Asme Div 1 Vs Div 2 211t59

ASME B&PV CODE SECTION VIII PRESSURE VESSEL DESIGN A COMPARISON – DIVISION 1 versus DIVISION 2 Dwight V. Smith Colt Engineering Corporation 400, 10201 Southport Rd., S.W. Calgary, Alberta, Canada T2W 4X9 Tel: 403-258-8000 Fax: 403-258-5899 E-Mail: [email protected]

ABSTRACT Historically, the ASME B&PV Code, Section VIII, Division 2, Alternative Rules for Construction of Pressure Vessels (Div.2), ASME [1], was usually considered applicable only for large, thick walled pressure vessels. Otherwise, ASME B&PV Code, Section VIII, Division 1, Rules for Construction of Pressure Vessels (Div. 1), ASME [2], was typically applied. A case can also be made for the application of the Div. 2 Code Section for some vessels of lesser thicknesses. Each vessel should be closely evaluated to ensure the appropriate choice of Code Section to apply. This paper discusses some of the differences between the Div. 1 and Div. 2 Code Sections, summarizes some of the main design requirements of Div. 2, and presents a case for considering its use for design conditions not usually considered by some, to be appropriate for the application of Div. 2 of the ASME Code. INTRODUCTION In the past, it was generally accepted that Div. 2 designs could result in considerable cost savings, Lengsfeld [3], through reduced material thicknesses only for large, high pressure ASME Code vessels. This was primarily because of the higher allowable design stresses permitted for Div. 2, ASME [4], resulting in thinner materials. Savings were realized by lower costs for material, fabrication and associated infrastructure. The reduced costs could be substantial where thick materials were involved. Thus, only where cost savings were large and obvious, was there a tendency to utilize a Div. 2 design. Given an increasingly competitive market place, and considering the potential for cost savings, Div. 2 designs for lesser thicknesses may be prudent. Although Div. 1 specifies relatively lower allowable design stresses, in recognition of advances over the years in steel making technology and non-destructive examination techniques, Div. 1 has recently been revised to include a design factor of 3.5, as opposed to 4.0. As a result, the higher Div. 1 allowable design

stresses now permitted, reduce the thickness advantage of Div. 2 designs, which utilize a design factor of 3.0. However, a current in-progress ASME review of Div. 2, may also result in higher allowable design stresses in future editions of that Code, creating additional opportunities for increased economy through use of Div. 2 vessel designs. Div. 1 stresses reflect the simplified design methods of Div. 1 which are commonly referred to as ‘design by rules’, as opposed to Div. 2, where engineering design is referred to as ‘design by analysis’. Div. 2 recognizes the technical progress made over time, Canonico [5], with respect to design methods, material manufacture and non-destructive examination techniques, which permit the use of higher allowable design stresses consistent with the following: • combined stresses under all loading conditions are calculated more accurately • vessel materials undergo more extensive examination and testing • welded ts are subjected to increased non-destructive examination Div. 2 generally dictates a more rigorous engineering design approach, however, it does provide simplified rules for design in Part AD. The vessel designer must utilize these rules, as applicable, in which case some aspects of the design may not be much more complex than that of a Div. 1 design. Although a Div. 2 design requires additional engineering, including Fatigue Analysis evaluation, technical documentation and inspection, as well as invoking certain in-service restrictions, the use of thinner materials with the associated cost savings can easily offset the additional costs of these items. When the additional requirements of a Div. 2 design are closely evaluated, it is apparent that they are not necessarily difficult or expensive to implement. Finally, the availability of computer software programs today allow for ease of design of either Div. 1 or Div. 2 pressure vessels.

