1.3 Design Codes

The codes and standards dictate the minimum requirements and give general guidance for design and construction; any extension beyond the minimum code requirement will be determined by agreement between the manufacturer and customer.

  • Design Codes for Pressure Vessels: The standard used in North America (and most commonly referenced internationally) is the ASME Boiler and Pressure Vessel Code. It contains twelve sections. Most chemical plant falls within the scope of Section VIII Division 1.

    The design guidelines as given by Bureau of Indian Standards in IS 2825 is stipulated to be used in India.

  • Design Codes for Storage Tanks: American Petroleum Institute’s: API-650 (Older code was API-12C), API-620. Indian Code: IS-803

  • Commonly Used Materials:

    • Specified in terms of UNS / ASTM / IS code numbers.

    • Plate Materials:

      • Mild steel: A 36, A 283, A 515 / A 516 Gr 60/70 (for pressure vessels), IS 2062, IS 2002 (for pressure vessels)

      • Stainless steel: 304, 316, 304L, 316L. L refers to the low carbon steels.

    • Pipe Materials: A 106, IS 1239

    • Forgings: A 105

  • Design Pressure: For vessels under internal pressure, the design pressure (sometimes called maximum allowable working pressure) is taken as the pressure at which the relief device is set. This will normally be 5 to 10% above the normal working pressure.

    Vessels subject to external pressure should be designed to resist the maximum differential pressure that is likely to occur in service. Vessels likely to be subjected to vacuum should be designed for a full negative pressure of 1 bar.

  • The criteria used in establishing the allowable stresses in the ASME code
    Allowable stress is the lowest value of stresses obtained from:

    • 25% of the specified minimum ultimate tensile strength at room or operating temperature,

    • 62.5% of the minimum expected yield strength for 0.2% offset at room or operating temperature.

    • Many engineers use a “factor of safety” of 3 for structural steel and a factor of safety of 6 for gray cast iron, based upon the ultimate strength, when designing structural parts. \[\text{Allowable stress} = \frac{\text{ultimate strength}}{\text{factor of safety}}\]

  • Joint Efficiency

    No radiography 70%
    Spot radiography 85%
    100% radiography 100%


    Joint efficiency is 100% for seamless heads.

  • Thickness calculation using code formulas:

    Circumferential, longitudinal and radial stresses are acting due to internal pressure of the vessel. Thickness calculation is as per the controlling stress acting on the part of the vessel. Simple formulas as given earlier (Eqns.1, 2, & 3) ignores the radial stress. Whereas, Codes recognizes that the radial stress may not be negligible, and adjustments have been made in the appropriate formulas. The following are as per ASME Sec VIII Div 1.

    • Cylindrical Vessels:

      • Circumferential stress (Longitudinal joints) \[t = \frac{PR}{fJ-0.6P} = \frac{PD}{2fJ-1.2P}\]

      • Longitudinal stress (Circumferential joints) \[t = \frac{PR}{2fJ+0.4P} = \frac{PD}{4fJ+0.8P}\]

    • Spherical Vessels: \[t = \frac{PR}{2fJ-0.2P} = \frac{PD}{4fJ-0.4P}\] where \(R\) is the inside radius; \(D\) is the inside diameter.

  • A cylindrical vessel under internal pressure tends to retain its shape in that any out of roundness or dents resulting from shop fabrication or erection tend to be removed when the vessel is placed under internal pressure. Thus any deformation resulting from internal pressure tends to make an imperfect cylinder more cylindrical. However, the opposite is true for imperfect cylindrical vessels under external pressure, and any imperfection will tend to be aggravated with the result of possible collapse of the vessel. For this reason, a given vessel under external pressure in general has a pressure rating only 60% as high as it would have under internal pressure.