Welding of High Pressure vessels and pipelines Procedure Roadmap

This guide has been prepared to assist with the preparation of welding procedures required for a high pressure vessel. A overview of the following areas will be discussed giving a basic roadmap to creating a new weld procedure: Preparation
Provisional weld procedure
Preparation of material
Preparation of equipment
Heat treatment
Qualification test
Inspection
Documentation
Production

Preparation

Specification details of the material to be welded are gathered including the thickness of material, purpose of vessel and standard of weld to be achieved. The chemical composition and mechanical properties of the material, including yield and tensile strength should be tested and recorded. Results should be compared to the specification given by the manufacturer of the material. The most appropriate welding method, joint type and filler material should be determined taking account of the outcome required for quality, environment and reliability.

Provisional Welding Procedure
The provision procedure is set up to provide an opportunity to test the welding variables for a specific application. This assures repeatability is achievable by properly trained welders.

Preparation of Material
The edges or surfaces to be joined by welding shall be prepared prior to welding. Pipe rarely will be received in a condition suitable for welding. Generally, there will either be an oil or rust coating, or a coating to prevent corrosion. This could include paint, primer, varnish, epoxy or tar all of which are undesirable for welding. Cleanliness is critical to prevent defects leading to rejected welds and costly repair, so these coatings must always be removed. Both pipe ends must be cleaned, on the inside and the outside, at least 25mm beyond the edge of the bevel. All surfaces and edges shall be free from cracks or any other defects which would adversely affect the quality of the weld. All moisture, grease or other foreign material that would prevent proper welding or produce dangerous fumes, should be removed. All surfaces to be welded should be wire brushed prior to welding.

Preparation of equipment
Welding material should be stored in suitable conditions that will keep them dry and free from surface rust and other contaminants. All rods should be of the same chemical composition and match that stated by the provisional welding procedure. Any damaged rods which have been compromised should be discarded and not used. Welding equipment should be fit for its purpose complying with the appropriate
Standards. The equipment should be of a power commensurate with the provisional welding procedure requirements. All equipment should be maintained in a serviceable condition. Any measuring instruments used for testing and inspection should be calibrated in line with the manufacturers recommendations.

Heat Treatment
Good welding practice is essential in preventing the weld cracking, which includes preheating when needed. Any preheat specified is to be maintained during welding. If welding is interrupted and the temperature of the base metal falls below the minimum preheat temperature, then arrangements will be made to preheat again prior to restarting the weld.

Qualification tests
The most experienced and competent welder available should be selected to conduct the qualification test. The selected welder current certificates and qualifications should be checked to be valid and in date. Before beginning to weld the procedure should be checked to assure that the correct electrode is being used. This includes the diameter and the classification of the electrode. A qualified welding inspector should witnessing the qualification tests process to confirm compliance with the requirements of the relevant