Introduction to Pressure Vessels
- Vessels, tanks, and pipelines that carry, store, or receive fluids are called pressure vessels.
- A pressure vessel is defined as a container with a pressure differential between inside and outside. The inside pressure is usually higher than the outside, except for some isolated situations.
- The fluid inside the vessel may undergo a change in state as in the case of steam boilers or may combine with other reagents as in the case of a chemical reactor.
- Pressure vessels often have a combination of high pressures together with high temperatures and in some cases flammable fluids or highly radioactive materials. Because of such hazards, it is imperative that the design be such that no leakage can occur.
- In addition, these vessels have to be designed carefully to cope with the operating temperature and pressure.
- It should be borne in mind that the rupture of a pressure vessel has the potential to cause extensive physical injury and property damage. Plant safety and integrity are of fundamental concern in pressure vessel design.
Pressure Vessel parts :
Following are the main components of pressure vessels in general:
1-Shell: The primary component contains pressure. Pressure vessel shells in the form of different plates are welded together to form a structure that has a common rotational axis. Shells are either cylindrical, spherical, or conical in shape.
2-End Closures ( Heads): All the pressure vessels must be closed at the ends by heads (or another shell section). Heads are typically curved rather than flat. The reason is that curved configurations are stronger and allow the heads to be thinner, lighter, and less expensive than flatheads. Heads can also be used inside a vessel and are known as intermediate heads. These intermediate heads are separate sections of the pressure vessels to permit different design conditions.
3-Nozzle: A nozzle is a cylindrical component that penetrates into the shell or head of the pressure vessel. They are used for attaching piping for flow into or out of the vessel, attaching instrument connection (level gauges, thermowells, pressure gauges), and providing access to the vessel interior at manway or providing for direct attachment of other equipment items (e.g. heat exchangers).
4-Support (Saddle): Support is used to bear all the loads of pressure vessels, earthquake, and wind loads. There are different types of supports, which are used depending upon the size and orientation of the pressure vessel. It is considered to be the non-pressurized part of the vessel.
Types of Supports :
Saddle Support :
- Horizontal drums are typically supported at two locations by saddle support.
- It spreads over a large area of the shell to prevent excessive local stress in the shell at the support point.
- One saddle support is anchored whereas the other is free to permit unstained longitudinal thermal expansion of the drum.
- Small vertical drums are typically supported on legs that are welded to the lower portion of the shell.
- The max. ratio of support leg length to drum diameter is typically 2:1
- Reinforcing pads are welded to the shell first to provide additional local reinforcement and local distribution.
- The number of legs depends on the drum size and load to be carried.
- Support legs are also used for Spherical pressurized storage vessels.
- Cross bracing between the legs is used to absorb wind or earthquake loads.
- Vertical pressure vessels may also be supported by lungs.
- The use of lugs is typically limited to pressure vessels of small and medium diameter ( 1 to 10 ft )
- Also moderate height to diameter ratios in the range of 2:1 to 5:1 .
- The lugs are typically bolted to horizontal structural members in order to provide stability against overturning loads.
Skit Support :
- Tall vertical cylindrical pressure vessels are typically supported by skirts.
- A support skirt is a cylindrical shell section that is welded either to the lower portion of the vessel shell or to the bottom head ( for cylindrical vessels).
- The skirt is normally long enough to provide enough flexibility so that radial thermal expansion of the shell does not cause high thermal stresses at its junction with the skirt.
Applications of Pressure Vessels
- Industrial compressed air receivers
- Domestic hot water storage tanks
- Diving cylinders (Scuba diving)
- Recompression chambers
- Distillation towers
- Autoclaves (In medical industry to sterilize)
- Oil refineries and petrochemical plants
- Nuclear reactor vessels
- Pneumatic and Hydraulic Reservoirs
- Storage vessels for liquefied gases such as ammonia, chlorine, propane, butane, and LPG.
