Title: Pressure Vessels
Author: David Chen
Steward: Fengqi You
Date Presented: January 13, 2014 /Date Revised: January 14, 2014
Codes for pressure vessels can be found in the ASME Boiler and Pressure Vessel Code (ASME BPV code). While there is no formal definition, generally any closed vessel over 150 mm in diameter and that will experience a pressure difference of greater than 0.5 bar can be classified as pressure vessels. Types of equipment that can fit these descriptions include many reactors, separation columns, flash drums, heat exchangers, surge tanks, and storage vessels. Pressure vessels with a wall-thickness:diameter ratio of less than 1:10 can be classified as thin-walled, and the rest, thick-walled.(Towler)
Generally, chemical engineers will not be directly involved in detailed mechanical design of pressure vessels. This will be handled by mechanical engineers with experience in the field. However, chemical engineers will need to understand basic concepts of pressure vessel design in order to estimate costs and communicate specifications to those who will carry out the design (Towler/UOP).
Designs and Codes
Many countries have codes and standards concerning pressure vessels. Compliance is usually legally required. The codes provide guidance on design, materials of construction, fabrication, inspection, and testing. In North America, the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BPV Code) is used. There are twelve sections, and section VIII has three subdivisions. The section titles are listed below. Other sets of codes exist for storage tanks, fittings, and piping. It is important to always use the most recent revisions in design. (Towler 3-5)
TABLE "American Society of Mechanical Engineers Boiler and Pressure Vessel Design Codes"
I Rules for construction of power boilers
III Nuclear power plant components
IV Rules for construction of heating boilers
V Nondestructive examination
VI Recommended rules for the care and operation of heating boilers
VII Recommended guidelines for the care of power boilers
VIII Rules for the construction of pressure vessels
D2 Alternative rules
D3 Alternative rules for the construction of high pressure vessels
IX Welding and brazing qualifications
X Fiber-reinforced plastic vessels
XI Rules for in service inspection of nuclear power plant components
XII Rules for construction and continued service of transport tanks
Between -30 and 345 ⁰C an allowance of 25 ⁰C above maximum operating temperature should be included. Above this, an even higher design allowance is used (Turton pg 1). Maximum allowable stress is highly dependent on temperature, because metals weaken with increasing temperature. The vessel should not operate at higher temperature than the highest at which the maximum allowable stress was evaluated.
There is also a minimum temperature for which the vessel can be guaranteed to operate safely. The minimum esign metla temperaure (MDMT) is the lowest temperature that can be expected in the vessel. In specifying the maximum and minimum temperatures, disturbances caused by upstream processes need to be taken into account.