TOFD and RT
Thread Content
1. In the new standards for spherical tanks, TOFD is now preferred for the butt welds of carbon steel spherical tanks. Is TOFD also preferred for other pressure vessels designed according to GB150? Will there be any revisions to GB150 in the future? 2. The thickness of spherical tanks generally does not exceed 50; has the choice of TOFD been made with the fact that the thickness of spherical tanks will not be too great in mind? If it is a high-pressure vessel with a cylinder wall thickness of 200 mm (made of forgings), should TOFD also be given priority? Can RT be used to inspect welds of such thickness?
It seems that more experience is still needed for the use of TOFD in China. The quality of the film is greatly influenced by human factors.
From the perspective of promoting cleanliness, pollution-free operations, and high efficiency, TOFD should be widely adopted. However, the capabilities of various non-destructive testing organizations vary; practical experience on the part of TOFD operators is extremely important, so its adoption should take place gradually. Furthermore, the use of TOFD for medium and thick plates is even more significant.
A new clause has been added in GB12337-2014: Gamma-ray panoramic exposure radiographic inspection is not recommended. According to the Large Pressure Vessels Code: when 100% gamma-ray panoramic radiographic inspection is used for the butt welds of large pressure vessels, an additional 50% of local X-ray or TOFT inspection must be carried out. If any defects exceeding the specified limits are found, 100% X-ray or TOFT inspection must be performed.
Can TOFD + surface inspection replace RT?
It depends on the energy that the flaw detection equipment can provide. The maximum thickness of steel that can be penetrated by X-rays emitted from an X-ray tube (with an electron acceleration voltage of 400 KV) is approximately 90 mm. The X-rays generated by a 20MeV linear accelerator can penetrate a maximum steel thickness of about 600 mm. The penetration capacity of a typical portable X-ray machine (320KV) does not exceed 60 mm.
The TOFD testing standards specify details such as the testing surface and the handling of testing blind areas; surface testing serves as a complement to the TOFD method for addressing surface blind areas.
As a requirement for the design documents, it is sufficient to specify the need for TOFD testing; regarding the handling of blind areas, the standard procedures shall be followed.
In my opinion, thin plates are not suitable for Toft testing, as there is a 1 mm blind zone on each side, which results in poor reliability of detection； I talked to manufacturers of high-pressure reactors, and they shared the same opinion. Toft’s method is actually easier to implement than Ray’s, but the evaluation of the profiles shows that its score is higher than Ray’s ； However, the current requirements for standard radiation treatment are too stringent and even inconsistent: 1) According to clause 5.9.3 of the NB/T47013.2-2015 standard, the exposure time using Ir192r radiation should not exceed 8 hours, and it is not allowed to use multiple radiation sources simultaneously ； At present, it is difficult for the single-source intensity of r-source to meet the requirements. 2) The old standard, JB4730.2—2005 for radiographic inspection, has been repealed: it does not contain these specific requirements. 4. Specifications for steel spheres – GB12337-2014: Clause 8.6.1.3 specifies that radiography should be used for stainless steel storage tanks, while Toft is recommended for other types ； For example, there is the issue of where to place the quality meter: according to clause 5.12.1.b) of NB/T47013.2-2015, for single-wall radiography, if it is not possible to place the quality meter on the side of the radiation source, it is allowed to be placed on the film side (except in the case of panoramic exposure of spherical tanks). Since panoramic exposure is used for spherical tanks in field conditions, it is required that the quality meter not be placed on the film side. This issue poses significant difficulties in carrying out field radiography.
If the thickness exceeds 90 mm, is gamma radiation the preferred option or X-rays?