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Pressure Vessel Design

 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.


VESSELS OPERATING AT LOW TEMPERATURE RANGES

Pressure vessels constructed in such a manner that a sudden change of section producing a notch effect is present, are usually not recommended for low temperature range operations. The reasons are that, they may create a state of stress such that the material will be incapable of releasing high localized stress by plastic deformation. So the material used for low temperature operations are tested for notch ductility.
Carbon steels can be used down to 60 C. notch ductility is controlled in such materials through proper composition steel making practice, fabrication practice and heat treatment. They have an increased manganese carbon ratio. Aluminum is usually added to promote fine grain size and to improve notch ductility.
Ductility of certain materials including carbon and low alloy steels is considerably diminished when the operating temperature is reduced below certain critical value. This critical value is usually described as the transient temperature. It depends up on the material, method of manufacture, previous treatment and stress system present. Below transition temperature fracture may take place in a brittle manner with little or no deformation, whereas at temperatures above the transition temperature, fracture occurs only after considerable plastic strain or deformation.

VESSELS OPERATING AT ELAVATED TEMPERATURES

Embitterment of carbon and alloy steel may occur due to service at elevated temperatures. In most instances brittleness is manifest only when the material is cooled to room temperature. This is inhibited by the addition of molybdenum and also improve tensile and creep properties. Two main criteria in selecting steel for elevated temperatures are metallurgical strength and stability. Carbon steels are reduced in their strength properties due to rise in temperature and are liable to creep. So the use of carbon steel is generally limited to 500 c.
The SA-283 steels cannot be used in applications with temperatures over 340 C .The SA-285 steels cannot be used for services with temperatures over 482 C. However both SA-285 and SA-212 steels have very low allowable stress, at the higher temperatures.

MATERIAL SPECIFICATION

Plain carbon and alloy steel plates are usually used where service conditions permit because of the lesser cost and greater availability of these steels. Such steels may be fabricated by fusion welding and oxygen cutting if the carbon content does not exceed .35%. Vessels may be fabricated of plate steels meeting the specifications of SA-7,SA-113 grade c and SA-283 grade A,B,C and D provided that

1. The operating temperature is between –28 and 360 C.
2. The plate thickness does not exceed 1.5 cm.
3. The vessels do not contain lethal liquids and gases.
4. The steel is manufactured by the electric furnace or open hearth process.
5. The material is not used for unfired steam boilers.

One of the most widely used steel for general purpose in the construction of pressure vessels is SA-283, grade C. This steel has good ductility and forms welds and machines easily. It is also one of the most economical steel suitable for pressure vessels. However its use is limited to vessels with plate thickness not exceeding 1.5 cm.
For vessels having shells of greater thickness, SA-285 grade C is most widely used in moderate pressure applications. In the case of high pressure or large diameter vessels, a high strength steel may be used to advantage to reduce the wall thickness. SA –212, grade B is well suit for such applications and requires a shell thickness of only 79% of that required by SA-285, grade C. This steel also is fabricated but is more expensive than other steels.
Now many new series of materials like low alloy, high alloy steels, high temperature and low temperature materials are available which can be selected to suit the requirements of every individual need of the process industry
The important materials generally accepted for the construction of pressure vessels are indicated here. Metals used are generally divided into 3 groups as

A) Low cost: - Cast iron, cast carbon and low alloy steel, wrought carbon and low alloy steel.
B) Medium cost: - high alloy steel (12% chromium and above) aluminum, nickel, copper and their alloys, lead.
C) High cost: - platinum, tantalum, zirconium, titanium silver.

Materials mentioned in b and c group are sometime used in the form of cladding or bonding for materials in group a. Non metallic linings such as plastics, rubber etc can also be used.

Vessels with formed heads are commonly fabricated from low carbon steel wherever corrosion and temperature considerations will permit its use because of the low cost, high strength, ease of fabrication and general availability of mild steel. Low and high alloy steel and nonferrous metals are used for special service.

Steels commonly used fall into two general classifications.

1) Steel specified by ASME code.
2) Structural grade steels, some of which permitted by the ASME

CLOSURES FOR PRESSURE VESSELS


All formed heads are fabricated from single circular flat plate by spinning or by drawing with dies in a press. Although the cost of heads formed from flat, plates involves additional cost of forming, the use of formed heads as closures usually more economical than the use of flat plates as closures except for small diameters.
A variety of formed heads are used for closing the ends of cylindrical vessels. These include flanged only heads, flanged and shallow dished, Toro spherical, elliptical, hemispherical and conical shaped heads. For spherical purposes flat plates are used to close a vessel opening, however flat heads are rarely used for large vessels.
For pressures not covered by the ASME code, the vessels are often equipped with standard dished heads, whereas vessels that require code construction are usually equipped with standard dished heads whereas vessels that require code construction are usually equipped with either the ASME dished or elliptical dished heads. The most common shape for the closure of pressure vessels is the elliptical dish. Most chemical and petrochemical processing equipments such as distilling columns, disrobers, absorbers, scrubbers , heat exchangers, pressure-surge tanks and separators are essentially cylindrical closed vessels with formed ends of one type or the other.
As mentioned above , the most common types of closures for vessels under internal pressure are the elliptical dished head( ellipsoidal head) with a major to minor axis ratio equal to 2.0:1.0 and the Toro spherical head in which the knuckle radius is equal to 6% or more of the inside crown radius(ASME standard dished head)

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