PV de-rating
PV de-rating
(OP)
We are running some 30+ year old PV (VIII-1). One of them is getting thin (shell thickness under min. value for MAWP)> I 'm thinking about de-rating the vessel to a lower MAWP.
How do I go about doing this ?
Your help would be greatly appreciated !!
How do I go about doing this ?
Your help would be greatly appreciated !!





RE: PV de-rating
Typically, an in-service pressure vessel falls under the domain of the National Board Inspection Code (NBIC). A de-rating of MAWP would fall under the definition of an alteration (see Appendix 4).
In any case, I would expect that the Jurisdiction will provide specific guidance in requirements for stamping a new nameplate with the lower MAWP. I would expect that some type of engineering analysis will be required by the Jurisdiction to determine the cause of the wall thinning, and justification of the de-rated MAWP. I would expect you will have to ultrasonically test the vessel wall thickness and perform a hydrotest at 1.5 X (derated MAWP).
RE: PV de-rating
RE: PV de-rating
The inspector may want you to repair it, derate it, or scrap it, who knows.
ER
RE: PV de-rating
API 579 allows you to assess defects on pressurised components (Fitness for service) to check whether they may be still used at the original conditions. Also there are provisions to re-rate the components if they fail in the assessment. It covers ASME VIII and the piping Codes. That's the way to go!!
RE: PV de-rating
RE: PV de-rating
The AIA should also tell you which code to use and to what extent. API 579 is good for assessment of localised defects if you have them. For general corrosion such an assessment might not be necessary. It is always good to find a cause of corrosion before rerating and try to mitigate it to extend the life of equipment.
RE: PV de-rating
When rerating/derating a pressure vessel you should contact your Jurisdiction to find the proper code to use (NBIC or API)for the the rerate/dererate. It also will help to have the original code and year of construction. I spend most of my time rerating pressure vessels and I have most all of the old ASME Sec VIII books. So if I can be of any help to you let me know.
QA Scott
RE: PV de-rating
QAScott,
I would like to check an existing vessel to see if it was conservatively designed and can be rerated for a slightly higher pressure.
This is a 40 yr old vessel, but hasn't used half the corrosion allowance. I think I know what the answer will be, but let me ask anyway - What procedure is best for evaluating a rerate?
I am assuming the calculation needs to be based on the Section VIII code for the year it was built, but will a calc based on current code be accepted?
Would it be acceptable to reduce the corrosion allowance to the difference between the minimum surveyed thickness and the original fully corroded thickness?
RE: PV de-rating
What rerate code will you be using NBIC or API 510? This wiil help me address the issue of what ASME Sec VIII edition to use for this rerate. You can reduce the corrosion allowance as needed for this rerate to get the out come you are looking for. Just do not forget to address all loading on this vessel when doing the rerate.
QAScott
RE: PV de-rating
Most jurisdictions will accept calc's based on the original code of construction, either the original edition or the latest edition. In fact, I believe there's an NBIC interpretation out there somewhere that says so; I don't have the time now to track it down. So chances are you can use VIII-1, 2001 A03. Trick question: How do you know your vessel was built to Div. 1? Just use the allowable stress from the original edition of the code of construction. Hopefully you have a copy of the original code. If not, post the material and temperature and I can get you the allowable stress.
jt
RE: PV de-rating
"The ASME Boiler and Pressure Vessel Code Section VIII, Division 1, 1999 Addenda, and Code Case 2290 provide higher allowable stresses than those given in the previous editions of the code. This has permitted rerating of pressure vessels that could not be rerated before, or allowed pressure vessels that have thinned below the originally specified corrosion allowance to remain in service. However, the pressure vessel designers should be cognizant of certain restrictions prior to undertaking the task of performing the necessary calculations for rerating a pressure vessel, or determining the minimum required thickness of a pressure vessel for continued service."
RE: PV de-rating
I keep seeing people refer to the "original code of construction" mentioned in the NBIC as having to go back to the specific edition/addenda of the construction code that the object was built.
Let me clear this matter up - the phrase "original code of construction" does NOT refer to the specific edition/addenda of the construction code that the object was built. It refers to the code of construction document itself. This information is based on interpretation 95-19 in the NBIC. Thus, the original code of construction for a vessel built under ASME Section VIII, Div 1 is ASME Section VIII, Div 1, period.
RE: PV de-rating
I agree with you 100%.Original code of construction is just that, and not the a specific edition/addenda.
QAScott
RE: PV de-rating
I fully agree with you. I didn't realize I was unclear on that in my post above when I said "...either the original edition or the latest edition. In fact, I believe there's an NBIC interpretation out there somewhere that says so..."
I guess the only thing I left out was that you could also choose an intermediate edition (though I'm not quite sure why anyone would do that...).
jt
RE: PV de-rating
Larry
RE: PV de-rating
NBIC Interpretation 98-14 Question 5
API-579 A.2.3(b)(1)
Additionally, API-579 allows you to use Section VIII Div 2 stress values if the area of concern is located in a cylindrical, conical, or spherical shell, away from a weld or any other discontinuities.
API-579 A.2.3(d)
RE: PV de-rating
The API-579 Fitness For Service approach makes sense in my opinion as something you do to make it to the next opportunity you have to bring the vessel into Section VIII compliance. Thus, I believe it is inappropriate to take a FFS approach as a permanent solution.
The NBIC may allow rerates based on increased allowable stresses - but the jurisdictions may not. California is one jurisdiction which "will not entertain" requests to rerate pre-1999 vessels using the new allowables. I'm curious as to how the other jurisdictions are with this issue. Has anyone done a survey or have any direct knowledge?
jt
RE: PV de-rating
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: PV de-rating
Keep in mind that the NBIC, like any other ANSI Standard, is intended to provide general technical guidance for Inspectors, Users and Jurisdictions. As with any Code, all scenarios cannot be accounted for, and this is where the Jurisdictions or Insurance Agencies have to step in and provided specific guidance. I have no problem using API 579 FFS to assure continued operation or even to support a re-rate. Component re-ratings need to be justified by adequate engineering calculation, that in my opinion (not the NBIC main committee) could include API 579 plus other technically sound approaches.
Since Jurisdictions are like small governing bodies, they can be all over the place on this very issue. If you would really like to know how many Jurisdictions have approved vessel re-rates, I would suggest contacting Mr. Robert Ferrell at the National Board. His number is 614-888-8320 X240.
RE: PV de-rating
RE: PV de-rating
The Foreword of API-579 contains the following sentence:
"This publication is intended to supplement and augment the requirements in API 510, API 570 and API 653: (i) to ensure safety of plant personnel and the public while older equipment continues to operate; (ii) to provide technically sound fitness-for-service assessment procedures to ensure that different service providers furnish consistent life predictions; and (iii)to help optimize maintenance and operation of existing facilities, maintain availability of older plants, and enhance the long-term economic viability."
The term "life predictions" says to me that 579 is intended to be used as a method to predict what the remaining life of a piece of equipment might be, whether that is 2 months to the next outtage or 30 years.
The phrase "enhance the long-term economic viability" also seems to state that the methods in this document can be used to support the usage of equipment over a long period of time.
I completely agree that the first step is to check with your jurisdiction to see what methods or documents they allow for rerating.
RE: PV de-rating
The most important benefit of API 579 as per me is "--- to ensure that different service providers furnish consistent life predictions;" Each type of defect and how to assess is detailed in 579. Now the user can ensure that standard methods are used by specifying API 579 for FFS and can be confident of FFS result.
Remember- 'Factor of safety is factor of uncertainty'!
RE: PV de-rating