Use of ASME Sect VIII Div 2 Pressure Vessel Design

1. Purpose

This procedure outlines the advantages and disadvantages of specifying ASME Section VIII Division 2 for the design and fabrication of pressure vessels. Guidelines are provided to aid in the process of deciding when to specify the use of Division 2. Design Engineer and Equipment Specialist accountabilities related to the specification of Division 2 are described.

2. General

2.1 There are three different versions of the ASME Boiler and Pressure Vessel Code, Section VIII, "Rules for Construction of Pressure Vessels". They are identified as Division 1, Division 2, and Division 3. Division 1 is the most commonly used version and is specified for the majority of Inflection Point Engineering’s applications. Division 2 requirements are more stringent however under certain circumstances it is the preferred version. Division 3 is intended for very high pressure applications (>10,000 psig) and is not applicable to refinery or petrochemical use.

2.2 Division 2 was completely rewritten and fundamentally revised in 2007. Use of the 2004 version is permitted until July 2009, and possibly later. Very few, if any, purchasers are specifying use of the 2007 edition, opting for the familiar 2004 edition instead. Therefore, this procedure, and Inflection Point Engineering’s specifications, continues to be based upon use of the 2004 edition of ASME Section VIII, Division 2.

2.3 The primary advantage of Division 2 is that it allows the use of higher allowable stresses (called stress intensities). Higher allowable stresses result in thinner pressure vessel shells. The savings in material, forming, welding, nozzle installation, shipping, erection, and structure/foundation costs may be quite significant. These savings will accumulate more rapidly for some materials (low chrome, stainless steel) than others (carbon steel). Thinner shells may also have metallurgical benefits, e.g., more consistent properties through the thickness, and fewer and lower residual stresses. However, permitting higher allowable stresses is not without expense. For example, Division 2 requires a rigorous stress analysis that includes the effects of fatigue, thermal gradients, local stress concentrations, welding, transient as well as steady state conditions, etc. Such an analysis is far more detailed than that required by Division 1. Also, Division 2 requires more extensive non-destructive examination (x-ray, ultrasonic, etc.) and more rigorous quality control on material and workmanship.

2.4 The final decision to use Division 2 is often economic, i.e., it is based upon the relative costs and savings. A great deal depends on the individual fabricator's experience with the Division 2 requirements. Occasionally, engineering considerations (e.g., high pressure) will require the improved analysis and quality control of Division 2, in which case Division 2 shall be specified regardless of cost. The specification of Division 2 greatly limits the number of potential fabricators. The expenses of acquiring and maintaining the analytical, fabrication, handling, testing, inspection, and examination capabilities for the large, thick wall equipment most commonly fabricated in accordance with Division 2, as well as obtaining an ASME Division 2 stamp, discourages many fabricators from entering this phase of the market.

3. Criteria

Use the following criteria to determine if Division 2 is appropriate:

3.1 Shell thickness

a. Use Division 1 to estimate the required shell thickness for the design pressure and temperature conditions. The required shell thickness is proportional to both the design pressure and the inside diameter. A thick shell may result from high pressure or large diameter or both. Thickness is also inversely proportional to the allowable stress which is a function of the design temperature. A thick shell results from a low allowable stress, which occurs at elevated temperature.

b. Using the estimated shell thickness determined above, apply the following guidelines to decide whether to specify ASME Section VIII Division 1 or Division 2. These guidelines are based upon information from vessel fabricators regarding the relative economies of Division 1 or Division 2 fabrication.

(1) If the shell is more than 4 inches thick, specify Division 2. Hydrocracking reactors are often in this range.

(2) If the shell is less than 2 inches thick, Division 1 is typically more economical. Most vessels fall into this category. Use Division 2 only if Section 3.2 applies.

(3) For shells between 2 and 4 inches thick, the final decision depends upon the fabricator's experience and costs (except as noted in Section 3.2). Normally the Inflection Point Engineering Project Specification gives the choice of Division 1 or 2.

(4) Sometimes the thickness of other vessels will affect the practicality of specifying Division 2. For example, if one vessel is 2¼ inches thick and all the other vessels are well below 2 inches, use only Division 1. Requiring a specialized Division 2 evaluation would be impractical and the preferred fabricator might not even have a Division 2 stamp.

(5) Figure 1 (based upon a Division 1 allowable stress of 17,000 psi) gives an approximate indication of whether Division 1 or Division 2 shall be specified.

3.2 Internal Design Pressure

When the internal design pressure exceeds 3000 psi, specify Division 2 regardless of the wall thickness. The analysis, quality control, and testing requirements of Division 2 address safety issues and economic concerns related to the great amount of energy released in the event of a failure. Additionally, Division 1 is limited to design pressures of 3000 psi and below (reference ASME Section VIII, Division 1, U-1(d)).

3.3 Design Temperature

Division 2 contains allowable stresses for materials only up to the point where creep criteria begin to govern. This occurs at temperatures above approximately 700F for carbon steel, 900F for low alloy materials, and 1000F for high alloy materials. The analysis rules are also applicable only below the creep range. To obtain a detailed stress analysis and high quality control, Division 2 may be specified at higher temperatures. However, the allowable stresses will be identical to those listed in Division 1, which means there will be no thickness savings.

