Design strength , pressure vessels

Step 5 Since the seismic design for pressure vessels is based on allowable stress rather than ultimate strength, the base shear may be reduced by a factor of 1.4. 

Two basic modes of failure are assumed for the design of pressure vessels. These are (a) elastic failure, governed by the theory of elasticity and (b) plastic failure, governed by the theory of plasticity. Except for thick-walled pressure vessels, elastic failure is assumed. When the material is stretched beyond the elastic limit, excessive plastic deformation or rupture is expected. The relevant material properties are the yield strength and ultimate strength. In real vessels we have a multiaxial stress situation, where the failure is not governed by the individual components of stress but by some combination of all stress components. 

In the design of process vessels and pressure equipment, two basic modes of failure may be assumed elastic failure based on the theory of elasticity and plastic failure based on the theory of plasticity. Except for thick-walled vessels, elastic failure is usually assumed for the design of pressure vessels. It is considered to occur when the elastic limit of the material is reached. Beyond this limit, excessive deformation or rupture is expected. These limits are usually measured in terms of tensile strength, yield strength, and, to some degree, rupture strength. 

A difference between tank cars and most pressure vessels is that tank cars are designed in terms of the theoretical ultimate or bursting strength of the tank. The test pressure is usually 40 percent of the bursting pressure (sometimes less). The safety valves are set at 75 percent of the test pressure. Thus, the maximum operating pressure is usually 30 percent of the bursting pressure. This gives a nominal factor of safety of 3.3, compared with 4.0 for Division 1 of the ASME Pressure Vessel Code. 

Brittle fracture is probably the most insidious type of pressure-vessel failure. Without brittle fracture, a pressure vessel could be pressurized approximately to its ultimate strength before failure. With brittle behavior some vessels have failed well below their design pressures (which are about 25 percent of the theoretical bursting pressures). In order to reduce the possibility of brittle behavior. Division 2 and Sec. Ill require impac t tests. 

For the EPR design, the ASME pressure vessel code requires design to be done at two-thirds of the alloy s yield strength (see Fig. 

For EPSR design, the stress level to contain an explosion is set at the yield strength, a design factor of 1. Thus, for an alloy, the design stress level would be about 1.5 times the ASME code design stress. So a pressure vessel rated at 6 bar for the ASME code (EPR) would have an EPSR rating of 9 bar. 

The national pressure vessel codes and standards require that all pressure vessels be subjected to a pressure test to prove the integrity of the finished vessel. A hydraulic test is normally carried out, but a pneumatic test can be substituted under circumstances where the use of a liquid for testing is not practical. Hydraulic tests are safer because only a small amount of energy is stored in the compressed liquid. A standard pressure test is used when the required thickness of the vessel parts can be calculated in accordance with the particular code or standard. The vessel is tested at a pressure above the design pressure, typically 25 to 30 per cent. The test pressure is adjusted to allow for the difference in strength of the vessel material at the test temperature compared with the design temperature, and for any corrosion allowance. 

Many users have reported satisfactory performance of annealed or normalized and tempered steels produced before 1969, as shown in Figure 1. These steels have been used for pressure-retaining equipment at design stress levels allowed by the 1969 or earlier editions of commonly-accepted codes (such codes include the ASME Code, Section Vlli, Division 1 the standards of the American National Standards Institute and, for the lower-strength materials, those of Deutsche Industrie-Normen). However, pressure vessels in hydrogen service have also been constructed using the higher allowable stresses permitted in either Section VHI, Division 2, or modifications of Section III of the ASME Code. Quenched and tempered or normalized and tempered steels have normally... 

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