All pressure vessel design and manufacturing are subject to the ‘Pressure Equipment Regulations’ No. 734 and shall be categorized and submitted to the applicable conformance assessments of SANS 347. These include the list of pressure vessel codes in SANS 347 Annex A.
All these codes follow more or less the same recipe to ensure equipment is safe for use. Industry specific rules can be directly related to the historical design process. Some improvements came about by reacting to accidents related to unknown material behaviour. Fatigue, creep and thermal cycling and various corrosion mechanisms have all been studied because of some form of failure. Over the past 100 years industrial accidents due to design inadequacy has been eliminated. All the major failure modes have been characterized and specific rules developed to stop each failure.
Pressure vessel codes include the rules for the major aspects of the design and manufacturing process i.e., material specifications, design rules for minimum thickness requirements, manufacturing and quality assurance rules and finally pressure testing. When all the rules are followed together the design team can be assured that the equipment would be safe for operation.
A case in point is a presentation of a component case study where failure due to plastic collapse is considered. Any person playing with a paper clip will have realized that bending apart the two fingers of the clip does not destroy it completely. Some plastic deformation is therefore acceptable in industrial equipment as well. How to limit this is neatly addressed by procedures for the non-linear evaluation of components for protection against plastic collapse.
The code procedure follows that a design be evaluated at the design parameters and then overloaded by the load factor β. If the component can carry this factored load with limited plastic deformation, it is classified as acceptable for protection against plastic collapse. The load factor β is summarized for various pressure vessel design codes below:
|Code||Load Factor β|
|ASME VIII-2 class 1||3.0|
|ASME VIII-2 class 2||2.4|
|ASME VIII-2 pre-2007||3.0|
|ASME VIII-1 post-1999||3.5|
|ASME VIII-1 pre-1999||4.0|
|BS PD 5500||2.5|
The specific component under consideration forms part of a swing bolt assembly that constrains a blind flange for process pressure equipment. Design loads for the blind flange and bolt connection are calculated with reaction forces of the clamp applied on the contact face. Three load cases are considered at load factors 100%, load factor β and maximum component load limit. The model is shown below and consist of linear hexahedral elements.
Design case results show maximum stress in the radius next to the contact face well below the maximum allowable stress for the specific material at the design temperature.
Load factor β results show equivalent plastic strain to be 0.27%. This value is well below the deformation limit of the material and indicates that the current design is capable of higher loads.
Maximum load factor results again show equivalent plastic strain with a local maximum of 30% located in the radius next to the contact face. The final load factor reached for this load was 4.55 or 455% of the design load.
This design is therefore acceptable for protection against plastic collapse. Similar procedures are followed for every failure mode until all requirements of the design section of the code is satisfied.
Rules and guidelines for manufacturing and quality assurance similarly ensure structural integrity and the absence of significant defects. The final design verification takes the form of a pressure test and encompasses all the previous requirements.
If the vessel passes the pressure test, it proves that the design calculations and simulation was performed correctly. Also, it shows that the manufacturing process is adequate, correct materials where used, and no defects are present. The pressure test is typically performed at a slightly higher pressure than the design. This over stress on some components results in compressive residual stresses that increase the design life of the vessel.
When a vessel passes the pressure test a client can be assured that the equipment will perform as required safely.
The rules defined in pressure vessel codes are the result of years of design refinement for industry specific load conditions and material behaviours. The Structa group of companies employ specialized personnel with extensive experience in every aspect of pressure vessel code requirement. This ensures that the code is applied as intended and delivers the assurance to our clients that all pressure equipment designed and manufactured by the group complies with the Pressure Equipment Regulations and are safe for use.