Welding is a primary piping fabrication activity. It is critical to ensure high quality welds to ensure safe plant operation. Various methods are used to make sure the welds are of highest quality such as referring best international standards, creating approved welding qualification procedures etc.
In process industry, most widely used codes are :
- ASME Codes such as B31.1, B31.3, B31.4, B31.8 etc.
- ASME Boiler and Pressure Vessel Code.
Some countries have their own local codes applicable to certain systems such as in India, steam piping must confirm to IBR regulations.
Normally in hydrocarbon industry, carbon steel, alloy steel and stainless steel are the base materials to be welded. Project welding specification is a primary document that must be referred by welding contractor while carrying out his work. This specification includes all the requirements of welding for the particular project.
Shop Welds and Field Welds
Piping isometric drawing is marked with shop welds and field welds. Shop welds are preferred as much as possible as weld quality can be tightly controlled in shop. But maximum transportable spool size must be considered as well as any field mismatch of dimensions.
Weld MTO is specified in inch-dia which is number of welds multiplied by pipe diamter.
Welding consummables include electrodes, filler wires, argon gas etc. Quality of welding consummables must be ensured as per project welding specification. Normally argon gas is required to be of 99.995 percent purity.
Qualification of Welders
Welders must be qualified as per applicable code e.g. B31.3
Welding Contractor’s Scope of Work
All the welding equipment like transformers, thermocouples, pyro-meters, automatic temperature recorders, heat treatment equipmets etc. is provided by welding contractor. All the weld testing equipment like radiographic equipments, radiographic films, darkroom facilities, dye-penetrant testing, magnetic particle testing, ultrasonic testing etc. are provided by the welding contractor.
Generally used welding processes are SMAW and GTAW.
SMAW : Shielded Metal Arc Welding Process
GTAW : Gas Tungsten Arc Welding Process
Which welding process to be emplyed where is given in Project Welding Specification.
Check out these videos from youtube to have a look of these methods.
Before starting welding, it is essential to carry out edge preparation procedure for different types of base materials. It involves cleaning of edges, alignment of edges and ensuring proper spacing between them as per applicable code. Tack welding, if required is done. It is essential to protect work pieces from weather during welding. Proper welding technique is usually specified in project welding specification. Welding procedure qualification record should be submitted by Welding Contractor to the Client.
Weather conditions during welding
The parts being welded and the welding personnel should be adequately protected from rain and strong winds. In the absence of such a protection no welding shall be carried out. During field welding using GTAW process, particular care shall be exercised to prevent any air current affecting the welding process.
Preheating and Post Weld Heat Treatment
Preheating and Post Weld Heat Treatment need to be carried out as per project welding specification.
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Cleaning of the Weld Joint.
All weld joints shall be free from adherent weld spatters slag, sward, dirt or foreign matter. This can be achieved by brushing. For stainless steels, brushes with only stainless steel bristles shall be used.
Inspection and Testing of Welds
After welding and PWHT, Inspection and Testing of welds is done as per various testing and inspection methods specified in project welding specification. Some of the testing methods employed are :
- Visual Examination
- Magnetic Particle Examination
- Liquid Dye Penetration Examination
- Radiographic Examination
- Ultrasonic Examination
Hardness requirements for welds shall be as per the Welding Specification Chart/Non Destructive Examination Specification included in welding contract. Hardness testing shall be carried out by Vickers Hardness Tester during welding procedure qualification and shall be cross sectional.
For production welds, hardness testing shall be carried out by portable digital hardness testers. Poldi hardness tester shall not be permitted. Contractor shall produce documentary evidence/calibration certificate to the Owner’s Inspector and obtain approval of the hardness testing equipment.
Hydrostatic and pneumatic tests shall be performed as per the requirements laid down by respective flushing and Testing specification/applicable codes to demonstrate the soundness of
the welds. The tests shall be conducted after fulfilling the requirement of visual examinations radiography etc., and after the entire work has been certified by the Owner’s inspector to be fit
for being subjected to such tests.
Repairs of Welds
Repairs of welds may be done if defects are within certain limits.
Defects ascertained, through the inspection methods, which are beyond acceptable limits shall be removed after the joint is completely radiographed by the process of chipping and grinding.
When the entire joint is judged unacceptable, the welding shall be completely cut and edges suitably prepared as per required alignment tolerances. The re-welded joint shall again be examined following standard practices.
No repair shall be carried out without prior permission of the Owner’s inspector. Repairs and/or work of defective welds shall be done in time to avoid difficulties in meeting the construction schedules