c.
Stress Analysis. The above procedure, basically, forms the first
phase of a design by analysis. A configuration has been established and
dimensions specified. Notice that the vessel has been designed to be
"smooth." That is, there are no gusset plates, fillet welds, added
reinforcing plates, etc. An attempt has been made right from the start to
eliminate any sharp notches or other severe highly localized stress areas.
It is to be understood that all welding practice must follow that defined in
Section VIII, Division 2 and also Section IX, "Welding Qualifications" of the
ASME Code and NAVSHIPS 0900-000-1000. Further, to ensure a good design all
welds must be ground smooth and in the case of butt welds, ground smooth and
flush and all imperfections removed. The next step in the design by analysis
procedure is to carry out a detailed stress analysis where necessary, and if
so desired and possible, apply the few areas of the Code where a detailed
analysis is not required. These areas are restricted to nozzle designs
conforming to all of the following:
o
Subject only to internal pressure loading
o
Provisions of Paragraph AD-540.1, 540.2(a) and (b)
o
Figure AD-540.1
o
Provisions of Paragraph 4-600(a), 4-611, 4-612,
4-613, or if experimental data is available for
similar but slightly different nozzle configurations
the provisions of Paragraph 4-622 may be
applied.
(1) Loading Conditions. The only loading conditions applied in
this analysis were the maximum design pressure of 1000 psig and the dead
weight load of the sphere full of seawater. This was done for simplicity.
The designer is cautioned that in the case of an actual vessel, he must
consider all loads specified in the User's Design Specification. These can
include thermal loads, pipe loads and seismic loads among others.
(2) Requirement for a Fatigue Evaluation. Paragraph AD-160.2 re-
quires that all parts of Condition A or all parts of Condition B must be
satisfied for such a vessel to be exempt from a fatigue evaluation. The
first part of Condition A is not met. This part requires that the expected
(design) number of full range pressure cycles including start up and shut
down plus other types of pressure and temperature gradient cycles must not
exceed 1,000 cycles. The pressure design basis of this chamber is 10,000
cycles of pressure from 0 to 1,000 to 0 psig, thus violating this
restriction. Part (a) of Condition B requires that the expected design
number of full range pressure cycles, including start up and shut down, does
not exceed the number of cycles in the applicable fatigue curve for the
material in Appendix 5 corresponding to a Sa value of 3 times the Sm
value for the material at the operating temperature. Sm was previously
established as 23,200 psi. 3 Sm is 69,600 psi. Entering Figure 5-110.1,
shown here as Figure 2-8 with a Sa value of 69,600, it is found that the
number of cycles of permissible operation is approximately 1,700 cycles.
Thus Condition B is not met and the vessel must be evaluated for fatigue.
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