ASME Codes for Pressure Vessels
Pressure vessels are designed to operate safely at a specific pressure and temperature, technically referred to as the “Design Pressure” and “Design Temperature”. A vessel that is inadequately designed to handle a high pressure constitutes a very significant safety hazard. Because of that, the design and certification of pressure vessels is governed by design codes such as the ASME Boiler and Pressure Vessel Code in North America, the Pressure Equipment Directive of the EU (PED), Japanese Industrial Standard (JIS), CSA B51 in Canada, Australian Standards in Australia and other international standards like Lloyd’s, Germanischer Lloyd, Det Norske Veritas, Société Générale de Surveillance (SGS S.A.), Lloyd’s Register Energy Nederland (formerly known as Stoomwezen) etc.
- It is a standard that provides rules for the design, fabrication, and inspection of boilers and pressure.
- This establishes and maintains design, construction, and inspection standards providing for maximum protection of life and property.
- ASME Section VIII: Boiler and Pressure Vessel Code (BPVC)
- Division 1 – Rules for Construction of Pressure Vessels
- Division 2 – Alternative Rules
- Division 3 – Alternative Rules for Construction of High-Pressure Vessels
General Materials for Pressure Vessels
The materials that are used in pressure vessel construction are:
- Nonferrous materials such as aluminum and copper
- Metals such as titanium and zirconium
- Nonmetallic materials, such as plastic, composites, and concrete
- Metallic and nonmetallic protective coatings
Various materials have some typical characteristics as below:
- Carbon steel: strength & moderate corrosion resistance
- Low-alloy steels: strength at high temperatures
- Stainless steels: corrosion resistance
- Nickel alloys: corrosion resistance
- Copper alloys: seawater resistance
- Aluminum: Light, low-temperature toughness
- Titanium: seawater, chemical resistance
- Refractories: very high temperatures
- Non-metallic: corrosion & chemicals
Factors Affecting Selection of Material:
Factors Affecting Selection of Material is as follows:
- Process fluids (i.e. a plastic might be perfect for the fluid corrosiveness, but will melt when the operators ‘steam’ the equipment during cleaning)
- Operating temperature
- Operating pressure
- Fluid Velocity
- Contamination of product
- Required life of the equipment (May choose to incur shorter life and replace more often)
- Cost of the materials of construction (base material + fabrication costs)
Classification of Pressure Vessels – Types of Pressure Vessels
Based on Wall Thickness:
1) Thin Wall Vessel
2) Thick Wall Vessel
Based on Geometric Shapes:
1) Cylindrical Vessels
2) Spherical Vessels
3) Rectangular Vessels
4) Combined Vessels
Based on Installation Methods:
1) Vertical Vessels
2) Horizontal Vessels
Based on Operating Temperature:
1) Low-Temperature Vessels (less than or equal to – 20° C)
2) Normal Temperature Vessels (Between – 20° C to 150° C)
3) Medium Temperature Vessels (Between 150° C to 450° C)
4) High-Temperature Vessels (more than or equal to 450° C)
Based on Design Pressure:
1) Low-Pressure Vessels (0.1 MPa to 1.6 MPa)
2) Medium Pressure Vessels (1.6 MPa to 10 MPa)
3) High-Pressure Vessels (10 MPa to 100 MPa)
4) Ultra High-Pressure Vessels (More than 100 MPa)
Based on Technological Processes:
1) Reaction Vessel
2) Heat Exchanger Vessel
3) Separation Vessel
4) Storage Container Vessel
Difference Between Thin Shell and Thick shell Pressure Vessels
- The pressure vessels, according to their dimensions, may be classified as thin shells or thick shells.
- If the wall thickness of the shell (t) is less than 1/10 to 1/15 of the diameter of the shell (d), then it is called a thin shell. On the other hand, if the wall thickness of the shell is greater than 1/10 to 1/15 of the diameter of the shell, then it is said to be a thick shell.
- Thin shells are used in boilers, tanks, and pipes, whereas thick shells are used in high-pressure cylinders, tanks, gun barrels, etc.
- Another criterion to classify the pressure vessels as thin shells or thick shells is the internal fluid pressure (p) and the allowable stress (σ t).
- If the internal fluid pressure (p) is less than 1/6 of the allowable stress, then it is called a thin shell. On the other hand, if the internal fluid pressure is greater than 1/6 of the allowable stress, then it is said to be a thick shell.
