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Design Pressure Selection

Process design has requested that a vessel be designed to accommodate a new process. The vessel must be a jacketed vessel. The process is stable and the normal operating pressure of the inner vessel is 425 psi and the normal operating pressure of the jacket is 500 psi. During startup, the inner vessel must be pressurized to 350 psi before the pressure can be applied to the jacket. Once the pressure in the inner vessel reaches 350 psi, the outer chamber is pressurized to 425 psi and then both chambers are slowly pressurized up to their operating pressures while maintaining a differential pressure between chambers of 75 psi or less. During shutdown the process is reversed. The process is relatively stable and computer controlled. Select a set of design pressures for the vessel.

Solution:
The inner vessel is exposed to both internal and external pressure. The maximum is operating external pressure on the inner vessel during startup, normal operation, and shutdown is 75 psi. Therefore, the external design pressure should be 75 psi plus an allowance. Utilizing the previous recommendation:

inner vessel external design pressure = 75 psi + 25 psi = 100 psi.

The maximum internal pressure in the inner vessel must be based on the internal pressure during the startup and the differential pressure during normal operation. In both cases this valve is 350 psi.

inner vessel internal design pressure = 350 psi + (0.10)(350 psi) = 385 psi.

The jacket is subjected to only internal pressure . The maximum internal operating pressure is 500 psi.

jacket internal design pressure = 500 psi + (0.10)(500 psi) = 550 psi

This shows that design pressure and temperature are highly dependent on operating procedures.