

 

[TD2K]

My mechanical engineer will typically want to have the flanges as the 'weak' link in the system if we are setting up a new piping specification. It's not uncommon for people to look at a system and say, for example, 'it has class 300 flanges thus it's good to 740 psig' depending of course on the materials and design temperature.

However, at higher pressures, long pipelines, etc this might be an expensive option. I know one system where the ball valve's bodies were the weak pressure link and another where the pump's casing design pressure was the limiting element in the system.

If you are dealing with a larger piping that needs, for example, 1000 psig design pressure, it might be an expensive 'adder' to design it for the full 1440 psig for class 600 flanges.

Within a plant's battery limits I find that the piping systems are usually designed so that the flanges are the lowest rated element but I would not say it's always true.

 

[BigInch]

TD2K,

You're right on target!

There are just two basic things that set the wall thickness for pipeline work.

1) How much it costs, and
2) Economics.

Actually, and seriously, this is only true when you need enough pipe to equal a minimum mill order. Buying directly from a pipe mill allows you to roll your own wall thickness, so we are not limited to using any particular standard wall of a pipe suppliers list and can spec our very own, down to the last 1/1000 inch.

You give a perfect example of pipeline economic decision making, when you mentioned that any additional pressure costs money. Designing pipe to ANSI#600 (1460 psig), instead of 1000 is a difference of 460 psi. For pipeliners, that 460 psi is about 64 X over what qualifies as extra, where 1 mil of wall thickness is probably going to equal around 7.2 psi of additional operating pressure.

There's a reason why its cheaper to transport just about anything via pipeline! You found it.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

on my current job, all valves use 600# ANSI flanges or higher. For most of my system, the highest MAWP for the pipeline is 9930 kPa, but for the class 150# and 300# lines, the valvs flanges are still 600# ANSI. Of course, for the higher pressure sections, higher rated flanges are used.

The primary reason for this standard is to reduce the number of different types of valves that the field services guys need to carry - less change of showing up with the wrong valve.

So, it is tough to make a general statement regarding flange vs. piping pressure rating. It really varies.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[eguentsch]

You are all making real good points.

The reason why I brought this discussion up is that I was reviewing a relatively simple skid package consisting of several spools, flanges, valves etc with regard to ASME B31.3.

One of the things I always check first is the MAWP of the piping, fittings etc.

In this package I was reviewing, the designer assigned one MAWP per spool (containing valves and fittings) and told me he would always look up the flange numbers and then use those, since "everyone" would know that the flange rating "always" is lower than the pipe rating.

Looking at the formula that I initially posted, it is very clear that this statement is not always true when working with corrosion allowances of 1/8" for certain pipe schedules.

In the end, I found several spools where the schedule needed to be changed or where the corrosion allowance needed to be switched from 1/8 to 1/16.

Unfortunately for me, this meant I also had to go through all the older packages and check those.......

 

[BigInch]

"switched" [COLOR=white red] ?[/color] ...Good one.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com