DESIGN OF PRESSURE VESSEL TO CODE SPECIFICATION

American, Indian, British, Japanese, German and many other codes are available for design of pressure vessels. However the internationally accepted pressure vessel code is American Society Of Mechanical Engineers (ASME).

Various codes governing the procedures for the design for fabrication, inspection, testing, and operation of pressure vessels have been developed, partly as a safety measure. These procedure furnish standards by which, any state can be assured of the safety of the pressure vessel installed within its boundaries. The code used for unfired pressure vessel is section 8 of the ASME boiler and pressure vessel code. It is usually necessary that the pressure vessel equipment be designed to a specific code in order to obtain insurances on the plant in which the vessel is to be used. Regardless of the method of design, pressure vessels within the limits of the ASME code specifications are usually checked against the specifications.

DEVELOPMENT AND SCOPE OF ASME CODE

In 1911, American society of mechanical engineers, established a committee to formulate standard specifications for the construction of steam boilers and other pressure vessels. This committee reviewed the existing Massachusetts and Ohio rules and conducted an extensive survey among superintends of inspecting departments, Engineers, fabricators and boiler operators. A number of preliminary reports were issued and revised. A final draft was prepared in 1914 and was approved as a code and copyrighted in 1915.
The introduction to the code started that public hearings on the code should be held every two years. In 1918, a revised edition of the ASME code was issued in 1924,the code was revised with the addition of the new section 8, which represented a new code for unfired pressure vessels.

THE API-ASME CODE

In 1931 a joint API-ASME committee on unfired pressure vessels was appointed to prepare a code for safe practice in the design, construction, inspection and repair of unfired pressure vessels.

SELECTION OF THE TYPE OF VESSEL

The first step in the design of any vessel is the selection of the type best suited for the particular service in question. The factors influencing this choice are:

1. The operating temperature and pressure.
2. Function and location of the vessel.
3. Nature of the fluid.
4. Necessary volume for storage or capacity for processing.
It is possible to indicate some generalities in the existing use of the common type of vessels. For storage of fluids at atmospheric pressure, cylindrical tanks with flat bottoms and conical roofs are commonly used. Spheres or spheroids are employed for pressure storage where the volume required is large. For smaller volume under pressure cylindrical tanks with formed heads are more economical

TYPE OF VESSELS
OPEN VESSELS

Open vessels are commonly used as urge tanks between operations , as vats for batch operations where materials may be mixed and blended as setting tanks, decanters, chemical reactors, reservoirs and s on. Obviously this type of vessel is cheaper than covered or closed vessel of the same capacity and construction. The decision as to whether or not open vessels may be used depends upon the fluid to be handled and the operation.

CLOSED VESSELS

Combustible fluids, fluids emitting toxic or obnoxious fumes and gases must not be stored in closed vessels. Dangerous chemicals such as caustic are less hazardous if stored in closed vessel. The combustible nature of petroleum and its products associates the use of closed vessels and tanks throughout the petroleum and petrochemical industries. Tanks used for the storage of crude oils and petroleum products and generally designed and constructed as per API specification for weld a silo storage tanks.

CYLINDRICAL VESSEL WITH FLAT BOTTOMS AND CONICAL OR DOMED ROOFS

The most economical design for a closed vessel operating of atmospheric pressure is the vertical cylindrical tank with a conical roof and a flat bottom resting directly on the bearing soil of the foundation composed by sand, gravel or crushed rock. In case where it is desirable to use a gravity feed, the tank is raised above the ground, and the flat bottom may be supported by columns and wooden joints or steel beams.

CYLINDRICAL VESSELS WITH FORMED ENDS

Closed cylindrical vessels with formed heads on both sides used where the vapour pressure of the stored liquid may dictate a stronger design , codes are developed through the efforts of the American petroleum institute and the ASME to govern the design of such vessels . These vessels are usually less than 12 feet in diameter. If a large quantity of liquid is to be stored, a battery of vessels may be used.

SPHERICAL AND MODIFIED SPHERICAL VESSELS

Storage containers of large volumes under moderate pressure are usually fabricated in the shape of a sphere or spheroid. Capacities and pressures used in this type of vessel vary greatly for a given mass; the spherical type of tank is more economical for large volume, low pressure storage operation..

VERTICAL vs HORIZONTAL VESSELS

In general functional requirements determine whether the vessel shall be vertical or horizontal. e.g.: distilling columns, packed towers which utilize gravity require vertical installation.
Heat exchangers and storage vessels are either horizontal or vertical. If the vessel to be installed outdoor wind loads etc are to be calculated to prevent overturning, thus horizontal is more economical. However floor space, ground area and maintenance requirements should be considered.

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Engineering Zone

Pressure Vessel Design