Critical systems of the desalination plant are designed to work at the pressures of 65 – 75 Bar in highly corrosive environment. The materials of choice are super-duplex and super-austenitic steels like 254 SMO.
Such bespoke systems are assembled from components – piping and equipment pieces, pumps, valves, etc. – ordered from different manufacturers.
A substantial part of the technical specifications included in the purchase orders is devoted to casting and welding quality assurance (CWQA) as defects in manufacturing are the main reason for such systems failure.
While requirements for the manufacturing process and inspection are well defined by the industry standards (totaling 13), their implementation planning and execution are obstructed by an extraordinary volume of work and data to be collected.
The table below details the CWQA scope for a desalination megaproject of 250 MLD, generated by the PlantDesigner software. The data show that in the worst-case scenario, the project schedule may provide room for all 1632 non-destructive test (NDT) areas.
| Category | Weld/Cast areas | NDT type | Coverage |
|---|---|---|---|
| Metal piping | 733W | VI + DPT + RT | 10-100% |
| RO manifolds | 558W | VI + DPT + RT | 10-100% |
| RO pumps | 257C | VI + MPT + RT | 100% |
| intake pumps | 72W + 12C | VI + DPT + UT | 10-100% |
While visual inspection (VI) is simple and quick, dye-penetrant (DPT), magnetic particle (MPT), radiographic (RT) and ultrasonic (UT) tests require time, equipment, and expertise in results' interpretation. Labor costs are a significant factor.
The typical NDT report (matching ASME Section V, Article 2) includes three sections - acceptance criteria, equipment and materials, and methods followed by the list of records and a gallery of the marked-up weld images. The NDT report is a go/no-go point in the order payment schedule.
In practice, to decrease the impact of NDT on the project budget and schedule, companies apply NDT only to about 20% of randomly selected welds. If defects are found, the percentage is raised to 50 – 100%. (Normally, progressive sampling is combined with “spot check” where a certain length or number of welds per welder or batch are examined.) Repairing defects impacts not only the payment schedule but the manufacturing one too as they should all be witnessed (by the third party), properly analyzed, and documented. That follows by the process correction.
The biggest differentiator in this practice is the manufacturer's history. For new entrants, the safer option may be 100% for (VI + DPT) and 30% for RT. For manufacturers with long-standing good reputation the NDT coverage may be reduced to 10 – 15%.
The manufacturer history is a key to the CWQA optimization. At minimum, the manufacturer’s database should include the following information.
- Personnel qualifications and training history
- Manufacturing standards
- Quality plans
- Weld records describing welding parameters and material chemical composition, post-weld heat treatment and checks
- Casting records describing design, preparation, process, post-casting treatment and checks
Building the CWQA software managing both manufacturing and manufacturer history is a lengthy development cycle. Currently, crenger.com offers web interface for NDT results submission and report generation.