What is RT-1 ,RT-2,RT-3 and RT-4?

RT1, RT2, RT3, and RT4 are codes used in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code to indicate the level of radiography required for butt welds in pressure vessels.

What is RT 1?

RT1 requires full-length radiography of all butt welds. Joint efficiency is 1. 

This is the most stringent radiography requirement. It requires full-length radiography of all butt welds, regardless of the pressure rating of the pressure vessel or the type of weld. This ensures that all welds are thoroughly inspected for defects.

What is RT 2?

RT2 requires full-length radiography of category A and D butt welds, and spot radiography of category B and C butt welds. Joint efficiency is 0.85. 

This requirement is less stringent than RT1. It requires full-length radiography of category A and D butt welds, and spot radiography of category B and C butt welds. Category A and D butt welds are considered to be the most critical, so they are required to be fully radiographed. Category B and C butt welds are considered to be less critical, so they are only required to be spot radiographed

What is RT 3?

RT3 requires spot radiography of all butt welds. Joint efficiency is 0.7. 

This is the least stringent radiography requirement. It requires spot radiography of all butt welds. This means that only a small portion of each weld is radiographed. This is a less reliable method of detecting defects, but it is often used for pressure vessels with lower pressure ratings.

What is RT 4?

RT4 does not require radiography of any butt welds. Joint efficiency is 0.5. 

This code indicates that no radiography is required for any butt welds. This is only allowed for pressure vessels with a pressure rating of 150 psi or less and all welded joints made with backing rings.

About radiography test as per ASME section viii division 1

The radiography requirements are designed to ensure the integrity of pressure vessels by detecting any defects in the welds. Radiography is a non-destructive testing method that uses X-rays or gamma rays to create images of the inside of a material. This allows inspectors to see any flaws or cracks in the weld that would not be visible to the naked eye.

The RT codes are used on the nameplate of the pressure vessel to indicate the level of radiography that was performed. This information is important for the owner and operator of the pressure vessel to know, as it helps them to assess the risk of failure and to plan for maintenance and repair

The radiography requirements are specified in Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code. This section also includes other requirements for the design, construction, and inspection of pressure vessels.

For joint efficiency check table number UW 12 in ASME section 8 div 1

I hope this helps.