Pressure Vessels. Conical Sections
- Posted by: César Boró Martín
- Category: Pressure Vessels
Conical transitions are really usual in several industrial sectors so that people minimally related to design or manufacturing of pressure vessels are used to deal with them. The shape or these conical sections is mainly imposed by the process in which vessel is immersed. In order to satisfy properly the process needs, the conical part could take different forms. The principal classification criterion is established by the external configuration of the cone, which could be either concentric or eccentric, as it is shown as follows.
It should be reminded that this conical element must be calculated to ensure that it withstands all loads to which it will be subjected throughout its useful life, including manufacturing, transport, pressure vessel lifting up to its service location, operation and subsequent discommissioning activities. Based on calculation, designer could determine that the thicknesses of the involved components that have been firstly considered are insufficient, so designer will have to strengthen the affected zone. For this, the most commonly implemented solutions consist of increasing the thickness of the conical section or cylindrical element attached to it, considering stiffening rings, establishing knuckles for join between conical and cylindrical parts or combining several of them.
Whenever the target of a conical transition involves to give continuity between two cylindrical elements in a pressure vessel, it can be easily seen that the junction between each cylinder and the cone will cause an abrupt discontinuity in the configuration of the pressure retaining boundary which entails a consequent stress concentration and an unbalanced force at the join, unless this undesired effect is somehow attenuated. For this purpose, at the junctions between cylindrical and conical components, a knuckle will be used. This knuckle reduces, as far as possible, the change in the shape of boundary and therefore the existing stresses will be lowered as well. The more the numerical value or the knuckles increases, the more the affecting stresses decrease.
The effect achieved by these knuckles is, as expected, inversely related to the needs to implement thicknesses increments or to add stiffening rings because of the reduction in stress level, as mentioned above. In practice, other secondary variables should be taken into account, such as constructive possibilities, the manufacturing schedule or the derived economic cost before choosing the definitive solution. There is talk of secondary variables since the pressure vessel structural integrity will always be more important than any other considerations, especially to vessels that must withstand high pressure, really high or low temperatures or with fatigue behavior where is strongly recommended to avoid high stresses due to geometric discontinuities so that knuckles are the most advisable solution.
Regarding the premises to be taken into account to develop the convenient calculations of this kind of pressure vessels, it should be noted that the join between cone and cylinder at the large diameter side, when it is affected by internal pressure, must withstands, usually, compression effect. Likewise, the cone-cylinder join at small diameter side, affected by internal pressure, must withstands, usually, tension effect. Nevertheless, if internal pressure is exchanged for external, the large end side will be affected, generally, by a tension effect while the small end side, by one of compression. An additional comment should be considered in relation to vertical pressure vessels calculation and its conical transition during transport and subsequent lifting, since these vertical vessels are usually transported horizontally and then lifted to their service location from horizontal position. In no case should this process be overlooked in calculation development and the designer has to ensure the structural integrity of the entire equipment, especially considering the conical part. Considering the calculation method, regardless of the code used, ASME VIII, EN 13445 or AD Merkblatter, the design requirements are similar and, therefore, the obtained results as well.
At least the following points must be taken into account to design a conical transition:
- Small diameter shell: thickness calculation required by internal and/or external pressures effect.
- Conical section: thickness calculation required by internal and/or external pressures effect.
- Large diameter shell: thickness calculation required by internal and/or external pressures effect.
- Cone-cylinder joins: required reinforcement calculation.
- Knuckles (if any): determination of requirements in order to avoid overstresses.
From the manufacturing point of view, it is necessary to bear in mind that the stress concentration, as mentioned above, in the area of junction between the cone and the adjacent elements could be increased its harmful effect because of an improper alignment in the welding between both components. If the conical transition has knuckles, which is widely preferred in industry, the circumferential seams that attach the cone to other elements are located outside the zone of geometric discontinuities and this arrangement causes a better alignment in the weld joint. It should be noted that a conical section with knuckles implemented is a more expensive solution. Regarding a method to manufacture the knuckles, these could be obtained from portions of elliptical or spherical heads that have the same thickness and shape as required. Often knuckles are manufactured in the form of toroidal rings which have the same thickness as conical part.