Pressure Vessels

Chapter 1: What Are Pressure Vessels?

Pressure vessels are enclosed containers that hold liquids, vapors, and gases at a pressure significantly higher or lower than the ambient pressure. They are widely used in various industries such as petrochemical, oil and gas, chemical, and food processing industries. Equipment such as reactors, flash drums, separators, and heat exchangers are examples of pressure vessels.

Several standards and regulations governing every aspect of pressure vessels. The ASME Boiler and Pressure Vessel Code (BPVC) is the most popular set of universally acknowledged standards governing the design, construction, installation, testing, inspection, and certification of boilers, pressure vessels, and nuclear power plant components. The ASME BPVC Section VIII is the code dedicated to pressure vessels and has three divisions:

• Division I encompasses all pressure vessels intended to operate at internal or external pressure that may exceed 15 psig. Such vessels may be fired or unfired, and the pressure can be obtained from an external source or as a result of direct or indirect heating. Design-by-rule approach is adapted by the engineers. Division I is predicated by normal stress theory.
• Division II covers all pressure vessels intended to operate at internal or external pressures up to 10,000 psig. Division II requirements on the materials, design, and non-destructive examination are more rigorous than Division I. It requires more detailed calculations. However, it allows pressure vessels to be subjected to higher stresses. Design-by-analysis is adapted by the engineers. Unlike Division I, it is predicated by maximum distortion energy theory.
• Division III states the mandatory requirements and prohibitions for pressure vessels intended to operate above 10,000 psig.

Another standard is the API 510 - Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration which deals with the maintenance, inspection, and repair of operating pressure vessels. It aims to revisit and preserve the integrity of the pressure vessels in service.

A pressure vessel must be operated below the maximum allowable working temperature and pressure, the pressure vessel’s safety limits. All activities involving pressure vessels must be carried out by qualified personnel because the accidental release or leakage of its contents poses a threat to the pressure vessel’s surrounding environment.

Chapter 2: Types of Pressure Vessels

Pressure vessels may be classified according to their purpose or geometry.

Types of Pressure Vessels According to its Purpose

• Storage Vessels. Storage vessels are pressure vessels that temporarily hold liquids, vapors, and gases. The vessel may be used to contain fluids in a later process, or for storing finished products such as compressed natural gas (CNG) and liquid nitrogen.

  

  
  

• Heat Exchangers. Heat exchangers are used to transfer heat between two or more fluids. They are commonly used in the food, pharmaceutical, energy, and bioprocessing industries. The operation of heat exchanger equipment depends on the thermal and flow properties of the fluids involved in heat exchange, and on the thermal property of the conductive partition (for indirect contact heat exchangers). Materials in a heat exchanger experience stress from the temperature difference of the hot and cold fluids, and the internal pressure containing the fluids.

  

  
  

• Boilers. Boilers are heat transfer equipment that utilizes fuel, nuclear or electrical power as sources of heat. They are typically composed of an enclosed vessel that allows heat transfer from the source to the fluid. They are primarily used to heat liquids. Oftentimes, phase transformation of the fluid from liquid to vapor phase occurs inside the boiler. The vapor generated by the boiler is used for various heating applications and in power generation. Steam boilers generate steam at an elevated pressure to accelerate the blades of the turbine. Hence, the boiler vessel must have high strength to endure such high pressures and thermal stress. For the majority of materials, strength decreases with increasing temperature.

  

  
  

• Process Vessels. Process vessels are a broad classification of pressure vessels. These are containers where industrial processes occur, such as mixing and agitation, decantation, distillation and mass separation, and chemical reaction. The change in the internal pressure of a process vessel depends on the nature of the process carried out and the transformation of the substances involved. Among the special types of process vessels are the following:

  • Distillation columns allow the separation of a mixture of liquids based on the difference in their volatilities. There are two types of distillation processes. The type of distillation process will greatly influence the design of the pressure vessel:

    • Flash distillation involves heating a highly pressurized liquid mixture stream followed by separation of the vaporization of the more volatile component inside a flash chamber. The heated mixture first passes through a valve, and the pressure drop across the valve will result in the partial vaporization of the fluid. The vapor will be collected in the overhead of the flash chamber, while the liquid will settle at the bottom.

    • In column distillation or fractional distillation, one or more liquid mixture streams enters in the column at one or more points. As liquid stream flows down the column through the holes of the column internals, it comes in contact with the rising vapor coming from the bottom of the column. The column internals such as trays, plates, and packings provide the surface for mass transfer between the liquid and the vapor phases. The height of the column vessel depends on the number of trays or height of packings contained inside the vessel.

