Design
DESIGN OF A PRESSURE VESSEL
The most commonly used pressure vessels are cylindrical pressure vessels. Let us see about the design of these cylindrical pressure vessels. For designing the cylindrical pressure vessels we have to consider two designs they are
Process design
Mechanical design
Now let us see about the mechanical design of the horizontal pressure vessels. Before going to mechanical design of the horizontal pressure vessels, we should know about the terminologies used in the mechanical design. The terminologies are
VESSEL:
A container or structural envelope in which materials are processed, treated, or stored; for example, pressure vessel, reactor vessel, agitator vessel, and storage vessels (tanks).
ALLOY:
Any of a large no. of substances having metallic properties consisting of two or more elements; with few exceptions, the components are usually metallic elements.
STABILITY OF VESSEL:
(Elastic stability) The strength of the vessel to resist buckling or wrinkling due to axial compressive stress. The stability of a vessel is severely affected by out of roundness.
YIELD POINT:
The lowest stress at which strain increases without increase in stress. For some purpose it is important to distinguish between the upper yield point, which is the stress at which stress-stain curve first become horizontal, and the lower yield point, which is the somewhat lower and almost constant stress under which the metal continues to deform. Only a few materials exhibit a true yield point; for some materials the term is sometimes used as synonymous with yield strength.
TENSILE STRENGTH:
The maximum stress a material subjected to a stretching load can withstand without tearing.
SPECIFIC GRAVITY:
The ratio of the density of a material to the density of some standard material.
FATIGUE:
Tendency of materials to fracture under many repetitions of a stress considerably less than the ultimate static strength.
ELASTIC:
Capable of sustaining stress without permanent deformation; the term is also used to denote conformity to the law stress-strain proportionality. An elastic stress or elastic strain is a stress or strain within the elastic limit.
ELASTIC LIMIT:
The least stress that will cause permanent set.
MODULUS OF RIGIDITY:
The rate of change of unit shear stress with respect to unit shear strain, for the condition of pure shear within the proportional limit.
MODULUS OF ELASTICITY (YOUNG’S MODULUS):
The rate of change of unit tensile or compressive stress with respect to unit tensile or compressive strain for the condition of uniaxial stress within the proportional limit. For most, but not all materials, the modulus of elasticity is same for tension and compression. For non-isotropic materials
HEAT TREATMENT:
Heat treating operation performed either to produce changes in mechanical properties of the material or to restore its maximum corrosion resistance. There are three principle types of heat treatment; annealing, normalizing, and post weld heat treatment
IMPACT TEST:
Determination of the degree of resistance of a material to breaking by impact, under bending, tensile and torsion loads, the energy absorbed is measured by breaking the material by a single blow.
OPERATING PRESSURE:
The pressure at the top of a vessel at which it normally operates. It shall not exceed the maximum allowable working pressure and it is usually kept at a suitable level below the setting of the pressure relieving devices to prevent their frequent opening. (Code UA-60)
OPERATING TEMPERATURE:
The temperature that will be maintained in the metal of the part of the vessel being considered for the specified operation of the vessel. (Code UA-60)
MAXIMUM ALLOWABLE WORKING PRESSURE:
The maximum gage pressure permissible at the top of a completed vessel in its operating position for a designed temperature. This pressure is based on the weakest element of the vessel using nominal thickness exclusives of allowances for corrosion and thickness required for loading other than pressure. (Code UA-60)
DESIGN PRESSURE:
The pressure used in determining the minimum permissible thickness or physical characteristics of the different parts of the vessel. (Code UG-60)
DESIGN TEMPERATURE:
The mean metal temperature (through the thickness) expected under operating conditions for the part considered. (Code UG-20)
STRESS
Stress is load acting per unit area. Internal force exerted by either of two adjacent parts of a body upon the other across an imagined plane of separation.
1) When the forces are parallel to the plane, the stress is called shear stress;
2) When the forces are normal to the plane the stress is called normal stress;
3) When the normal stress is directed toward the part on which it acts is called compressive stress;
4) When it is directed away from the part on which it acts it is called tensile stress.
RESIDUAL STRESS:
Stress remaining in a structure or member as a result of thermal or mechanical treatment, or both.
STRAIN:
Any forced change in the dimensions of a body. A stretch is a tensile strain; a shortening is a compressive strain; an angular distortion is a shear strain. The word strain is commonly used to connote unit strain.
CREEP:
Continuous increase in deformation under constant or decreasing stress. The term is usually with reference to the behavior of metal under tension at elevated temperatures. The similar yielding of a material under compressive stress is usually called plastic flow or flow.
CORROSION:
Chemical erosion by motionless or moving agents. Gradual destruction of a metal or alloy due to chemical process such as oxidation or action of a chemical agent.
HYDROSTATIC TEST:
The completed vessel filled with water shall be subjected to test pressure which is equal to 1 ½ times the maximum allowable working pressure to be marked on the vessel or 1 ½ the design pressure by agreement between the user and the manufacturer. (Code UG-99)
POSTWELD HEAT TREATMENT:
Heating a vessel to a sufficient temperature to relieve the residual stresses which are the result of mechanical treatment and welding. Pressure vessels and parts shall be post weld heat treated.
JOINT EFFICIENCY
A numerical value expressed as the ratio of the strength of a riveted, welded, or braze joint to the strength of the parent metal.
THIN CYLINDER:
If the ratio of the thickness to the internal diameter i.e. t/d is less than about 1/20, the cylinder is assumed to be thin cylinder.
THICK CYLINDER:
If the ratio of thickness to the internal diameter i.e. t/d is greater than 1/20, the cylinder is assumed to be thick cylinder.
REQUIRED THICKNESS:
The “required thickness” is that computed by the formulas in this division, before corrosion allowance is added.
NOMINAL THICKNESS:
The “nominal thickness” is the thickness selected as commercially available, and as supplied to the manufacturer; it may exceed the design thickness.
DESIGN THICKNESS
The “design thickness” is the sum of the required thickness and the corrosion allowance