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PV de-rating
10

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 !!

RE: PV de-rating

3
First off, I would contact your Inspector or Authorized Inspection Agency that insures the vessel and contact your Jurisdiction to inform them of your intentions to permanently de-rate the vessel.

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

2
For vessels that we rerate we follow the rules of API-510, Pressure Vessel Inspection Guide: Maintenance Inspection, Rating, Repair, and Alteration.

RE: PV de-rating

If you own the vessel,(owner/user) you can derate it by replaing the safety valve and limit controls with a lower pressure.(if apply to your vessel)

The inspector may want you to repair it, derate it, or scrap it, who knows.
ER

RE: PV de-rating

raduval,
        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

When you contact the AIA they will most probably require recalculation of entire vessel wall thickness and don't forget about the nozzle reinforcement calcs.

RE: PV de-rating

When you contact the AIA they will most probably require recalculation of entire vessel wall thickness and don't forget about the nozzle reinforcement calcs.

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

raduval,

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

pipesnpumps,

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

pipesnpumps-

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

2
We are in the process of rerating most our pressure vessels as part of load up of our plant. I saw the following information from a consultant which may be of use to you.

"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

jte and others;
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

metengr,

I agree with you 100%.Original code of construction is just that, and not the a specific edition/addenda.

QAScott

RE: PV de-rating

metengr-

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

Using any ASME VIII-1 as original Code of construction was fine, until they released the 1999 edition.  If you use the 1999 or more recent edition for a pre-1999 vessel, make sure you use allowables in pre-1999 Codes.  The allowables changed in the newer Code to recognize advances in material/fabrication quality.

Larry

RE: PV de-rating

Actually both NBIC and API allow you to use the 1999 addenda stress values as long as the vessel was built in 1968 or later.  There are a few other limitations, including lethal and cyclic service, but basically both allow you to calculate new tmins based on the new stress values if you wish to do so.

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

solinar (and metengr?)-

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

It is my understanding that the Commonwealth of Virginia does not allow re-rates with the higher ASME VIII allowable stresses.

Steve Braune
Tank Industry Consultants
www.tankindustry.com

RE: PV de-rating

When the NBIC was revised in 2001 to reflect the change in allowable stress values in the ASME B&PV code, this created a real hornets nest regarding repair/alteration activities especially for in-service, vessel re-ratings. As it turns out, the committee finally moved forward with permitting the use of higher allowable stresses to re-rate a pressure retaining item when it added RC-3024 , and RD-3010, paragraphs a)-f).

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

For your information, the Stae of Indiana does not permit either.

RE: PV de-rating

In my opinion, API-579 is not presented as a short term stopgap to last you until the next outtage.

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

Solinar is absolutely correct. API 579 also specifies methods for monitoring where applicable. You can continue unless other defects are found as in normal case. Of course many users will do the repair or replacement, if it is not difficlut, at the next opportunity.
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

pipesnpumps- AIA will not allow you to rerate (upward) ASME/NB vessels since their fabrication was performed in accordance to ASME code dated at the time of fabrication.  But AIA will allow you to derate as long as some of the comments by above contributors are met.  Also any major repair and calculations perform on such vessel will have to be done by that same code used at the time of fabrication.

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