Page 192 - Pressure Vessel Design Manual

170 Pressure Vessel Design Manual

Types of Stresses and Allowables
e Ss and Slo< 1.5s and 0.9Fy: circunzferential bending at
~
e SI to S4: longitidinal bending. lzorn of saddle.
Tension: SI, Ss, or 54 + (T, c SE
1. If a wear plate is used, t, may be taken as t, + t,v pro-
Compression: S2, S?, or S4 - ne < S, viding the wear plate extends W1O above the horn of
the saddle. Stresses must also be checked at the top of
where S,=factor "B" or S or t,E1/16r
the wear plate.
whichever is less. 2. If stresses at the horn of the saddle are excessive:
a. Add a wear plate.
1. Compressive stress is not significant where RJt < 200 b. Increase contact angle 8.
and the vessel is designed for internal pressure only. c. Move saddles toward heads, A < R.
2. When longitudinal bending at midspan is excessive, d. Add stiffening rings.
move saddles away from heads; however, do not
exceed A > 0.2 L. e S12 < 0.5F, or 1.5 S: circumferential compressitie stress.
3. When longitudinal bending at saddles is excessive,
move saddles toward heads. 1. If a wear plate is used, t, may be taken as t, + t,v, pro-
4. If longitudinal bending is excessive at both saddles and viding the width of the wear plate is at least
midspan, add stiffening rings. If stresses are still exces- b + 1.56fi.
sive, increase shell thickness.
2. If the shell is unstiffened the maxinlum stress occurs at
the horn of the saddle.
e S5 to Sg < 0.8s: tangential shear. 3. If the shell is stiffened the maximum hoop compression
occurs at the bottom of the shell.
1. Tangential shear is not combined with other stresses. 4. If stresses are excessive add stiffening rings.
2. If a wear plate is used, t, may be taken as t, + t%", pro-
viding the wear plate extends lU10 above the horn of e ( + )SIB + g4 < 1.5 S: circumferential tension stress-shell
the saddle. st if ened.
3. If the shell is unstiffened, the maximum tangential e ( - )S13 - us < 0.5F~ circumferential compression stress-
shear stress occurs at the horn of the saddle. shell stifled.
4. If the shell is stiffened, the maximum tangential shear 0 ( - )SI1 - (T~ 0.9F,: circumferential compression stress in
occurs at the equator. <
5. When tangential shear stress is excessive, move saddles stifening ring.
toward heads, A 5 0.5 R, add rings, or increase shell
thickness. Procedure for Locating Saddles
6. When stiffening rings are used, the shell-to-ring weld
must be designed to be adequate to resist the tangential Trial 1: Set A = 0.2 L and 8 = 120" and check stress at the
shear as follows: horn of the saddle, Sg or SIo. This stress will govern for
most vessels except for those with large WR ratios.
Q lb allowable shear Trial 2: Increase saddle angle 6 to 150" and recheck stresses
St=-: < at horn or saddle, Sg or Slo.
nr in. circumference in. of weld Trial 3: Move saddles near heads (A = W2) and return 8 to
120". This will take advantage of stiffness provided by the
Sll + 01~ < 1.2.5 SE: additional stress in head. heads and will also induce additional stresses in the heads.
Compute stresses S4, Sa, and Sg or S~O. A wear plate may
be used to reduce the stresses at the horn or saddle when
1. Sll is a shear stress that is additive to the hoop stress in the saddles are near the heads (A < W2) and the wear
the head and occurs whenever the saddles are plate extends W1O above the horn of the saddle.
located close to the heads, Ai0.5 R. Due to their Trial 4: Increase the saddle angle to 150" and recheck
close proximity the shear of the saddle extends into stresses S4, Sg, and Sg or SI(). Increase the saddle angle
the head. progressively to a maximum of 168" to reduce stresses.
2. If stress in the head is excessive, move saddles away Trial 5: Move saddles to A=0.2L and 8=120" and design
from heads, increase head thickness, or add stiffening ring stiffeners in the plane of the saddles using the equa-
rings. tions for SI,? and s14 (see Note 7).