Issues regarding magnetic particle testing after the pressure test of spherical tanks
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According to GB50094, magnetic particle testing must be carried out on all welds after the hydrostatic test but before the airtightness test. The material used for our spherical tanks is Q345R, and its tensile strength is not high enough to require magnetic particle testing; therefore, per the standards, such testing is not necessary. However, the design documents specify that 100% magnetic particle testing should be performed on Class A joints after the hydrostatic test. So which welds need to undergo magnetic particle testing?
If the specifications require it not to be done, can you issue a work order for the design?
Since magnetic particle testing is required by the design, the designer must have had something in mind, and that is specified in the drawings. My opinion is to do it. All the butt welds on the spherical tank shell are Class A welds. Additionally, the weld connecting the manhole to the shell is also a Class A weld.
According to GB50094-98 5.4.1.1, after radiographic or ultrasonic inspection of spherical storage tanks, magnetic particle testing is performed on the inner and outer surfaces of the shell butt welds prior to the pressure test. After the pressure test, in accordance with clause 5.4.2, spherical storage tanks are subject to a magnetic particle or penetrant re-inspection following the pressure test; the proportion of the weld length to be inspected shall be at least 20%. The areas to be inspected include intersecting weld joints, the inner and outer surfaces of the welds between the nozzles and the shell plates, reinforcement rings, gaskets, struts, and the outer surfaces of other fillet welds, as well as the welds performed by each welder and the areas where welding fixtures left marks. Additionally, areas on the shell that have defects and were ground after welding are also to be inspected.
Firstly, GB50094-98 for magnetic particle testing does not specify any limits regarding tensile strength; magnetic particle inspection is essential during the installation process. Surface quality is extremely important. Secondly, it’s hard to understand why RTUT is done first and then MT; it doesn’t make much sense.
According to the latest version of GB50094-2010, after heat treatment and hydrostatic testing, magnetic particle inspection for spherical tanks must first meet the requirements specified in the design drawings; secondly, for those with a strength greater than 540, 100% surface inspection is required. This represents an improvement over previous standards. Regarding the issues mentioned earlier, RT inspection is carried out at 100%, followed by re-inspection using UT, as ultrasound is more sensitive to detecting cracks than X-rays, hence re-inspection is necessary. For near-surface ferromagnetic materials, only magnetic particle inspection can be used, as RT and UT are not suitable for this purpose. Surface inspection can only be done using magnetic particle testing. In essence, a combination of all three methods is used to ensure the quality of the welds
I would like to ask, how many times is magnetic particle testing required? According to GB50094, it is required to be done three times: once before heat treatment, once after heat treatment but before the hydrostatic test, and once again after the hydrostatic test. I’m not sure if this understanding is correct
Look at the welding process; the welding process diagram clearly indicates the type of weld.