Precautions for pressure vessel design
Pressure vessel: refers to the closed equipment that contains gas or liquid and carries a certain pressure. Its scope is defined as the fixed vessel and mobile vessel that the maximum working pressure is greater than or equal to 0.1MPa (gauge pressure), and the product of pressure and volume is greater than or equal to 2.5MPa · L, and the maximum working temperature is higher than or equal to the standard boiling point.
Structure of pressure vessel:
- 1. Single layer type;
- 2. Multi layer type: high safety, but many production processes, low labor productivity;
- 3. Plate winding: it is not necessary to wrap the layer by layer and weld the welds of each layer;
- 4 Groove winding type: the groove steel belt is meshed layer by layer, which can make the steel belt layer bear part of the axial force of the container; there is no girth weld running through the whole wall thickness on the cylinder; it has high use safety; but it needs special rolled groove steel belt and special machine tool;
- 5. Hot sleeve type;
- 6 Forging welding type: it has become the main structural form of light water reactor pressure vessel, petroleum industry hydrogenation reactor and coal conversion reactor.
Pressure vessel design:
According to the given process design conditions, in accordance with the provisions of the current standards and specifications, on the premise of ensuring safety, select materials economically, correctly and reasonably, and design the structure, strength (rigidity) and sealing.
- 1. Structural design: determine a reasonable and economic structural form to meet the requirements of manufacturing, inspection, assembly, transportation and maintenance;
- 2. Strong (rigidity) design: determine the structure size to meet the requirements of strength or rigidity and stability, so as to ensure the safe and reliable operation of the vessel;
- 3. Sealing design: select appropriate sealing structure and materials to ensure good sealing performance.
Delimitation scope of pressure vessel:
- 1. Pressure components: components in the vessel that directly bear pressure load (including internal pressure and external pressure), such as vessel shell components, opening reinforcement ring, external pressure reinforcement ring, etc;
- 2. Non pressure components: parts directly welded with pressure components to meet the use requirements, which do not bear pressure load (only bear gravity load), such as bearings, lifting lugs, base plates, etc;
- 3. The scope of pressure vessel is clearly defined in GB150 and the code, which refers to the shell and its pressure parts (pressure parts) connected as a whole.
The basic principles of welding structure design of pressure vessel:
Butt joints shall be used as far as possible: it is easy to ensure the welding quality.
All longitudinal and circumferential welding joints and butt welding joints on convex head shall be used except butt joints. Butt joints shall be used as far as possible for welding structures at other positions.
For example: for fillet weld, use butt weld instead [figure 1 (a) is changed to (b) and (c)].
It reduces the force concentration, facilitates the nondestructive testing and ensures the internal quality of the joint.
Figure.1 Corner joint and butt joint of vessel connecting pipe
The structure of full penetration shall be adopted as far as possible, and no incomplete penetration defects are allowed:
Incomplete penetration: refers to the phenomenon that the base metal and the weld metal are partially incomplete fused and leave gaps. The initiation point of brittle failure caused by incomplete penetration may also induce fatigue failure under alternating load.
Improvement: select appropriate groove form, such as double-sided welding; when the diameter of the vessel is small and it is unable to clear the root from the inside of the vessel, single-sided welding and double-sided forming butt joint shall be selected, such as backing with argon arc welding, or groove with backing plate, etc.
Minimize the stress concentration at the weld:
- The joint is often the origin of brittle failure and fatigue failure, therefore, in the design.
- The stress concentration must be minimized when welding the structure.
Measures: equal thickness welding shall be adopted as far as possible. For butt joint of unequal thickness steel plate, the thicker plate shall be thinned and transited according to a certain slope, and then welding shall be carried out to avoid sudden change of shape and reduce the stress concentration. Generally, when the thickness δ 2 of the thin plate is not more than 10 mm, the thickness difference between the two plates is more than 3 mm; or when the thickness δ 2 of the thin plate is more than 10 mm, the thickness difference between the two plates is more than 30% of the thin plate, or more than 5 mm, the edge of the thick plate shall be thinned according to the requirements of Figure 2.
Fig.2 Butt joint with unequal plate thickness
Common welding structure design of pressure vessel:
Main content: select appropriate weld groove to facilitate welding materials (welding rod or welding wire) to extend into groove root to ensure full penetration.
Groove selection factors:
- Minimize the amount of filler metal;
- Ensure penetration and avoid all kinds of welding defects;
- Facilitate welding and improve labor conditions;
- Reduce the welding deformation and residual deformation, and select the groove form symmetrical along the thickness as much as possible for the welding of thicker components, such as x-groove, etc.
Problems caused by opening:
Weakening the strength of the wall and generating high local stress.
Pressure vessels meeting one of the following conditions may not be provided with inspection holes:
- 1. Pressure vessel with cylinder di ≤ 300mm.
- 2. The container is provided with a detachable head, cover plate or other covers that can be opened and closed, and their dimensions are not less than the provisions;
- 3. Pressure vessels without corrosion or slight corrosion and without internal inspection and cleaning;
- 4. Pressure vessel for refrigeration device;
- 5. Heat exchanger.
If it does not belong to the above five situations. However, if the inspection hole cannot be opened due to some special reason, the following measures shall be taken:
- 1. 100% NDT shall be conducted for all longitudinal and circumferential welds of the vessel;
- 2. Indicate the calculated thickness on the design drawing, and focus on the thickness measurement inspection during the use or inspection of pressure vessel;
- 3 shorten the inspection cycle accordingly.
Sealing design of bolted flange connection:
There are two key problems to be solved in the design of bolted flange connection:
- 1. Ensure that the connection is “tight without leakage”;
- 2. The flange shall have enough strength to avoid damage due to stress.
In practical application, leakage is the main problem, and there is little damage due to insufficient strength.
Sealing performance: compression surface and gasket.
Source: China Pressure Vessels Manufacturer – www.secmachinery.com
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