Page 109 - Pressure Vessel Design Manual

90 Pressure Vessel Design Manual

An economic UD ratio is between 1 and 10. L/D ratios Given
greater than 10 may produce the lowest surface-area-to-
volume ratio but should be considered impractical for most V, volume
applications. Obviously plot space is also a consideration in P, pressure
ultimate cost. In general, the higher the pressure the larger C, corrosion allowance
the ratio, and the lower the pressure the lower the ratio. As S, allowable stress
previously stated, the optimum UD ratio for an E, joint efficiency
atmospheric drum is 1. Average pressure vessels will range
between 3 and 5. Method 1
Two procedures are included here and are called Method
1 and Method 2. The two procedures, though similar in 1. Calculate F1.
execution, yield different results. Both methods take into 2. From Fig. 2-58, using F1 and vessel volume, V, deter-
account pressure, corrosion, joint efficiency, and allowable mine the vessel diameter, D.
stress. Even with this much detail, it is impossible to deter- 3. Use D and V to calculate the required length, L.
mine exactly what proportions will yield the lowest overall
cost, since there are many more variables that enter into the
ultimate cost of a vessel. However, determining the lowest
weight is probably the best parameter in achieving the lowest Method 2
cost.
The procedure for determining the optimum WD ratios 1. Calculate Fz.
for the two methods is as follows: 2. From Fig. 2-59 determine WD ratio.
3. From the UD ratio, calculate the diameter, D.
4. Use D and V to calculate the required length, L.

v (cu. ft.) P (PSIG) Method' D (ft) L (ft) t (in.) w (Ib) UD
1500 150 I 1 I 7.5 I 34 I 0.5625 I 20,365 I 4.5 I
I 2 I 8.5 I 23.6 I 0.625 I 20,086 I 2.8 I

300 1 6 53 0.8125 35,703 8.8
2 7.5 31.5 0.8125 28.668 4.2

'Methods are as follows, based on graphs: Method 1: K. Abakians, Hydrocarbon Processing, June 1963. Method 2: S.P. Jawadekar, Chemical Engineering, Dec. 15, 1980.