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2022

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06

Design service life of pressure vessel

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Design life refers to the expected service life of the container. The "Regulations on the Safety Technical Supervision of Fixed Pressure Vessels" stipulate that the design life of the pressure vessel shall be marked on the general design of the pressure vessel (for vessels designed for fatigue analysis, the number of cycles shall be indicated).

Design life and service life 

Design life refers to the expected service life of the container. The "Regulations on the Safety Technical Supervision of Fixed Pressure Vessels" stipulate that the design life of the pressure vessel shall be marked on the general design of the pressure vessel (for vessels designed for fatigue analysis, the number of cycles shall be indicated).

The design life of the pressure vessel is not necessarily equal to the actual service life, it is only an estimate given by the designer based on the expected use conditions of the vessel. Its function is to remind the user that when the design life of the pressure vessel is exceeded, the necessary measures should be taken such as: frequently measuring the thickness and shortening the inspection period.

Under normal circumstances, the specific expected actual service life should be submitted to the design unit in writing by the design client (user). The designer then determines the corrosion allowance based on the service life and the corrosion rate of the medium to the equipment. In reality, the designer usually gives the design life, so: corrosion allowance = annual corrosion rate × design life.


 

Determining the design life 
 

In determining the design life of a container, the following factors should generally be considered:
1. Select appropriate materials and structural design;
2. Reasonable corrosion allowance;
3. Limit the possibility of creep deformation(high temperature conditions) or fatigue;
4. The cost of container construction;
5. The replacement cycle of the loading capacity, etc.

The recommended design life of the container is as follows:
1. General container, heat exchanger: 10 years
2. Fractionating towers, reactors, high-pressure heat exchangers: 20 years
3. Spherical container: 25 years
4. Important reaction pressure vessel (such as thick-walled hydrogenation reactor, ammonia synthesis converter, etc.): 30 years

For pressure vessels designed for fatigue analysis, the number of cycles of alternating loads during the design life shall be indicated in the design drawings.


 

Corrosion allowance 


Cases where no corrosion allowance needs to be considered:
1. When the medium has no corrosive effect on stainless steel;
2. Base material with reliable corrosion-resistant lining material;
3. The sealing surface of the flange;
4. Heat exchange tube for shell and tube heat exchanger, straining beam, spacer tube, cross baffle;
5. The outer surface of the container and its external components that can effectively prevent environmental corrosion with paint;
6. Tray deck, packing, etc.

Cases where corrosion allowance needs to be considered:
1. Corrosion allowance should be considered for the cylinder, head, connector tube, man (hand) hole and internal components that are in contact with the working medium;
2. If there are special provisions or actual use experience on the corrosion allowance in the process system specialty or engineering design documents, it shall be selected according to the provisions and experience;
3. For pressure vessels with uniform corrosion, the corrosion allowance shall be determined based on the product of the expected service life and the corrosion rate of the medium on the metal material;
4. The erosion, wear and local corrosion of the container shell or components by the medium should also be considered;
5. For components that are in contact with the media on both sides, different corrosion allowances should be selected according to different working media on both sides, and the two are superimposed as the total corrosion allowance of the component.

Note: The finish allowance includes the following:
a) Thinning in machining set by the manufacturer (such as heads);
b) Machining allowance (such as machining allowance for tube sheet and metal parts).
 

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