Engineers' Guide to Pressure Equipment - Index of

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Engineers’ Guide to Pressure Equipment
i) Seamless ii) Welded (joint effeciency 3)
iii) The practical desgin
Fig. 3.2 Welded joint efficiency – the principle
3.2.3 Pressure vessel cylinders
Vessel cylinders are usually made from flat plates that are rolled and then
welded along their longitudinal joints. Circumferential joints are used to
attach end pieces (dished ends or ‘heads’) to the cylinder, and to weld
together rolled plates for a long vessel. Weld types and efficiencies usually
differ for longitudinal and circumferential joints. The joint stresses in a
vessel must, therefore, be designed to satisfy both requirements.
Typically, this is expressed in a form something like (see Fig. 3.3).
• Circumferential stress, S c = (pD/2t ) ≤ η � S
where η � = longitudinal joint efficiency and p = pressure
and
• Longitudinal stress, S � = (pD/4t) ≤ η c S
where η c = circumferential joint efficiency and p = pressure
The symbol σ c is sometimes used instead of S c, and σ � instead of S �
p = pressure
In these formulae, most vessel codes take the diameter, D, to be the mean at
the wall mid-surface (D = D i + t), although they often quote the equation
conveniently in terms of D i (the inner diameter). As the circumferential
stress is twice the longitudinal stress, the circumferential strength is usually
the controlling parameter, provided that η c is greater than half η � (which is