K. R. Ferguson
Chevron U.S.A. Inc.
Midland, Tex.
A. G. Stutheit
Chevron U.S.A. Inc.
Lafayette, La.
Complete in situ weld overlay to repair large process vessels in a hot potassium carbonate CO2 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 quality-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 CO2-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 C. (450 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:
- Sacroc would shut down a given process train, then drain and wash the vessel.
- The contractor would enter the vessel and remove all bubble cap trays and internal hardware attached to the vessel walls (that is, tray support rings, downcomer bars, etc).
- The contractor would remove previously applied strip-lining and repair existing corrosion damage in the vessel walls.
- The contractor would sandblast the inside of the vessel to a National Association of Corrosion Engineers (NACE) "white metal" finish.
- The contractor would apply a single layer 309LSi stainless-steel weld overlay to the entire internal surface of the vessel.
- The contractor would apply a single layer 309LSi weld overlay to all nozzles where possible. Nozzles and couplings too small for overlay would receive liners or be replaced with stainless steel.
- Upon completion of the overlay, the contractor would install new 304L stainless-steel tray support hardware and new "valve trays" and prepare the vessel for return to service.
The necessity, intent, and details for conducting a "pretest" were also defined in the first section of the specifications.
The second section of the specifications dealt entirely with the required physical characteristics of the overlay (thickness, chemistry, etc.), the mechanics of overlay deposition, treatment of nozzles, the quality-assurance program, and the overlay guarantee.
The physical characteristics defined for the finished overlay and the mechanics of deposition are shown in Table 1.
Rigorous quality assurance (QA) was considered essential to the project's success. A program was developed which involved both Chevron and contract personnel. Strict compliance with this program was required.
Both Chevron and contract QA inspectors were empowered to shut down overlay process work whenever this QA plan was not being followed or if QA parameters were found to be out of range.
The QA plan consisted of the following:
- Preproduction "workmanship samples" were evaluated by Chevron's materials lab to confirm the quality of the contractor's work.
- The base metal surface preparation was carefully examined by a team of both Chevron and contractor inspectors before the start of the overlay work.
- "In-process" chemistry analyses of the resulting overlay were conducted on a minimum of 30 "chip" samples from each vessel shell and 3 samples from each head.
- The ferrite content of each operator's welds was checked at least four times per shift using Severn gauges.
- The welding parameters of each welding operator were checked at least twice per shift.
- Constant visual checking of weld quality was conducted by both Chevron and the contractor's quality inspectors.
Each contractor was required to submit, as part of the bid, a written quality guarantee covering the finished overlay. This guarantee provided for the repair of all overlay defects (such as cracks and pinholes) as a condition for Chevron's acceptance and subsequent payment for work.
Liquid-dye penetrant analysis of the finished surface was established as the method of final inspection, and selective flaw removal and overlay replacement became the method of repair.
In addition, overlay contractors were required to supply, as part of their bids, information on numerous technical and logistical considerations which would affect the project.
Here is a sampling of these questions:
- Does the hydrocarbon service or the vanadium or sulfide-scale history of the vessels pose any special problems to weld overlay?
- What are the special safety considerations, given that the adjacent train will be operational during the overlay process?
- What problems does the contractor have with Chevron's performing additional construction work (such as piping replacement and platform and ladder work) external to the vessel during the overlay process?
- What will be the job description and qualifications of the contractor's dedicated quality inspector?
- Given the confined work space inside the vessel, will the contractor tolerate a Chevron inspector on site at all times?
- What will be the actual frequency and method of testing for overlay composition and integrity?
- How many welding units would the contractor propose to use? And what would be the resulting time to overlay?
- How does the contractor view the logistics of gutting, cleaning, overlaying, and refitting the vessels, and what kind of schedule would the contractor propose?
What prospective contractors answered to these and other questions gave Chevron valuable insight into the technical and project expertise of the contractors and helped further to define the mechanics of the project itself.
Many of these questions would be later resolved during the field pretest.
FIELD "PRETEST"
Because of the project's size and its involving a relatively new technology, Chevron decided to conduct a "pretest" of the process in a controlled environment similar to what would be encountered at the Sacroc-Sun facility during the actual project.
The abandoned North Snyder gas plant (NSGP) absorber was ideal for such a pretest because it was approximately the same size as each of the Sacroc-Sun vessels, had experienced the same operating conditions as the candidate vessels, and was available for unobstructed testing.
All aspects of the final project would be tested on a small scale, including support services and overlay technology. Because any knowledge gained from the pretest would reduce problems during the actual overlay process, the pretest was a planned exercise and its cost included in the original project appropriation.
The weld overlay pretest was scheduled for a month before the actual start of production overlay. This was to allow sufficient time for modifications in the overall project logistics.
