Q. Pressure and Temperature in Unfired pressure vessels?
The design pressures and coincident maximum and minimum metal temperature, shall be determined by carefully considering all operating phases (including liquid head) that the vessel may experience during the specified project life, such as:
- Initial Startup
- Normal operations
- Temporary operations
- Emergency shutdown
- Emergency operations
- Normal shutdown
- Startup following a turnaround or an emergency shutdown
- Cleaning, steam out and decontamination
- Regeneration
- Upset conditions
- Environmental restraints on relief venting
The margin above the maximum anticipated operating pressure selected to establish the design pressure and coincident maximum metal temperature, must be carefully considered for each vessel component as a function of the overall objective, with respect to pressure relief, coupled with the uncertainties in determining what actual pressures will be developed. For example, where minimization of severely flammable or acutely toxic environmental hazards is a controlling design requirement, the establishment of a design pressure and associated Maximum Allowable Working Pressure (MAWP) that will provide containment without actuation of the pressure relief device, may be a consideration.
This margin is also dependent upon the operational characteristics of the pressure relief device. For example, when the maximum anticipated operating pressure of a gas/vapour service can be identified with confidence, and metal-seated, direct spring-operated valves will be used, the design pressure is frequently established by dividing the maximum anticipated operating pressure by 0.90. However, when a pilot operated pressure relief device is used, the design pressure is sometimes established by dividing the maximum anticipated operating pressure by a factor as high as 0.98. option. Note that prior jurisdictional acceptance may be required, and that this Code Case Number shall be shown on the Manufacturer's Data Report. Likewise, with permission from the authority having legal jurisdiction over the installation of pressure vessels (should one exist), the advantages of using the provisions of Code Case 2203, entitled ‘Omission of Lifting Device Requirements for Pressure Relief Valves on Air, Water over 1400F, or Steam Service', should be considered. Expiry dates of each of these code cases should be checked.