11 replies to this topic

[sheiko]

Hello,

I am working in an EPCM company

One basic question:

Shall process engineers size safety-relief valve of storage tank or shall we let it to vendors by only specifying the relieving flowrate (and other required data...) in the process data sheet?

I am used to do the sizing in the case of pressure vessels (API 520, 521) but i have heard that for storage tank it is different. Is it true? Any references will be welcomed.

[Art Montemayor]

Sheiko:

My practice for the last 48 years has been to issue specific, accurate, and checked specification sheets for storage tank relief valves to suppliers and requiring them to size and select the required size of relief device specified. I always require that the supplier either issue the sizing calculations or indicate the sizing method used (graphs, tables, etc.).

I ALWAYS check the calculations or generate my own in order to check the sizing and identify the contingency amount of capacity allowed in the valve.

In the end, the bottom line is WE, the owner's engineering representative are the responsible parties for seeing to it that the proper relief device is installed - correctly. Once the owner and its engineering representatives accept the installed equipment and proceed to operate with it, it is the owner (& his representatives) that are legally liable for the ultimate results of all the accepted equipment. I always make sure that I will never accept or foster defective or ill-designed equipment. That's my job.

I make SURE that the EXPERT is identified as the manufacturer; I make sure that the expert's calculations are to my satisfaction and approval.

[sheiko]

Dear Art, thank you for this quick answer.

1/ If my understanding is correct, you don't put the nozzle size on the process data sheet but calculate it for checking the vendor results. Right? (sorry if i tend to reformulate things, but it is to be sure i have understood. Moreover English is not my native language...).

2/ Shall we do the same for pressure vessels or it is required to size the nozzles in this case?

[Art Montemayor]

Sheiko:

I believe you are working for an engineering contractor in design, procurement, and possibly construction. I, too, have gone through that experience and can confidently state that no engineering company is going to expect that their staff or employees are the experts on the specific purchased equipment that the company does not have a design and fabrication patent on. This includes such items as pressure relief valves, pumps, compressors, electric motors, etc., etc.. My point here is to identify who, in the scheme of things, is logically and legally considered the “expert”. This often becomes a very important point in the business of engineering – especially when one is drafting and/or signing a contract for a project. I have served as an “expert” witness in some legal proceedings in the past and lawyers have always been quick to identify who is the considered and accepted expert in the subject matter being debated.

I have always applied the same criteria and logic in my engineering career and have found that every engineering organization that I’ve been associated with agree with this principle. I – and every engineering manager that I’ve been associated with – always consider that in the case of specific equipment designed and fabricated by others, the primary recognized responsibility to identify the capacity and critical characteristics of the said equipment fall on the original designer and fabricator. Therefore, I consider it foolish and fool hardy to anticipate or calculate the “correct” orifice size of a Farris or Consolidated pressure relief valve before having the fabricator do so. I have always had engineers under me check and confirm to make sure that they agree with the sizing and other characteristics, but I don’t hold them responsible for the original design. I don’t consider this to be logical or trustworthy, even though the same sizing computer program is being used by both. If there is going to be a math or typo mistake in the calculations, I want the expert to generate it and my staff to find and correct it.

I apply the same logic to pumps and compressors. My process calculations have often resulted in totally different line sizes than those furnished by the pump or compressor fabricators. This doesn’t mean that one (or both) of us is wrong. It just means that for some specific engineering reason – unbeknownst to me – the fabricator calculates a different size. As long as that size is safe and works – as confirmed and approved by me – I feel secure that the applications is sound.

The same applies to some pressure vessels – but not all the time. Some pressure vessels are very fabricator-design specific – such as special filters, separators, reactors, and even some special exchangers. However, the ordinary “pots and pans” – such as storage tanks, accumulators, surge vessels, and other process vessels – are subject to criteria by the engineering company applying them to a specific process. In that case, the engineering design company’s engineer who specifies the pressure vessel should take the responsibility of “expert” in the process design – which means that he/she may have to take the responsibility of specifying the critical sizes of some nozzles or special internal devices such as distributors, baffles, structures, etc. This has been the case in situations where the engineering company owns the specific design of an ammonia converter, for example.

I hope I have specifically described and defined what I mean by expert design and the responsibilities expected of engineers in design and construction. Although we take full responsibility for the ultimate design, construction, and operation of a facility it doesn’t mean that we have to generate every detail ourselves. We also manage (& always check) others in producing some of the details that we require for the total project(s).

[sheiko]

I make SURE that the EXPERT is identified as the manufacturer;

Art,

"Manufacturer" in PED (Pressure Equipment Directive in Europe) terms does not have to be the company that actually fabricates the equipment. In every project, it has to be decided who will take the PED role of "manufacturer". In the case of the design of a chemical plant, this can be the client (operating company), the EPCM contractor, a Project Management Consultant or the actual equipment manufacturer, or any other that takes the responsibility for compliance with PED. (see gvdlans post http://www.cheresour...showtopic=7247)

So, if i reformulate your wording as "make SURE that the MANUFACTURER is identified and is the EXPERT;", does it make sense?

Thank you in advance

[Art Montemayor]

Sheiko:

Yes, I agree with the re-phrasing "make SURE that the MANUFACTURER is identified and is the EXPERT."

