The regulations of dished bottoms
The custom-made dished bottoms
are elements used for closing pipes and tanks at the ends. Their correct design is essential to avoid the problems that can result from an incorrect distribution of efforts and end effects.
To do this, reference standards have been created and the main ones are two:
- ASME B16.9
standard: the American regulation allows the creation of bottoms up to diameters equal to 12 "and thicknesses reaching 12.7 mm, with exceptions deriving from the use of particular materials (up to 24" in diameter with steel carbon);
standard: the European standard allows to reach the same diameters as the American standard, but the thicknesses allowed for stainless steel are not more than 4 mm.
Following the regulations is necessary to respect the criteria of good design and to avoid failures or malfunctions which can also lead to important consequences, considering the applications for which these components are made.
The realization of dished bottoms
The construction of custom-made dished bottoms can be carried out using two main techniques:
1. Deep drawing;
The drawing is a plastic deformation process that leads a sheet to assume the classic bomb shape characteristic of the bottoms. The operation takes place by fixing a sheet with a special blank holder and making a punch act, with increasing force as the displacement increases, so as to deform it against a special matrix. A fundamental control parameter is the thickness of the drawn piece that is obtained. Although theoretically it should remain constant, in practice it tends to thin: if it is too small, you must avoid any problem by acting through the use of successive steps with less depth than this. In addition to what has been said, a certain importance is to be given to both the material and the lubricant that is used: for this reason it is always necessary to carry out preliminary tests before using the process at the production level.
Forging, on the other hand, is a hot plastic deformation process that is carried out by bringing the sheet to temperature and, subsequently, subjecting it to the action of a press. This operation is never, generally, the final operation but serves mainly as a phase of initial roughing of the form that must be completed with over operations. An advantage of forging is the orientation of the structure of the material that follows the profile created: this characteristic aspect allows to significantly improve the resistance characteristics of the component.
Once these operations are completed, a final finishing and trimming phase always follows to further improve both the aesthetic aspect and the mechanical characteristics. In addition, heat treatments are carried out to relax residual stresses which are particularly harmful for the component's resistance in use.
The main types of stresses for caps
Caps, despite being structurally simple components from a constructive point of view, are also faced with loads of considerable size which can compromise their functionality in operation if they are not envisaged and studied in the preliminary design phases. The role of the structural designer of reference is, in fact, to understand if there are certain loads to which the cap will be subjected and, referring to the previously mentioned standards, he must understand if there are margins for any type of failure.
Among the main loads that a cap may have to face are:
- water hammers: hydraulic load often found inside the pipes and which can pose a risk for the structural integrity of the system itself;
- overpressures: in the event of malfunctions or various problems, there may be a sudden increase in pressure in the area of the caps and this can lead to yielding or leaks;
- fatigue loads: especially in discontinuous systems this can be a noticeable stress. Even if the single stress value does not in itself represent a problem for the seal, the repetition of a large number of cycles can lead to breakage.
Another aspect that must be taken into consideration is that although there are safety factors in the design phase, a product with defects such as residual stress, cracks, unsuitable treatments, banding, corrosion and other defects of various kinds, may not resist as calculated in the study phase: therefore, the correct production of the caps assumes importance.