In proposing a Div. 2 design for a given vessel, the end must be consulted and the applicable Jurisdictional requirements must be understood. The requirements for monitoring, for example of cyclic conditions, as well as for future inspection, can add costs that are not acceptable to some end s. In addition, some Jurisdictions stipulate more stringent requirements regarding field repairs to Div. 2 vessels. For example, detailed analysis and repair procedure preparation and approvals can add significantly to scheduled shutdown times. A given Jurisdiction may also have more stringent registration requirements, such as stating that the design firm and/or the design engineer be ed in that Jurisdiction. The above should be clarified prior to initiating a Div. 2 design proposal. It should be noted that the advantages of a Div. 2 design for SA516-Gr 70 materials are most apparent for design temperatures of 300°F or less. At higher temperatures, the allowable Div. 2 design stresses for SA516-Gr 70 materials are lower, where those for Div. 1 are fairly consistent up to much higher temperatures. Some design condition scenarios can actually result in a Div. 1 design thickness being less than that of a Div. 2 design. This paper relates primarily to SA516-Gr 70 materials. Other types, such as high strength materials may provide much greater economy through the use of a Div. 2 design. It should not be assumed that, because a particular vessel design does not require what is traditionally considered ‘heavy wall’ material, that a Div. 2 design would not be economically advantageous. Each case should be individually evaluated. Finally, although a Div. 2 design may be considered for reasons of economy, Rapoport [6], certain specific design conditions may also dictate its use. These include: • cyclic temperatures and pressures • severe operating temperature gradients • complex geometrical configurations REQUIREMENTS FOR ASME SECTION VIII, DIV. 2 ’s Design Specification A “’s Design Specification” is required for vessels designed to Div. 2. This document is usually prepared by the engineering company on behalf of the end , and contains details such as the operating and design conditions, materials of construction, Fatigue Analysis evaluation, method of and location of installation, along with reference to the fabrication drawings and the process design data. The Specification is issued to ensure compliance with the requirements of the latest applicable Edition and Addenda of the ASME Code, Section VIII, Div. 2 and must be stamped by a ed Engineer. This document is not usually complicated or expensive to prepare. Manufacturer’s Design Report A “Manufacturer’s Design Report” is also required. This document is prepared by the Manufacturer and contains design and fabrication details/drawings related to the vessel. The information in the report is a compilation of vessel related data that already exists as a result of the design and manufacture of the vessel. This information is also required by most Clients for a Div. 1 vessel. This document also requires a ed Engineer’s stamp and Fabricators of Div. 2 vessels usually have qualified Engineering personnel on staff. A Fabricator licensed and experienced in the design and fabrication

of Div. 2 vessels will not have problems generating this document, nor should it add significant cost. Fatigue Analysis Although Fatigue Analysis, where required, can be expensive, many vessels consist of conventional carbon steel materials with simple geometric configurations, where Fatigue Analysis is not required. The decision as to whether or not a Fatigue Analysis is required for a particular vessel involves a number of criteria and is documented in the ’s Design Specification. Even in cases where an analysis is required, the cost should be evaluated relative to savings in other areas, as an analysis is not necessarily overly complex and may be readily carried out by Engineers experienced in this area. It should be noted that a Fatigue Analysis may also be required for a Div. 1 vessel. Inspection Inspection requirements for a Div. 1 vessel may differ little, if at all, from those invoked by Div. 2. Many industry specifications, such as those for vessels in sour service, stipulate inspection requirements exceeding those of Div. 1. In some cases the specified inspection requirements even exceed those of Div. 2. Therefore, the engineering company, or end specified inspection details, for a particular vessel, must be evaluated against the requirements of the applicable ASME Code Division, to assist in the decision as to the appropriate Code Division to apply. PURCHASING/FABRICATION The fabrication costs for a Div. 2 vessel can actually be less than those for Div. 1. The thinner material requirements may result in reduced material purchase price, as well as a reduction in preparation, handling and welding costs. INSTALLATION/INFRASTRUCTURE/SHIPPING COSTS Because of the reduced material thickness and weight, installation and infrastructure costs can also be reduced. Therefore, the total installed cost of a vessel may be reduced further through savings related to concrete foundations, structural steel and lifting cranes. Shipping costs should also be lower, and in fact, fabricators who would not otherwise be able manufacture or ship a vessel designed to Div. 1, due to weight restrictions, may otherwise be able to provide a quotation for Div. 2 vessel with materials of lesser thicknesses. A WORKABLE EXAMPLE Following is an example of a vessel that historically, might not be considered as a candidate for a Div. 2 design. (ref. Table 1, “WORKABLE EXAMPLE PARAMETERS” and Cost & Thickness Summary Graphs, Figures 1 and 2, respectively): Design Conditions: Material: SA 516-70 Pressure: 450 psi Temperature: 300°F Heads: 2:1 SE

ID: 14 ft. Length: 100 ft. Jeff: 1.0 CA: 0.1250 in.

Div. 1

Allowable Stress (psi)

Calc t (in.)

Actual t (in.)

20,000

2.041

2.0625

Calc ∆t (in.)

0.2190 Div.2

22,500

1.822

1.8750

* Cost savings based on $2.50/fabricated lb.

Actual ∆t (in.)

