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
Different temperature allowances are used above and below normal operating tempratures. For temperatures between -30 and 345 ⁰C, Turton gives a maximum allowance of 25 ⁰C above maximum operating temperature should be included. Above this, an even higher design allowance is used (Turton pg 1). Towler/UOP gives 50 ⁰F above the maximum operating temperature and -25 ⁰F below the minimum. (Towler/UOP)
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. Metals may become brittle at very low temperatures (Towler/UOP).The minimum design metla temperaure (MDMT) is the lowest temperature that can be expected in the vessel. (Towler)
In specifying the maximum and minimum temperatures, disturbances caused by upstream processes and external factors need to be taken into account. These disturbances may include:transient conditions, upsets, auto-refrigeration, climate, other cooling factors. (Towler pg 8-9, Towler/UOP)
Steel is the most common material used in construction of tanks and pressure vessels. Other construction materials include other alloys, wood, concrete, or fiber-reinforced plastics (some low-pressure applications).
Materials must be chosen that will be able to resist deformation and failure at the process temperature and pressure, and be compatible with the internal material. (Peters, pg 552/ Towler pg 8-9/ Towler/UOP) Other factors for selection include ease of fabrication, availability of parts, and cost. (Towler/UOP)
Nondestructive testing methods are ways of evaluating the integrity of a vessel without compromising it. Inspections need to be carried out for new vessels and regularly once operation begins.
The simplest is a visual inspection for cracks or defects on the surface. It is also the cheapest, requiring only an inspector.
Radiography is used to detect subsurface cracks and defects. It is difficult and expensive, and may require specialized inspectors. It is required by the code in certain cases.
Ultrasonic detection can be used during operation to detect wall thinning.
The ASME BPV Code requires pressure testing with an inspector present before vessels can be approved.
Both hydraulic and pneumatic pressure tests are used. Hydraulic testing is preferred to pneumatic because it is safer.