## ASME BPVC VIII Div 1 Max Allowable Stress

## ASME BPVC VIII Div 1 Max Allowable Stress

(OP)

I have a copy of the BPVC VIII Div 1, and a Professional Engineering study book. I can use these two to calculate minimum allowable pressure vessel thickness. However, I have not found what the ASME max allowable stress is for a broad range of materials. Searching the internet, I have found a few [unverified] sources stating that, for Div I, the max allowable stress is the materials UTS/4 (eg if UTS is 100,000 kPa, then the max allowable stress for ASME calculations is 25,000 kPa).

Is the UTS/4 rule of thumb valid? If so, why would the max allowable stress be based on ultimate tensile stress and not yield stress? It seems to me that using yield stress as a benchmark for max allowable stress would allow for greater accuracy in regards to predicting the minimum allowable material thickness.

Is the UTS/4 rule of thumb valid? If so, why would the max allowable stress be based on ultimate tensile stress and not yield stress? It seems to me that using yield stress as a benchmark for max allowable stress would allow for greater accuracy in regards to predicting the minimum allowable material thickness.

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

I'm not so concerned about WHAT the max allowable stress is. I am curious as to WHY the max allowable stress is what it is. Why base the max allowable stress on UTS and not YS?

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

This is discussed in more detail in ASME II-D, Appendix 1.

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

ASME B&PV code committee adopted design by rule years ago to provide suitable margin for safe, long term design of boilers and pressure vessels in service. With that said, ASME B&PV code rules are strength based design using UTS/3.5 or 67% of YS, whichever is

lowerfor time independent service.## RE: ASME BPVC VIII Div 1 Max Allowable Stress

I was not aware of the 67%YS requirement. That fully satisfies my curiosity:)

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

UTS would correspond more or less to the bursting pressure.

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

The UTS requirements safeguards against a burst failure, and the YS requirement safeguards against a low or mid cycle fatigue failure.

The UTS/3.5 requirement may be complete in a

practical sense, in that perhaps no available pressure vessel steel will have a low cycle fatigue failure when the applied stress is under UTS/3.5.However, the UTS/3.5 requirement is incomplete on a

logical & theoretical basis. Given my limited experience in pressure vessel design, I prefer to have the logically complete set of requirements.## RE: ASME BPVC VIII Div 1 Max Allowable Stress

You are aware that there exists rules in the pressure vessel codes that deals with fatigue, from the lowest of cycles to the highest of cycles? And that there are multiple other failure modes besides burst?

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

I concur with your comments. I don't think you will get a technical basis for the claims.

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

"Low cycle" and "mid cycle" are relative terms. What I consider to be a low cycle for my products is likely different than what others may consider low for other products.

I have in my mind other failure modes besides burst, because I consider burst pressure to be the failure that occurs around UTS on virgin material. There is also a fatigue failure, which I do not consider burst even though the event may be a high energy event like a burst. There is also a corrosion failure, which you may argue is burst because the stress is now at UTS given the thinner material... but I am still going to use different wording because the process leading to the failure is different.

If the ASME BVPC did not have a requirement that was linked to YS, then I would be disappointed. However, my curiosity and question has lead to a learning event for me.

That is my thought process. My thought process has proven valuable, because I discovered the 67% UTS requirement... which I suspect should exist based on my thought process. I am not going to argue any more.

## RE: ASME BPVC VIII Div 1 Max Allowable Stress

First, regarding a supposed "endurance limit". Such a beast just doesn't exist, either in the literature nor in the pressure vessel code. Both the smooth bar and welded joint S-n fatigue curves have a negative slope at the maximum cycles (10^11 for smooth bar and 10^7 for welded joint). And for the smooth bar curve, the stress amplitude at 10^11 is around 7ksi for carbon steels.

Second, but again using the smooth bar curves, if you take a typical carbon steel such as SA516-70, with a minimum specified yield of 38ksi, at 38ksi stress amplitude (not range, but amplitude), the allowable cycles is roughly 1000 cycles. 90% is 34.2ksi, which results in roughly 1200 cycles. At 2/3 yield (25.3ksi), the allowable number of cycles is roughly 15000 cycles. However, the general membrane stress in a pressure vessel is often much less compared to the peak stress, which could be 3 or more times higher. And the cycle life at 50% yield (19ksi) is around 2e6 - not infinite life by a long shot.

Third, fatigue is absolutely nothing like burst. It's a crack, and considering that most vessel designs are leak before break, a through wall crack will result in a leak, which is absolutely nothing like a burst.

The 2/3 margin on yield, if you actually research other industries and other codes, ebbs end being a fairly typical margin: structural codes use it, for example. It provides a margin against gross section yielding, which is somewhat relevant for materials that exhibit a yield plateau, and less so for materials that don't.

Finally, before anyone feels the bed to remind me that the ASME fatigue curves have a supposed margin of 2 on stress and 20 on life, let me remind you that those margins are on the mean of the data (that contains substantial scatter), and actually best approximates a minimum curve.

Hopefully this has been instructive for you and you learn something.