This article from our guest expert, Todd Sprentall, discusses the design for swing bolt closure pins, its basis in ASME Section VIII, and the assembly of the typical swing bolt.
ASME Section VIII Division 1, Appendix II – Swing bolt closure pin design philosophy: Shear only or with bending?
Many fabricators around the globe employ swing bolt closures on their vessels, whether they are shop fabricated or purchased from another vendor. The closures are simple and effective and allow for fairly rapid access to a vessel’s internals at a reasonable cost when compared to ASME full section quick opening closures. Designs vary depending upon the application and the fabricator and appendix I rules are also used in certain designs.
The one common element to all designs is the lug/pin/bolt system. ASME Section VIII Division 1 details the size of the number of the bolts but design of the pins & lugs are left to the designer and/or local jurisdictional guidelines.
Once again, the ASME has input and in Section IID, Table 1A general note (c), guidance is given as to an allowable stress multiplier for a member in restricted shear such as a dowel bolt (or one could reasonably assume, a swing bolt pin). This appears to be the consensus taken by North American governing bodies and pins are sized by shear calculations alone, using standard formulas provided in pressure vessel handbooks and design manuals such as those by Megesy1 and Moss2. Bolt lugs are also designed using formulas provided by these manuals but are not the topic of discussion here.
Now comes the fun part (if we weren’t going to have some fun, I would have stopped after the last paragraph!). Since these bolts are allowed to swing – hence the name swing bolt – there must be some space between the lug and the bolt. That means that the pin cannot be considered to be purely in restricted shear, can it?
The consensus is that it cannot, and that means that bearing stress on the lugs (once again, not exactly the topic here) and bending stress on the pins must be considered.
Typical Swing Bolt Assembly
The point from which to calculate the bending moment, subsequent stress, and the allowable material stress to be used are the next issues to be addressed.
To address the first, a point through the thickness of the lug, at which a minimum bearing area exists to support the local load, given the allowable stress of the lug material, and the pin diameter can easily be calculated.
The distance between this point and the face of the bolt can be used in conjunction with the local load to calculate the bending moment using M=Fd. Once the bending moment is known, the bending stress is easily (once again) calculated using σ=My/I, where “y” is the distance from extreme fiber to pin centroid and “I” is the second moment of inertia of the pin.
The second question is not so easily answered as there is no clear cut guidance given for material in bending by ASME Section VIII Division 1 or Section IID. The obvious choice would be to use the allowable stress (Sa) and call it good. But is that being overly conservative in this case?
The author has seen a 0.66Fy (yield stress) factor used as well, but then should you obtain Fy from ASME Section IID, from the material test report, somewhere else? The simple fact is that there is no clear cut guidance provided.
The end result is that the bending case should be evaluated by the astute designer even though local jurisdictional authorities may not require it. Ultimately, it is the responsibility of the designer and the professional engineer verifying the design to ensure the pins are safely designed.
1. Pressure Vessel Handbook 14th Ed., Eugene V. Megyesy, PV Publishing Inc., 2008
2. Pressure Vessel Design Manual 4th Ed., Dennis R. Moss, Elsevier, 2013