Weld overlay performed during the pretest was judged complete upon evidence of a continuous, corrosion-resistant surface with a high level of consistency.
For a more or less real time assessment of the deposited overlay quality, the overlay samples were shipped by air to an analytical laboratory for priority analysis and facsimile communication of results.
The on site portion of the pretest lasted 5 days. Project supervisors believed it fulfilled original expectations and justified its cost.
All project supervisors were involved at some time with the pretest.
SAMPLE VERIFICATION
Part of Chevron's technical review aimed at determining acceptable "production chemistry" for a single-layer weld overlay by the contractor so that any necessary adjustments might be made before final project work.
In addition, because the overlay would be a nonstabilized material, there was concern about susceptibility to polythionic acid stress corrosion cracking.1
Analysis work conducted by Chevron's materials lab following the pretest verified the quality and consistency of the samples taken during the pretest. The lack of sensitization in both the as-welded and postweld heat-treated condition was confirmed by two ASTM tests (A262 Practice C and A262 Practice E) and a U-bend test.
Tables 2, 3, and 4 summarize the results of nondestructive and destructive tests, chemical analysis, and corrosion tests performed on the contractor's workmanship (welding qualification) and pretest samples.
This project was labor intensive, requiring the service of 8 full-time Chevron employees. In addition, 125 contract personnel were involved in the project at one time or another.
Assembly of the project supervisors received careful attention because of the project's complexity, its schedule, and its safety considerations.
The complexity arose from the use of a new technology, work in a very confined space, and several tasks occurring at one time.
The project was run on an intense schedule: 24 hr/day, 6 (occasionally 7) days a week.
Safety was a prime concern because potentially dangerous work was being performed in close quarters, adjacent to a live processing train.
PROJECT TEAM
Given this combination, it was imperative to choose competent, experienced personnel who could work under extreme and prolonged pressure. Fig. 1 depicts the organizational structure of the project team.
The project manager was supported by a drafter, a Sun plant operating liaison, a process and safety engineer, and a purchasing agent. A project construction representative was in charge of all field operations and reported to the project manager as well.
The line supervisors, including the overlay and piping supervisors on day shift and the overlay supervisor on night shift, reported to the project construction representative. The dedicated safety supervisors on both day and night shifts reported to the project safety engineer. A total of 13 contracts were let directly by Sacroc, with another 6 subcontracts let by various primary contractors. With the exception of the welding wire included in the overlay contract, these contracts were for labor and service.
The four major contracts were vessel weld overlay, vessel internals handling-vessel preparation, crane and rigging, and piping fabrication and replacement. George P. Reintjes Co., Houston, provided the basic overlay expertise for the project; Wyatt Field Services Co., Houston, provided necessary vessel support work and expertise.
All project management was handled from a temporary office trailer just outside the plant.
Keying the success of this project were the daily supervisors' meetings held each morning during shift change. Chevron and contract supervisors from both shifts were required to attend and contribute at this meeting. It dealt (at a minimum) with progress to date, plans for the coming day, and safety matters.
Appropriation for this project was approved Mar. 1, 1988, and engineering and drafting work started immediately and continued until actual overlay work began on the first absorber column. This date was set by the time required to obtain enough welding wire to overlay both columns. The actual weld overlay and piping work began on the first column (100 Train) on Oct. 3 and was completed Dec. 3. The upgrading of the 200 Train began Jan. 3, 1989, and was completed Feb. 28.
The month's delay between the two trains resulted from startup and stabilization of the first train. It should be noted that the 200 Train was completed in 8 weeks compared with 9 weeks for the 100 Train (11% reduction) and that each phase in the 200 Train took 10-20% less time than it did on the 100 Train, mainly as a result of the learning curve traversed on the first train.
SAFETY PROGRAM
Always, the overlay work was performed immediately next to an operating process train. Other Chevron facilities were polled for past experiences of hot work being performed in operating areas. The conclusion was that such work is mostly done in a turnaround (or 100% shut-down) or at least in a more isolated environment.
Therefore, Sacroc and Sun were challenged to perform 4 months of hot work in an operational plant area.
The project safety program's chief concern was to protect all personnel from the many inherent hazards of working in this facility.
Receiving the most attention were H2S, fire, explosion, and hot potassium carbonate solution. The contractors had developed their own work-safety programs in addition to enforcement of all Chevron requirements.
The safety program was defined in a project safety manual created specifically for this project. This plan outlined evacuation plans, first aid and CPR data, and call lists. The plan's primary goals were the following:
- Chevron's key responsibility in emergencies is to ensure that all personnel are properly evacuated.
- Sun (as plant operator) would be the key party involved in rescue operations, emergency containment, and plant control.
