Designing to minimum required by B31 Codes

[JayStern]

Hello,

I have a client that requests all designs submitted to be the minimum required by code. It is first cost that drives their decisions. When working with this client, I find myself having to defend standard design practices (example of what they want: non standard corrosion allowances, lighter schedules on small diameter pipe, fewer weld inspections, etc). Most of our standard piping specifications have to be rewritten in order to be reduced to the minimum required by CFR, API, and ASME. Our standard piping specifications are comparable to those used by large corporations in the petrochemical field in that they exceed the minimum requirements of the relevant codes. Whenever I feel that design considerations are marginal, I document the potential for problems in writing and send the information on the client. When working for this client, I do not have the comfort level that I enjoy with most of my designs. I am sure that if there is a liability problem down the road, I will be in the line of fire. What is the most compelling reason not to design to minimum required by code, but instead, use experience to know when to exceed minimum code requirements? Should this client be dropped because they are more of a liability than an asset?

Jay

 

[hacksaw]

if you have a melt-down, who is the customer going to accuse of poor engineering practice?

if you have any doubts, then wait until he gets his hired gun on the stand to show how you sold him a substandard design....

ultimately he'll respect you for taking a stand that is in his interest, but you may have a sales job to do.

 

[Scipio]

That depends, when your company got the job was it based on the client's 'minimum required' or was it based on your company standard practice? Bear in mind minimum code requirements usually still have a safety factor of 2.5 to 5 depending on the code. Properly engineered, the only problem I see with meeting bare requirements is that the requirements could toughen up a year from now, causing problems with future modifications to the design.

Proper engineering is where I see the risk, you're in a company where everyone is used to seeing things done by the company standard. Start deviating from that on a case-by-case basis, and nobody knows if that 2" sched 40 pipe is OK or if it's a mistake, so some of the backups might fail. The thing to bear in mind is that while meeting minimum requirements may save some material cost up front, it's likely to cause a lot of schedule problems, and drive up the engineering cost of the project.

On the technical side, it can also lead to some unforseen problems, as well. Changing to a lighter schedule on small bore piping, for instance. Now you'll have to proceed case by case on wall thickness if you've got butt-welded, socket-welded or NPT joints. Chances are also good your company's standard tables (assuming you have them) regarding things like impact-testing requirements, branch connections, and spans between pipe supports are of questionable value as well, as they're all based on some combination of predictable combination of wall thickness & pipe diameter.

Of course, it depends on the intricacy of the job - big difference if you're designing the piping for a modular 2000 horsepower, 1500# sour recip compressor, or running a 2" drain line 20' from a separator to an atmospheric holdup tank!

 

[JayStern]

Scipio,

Unfortunately, the projects are of medium complexity with design pressures between 285 to 2220 psi, reciprocating compressors in some lines, and flammable gases. There is a serious potential for loss of life or significant monetary loss should a system fail. I am finding myself recalculating wall thickness for piping and coming up with marginal corrosion allowances (less than 1/16 inch in some cases). ASME B31.3 just says you have to account for corrosion. In most projects, the clients spec realistic/conservative allowances. This client specs marginal allowances. I don’t like it. It is difficult to predict potential vibration problems that could occur with the light schedule small diameter pipe when large reciprocating compressors are in the system. It may be years before metal fatigue issues arise. I need some ammunition to help sell/scare the client into accepting good engineering practices. Any advice will be helpful.

Thanks

Jay

 

[TBP]

I doubt that this client will care about anything more than his financial numbers for the next quarter. I have no doubt whatsoever that if anything fails, for any reason, ever, they'll sue everybody from the designer, to the contractor, to the material supplier, to the guy that drove the coffee truck, to the guy who provided the "thunder shacks" for the constuction crew. Hopefully, the coffee truck guy and the thunder shack guy are different people

At least your clients realize that there are applicable codes. Come on out to "General Industry Land" if you want to truely see the wild west in action. I've lost track of the number of times I've been told (and forcefully, I might add) that "There's no code for compressed air piping." Who's informing me of this situation? Contractors, maintenance foremen, project & plant engineers, consulting engineers... The lack of code awareness used to startle me. Not any more. It's just the way it is. There is some pretty scary stuff out here.

 

[DSB123]

JayStern,
All I can add is to reiterate a paragraph from ASME B31.3(and similar in other Codes):- "The Designer is cautioned that the Code is not a design handbook; it does not do away with the need for the designer or for competent engineering judgement"
For me that's all you need to quote to your Client and remember to provide the minimum requirements of the Code does not necessarily result in a "good engineering solution" and as an engineer you have a duty of care to provide a "fit for purpose" design. If you are uncomfortable then don't let the Client bully you- stick to your guns.