Types of End Closures
- Formed heads are used as end closures for cylindrical pressure vessels
There are two types of end closures:
1. Domed heads:
2. Conical heads
Design of Pressure Vessel :
Stresses in a Thin Cylindrical Shell due to an Internal Pressure
The analysis of stresses induced in a thin cylindrical shell is made on the following assumptions:
1) The effect of the curvature of the cylinder wall is neglected.
2) The tensile stresses are uniformly distributed over the section of the walls.
3) The effect of the restraining action of the heads at the end of the pressure vessel is neglected.
When a thin cylindrical shell is subjected to internal pressure, it is likely to fail in the following two ways:
1) It may fail along the longitudinal section (i.e. circumferentially) splitting the cylinder into two troughs, as shown in Fig
2) It may fail across the transverse section (i.e. longitudinally) splitting the cylinder into two cylindrical shells, as shown in Fig.
Thus the wall of a cylindrical shell subjected to internal pressure has to withstand tensile stresses of the following two types:
(a) Circumferential or hoop stress, and
(b) Longitudinal stress.
Circumferential or Hoop Stress
σ = pd / 2t
Where, p = Intensity of internal pressure,
d = Internal diameter of the cylindrical shell,
l = Length of the cylindrical shell,
t = Thickness of the cylindrical shell, and
σ = Circumferential or hoop stress for the material of the cylindrical shell.
σ = pd / 4t
Thick Cylindrical Shells Subjected to an Internal Pressure
- When the ratio of the inner diameter (d) of the cylinder to the wall thickness (t) is less than 10 to 15, the cylinder is called a thick cylinder.
- Hydraulic cylinders, high-pressure pipes, and gun barrels are examples of thick cylinders.
- The radial stress (σr) is neglected in thin cylinders, while it is of significant magnitude in the case of thick cylinders.
- There are a number of equations for the design of thick cylinders. The choice of equation depends upon two parameters: Cylinder material (whether brittle or ductile) and Condition of the cylinder ends (open or closed).
- In the design of thick cylindrical shells, the following equations are mostly used:
1. Lame’s equation,-
When the material of the cylinder is brittle, such as cast iron or cast steel, Lame’s equation is used to determine the wall thickness. It is based on the maximum principal stress theory of failure, where maximum principal stress is equated to permissible stress for the material.
2. Birnie’s equation, –
In the case of open-end cylinders (such as pump cylinders, rams, gun barrels, etc.) made of ductile material (i.e. low carbon steel, brass, bronze, and aluminum alloys), the allowable stresses cannot be determined by means of maximum stress theory of failure. In such cases, the maximum-strain theory is used. According to this theory, failure occurs when the strain reaches a limiting value.
3. Clavarino’s equation and
This equation is also based on the maximum-strain theory of failure, but it is applied to closed-end cylinders (or cylinders fitted with heads) made of ductile material.
4. Barlow’s equation.
This equation is generally used for high-pressure oil and gas pipes.
- The standard method of construction for boilers, compressed air receivers, and other pressure vessels of iron or steel before gas and electrical welding of reliable quality became widespread was riveted sheets which had been rolled and forged into shape, then riveted together, often using butt straps along the joints, and caulked along the riveted seams by deforming the edges of the overlap with a blunt chisel.
- Hot riveting caused the rivets to contract on cooling, forming a tighter joint.
Manufacturing methods for seamless metal pressure vessels are commonly used for relatively small diameter cylinders where large numbers will be produced, as the machinery and tooling require large capital outlay. The methods are well suited to high-pressure gas transport and storage applications and provide consistently high-quality products.
Backward extrusion: A process by which the material is forced to flow back along the mandrel between the mandrel and die.
Cold extrusion (aluminum):
Seamless aluminum cylinders may be manufactured by cold backward extrusion of aluminum billets in a process that first presses the walls and base, then trims the top edge of the cylinder walls, followed by press forming the shoulder and neck.
Seamless cylinders may also be cold drawn from steel plate discs to a cylindrical cup form, in two or three stages.
Large and low-pressure vessels are commonly manufactured from formed plates welded together. Weld quality is critical to safety in pressure vessels for human occupancy.
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