However, let us not forget the details about how we want to deal with the potential supplier. If you intend to make certain specifications to your supplier, you should (in my opinion) give him/her the right to design according to his/her critieria such that he/she can guarantee what is supplied. You can, after the initial proposal, make the critical notes and proposed modifications in accordance with the supplier/manufacturer's approval and confirmation that the changes you make to his/her design does not jeopardize the safe and efficient operation of the equipment. This procedure, in my opinion, keeps the manufacturer recognized as the EXPERT - and the basic, responsible owner of the design that you have purchased. In this manner, there can be no mistaking who is taking the design responsibilities for the ultimate results.

I appreciate you bringing this subject up because it is becoming an important issue in this modern world of sub-contracting and "out-sourcing" of equipment and services. It is important - especially to young students - to understand just how involved engineers are in the world of business and legal proceedings. It is part of everyday engineering business and we have to make sure that we protect our professional results and standings.

[djack77494]

This is a very interesting topic. I also tend to think that each part of the total job has a responsible party and that they cannot shed or share their responsibility (unless they hire a competent contractor for that purpose). But I view the process as being multi-faceted, with different disciplines and parties performing different functions and accepting different degrees of respsonsibility. This is best illustrated by example.

Let's say we're designing a unit in a refinery. The technology provider (licensor) is responsible for providing a basic design for a technolgy that will meet the process requirements. If there are specific, non-obvious mechanical details that are part of the technology, they define these requirements. It is their responsibility to properly do the basic design, and they GUARANTEE that it will work IF it is built and operated as designed. The engineering contractor is responsible for properly implementing the licensor's design. He/she will specify equipment that can handle the material balance flows and process conditions defined by the licensor. But there is obviously MUCH more to things than that. The engineer must ensure that his design will work with the equipment and piping, etc. laid out as he lays it out. He must ensure that, for example, vessels are properly specified, since the licensor may say little about even major process vessels other than their operating conditions. The engineer must assure that the final design is safe and operable, and he should voice any concerns he may have to the licensor if those concerns relate to the basic engineering.

A vessel designer may receive a sparsely populated vessel datasheet from the engineer. He now turns that into detailed drawings that define how the vessel should be constructed. He (or perhaps an engineer overseeing his work) is responsible for the vessel's design. The fabricator is responsible for turning the drawings into a vessel that satisfies all the requirements of the design. Finally, the constructor is responsible for properly installing a sound vessel in the correct location.

This is a very simplified version of the total process. At each step there may be a requirement to issue further instructions for subsequent steps. Many others will actually be involved, and each "cog in the wheel" has a secondary responsibility to confirm that the instructions he has received from the prior steps are, in fact, able to perform as stated. Others should oversee the entire process.

[Qalander (Chem)]

Dear sheiko All said ,metioned by the Forum giants here is very true and bindingg.

However obviously the party who is having all this done on his/her premises and main expense has to ensure at all phases of design, fabrication construction/installation, pre-start up and guarantee test run stages that every little bit is ensured with needful safety.

Since ultimate responsibility rests with this company/party either to oversee directly or
through hiring reliable/ trust worty expertise to ensure on its behalf.

As such you may judge your own actual role/position to do and act.
I hope this helps
Best regards
Qalander

[sheiko]

Some pressure vessels are very fabricator-design specific such as special filters, separators, reactors, and even some special exchangers. However, the ordinary pots and pans such as storage tanks, accumulators, surge vessels, and other process vessels are subject to criteria by the engineering company applying them to a specific process. In that case, the engineering design company's engineer who specifies the pressure vessel should take the responsibility of "expert" in the process design which means that he/she may have to take the responsibility of specifying the critical sizes of some nozzles or special internal devices such as distributors, baffles, structures, etc.

Dear Art,

Yes am i working for an engineering company.

Re-reading your second post, i have noticed you consider storage tanks as part of "ordinary pots and pans".

So, if my understanding is correct, my company should be considered as the expert and take responsability for the design of the equipment (storage tank) but NOT for the design the relief device on it (as per the second paragraph of your second post). Am i correct?

[Art Montemayor]

Sheiko:

In my opinion, if your company designed, calculated, and specified the fabrication of the storage tank, then it is the responsible "expert" who stands behind the calculations and construction specifications as well as operating results.

Note that not ALL storage tanks are equals. I have dealt with LNG storage tanks, Methanol storage tanks, heated crude oil storage tanks, Furfural storage tanks, THF storage tanks, Liquid Oxygen storage tanks, etc., etc. Most - or all - of these cannot be considered "pots-and-pans". Service water, deaerated water, gasoline fuel, and other such mundane items are what I would consider "pots-and-pans". Those are the vesels that an engineering company usually designs on its own and has the fabrications contracted out to others.

If your tank is one that your company designed, calculated, and had fabricated then your company is the expert. I don't think your company is ever going to try to get into the business of designing and manufacturing Safety Relief Valves. When it does, however, then it would be the expert in question.

[sheiko]

Thank you so much Mr Art Montemayor (su apellido lo dice todo! )

[Qalander (Chem)]

Thank you so much Mr Art Montemayor (su apellido lo dice todo! )

Thanks sheiko/Art Montemayor Esq.
for the above interesting and useful discussion;on otherwise a rare topic.
Regards
Qalander