Repair or Alteration of Pressure Vessels
James C. Keenan
Senior Staff Engineer
Summary: This article was originally published in the Summer 2010 National Board BULLETIN.
The purpose of this article is to briefly identify some of the special requirements applicable to the repair or alteration of pressure vessels fabricated of quenched and tempered (UHT) materials and, in particular, the additional rules that should be utilized when repairs or alterations are made to these items.
The most frequent contact with these vessels as a repair organization or inspector is the repair or alteration of over-the-road liquid petroleum gas (LPG) transport vessels. The information contained in this article pertains to vessels fabricated utilizing Code Case 1204-11 or Section VIII, Div. 1, Part UHT, depending on the year built.
Repairs or alterations to these vessels should follow the rules of Part UHT of Section VIII, Div. 1, and the National Board Inspection Code (NBIC).
The following paragraphs include a summary of areas to be considered when repairs or alterations to quenched and tempered materials are encountered.
Design of Nozzles and Other Connections (UHT-18)
UHT-18 requires all nozzle welds to be full-penetration design. There are also specific requirements for nozzle materials, geometry, and attachment details.
Materials (UHT-5, UHT-6, UHT-18, UHT-86)
All material listed in Table UHT-23 shall be impact-tested as required by UHT-6 in the final heat-treated condition. In some cases additional drop weight tests may be required based on minimum design metal temperature.
The thickness limitations of the material specification shall not be exceeded.
Prior to the 1994 addenda, only UHT material could be used for head and shell sections joined to each other. However, addendum 94 revised UHT-5 (b) to now permit the joining of UHT materials to UCS or UHA materials in the head and shell sections.
Nozzles and reinforcement pads shall be made of material with a specified minimum yield strength within ± 20 percent of that of the shell to which they are attached; however, pipe flanges, pipe, or communicating chambers may be carbon, low-, or high-alloy steels welded to nozzle necks of the required material provided the rules of UHT-18 (b)(1 thru 4) or UHT-18 (c)(1 thru 4) are satisfied.
In general, UHT-28(a) requires all structural attachments and stiffening rings which are welded directly to pressure parts shall be made from materials of specified minimum yield strength within ± 20 percent of the materials to which they are attached. UHT-28 (b) modifies this requirement for certain materials. It’s important to note that this includes all attachments, permanent or temporary, that are welded to the pressure boundary.
Welding & Welding Procedure Qualification (UHT-17, UHT-20, UHT-30, UHT-82, UHT-83, UHT-84, UHT-85 & Section IX )
Section VIII, Div.1, Part UHT, identifies the rules for welding quenched and tempered materials. Section IX also has additional requirements for welding these materials:
- All category A, B & C and all other welded joints between parts of the pressure-containing enclosure which are not defined by the category designation shall be Type 1 of Table UW-12 unless otherwise exempted by UHT-17.
- Maximum joint offset values shall meet UHT-20 rather than UW-33 (a).
- UHT-30 requires the attachment of stiffening rings to be in accordance with UG-30.
- UHT-82 contains the specific requirements regarding the performance of welding and qualification of welding procedures for these materials. Repair organizations are cautioned to review these requirements and become familiar with them prior to performing any repairs and/or alterations.
Any surfaces cut by metal melting methods (i.e., torch cutting, arc gouging) that are not to be rewelded shall have 1/16-inch removed by grinding or other mechanical means and be inspected by the MT or PT method after grinding (UHT-83).
The requirements of UW-35 (a) and UW-51 (b) shall be met except for SA-517 material. The maximum weld reinforcement shall not exceed 10 percent of the plate thickness or 1/8-inch (3.0mm), whichever is less. Undercut or abrupt transitions are not permitted on groove, fillet, and butt welds (UHT-84).
Temporary welds shall be removed after they have served their purpose and shall be made using qualified welding procedures and welders. The base metal shall then be restored to a smooth contour. The removal area shall be examined by the MT or PT examination method for the detection of cracks. Repair welds, if required, shall be accomplished utilizing qualified welding procedures and welders and reexamined by MT or PT upon completion (UHT-85).
As stated in the introduction, the repair organization and inspectors’ involvement with these materials is generally with over-the-road LPG transport vessels being repaired or altered. In many cases the manufacturer’s data report identifies the head and shell material as T-1 or Code Case 1204 when stamped prior to 1968. After 1968, this material is identified as SA-517 Grade E or Grade F.
One final note: When selecting a WPS to perform a repair or alteration, it is necessary to perform a thorough review of the WPS to ensure it addresses all of the essential and supplemental essential variables required by both Part UHT and Section IX.
Nondestructive Examination (NDE) UHT-57
All welded joints of Type No. (1) Table UW-12 shall be radiographed for their full length in accordance with UW-51 after any corrosion-resistant alloy cover weld has been deposited.
Nozzle attachment welds shall be RT, PT, or MT examined as specified in UHT-57 (b). Corrosion-resistant overlay weld deposits shall be examined by the PT method.
All welds, including welds for attaching nonpressure parts to heat-treated steels shall be examined for cracks by the MT or PT method after the hydrostatic test, except as permitted by UHT-57 (d) and (e). If using MT, a method that will avoid arc strikes shall be used.
Post Weld Heat Treatment (PWHT) UHT-56
All vessels or vessel parts constructed of steels listed in Table UHT-23 shall be post weld heat treated when required by Table UHT-56.
All welding of connections and attachments shall be post weld heat treated whenever required by Table UHT-56 based on the greatest thickness of material at the point of attachment of the head or shell [(see UHT-56 (b) & (c)].
Caution: Even though UHT-82(g) may exempt these vessels from PWHT requirements the Department of Transportation (DOT) may require PWHT of tanks constructed in accordance with Part UHT (Ref: DOT, NTTC Spec. MC-331). If this is the case, welding procedures and welders must be qualified with PWHT. Some minor repairs may be exempted by DOT; in this case special bulletins have been issued by DOT that further exempts PWHT.
Do not stamp repair nameplate information directly on vessels with shell thicknesses less than ½-inch (13mm). Nameplates are preferred on vessels constructed by this part in all thicknesses in preference to stamping.
It is hoped this information will make both the repair organization and inspector aware of the additional or different requirements when repairing or altering vessels made of quenched and tempered steel. This information is not intended to be used in place of the Code. Individuals using this information must consult the Code for the specific requirements pertaining to these materials.
For all repairs or alterations to these vessels, it is essential the materials and heat treatment requirements be known. To accomplish this the repair organization must obtain a copy of the original manufacturer’s data report or ascertain the material type through testing. In most cases the data report will be available from the vessel owner. If National Board registered, it will be available from the National Board.
With safety in mind, repair organizations and inspectors are urged to become familiar with the additional code requirements prior to performing or authorizing repairs or alterations on these types of vessels.