Introduction: Pressure Vessels

Pressure Vessels Introduction

Any vessel which operates under pressure or in which pressure more than atmospheric is maintained can be called a pressure vessel. Such vessels are usually designed according to pressure vessel codes. The various types of equipment which may operate under pressure are reaction vessels, heater/cooler, heat exchangers, digesters, filters, etc. When the pressure is maintained inside the vessel it is called a vessel subjected to internal pressure. When pressure lower than atmospheric is maintained inside the vessel it is termed as subjected to external pressure or operating under vacuum.

As advances in chemical process technology are taking place, it is becoming more and more necessary to use high pressures and retain them more safely. Therefore the design of high-pressure vessels has evoked great interest and to design such vessels proper stress analysis and proper design procedure is necessary.

Some examples where pressure vessels are used:

1. In the manufacture of ammonia where the reaction between nitrogen and hydrogen occurs at a temperature of 450-500°C and 200-900 atm. pressure in the presence of (Fe + Mo) catalyst.
2. The manufacture of HNO₃ where compressed air and NH, reacts at 750°C and 7 kg/cm2 in the presence of Platinum-rhodium catalyst.
3. In the manufacture of urea where ammonia and CO₂, react at 175 C and at a pressure of 200 kg/cm2 to produce ammonium carbamate.
4. In the manufacture of methanol, the reaction between carbon monoxide and water-gas occurs at 250 kg/cm2 and at 220°C.
5. In the manufacture of aniline, the vapor phase reduction of nitro-benzene is carried out at 27°C and 1.5 kg/cm2.
6. In the manufacture of acetone catalytic de-hydrogenation of isopropyl alcohol is carried out at 3-4 kg/cm2 and at 500°C.

The vessel used for the operation may be cylindrical or spherical. Spherical vessels are better in strength and lower in cost but due to difficulties of fabrication cylindrical vessels are more commonly used. Liquid ammonia or petroleum products are usually stored in spherical vessels. They are called as Horton spheres.

When a vessel is subjected to internal pressure, the various stresses induced in the vessel are:

1. Hoop stress or circumferential stress
2. Longitudinal stress
3. Radial stress

For low values of operating pressure, the stress variation need not be considered. For high operating pressure, variation in all the three stresses is appreciable. Out of this tangential stress, variation is greatest and the design should be based on it. The thickness required for a vessel depends upon the operating pressure and permissible stress of the material.

Before the detailed design of a pressure vessel, the following parameters have to be considered

1. The type of fluid handled or being processed.
2. Type of service required (continuous, intermittent or fluctuating).
3. The geometry of vessel and type of vessel required for a process which can be fabricated economically for the process requirements
4. Availability of different materials of construction and their cost data

Process fluid, operating pressure, temperature and length of service decide the material of construction.

For vessels subjected to internal pressure, the design pressure is usually fixed at 110% of the normal operating pressure. In calculating design pressure at any point in the vessel the head due to the operating liquid, catalyst, etc. has also to be considered.

Vessels operating under vacuum are generally designed for full vacuum conditions. Design temperature is fixed generally at a value of 10-20°C higher than the maximum operating temperature. Dead loads like the weight of the vessel, the weight of internals, weight of contents, etc. and the forces like wind load, earthquake load, the reaction from platforms, piping, etc. are also considered.