Calc Wt (lbs)

Actual Wt (lbs)

482,000

487,100

0.1875 430,421

Calc ∆wt (lbs)

Actual ∆wt (lbs)

Calc Cost Saving*

Actual Cost Saving*

51,579

44,100

$128,947.00

$110,250.00

443,000

TABLE 1 WORKABLE EXAMPLE PARAMETERS

DISCUSSION OF WORKABLE EXAMPLE Table 1 shows a comparison of Div. 1 versus Div. 2, in of the differences in calculated thicknesses (Calc ∆t) and the differences in actual thicknesses (Actual ∆t). Actual thickness (t) values represent commonly commercially available material thicknesses. Similarly, the corresponding differences in calculated and actual weights, are also shown. Finally, the resulting calculated and actual cost savings are indicated. Figure 1, “Vessel Cost Summary”, shows the variation in cost relative to three factors; vessel length, vessel diameter and the Code Div. utilized. It can be seen that, as the diameter for a given length increases, the Div. 2 cost benefit also increases. Further, as the vessel length increases, the Div. 2 cost benefit also increases. This is because, as the vessel length increases, the vessel weight increase is greater for a Div. 1 design, than for a Div. 2 design. Greater weight corresponds to increased cost. Figure 2, “Vessel Thickness Summary”, shows the variation in thickness relative to three factors; diameter, pressure and Code Div. utilized. It can be seen that as the pressure for a given diameter increases, the thickness also increases. Further, as the vessel diameter increases, the thickness benefit (cost reduction) related to a Div. 2 design also increases. The data shows that even without considering reduced infrastructure requirements, the capital cost savings can be significant by utilizing a Div. 2 design. In this example, any additional requirements invoked by a Div. 2 design, might be justified from a cost perspective, even allowing for additional costs where a Vendor quotes higher pricing for a Div. 2 engineering design. It should be noted however that the number of qualified manufacturers for Div. 2 vessel work, is much lower than that for Div. 1. This means that a less competitive environment may exist for Div. 2 vessel manufacture. Smaller vessels or other design conditions may result in less cost savings per unit. However, individual evaluation of each case will maximize commercial benefits. This is particularly true where a number of vessels are involved, where the cumulative cost savings can be significant. In the example presented here, if two vessels were being purchased, the cost savings could approach a quarter of a million dollars, even without considering infrastructure cost savings. SUMMARY & CONCLUSIONS In addition to large, thick pressure vessels, in some cases significant cost savings may be realized by utilizing an ASME Code, Div. 2 design for large carbon steel vessels at lower design pressures/temperatures, with thinner materials. This is primarily true for design temperatures of 300°F or less. Although a Div. 2 design requires additional engineering, including Fatigue Analysis evaluation, technical documentation and inspection, as well as invoking restrictions on future field

repairs and monitoring, the use of thinner materials can offset the additional costs of these items. When the additional requirements of a Div. 2 design are closely evaluated, it is apparent that they are not necessarily difficult or expensive to implement. Lower weights and thinner materials should reduce fabrication and shipping costs, as well as costs for installation and infrastructure. Fabricators who would not otherwise be able to provide a quotation, due to inability to manufacture or ship a vessel designed to Div. 1, because of weight restrictions, may be able to provide a quotation. For certain pressure vessels, the use of an ASME Code, Section VIII, Div. 2 design can result in substantial cost savings, in of total installed cost. This is particularly true, for example, where large vessels will be field fabricated. However, even smaller, lower pressure vessels to be shop fabricated can result in significant cost savings, particularly on a cumulative basis. Each case should be evaluated on an individual basis to determine the most cost effective design Code, and where preliminary calculations indicate that a Div. 2 design may result in significant cost savings, Vendors should be requested to provide costs for both a Div. 1 and a Div. 2 option. REFERENCES 1. ASME International, ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, 2001 Edition, New York 2. ASME International, ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, 2001 Edition, New York 3. Lengsfeld M., Holman R., Lengsfeld P., 1995 “Economic Advantages of Division 2 Design For Vessels Per ASME Code Section VIII”, PVP 313-2, International Pressure Vessels and Piping Codes and Standards: Volume 2 – Current Perspectives ASME 1995 4. ASME International, ASME Boiler and Pressure Vessel Code, Section II, Part D, 2001 Edition, New York 5. Canonico D. A., 1999, “Adjusting the Boiler Code”, An article based on an address delivered to the 68th General t Meeting of the National Board of Boiler and Pressure Vessel Inspectors and the ASME Boiler and Pressure Vessel Code 6. Rapoport A., 1990, “ASME Code, Section VIII, Div.2 & its Comparison to Div.1”, Colt Engineering Corporation, Calgary, Alberta