The project safety manual was supported by a mock disaster training drill before the project began and in which all personnel participated. This drill was followed by a critical discussion. Other training programs included H2S safety (Texas RRC Rule 36), CPR, and first aid.
Each contractor supplied one individual who would serve continuously as project safety superintendent. In addition, contractor "hole watches" were stationed at each working level of the absorber column, maintaining visual and radio contact with those working inside the vessel. These persons also monitored H2S levels in the surrounding area.
Also, Chevron provided a continuous safety inspector on each working shift who ensured proper contractor work procedures, monitored the work area, and inspected safety equipment. This person also obtained general work, confined-space entry, and hot work permits from the Sun plant operator at every (12 hr) shift change as required by Chevron and Sun safety policies.
H2S and combustible gas levels were continually monitored throughout the plant.
Even though the plan was equipped with standard, on site safety equipment, additional equipment was installed to accommodate the project:
- A cascade breathing-air system to supply all workers in the column (10 masks) in the event of an emergency
- A "fire boss" trailer-mounted CO2 fire extinguisher
- Additional hand-held fire extinguishers
- Additional H2S monitors
- Additional wind socks
- A special project evacuation horn.
Success of the project safety program was demonstrated by three evacuation incidents occurring without mishap over the 4-month project duration. Prompt and proper actions by project personnel prevented each incident from becoming a disaster.
In fact, several incidents were logged during the project. All were industrial hazards (falling objects, cuts, sprains, eye protection, ventilation, etc.), but work was accomplished without serious incident because of protective equipment and strict adherence to rules and procedures.
SUPPORT SERVICES
The Sacroc-Sun absorber upgrade project involved performance of a major construction project within the confines of an operating-plant environment. Because the existing utility systems at the plant had been designed only for plant support, auxiliary utility systems had to be installed to supply project utility needs.
Furthermore, all required project support services had to be installed and operated within the severe space and safety constraints imposed by the plant. Chevron independently contracted for the following utility and service systems: electricity; compressed air; sanitation; waste storage and removal; crane, rigging, and materials handling; and scaffolding.
All of these support services were standalone systems, independent of any plant utilities. This factor allowed the project work to proceed without interfering or interrupting plant operations.
In addition, these support service contracts were administered by Chevron personnel, which freed both project contractors and plant operators to deal strictly with their respective aspects of the project.
The original design of the internal hardware in the Sacroc-Sun absorber columns consisted of 36 bubble-cap trays. The initial intent of the project scope was to re-use this tray hardware without modification.
During the initial phases of the project, however, Chevron's engineering technology department conducted an analysis to verify the process effectiveness of this internal design. This study was prompted by the changes in the composition of the process feed stream since the original design of these vessels.
This design study indicated that several rows of bubble caps on each tray level would have to be blanked off to allow optimum processing efficiency if the existing trays were to be re-used.
Prompted by the mechanical complexity and time-consuming nature of this tray blanking procedure, project engineers decided to investigate other options, particularly the complete replacement of internal hardware.
Bids for new hardware were solicited from several vendors. During the process, it became apparent that purchasing all new "valve-type" trays was more cost effective than attempting to modify existing trays and tray hardware.
In addition, tray replacement proved to be a far cleaner solution from an engineering standpoint because the new trays could be specifically designed for the existing and forecasted process conditions. Tray replacement was also more consistent with the overall project philosophy of providing a highly reliable and efficient finished product.
The new "valve trays" were constructed of 304 stainless steel and designed to fit exactly onto the hardware (tray rings and downcomer bars) utilized by the old bubble cap trays. Allowances in the outside diameter of trays accounted for the thickness of the weld overlay. The new trays were received at the job site before completion of overlay on the first column and installed without major difficulty in the finished column.
NBIC CRITERIA
Chevron chose to apply National Board Inspection Code (NBIC) criteria and requested that the prime contractor supply an R-Stamp repair certification even though pressure-vessel legislation requiring such action is not currently in place in Texas.
This was performed because of the magnitude of vessel work involved, the critical location of Sacroc unit vessels next to nonunit equipment, and as added assurance that all necessary steps would be performed in this critical project with difficult operating logistics.
Because the NBIC considers both weld buildup of wasted areas and a corrosion-resistant weld overlay to be "routine" repairs (Chapter III, Supplement 1, Subpart C), no reinspection and recertification of the vessel was required by the weld-overlay part of the project.
REFERENCE
- NACE Standard RP-01-70 (1985) revision), Item No. 53004, "Protection of Austenitic Stainless Steel from Polytgionic Acid Stress Corrosion Cracking during Shutdown of Refinery Equipment," National Association of Corrosion Engineers, Houston.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.