ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
ASME Section I and Section VIII Fundamentals – Part 2
The formulae in ASME Section I and Section VIII are used to determine the minimum required thickness and design pressure of piping, tubes, drums and headers using the Maximum Allowable Working Pressure (MAWP). However, Paragraph UG-31 states, that these formulae may be also used for calculating wall thickness of tubes and pipes under internal pressure.
- The ASME Boiler Code Section I, as well as Section VIII, requires longitudinal and circumferential butt joints to be examined by full radiograph.
- When the vessel design is required fully radiographed longitudinal butt-welded joint, the cylindrical shell will have a joint efficiency factor (E = 1.0). This factor corresponds to a safety factor (or material quality factor) of 3.5 in the parent metal.
- When the vessel design is required non-radiographed longitudinal butt-welded joints the vessel will have a joint efficiency factor (E = 0.7, which corresponds to a safety factor of 0.5 in., resulting in an increase of 43% in the thickness of the plates.
Pressure Vessels Maximum Allowable Stress Values:
- The maximum allowable stress values to be used in the calculation of the vessel’s wall thickness are given in the ASME Code for many different materials. These stress values are a function of temperature.
• Division 1: governs the design by Rules, is less stringent from the standpoint of certain design details and inspection procedures, and thus incorporates a higher safety factor of 3.5. For example, if a 60,000 psi tensile strength material is used, the Maximum Allowable Stress Value is 17,142 psi.
• Division 2: governs the design by Analysis and incorporates a lower safety factor of 2.5. Thus, the maximum allowable stress value for a 60,000 psi tensile strength material will become 24,000 psi.
• Many companies require that all their pressure vessels be constructed in accordance with Division 2 because of the more exacting standards. Others find that they can purchase less expensive vessels by allowing manufacturers the choice of either Division 1 or Division 2.
• Normally, manufacturers will choose Division 1 for low-pressure vessels and Division 2 for high pressure vessels. The maximum allowable stress values at normal temperature range for the steel plates most commonly used in the fabrication of pressure vessels are given in Table above.
ASME SECTION I - POWER BOILERS: Types, Design, Fabrication, Inspection & Repair:
- Provides requirements for construction of power, electric, and miniature boilers; high temperature water boilers used in stationary service; and power boilers used in locomotive, portable, and traction service.
- Rules allow the use of the V, A, M, PP, S and E symbol stamps. The rules are applicable to boilers in which the steam is generated at pressures exceeding 15 psig, and high temperature water boilers for operation at pressures exceeding 160 psig and/or temperatures exceeding 250 °F.
- This code covers Power Boiler Superheaters, Economizers, and other pressure parts connected directly to the boiler without intervening valves are considered as part of the scope of Section 1.
ASME SECTION I – Boiler Tubes up to and including 5 inches O.D. (125 mm):
- a) The minimum required thickness, according to ASME paragraph PG-27.2.1, use equation below:
- b) To calculate the Maximum Allowable Working Pressure (MAWP):
- t = Minimum Design Wall Thickness (in)
- P = Design Pressure (psi)
- D = Tube Outside Diameter (in)
- e = Thickness Factor (0.04 for expanded tubes; 0 = for strength welded tubes)
- S = Maximum Allowable Stress According to ASME Section II, Table 1A
Example 1 - Boiler Tube:
- Calculate the minimum required wall thickness of a water tube boiler 2.75 in O.D., strength welded (E, or, e = 0) into place in a boiler. The tube has an average wall temperature of 650°F. The Maximum Allowable Working Pressure (MAWP) is 580 psi gauge. Material is carbon steel SA-192.
- Note: Before starting calculations check the correct stress table in ASME Section II, Table 1A:
- Consider SA-192 = 11,800 psi – allowable stress – Div. 1.
For tubing up to and including 5 in O.D., use equation 1.1 above.
- P = [580 psi]
- D = [2.75 in]
- e = 0 (strength welded)
- S = [11,800 psi] at [650°F])
- t = 2.75 x 580 + 0.005 (2.75) + 0 2 (11,800) + 580
- t = 0.079 in.
- Note: Where the manufacturing processes produce only standard plate thickness, so should be used 1/8 in (3.2 mm) minimum.
ASME SECTION I – Piping, Drums, and Headers:
- The following formulae are found in ASME Section I, paragraph PG-27.2.2. The information for piping, drums, or headers may be given with either the inside I or outside D measurements.
- a) Using the outside diameter:
- b) Using the inside radius:
Example 2 – Steam Piping:
- Calculate the required minimum thickness of a seamless steam piping at a pressure of 900 psi gauge and a temperature of 700°F. The piping is 10.77 in O.D., (10 inches nominal) plain end; the material is SA-335 – P1, alloy steel. Allow a manufacturer’s tolerance allowance of 12.5%.
Note: Before starting calculations check the material stress table in ASME Section II, Table 1A: SA-335 – P1 = 13,800 psi – allowable stress – Div.1. Use equation 2.1:
- P = [900 psi]
- D = [10.77 in]
- C = 0
- S = [13,800] – (SA-335 – P1 alloy steel at 700°F)
- E = 1.0
- y = 0.4 (Ferritic steel less than 900°F)
- t = 900 x 10.77………. + 0 = 2(13,800)(1.0) + 2(0.4)(900)
- • t = 0.34 in.
Example 3 – Maximum Allowable Working Pressure (MAWP):
Calculate the Maximum Allowable Working Pressure (MAWP) for a seamless steel pipe of material SA- 209-T1. The outside pipe diameter is 12.75 in. (nominal diam. 12 in.) with a wall thickness of 0.46 in. The operating temperature is 850°F. The pipe is plain ended.
- D = 12.75 in (outside diameter);
- t = 0.46 in;
- C = 0 (3 to 4 inches nominal and larger);
- E = 1.0 (seamless pipe as per PG-9.1);
- y = 0.4 (austenitic steel at 850°F).
- SA-209-T1 at 850°F, (Section II, Table 1A, Div.1), S = 13,100 psi. Use equation 2.2:
- P = 2SE (t – C) = D – (2y) (t – C)
- P = 2 (13,100) (1.0) x (0.46 – 0) = 12.75 – (2 x 0.4) (0.46 – 0)
- P = 26,200 x 0.46 = 12.75 – 0.368 •
- P = 973 psi
Source: ASME International Boiler & Pressure Vessel Code
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