4. Design Engineer Accountabilities

4.1 Using the design conditions, vessel diameter, and material of construction, determine the approximate vessel thickness based upon the rules and allowable stresses of ASME Section VIII Division 1. Tool ” may be used for this calculation. Applying the guidelines in Section 3, determine if Section VIII Division 2 is likely to be used for the vessel design and fabrication. If not, proceed as with other vessels (this may include the special skirt to vessel junction details of paragraph 5.3).

4.2 If Division 2 may be used for design and fabrication of the pressure vessel, the Design Engineer shall provide the following information to the Equipment Specialist. This is in addition to the information normally provided for pressure vessels.

a. Vessel design life (default is 20 years)

b. The number of planned and accidental pressure cycles that exceed 20% of the design pressure (default is 7 full pressure range cycles and 12 additional cycles exceeding 20% of design pressure per year)

c. Identify any cyclical temperature or pressure loadings. Include the frequency, range, and magnitude of the cycles.

d. The maximum operating temperature differential across the vessel and its approximate distribution (if not uniform) (default is 100°F (56°C)).

e. Nozzles where the internal process fluid operating temperature differs from the vessel's operating temperature (higher or lower) by more than 50°F (28°C). Examples are purged level connections and hydrocracker quench nozzles.

f. Curves of the internal process fluid operating temperature and pressure plotted vs. time for normal start-up, normal shutdown, and any other operating, upset, or emergency cases (e.g., power failure or temperature excursion). See Figures 2, 3, and 4 for example curves. More than three curves may be necessary to describe all of the transient cases that the vessel designer needs to consider. Include the cases upon which the design temperature and pressure are based. These curves, which are uniquely developed for each process unit, are available from the Technology Specialist. Tool , develops the required curves for Hydrocracking and Hydrotreating equipment. Inflection Point Engineering does not provide heat transfer coefficients, therefore, the designer may conservatively use the provided temperatures as the inside surface temperatures. Division 2 of the ASME Code requires that the maximum range of stress intensity be determined based upon all operating conditions to which the vessel may be subjected. Thus, the variation of temperature and pressure with time, based on the actual anticipated operation of the unit, must be specified to the vessel designer. If the curves were not provided, Inflection Point Engineering would be accepting responsibility for the adequacy of a vessel design considering only the design steady state temperature and pressure condition.

5.0 Equipment Specialist Accountabilities

When the Design Engineer has determined that design and fabrication in accordance with ASME Section VIII, Division 2 will be included in the Inflection Point Engineering Project Specification(s), the Equipment Specialist shall perform the following tasks.

5.1 Refer to Inflection Point Engineering Standard Specification 3-17, “Pressure Vessels-ASME Section VIII Division 2” on the vessel sketch.

5.2 Include the operating temperature and pressure vs. time curves for normal start-up, normal shutdown, and all upset conditions in the project specification for the equipment. Place the curves on the pages immediately following the vessel sketch (for reactors, place the curves after the note pages) and include a reference to the curves on the equipment sketch. The curves allow the vessel designer to determine the transient thermal gradients and resultant stresses in the vessel shell.

5.3 Determine if the special skirt to vessel junction detail shown on Inflection Point Engineering Standard Drawing 3-102 is necessary. This detail is used when the shell thickness exceeds 4 inches (100 mm), when the design temperature exceeds 700ºF (370ºC) for carbon or killed carbon steel or 800ºF (425ºC) for low alloy steel, or when the vessel is subjected to cyclic temperature or pressure conditions. If Standard Drawing 3-102 is required, so indicate on the vessel sketch.

5.4 If cyclic design is indicated, include the frequency, range, and magnitude of the cycles on the equipment project specification.

5.5 If the operating temperature of the vessel is not uniform, or if there are nozzles operating significantly hotter or colder than the vessel shell, include this information on the project specification. Normally the temperature differential must exceed 100ºF (56ºC) to be significant unless a lower value is determined per Paragraph 5.6. The vessel designer uses this information to determine thermal gradients and the resultant stresses in the vessel shell. If no information is provided, Inflection Point Engineering Standard Specification 3-17 tells the designer that the shell temperature is essentially uniform.

5.6 If the Design Engineer has indicated a design life greater than 20 years, there are potential start-up / shutdown and / or pressure cycles in excess of the values shown in Paragraph 4.2b, or if the vessel’s shell is made of more than one material, evaluate the need for a fatigue analysis. Use the procedure of ASME Section VIII Division 2, Section AD-160.2 for this evaluation. If a fatigue evaluation is necessary, so indicate on the equipment project specification. When a fatigue analysis can be avoided if the maximum temperature differential between two points (as defined in AD-160.2) is limited to a value less than 100ºF (56ºC) so indicate on the equipment project specification. The vessel designer determines the temperature differentials in the vessel shell considering the information noted in paragraphs 5.2 and 5.5. If the maximum temperature differential is below the threshold value given by Inflection Point Engineering, no fatigue analysis is necessary. Unless a lower value is given in the Inflection Point Engineering Project Specification (see above) the threshold value is 100ºF (56ºC) per Inflection Point Engineering Standard Specification 3-17.

Figure 1 Use of ASME Section VIII Division

Figure 2 Start-up Internal Process Operating Temperature

Normal Start-up Internal Process Operating Pressure

Figure 3 Shutdown Internal Process Operating Temperature

Normal Shutdown Internal Process Operating Pressure

Figure 4 Upset Condition

Internal Process Operating Temperature

Upset Condition Internal Process Operating Pressure