ASME Sect VIII vs API 620

[sebjunioreng]

I've seen a few threads related to this subject but I just need to be clear on this.
We currently have a vessel that is designed to 5 psig and was designed to ASME sect VIII (design equations tell me this is the case). We are modifying a few things on this vessel and we are just looking into a few things to ensure that the vessel will be ok. The vessel was designed out of SA516-70 plate that was 1/8" thick + 1/8" corrosion allowance. Here are my questions:
1. Are we allowed to simply use ASME section VIII formulas if the vessel is not section VIII (it is, according to the pressure, an API 620 tank)? If we use section VIII formuals for design, is it ok to call it a API 620 tank and not get it certified?
2. If we can use section VIII, how can we do so knowing that the minimum thickness required by API 620 is 3/16" + corrosion allowance (therefore our vessel does not meet this criteria)? For ASME, it is 1/16" so we'd be ok there.
3. Is there anything within the code (API 620 or ASME section VIII div1) that would help with this?

I guess I'm basically asking how we (and the original manufacturer) can use section VIII design for a tank that doesn't fall under section VIII jurisdiction. Is ASME a more rigorous code? If so, how can we NOT get the vessel stamped according to section VIII if we don't meet the minimum design thickness of API620?

 

[JStephen]

Generally, state law will require that pressure vessels (defined as 15 PSIG and higher) be designed and constructed by ASME B&PV Code. For vessels under that pressure, there will not be a legal requirement to use any specific code. API-620 is set up for certain types of vessels, but is not automatically required on every tank that falls within its pressure ranges.

That leaves you free to build it to whatever code or standard you choose. It is generally reasonable to design to ASME even for lower pressures- ASME is more conservative in most cases. Just because ASME is used for the design, doesn't mean you have to use ASME inspection and testing requirements.

I might point out that pressure vessel shops are liable to have and be familiar with Section VIII, and not API-620, while tank erectors are liable to have and be familiar with API-620 and not Section VIII, and this will affect the preferred standard of construction.

There are some differences in the two codes. For example, API-620 will give you some unreasonable thicknesses of small-radius knuckles, whereas ASME gives reasonable results. And API-620 includes fairly straightforward rules for compression ring areas, which I seem to recall that ASME does not.

As far as the shell thickness and corrosion allowance issues are concerned, you might check into API-653 and their similar standard dealing with pressure vessels. While API-620 might specify minimum thickness + CA, that is not necessarily a requirement for continued operation. (And double check that the 3/16" is added to, and not inclusive of, the CA- I know API-650 is the latter, but don't remember about 620.)

 

[MJCronin]

seb,

I would like to second the remarks made by JStephen...

Unfortunately, his recommendations emphsize the mess that we are in today with low pressure vessels....

While your comments, concerns and questions are legitimate, I believe that you must remember that your structure is "between the codes" and you must simply provide good enginering design principals.

I have made the point in other engineering forums and other threads that there should be a nationally recognized and legitimized code for large pressure vessels operating between 2.5 and 15 psig.....Everybody simply uses a non-code stamped ASME VIII vessel because there is no reasonable alternative

Most of these low-pressure vessel applications are in the process chemical industry,......so, in my opinion, if the AIChE had any "testicular fortitude" at all, they could force the development of such a code.

(I was going to use a more graphic, anatomical expression above,....but I was afraid of being reported to the website authorities......Patty L. has repeatedly warned me before...)

My opinion only....

MJC

MJC

 

[jte]

sebjunioreng-

I'm a bit confused. You state that the (already been in service?) vessel was designed (and fabricated?) to Section VIII (Div. 1?) because you checked the formulas used. Not because the nameplate has a "U" stamp. Not because you are looking at a U-1 form. Sounds to me like you have a good situation. You have a low pressure tank "designed and built to Section VIII-1 but not stamped." Now, after it has left the shop and has been in service, you want to modify it. I'm not aware of any jurisdiction that would want to get involved, but check with yours. After that, check your insurance policy to be sure you don't invalidate that. After that, modify away! Use good engineering judgement and qualified welders and materials etc. etc.

To answer your questions directly, though:
1)The vessel is not included in the scope of VIII-1 per U-1(c)(2)(h). But you failed to slowly read U-1(c)(2): "...however, any pressure vessel which meets all the applicable requirements of this Division may be stamped with the Code U Symbol." In other words, while your vessel is not included, it is not excluded either. See Thread292-47571, Thread292-41922.

2) You do not have an API-620 tank. Don't worry about API-620 criteria.

3) See 1) and my first paragraph.

"I guess I'm basically asking how we (and the original manufacturer) can use section VIII design for a tank that doesn't fall under section VIII jurisdiction. Is ASME a more rigorous code? If so, how can we NOT get the vessel stamped according to section VIII if we don't meet the minimum design thickness of API620?"

Well, you may choose to design and construct to VIII-1. You may even stamp it if it meets all of the requirements. The ASME B&PV Codes are generally considered more rigorous than the API Standards. That last sentence is a beauty. You don't mix and match codes and standards. The minimum thickness of API-620 has nothing to do with a vessel designed to VIII. As for not stamping the vessel, all you have to do is ask the fabricator not to.

jt

 

[sebjunioreng]

jte et al,
Thanks for your responses. To clear up a few issues. The vessel was fabricated and designed by a shop here in Alberta, Canada in 1983. It has been in service here (in Alberta) since then with no issues. The design calcs that I have seen indicate that it was designed as a sect VIII div 1 vessel despite the pressure being below 15psig. The stamping on the vessel however says that the vessel is an API 650 vessel (?!).
We don't know what the design pressure of the vessel is as our files do not contain this (our record keeping was perhaps lacking back in the 80's). We do know that vessel was pneumatically tested to 5psig. The vessel collects a mixture of water + sand and sends the gases out the top to the flare line. We have a PSV on the tank set to 5 psig.
I have ran through the calculations (5 psig vapour pressure + liquid level) in ASME sect VIII div 1 formulas and have determined that the vessel thickness is adequate to withhold the pressure (roof, shell, and bottom cone). This is not really surprising since, like I said, we have been operating this vessel for 20 years.
This confusion of API 650 vs. API 620 vs. sect VIII really rose about when we were doing an inspection on the vessel and realized that, with the pressures involved, it should be an API 620 tank. But when we looked in the design calcs (of what there was) the only thing there was ASME sect VIII div 1 calcs which got us scratching our heads. Add to that the API 650 stamp and now we're really confused (the original vessel manufacturer no longer exists).

With the inspection due out soon, we will see what our thicknesses are now and we were concerned that the vessel would not be strong enough to hold back the liquid level + internal pressure in the years to come.
Basically, I was trying to figure out if I could use section VIII formulas to say "Yes, the vessel is good and we're satisfied that it will remain ok". But when this whole issue of API and ASME came up, we weren't sure we could even use ASME formulas.
Hope this clears some of the air. Thanks again.

Sebastien

 

[jte]

Sebastien-

Yup, you have a confusing situation! The API-650 stamp on the vessel really messes things up. One good resource for you would be ABSA at

http://www.albertaboilers.com/.

At least their interpretation of the situation will have some authority. My and everyone else's on this site are just opinions and interpretations.

The simplest thing to do is to just replace the equipment. You might find that you're money ahead with a new flare KO drum instead of fooling around trying to straighten out the mess, discover issues during inspection which then need to be rapidly repaired with (expensive) field labor.

jt