Regarding the calculated thickness indicated on the drawing
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The fixed capacity regulations stipulate that for pressure vessels that cannot undergo internal inspections due to structural reasons, the calculated thickness and the requirements for regular inspections during operation must be specified； I have two questions here: 1. In the case of no corrosion, is it necessary to specify the calculated thickness? 2. If it is necessary to specify the calculated thickness, sometimes, due to hole reinforcement, some excess amount that exceeds the calculated thickness is used for reinforcement, resulting in a minimum required thickness that is greater than the calculated thickness. In such cases, is it acceptable to not specify the calculated thickness instead and to specify only the minimum required thickness?
1. In the case of no corrosion, there is no need to specify the calculated thickness. 2. It’s okay to specify the minimum required thickness.
The calculated thickness of the casing should not be understood literally; it is necessary to take into account the minimum thickness required after reinforcement and proper support are in place. Therefore, even in the absence of corrosion (and considering only uniform corrosion on the protruding surfaces, there are still many other forms of corrosion that need to be taken into account), it is essential to specify the calculated thickness (the minimum thickness) for easy verification, thereby ensuring safe use throughout the design life of the equipment.
1. Since it is not possible to inspect the interior of the equipment, it is impossible to determine the extent of internal corrosion. Even in the absence of corrosion, other types of wear and damage such as erosion can still occur. To ensure that there are reference standards for wall thickness during maintenance and inspections, thereby enabling effective monitoring of the equipment’s condition, it is necessary to specify the calculated thickness even when there is no corrosion. 2. It is acceptable to specify the minimum required thickness; essentially, this has the same effect as specifying the calculated thickness.
1. The calculated thickness should be indicated on the pattern. 2. The calculated thickness indicated on the pattern does not mean that the equipment can continue to be used as long as the actual measured thickness is greater than or equal to the calculated thickness.
The procedure requires you to specify the purpose of calculating the thickness in order to facilitate monitoring of the shell thickness. You can specify the calculated thickness, and then state in the requirements for periodic inspections that the thickness near the openings for reinforcement must not be lower than the minimum thickness.