Page 33 - Pressure Vessel Design Manual

General Design 19

PROCEDURE 2-2

EXTERNAL PRESSURE DESIGN

Notation the critical factor rather than material strength. Failures can
~~ ~ occur suddenly, by collapse of the component.
A =factor “A,” strain, from ASME Section TI, Part External pressure can be caused in pressure vessels by a
D, Subpart 3, dimensionless variety of conditions and circumstances. The design pressure
A, = cross-sectional area of stiffener, in.2 may be less than atmospheric due to condensing gas or
R = factor “B,” allowable compressive stress, from steam. Often refineries and chemical plants design all of
ASME Section 11, Part D, Subpart 3, psi their vessels for some amount of external pressure, regarcl-
D = inside diameter of cylinder, in. less of the intended service, to allow fbr steam cleaning and
Do =outside diameter of cylinder, in. the effects of the condensing steam. Other vessels are in
]I1, = outside diameter of the large end of cone, in. vacuum service by nature of venturi devices or connection
D, = outside diameter of small end of cone, in. to a vacuum pump. Vacuums can be pulled inadvertently by
E = modulus of elasticity, psi failure to vent a vessel during draining, or from improperly
I = actual moment of inertia of stiffener, in. 4 sized vents.
I, = required moment of inertia of stiffener, in.4 External pressure can also be created when vessels are
I: = required moment of inertia of combined shell- jacketed or when components are within iririltichairibererl
ring cross section, in. 4 vessels. Often these conditions can be many times greater
L, = for cylinders-the design length for external than atmospheric pressure.
pressure, including k the depth of heads, in. When vessels are designed for bot11 internal arid external
For cones-the design length for external pres- pressure, it is common practice to first determine the shell
snre (see Figures 2-lb and 2-lc), in. thickness required for the internal pressure condition, then
L,, = equivalent length of conical section, in. check that thickness for the maximum allowable external
L, = length between stiffeners, in. pressure. If the design is not adequate then a decision is
I,, - T = length of straight portion of shell, tangent to made to either bump up the shell thickness to the next
tangent, in. thickness of plate available, or add stiffening rings to
P = design internal pressure, psi reduce the “L’ dimension. If the option of adding stiffening
P;, = allowable external pressure, psi rings is selected, then the spacing can be determined to suit
P, = design external pressure, psi the vessel configuration.
R,, = outside radius of spheres and hemispheres, Neither increasing the shell thickness to remove stiffening
crown radius of torispherical heads, in. rings nor using the thinnest shell with the Inaximum number
t =thickness of cylinder, head or conical section, in. of stiffeners is economical. The optimum solution lies some-
t,, =equivalent thickness of cone, in. where between these two extremes. Typically, the utilization
c( =half apex angle of cone, degrees of rings with a spacing of 2D for vessel diairictcrs up to abont
eight feet in diameter and a ring spacing of approximately
“D” for diameters greater than eight feet, provides an eco-
Unlike vessels which are designed for internal pressure nomical solution.
alone, there is no single formula, or unique design, which The design of the stiffeners themselves is also a trial and
fits the external pressure condition. Instead, there is a range error procedure. The first trial will be quite close if the old
to
of options a~ail~ble the designer which can satisfy the APT-ASME formula is used. The forinula is its follows:
solution of the design. The thickness of the cylinder is only
one part of the design. Other factors which affect the design 0.16D~P,LS
are the length of cylinder and the use, size, and spacing of Is = E
stiffening rings. Designing vessels for external pressure is an
iterative procedure. First, a design is selected with all of the Stiffeners should never be located over circurnferentlal
variables included, then the design is checked to determine weld seams. If properly spaced they may also double as insu-
if it is adequate. If inadequate, the procedure is repeated lation support rings. Vacuum stiffeners, if coinbined with
until an acceptable design is reached. other stiffening rings, such as cone reinforcement rings or
Vessels subject to external pressure may fail at well below saddle stiffeners on horizontal vessels, must be designed for
the yield strength of the material. The geometry of the part is the combined condition, not each independently. If at all