 

[bvi]

B31.3 provides minimum requirements for safety. Consideration of long term operability, reliability, etc are other considerations that should be incorporated. Many things (e.g. valve selection and valve trim) are not specified by the code. So, it is not clear to me how "meeting the minimum code requirements" effects your design decisions in these areas.

Regarding the basics such as pressure design, support and flexibility analysis, meeting the code is sufficient. However, if the specified corrosion allowance is too light, the net present value of the cost of repeated replacement (note that a generally accepted retirement thickness is 0.1 inches, regardless of the required thickness for internal pressure) of the piping will exceed the net present value of providing heavier piping in the first place. Depending upon the service, a 1/16 inch CA may or may not be sufficient. For small bore pipe, Perhaps you can handle CA by relating it to the client in terms of design life, based on anticipated corrosion rates (e.g. from NACE publications), considering perhaps a 0.1 inch retirement thickness.

Regarding vibration, I hope you are bracing your small bore branch connections around the reciprocating compressor. Otherwise, you are likely to have instances of vibration problems due to resonance and consequential fatigue.

 

[Scipio]

You've got some serious piping there, but again, meeting even minimum code requirements (where applicable) should still be a safe design. Again I stress that my concern would stem from the fact your company doesn't build to minimum code requirements as a matter of course, driving up engineering hours. Also, as I said, it depends a lot on how your company bid the job - if your company got the project(s) because they said they'd work to minimum requirements, then your client isn't the problem.

Personally, I'd walk from a client who expected me to calculate wall thicknesses for every joint of pipe in order to save a buck, it tells me they don't have much of an idea what life is like in the real world and things are only going to get worse as invoices start coming in.

 

[weldmasterdave]

Reading through the replies is interesting to see I am not alone in being shocked about the ignorance regarding relevant codes and standards. I myself am ignorant but am learning quickly out of need. Most codes are there for a reason which invariably arose out of a near miss or tragedy. Some codes and standards require adherance out of regulatory compliance with OSHA PSM regulations and EPA RMP regulations regarding highly hazardous chemicals. That should foster better adherance to sound engineering. Deviations from recognized and generally accepted good engineering principles is risky. As pointed out during a seminar I took a while ago, even though you can defend your actions, you will have a tough time on the stand. The thought that sticks with me and governs my decisions is if I am placing people at unnecessary risk.

 

[StressGuy]

Part of the issue with code awareness is that there are too freakin many of them. Personally, I think the various technical organizations that produce these thousands of pages ought to be doing a better job of raising awareness of the information that they have available.

I'd bet most of you, like myself, are ultimately limited in your awareness of what standards are even out there by the institutional knowledge that your company has picked up over the years and managed to pass on to you.

With some of the things I've seen, I have to wonder how much people are reinventing the wheel or, worse, duplicating mistakes of the past, becuase the knowledge isn't been widely disseminated.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

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

 

[ccfowler]

Well put, StressGuy! Code awareness is an all-too-common problem. hacksaw and TBP are surely right about this client probably wanting to sue anyone and everyone when the cheapskate design exhibits the slightest hint of a problem.

JayStern, this may well be a client that you can't afford. Can you and your firm afford the liabilities associated with the compromised designs that are being demanded? If you are to continue, it would probably be best to get lots of documentation signed by your clients regarding all of the skimping that is being demanded. Don't forget that injured third parties can use the signed documents demanding the minimal designs as proof of your complicity and willingness to compromise.

I would be at least as uncomfortable as you are. No matter what, your house, car, TV, and engineering career will be centered in the line of fire when the bad stuff starts flying.

Many years ago, an "old-timer" with much practical experience gave me some very good advice about codes and seeing them in a proper perspective. Paraphrasing his comments, "Codes are best seen as written in red (blood, not ink). They are mainly the result of people being maimed or killed. To a lesser extent, they are prompted by experiences of very large financial losses."

In general, I believe that code-minimum designs should be considered to represent a violation of the spirit and intent of the code if not its specific provisions. Thankfully, some codes contain specific statements to this effect.

 

[LSThill]

ccfowler (Mechanical) and Team members

Code awareness become a NBIC INSPECTOR, and work as Senior Staff Engineer for an International Company writing Best Engineering Standards for Refinery, LNG, and Offshore Platform and Top Side Facilities.

Leonard Thill
Leonard@thill.biz

http://www.thill.biz