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Dear All
I have joined this site today after seeing many valuable responses to question raised by users.I want to know how to calculate MAWP of ASME APP 2 FLANGE,as it's not given in the code moreover in COMPRESS design, MAWP value is shown but not the method.

Thanks
Rishi

As for any other component, the flange MAWP is the highest pressure that may be sustained by the flange without exceeding any ASME Code requirements.

For flanges these requirements include: (1) allowable stresses in flange at operating condition, (2) allowable stresses in flange at gasket seating condition, (3) allowable stresses for bolts at operating and gasket seating conditions, and (4) flange rigidity analysis (and there may be other requirements as well). Note that the allowable stresses correspond to 5 different calculated stresses in the flange and hub (5 each for operating and gasket seating conditions). Thus there are numerous Code requirements, any one of which may limit or govern the MAWP.

You might confirm the MAWP reported by your software by changing the input design pressure to equal the MAWP that is reported, then re-run the calculations. The report would then be based on the "design" pressure, which is the MAWP. You can then review the calculations to determine what is the governing condition for MAWP.

In response to my post someone sent a private message (PM) asking how to calculate the MAWP of the flange. I thought my reply was clear but I will provide more detail on this matter in the forum.

For the flange…or any component…the MAWP must be determined by considering all applicable Code requirements. For flanges these requirements include: (1) allowable stresses in flange at operating condition, (2) allowable stresses in flange at gasket seating condition, (3) allowable stresses for bolts at operating and gasket seating conditions, and (4) flange rigidity analysis (there may be other requirements as well). Note that the allowable stresses correspond to 5 different calculated stresses in the flange and hub (5 each for operating and gasket seating conditions). Thus there are numerous Code requirements, any one of which may limit or govern the MAWP.

Here is a key point: Note that many of the Code requirements applicable to the flange analysis are not directly related to each other (if related at all) and a mathematical formula expressing the relationship of each to the other is not possible. In other words, we cannot define a system of algebraic equations and then solve them as, say, a matrix or some such mathematical trick. Thus all Code criteria must be considered separately, there is no one single formula available to express the MAWP of the flange (unlike there is for a head, for example).

Because the Code criteria express unique requirements there may not be (and typically is not) a continuous curve plotting the MAWP of a particular flange given small changes in any single parameter. Instead, there will be discontinuities in the curve as different Code criteria take effect and govern the MAWP.

For example, it is possible to calculate the MAWP based on each of the individual Code requirements above (say, stresses in flange, or stresses in bolts, or flange rigidity). The governing MAWP would then be the smallest such MAWP for the flange. As small changes are made to that flange a point will be reached where a different Code criteria becomes effective in limiting the flange MAWP and sudden jumps, or discontinuities, in the MAWP will be observed.

This sort of analysis can be done by hand (hopefully with a hand calculator) but would be very tedious. Easier is to create a computer spreadsheet or computer program to perform all of the required Code calculations, then at least the labour of computation is relieved from the designer and he can concentrate on using mathematical tricks to search for the special "magic" pressure that is just such that at that pressure at least one of the Code criteria is at its limiting value (allowable stress, flange rigidity, etc) but no other Code criteria is exceeded.

It may even be hard enough just to find the MAWP for any particular given criteria (say, the pressure at which the bolts are fully stressed). Some of these questions may be solved easily enough by simple algebra, but others may not be so easy and will require some method of "finding the root" of an equation. ...and remember that while there are formulas for finding the root of a quadratic equation, or formula to third or fourth powers, it is been proven that there is no general algebraic formula to obtain the root of fifth degree equation (thank you Mr. Abel). But the field of mathematics provides a number of tools that could be used to try to "home in" on this magic pressure.

One method would be to create a plot of calculated stress (or bolt stress, or rigidity) as a function of pressure. Then you would have to find the point (pressure) at which the stress is equal to the allowable stress (or other governing criteria). This is a standard sort of mathematical problem. Repeat this for all of the Code requirements and the MAWP of the flange is the smallest such pressure.

If your spreadsheet calculates all the Code criteria, then you could manually keep increasing the pressure, watch the calculated results, and keep going until one of the criteria is "just met". Ooops, you went too far? Then just reduce the pressure a little bit.
Given a fancy-enough spreadsheet or computer program you could write routines to automatically determine the MAWP. One method is to use a binary search. The binary search is a standard method of finding the desired value (in this case the MAWP) and is very, very fast.

I think some people expect this to be a simple process with a single simple formula. But it is not. Maybe conceptually it is easy (I think it is). But the mechanics of actually computing the MAWP for the flange is not so easy.

A similar problem exists for nozzles: many unrelated Code criteria.

A closing remark: It's easy enough to find the MAWP for a head or a cylinder. The Code formulas for required thickness of the head is typically only a single formula, so solve this for pressure as function of thickness. Cylinders are more complicated in that there are two formulas for required thickness, based on circumferential and longitudinal stress; either may govern.

It is when you get to looking at flanges (or nozzles) that so many Code criteria must be considered. One must really have a great need for the MAWP information for a flange in order to have to resort to the solution by hand. Of course, many commercial software now include this function so MAWP is easy to obtain.

One other criteria that applies to all components and may limit the MAWP (even for heads, cylinders) is that the MDMT rating must meet the specified design MDMT. Consider carbon steels under UCS-66. The MDMT rating can use the coincident stress ratio to obtain a colder MDMT rating than the base curves. The coincident stress ratio is a function of pressure. If all Code requirements are otherwise met for the component, then the pressure limit (MAWP) may be governed by the coincident ratio, which must be small enough in order for the rated MDMT to meet the design MDMT.