Procedure 2-2 External Pressure Design

A = factor A, strain, from ASME Section II, Part D, Subpart 3, dimensionless As = cross-sectional area of stiffener, in.  

Dl = outside diameter of the large end of cone, in. Ds = outside diameter of small end of cone, in. 

E = modulus of elasticity, psi I = actual moment of inertia of stiffener, in.  

I s = required moment of inertia of combined shellring cross section, in.  

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On occasion the ASME Boiler and Pressure Vessel Committee is requested (o provide for a new material chart for external pressure design such as those in Appendix V of Section VIII, Division 1. The SGDE/SCD requires reliable dam upon which to base the construction of charts. The SG is not in a position lo develop or evaluate the requited data. Consequently, the SGDE recommends the following procedures to be followed in providing the SG with adequate and reliable data. 

Division 1. Below the creep range, design stresses are based on one-fourth of the tensile strength or two-thkds of the yield, or 0.2% proof stress. Design procedures are given for typical vessel components under both internal pressure and external pressure. No specific requkements are given for the assessment of fatigue and thermal stresses. 

Tanks that could be subjected to vacuum should be provided with vacuum-breaking valves or be designed for vacuum (external pressure). The ASME Pressure Vessel Code contains design procedures. 

To determine wall thickness and stiffening requirements for straight pipe under external pressure, the procedure outlined in ASME BPV Code Section VIII, Division 1, UG-28 through UG-30 shall be followed, using as the design length, L, the running centerline... 

Step 8 If the computed allowable external pressure is less than the design external pressure, then a decision must be made on how to proceed. Either (a) select a new thickness and start the procedure from the beginning or (b) elect to use stiffening rings to reduce the L dimension. If stiffening rings are to be utilized, then proceed with the following steps. 

For AS ME Code vessels the allowable compressive stress is Factor B. The ASME Code, factor B. considers radius and length but does not consider length unless external pressure is involved. This procedure illustrates other methods of defining critical stress and the allowable buckling stress for vessels during transport and erection as well as equipment not designed to the ASME Code. For example, shell compressive stresses are developed in tall silos and bins due to the side wall friction of the contents on the bin wall. 

Any out-of-round ness after fahricalion of a vessed designed for external pres.sure will redu e the strength of the vessel, llie oul-of-roundness results in increased stress concent rations, and the effect of external pressure is to aggravate the condition. Thus a shell of elliptical shape or circular shell, either dented or with flat spots, is less strong under external pressure than a ves.sei having a true cylindrical shape. The following procedure may be user to determine t he additional stress from elliptical aut-of.rraunilness. 

There will be no stress from intemal or external pressure for the skirt, unlike for the shell of the vessel, but the stresses from dead weight and from the wind or seismic bending moments will he a maximum. The same procedure may be us i- for designing the skirt as for designing the shell, which was described in Chapter 9. Note Subscript b refers to the base of the skirt. 

ASME provides necessary formulae to compute vessel sizes and membrane stresses of the vessel due to internal and external pressures it leaves to the designer to carry out analytical procedures for computing stresses due to other loads including combinations of loads. 

The ASME procedure for the design of cylindrical shells under external pressure is complicated because of the various parameters that must be considered. A summary of the procedure is shown in Fig. 8.12 as an aid to the designer. 

As a prelude to the design of the tube reactor (10), a kinetic study of the phenolysis procedure as a function of temperature was carried out on a larger scale. The equipment used was a stainless steel pressure reactor (Model 4501, Parr Instrument Company, Moline, Illinois). This reactor is fitted with an internal stirrer, an external electric heater, and a continuous sampling device. A mixture of the commercial ammonium lignin sulfonate (668 g) and molten phenol (1000 mL) was sealed into the reactor and heated to the designated temperatures. Approximately 3 hours were needed to heat the reactor from room temperature to 200 °C. A similar period of time was required to cool the reactor and its contents back to 22 °C after completion of a run. After a reaction period nominally lasting 2 hours, the unreacted phenol was steam distilled from the reaction mixture and the amount measured by comparative UV spectroscopy. The results obtained and summarized in Table IV show that a substantial amount of phenol becomes chemically combined with the renewable resource feedstock. 

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