Vessel Specifications 1

[jproj]

I work for a pressure vessel design company and read many technical specifications daily. I have noticed that many companies have conservative specifications.

A few that I have seen include:
1) The use of support rings (vacuum rings) is prohibited.
2) Flux Core Arc Weld is not allowed on pressure parts.
3) Steel manufactured in China shall not be used.

From what I've seen, many of these specifications are the result of an accident (e.g. tank blows up due to shoddy work / material).

My question is: why do companies specify ultra conservative methods of construction? The majority of manufacturers use (and have used) these methods without a problem for decades.

I can see how one would tend to be conservative after an accident, but why not find the root of the cause (e.g. not that the vessel imploded, but the vessel imploded because the vacuum rings were not welded on correctly)? If a weld procedure is to blame, change it. If a certain material composition is needed, verify that you are getting what you are paying for (chemical analysis etc.). I can't understand why someone would buy a 1.5" thick tank instead of a 0.5" thick tank with a couple vacuum support rings.

To me it seams that the price of developing a specification would far out-weigh the additional costs resulting from 100+ over-designed tanks.

Any thoughts?

jproj

 

[PSE]

My guess - liability. The perception of a flawed design could be construed as a greater problem than a good design poorly executed. The specifications may also be there so that additional manufacturing operations are not needed. Could it be more cost effective to use 1.5" thick material and not weld support rings rather than .5" material with? Consider the additional labor, inspection, possibility of failure.

 

[drawoh]

jproj,

How about the acceptability of failure?

If my bracket for holding a printed circuit board fails, the system rattles a bit, somebody fixes it, and I am embarrassed. If you are determined to reduce cost and weight, you may allow for failure in your design process. You can test the hell out of the thing.

If failure trashes your schedule, creates major financial loss or places people in danger, you should be more cautious.

JHG

 

[MJCronin]

jproj,

There seem to be some errors in your understanding of the business and marketplace..... The "quick buck" always has, and will continue to be God...

Companies have no intention of maintaing a knowledge or technical information base.....to companies this means that you have to hire a few experts, pay them, and keep them in a group called "corporate engineering".......most companies quickly disband such an "obvious expense" (!!!!?!???!!)

First, most companies have no memory or even any sense of responsibility....By the time that the engineer "job shopper" finished writing the specification or datasheet for that vessel or tank, he will either be fired or reassigned someplace else.....His MBA boss, who is 25 years old will be re-assigned to another department....

About this time, the process engineer discovers that full vacuum might be a good design condition....

By the time the tank or vessel reaches the field site, another temporary group will be assigned to install and test the unit. Sometimes at this point, modifications are made in the field without documentation or AI approval....

By the time that the vessel ages, and perhaps fails, another group will be evaluating and investigating the failure. Because the plant never incorporated any type of HAZOP or vessel inspection program, the failure will be covered up, or quickly forgotten.

The original job shopper is now doing something else, in another field, in another state. His MBA boss is now in "sales"

And the cycle repeats....

just my thoughts, experiences....

Anybody out there disagree or take issue with this ???

MJC

 

[BobPE]

MJcronin:

that is the truest meaning of industry exempt I have read yet(as a tool of the MBA that is, not as a choice of the engineer)!! I agree with you 100 percent.

BobPE