Unusual weld overlay project stressed quality, safety. (Vessel Corrosion Repair)(part 2)

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Complete in situ weld overlay to repair large process vessels in a hot potassium carbonate C02 treating plant has proven cost effective and technically feasible. Chevron U.S.A. Inc. employed the approach at its Sacroc unit near Snyder, Tex.

This is the second of three articles which discuss in detail the various planning, execution, safety, and quality-control considerations for success of this 6-month project. The first appeared in OGJ, Dec. 2, 1991, p. 54.

Lack of precedents

From a technological standpoint, this project had few precedents. In the petroleum industry, in fact, it appears to have had no direct precedent whatsoever, weld overlay having been used only for small area repairs and never for complete vessel cladding.

As a result, the project represented a unique application of a relatively new technology. Applying 29 metric tons of weld wire over 743 sq m (8,000 sq ft) of vessel surface required special attention to work specifications, manpower selection, preparation of safety and quality-assurance programs, and materials procurement. The project's unusualness made it difficult to find contractors both qualified and willing to attempt the job. Several had performed shop" overlay and cladding operations, but few had ever attempted such a high-risk field project.

Extensive research turned up four domestic contractors who had the potential expertise to perform the overlay project. With only one exception, these contractors were willing to visit and observe the project site and to present their solutions to the problem. All four contractors were requested to submit bids for the project.

The essential concerns with the proposed "overlay option" included suitability to purpose chemistry of the overlay), time to apply, cost of application, and history of use.

Each bid proposal had to address each of these concerns.

Specifications; QA

Because this project had no precedent from which to draw technical or procedural guidance, all specifications and contracting procedures had to be developed in-house. The specifications were developed in two sections. The first defined the project scope, logistics, and responsibilities of both Chevron and the selected overlay contractor. The second defined the technical aspects of the stainless steel weld overlay and the quafity-assurance procedures which would have to be satisfied by the contractor.

As defined in the first section of the specifications, the objective of this project was to weld overlay two existing carbon steel absorber columns in a hot potassium carbonate CO[sub.2]-removal facility, The dimensions of the columns are 3.7 m (12 ft) ID x 26.7 m (102 ft) long (seam to seam). The vessels have 2:1 elliptical heads.

Vessel pressure and temperature ratings are 4,238 kPa (600 psi) and 232 [degrees]C. (450[degrees] F.), respectively. Nominal W.T. is 730 mm (2.875 in.). Each vessel contained a total of 36 bubble captrays.

Following is the sequence of events determined necessary to accomplish this objective:

1. Sacroc would shut down a given process train, then drain and wash the vessel.

2. The contractor would enter the vessel and remove all bubble …