    

    
    
  • Decanters and gravity settlers allow the separation of a solid-liquid or liquid-liquid mixture. The denser component settles at the bottom of the vessel. This type of vessel typically has a narrow cross-sectional area or is oriented horizontally.

  • Industrial mixers are pressure vessels that are equipped with motor-powered blades to homogenize and emulsify a single or multiple substances. The substances mixed may be a pure liquid mixture, a semi-solid mixture, or a solid-liquid mixture. Agitating equipment operates at varying speeds depending on the extent of homogeneity. The mixing tank may be subjected to elevated temperatures and pressures, depending on the final product requirements.

    

    
    

  • Chemical reactors are enclosed pressure vessels used to contain and/or stir the reactants, products, and catalysts during a chemical reaction. They are equipped with agitators or stirrers to facilitate the blending among the reactants, thereby increasing the molecular contact among them. Baffles are installed to avoid the swirling of the fluid and create a desirable flow pattern inside the reactor.

• As the reactants are converted into products, the internal pressure increases if gaseous products are generated and increases even more at higher temperatures.

  The following are the types of chemical reactors which utilize a pressure vessel:

  • Jacketed reactors maintain the temperature of the reactants, products, and catalysts during a chemical reaction. A utility fluid (e.g., cooling water, steam) flows through the jacket that wraps around the vessel to cool or heat the contents of the reactor.

    The nature of the reaction is a critical consideration in designing reactors. Heat may be released (exothermic reaction) or absorbed (endothermic reaction) during a chemical reaction. Cooling or heating is important to provide favorable conditions for the reaction, thus maximizing product conversion and increasing efficiency, and to prevent uncontrolled increase or decrease in temperature during the reaction. Therefore, a jacketed reactor must be considered.

    

    
    

  • Packed bed reactors are cylindrical vessels that contain an immobilized bed of catalyst. The liquid or gaseous reactants flow from one side of the vessel, and the reaction takes place on the surface of the solid catalyst. Packed bed reactors provide high conversion per weight of catalyst and more contact area for the reactant and the catalyst. However, in these reactors, the cylindrical vessel must be able to support the weight of the catalyst bed.

  • Fluidized bed reactors also contain a bed of catalyst. In these reactors, the gaseous or liquid reactants pass through the bed at high velocities which suspend the solid catalyst inside the vessel and make it behave like a fluid. The fluidization of the catalyst allows thorough mixing of the reactants in all directions, resulting in attaining high reactant conversion and mass transfer rates, and uniform temperature across the reactor.

Types of Pressure Vessels According to its Geometry

• Spherical Pressure Vessels. Spherical pressure vessels are ideal for containing high-pressure fluids due to their strong structure, but they are difficult and expensive to fabricate. The internal and external stress is evenly distributed on the sphere‘s surface, which means there are no weak points. They have a smaller surface area per unit volume. Spherical vessels will consume less amount of material than the cylindrical vessel if a pressure vessel of the same volume will be fabricated. The smaller surface area of the spherical vessel will also have less heat transfer from the hotter body compared to other shapes.
  
  

  

  
  

• Cylindrical Pressure Vessels. Cylindrical pressure vessels are composed of a cylindrical shell and a set of heads. The cylindrical shell is the body of the pressure vessel. The heads serve as the end caps or enclosure to the shell to cover the contents of the vessel. The heads may have a flatter or more rounded profile. The latter reduces the weakness of the cylindrical vessel.

  Cylindrical pressure vessels are the most widely used vessel shape due to their versatility. They are much cheaper to produce than spherical vessels. However, they are generally weaker than spherical pressure vessels. They typically require thicker walls to achieve the same strength of spherical vessels bearing the same internal pressure.

  

  
  

  The following are the types of pressure vessel heads:

  • Hemispherical heads are ideal in handling high-pressure fluids and enclosing large-diameter vessels because the pressure is distributed equally across the head‘s surface. They have simple radial geometry and higher internal volume, but they are more difficult to fabricate and join to the shell. Compared to other head geometries, hemispherical heads require the least wall thickness that will handle the same internal pressure. The radius of a hemispherical head is equal to the radius of the cross-section of the cylindrical vessel. The depth of the head is half of the diameter.
    
    

    

    
    
  • Torispherical heads are suitable for handling pressures less than 15 bars. They are the easiest and cheapest to fabricate among the heads. They are used for pressure vessels with height restrictions because of their flatter profile. They are formed from part of a torus and part of a sphere. The transition of the cylinder and the dish is called the knuckle that is in toroidal shape. The knuckle radius is equal to the radius of the torus, and the crown radius is equal to the radius of the sphere.
    
    

    

    
    
  • Ellipsoidal heads have a depth that is a fraction of the width of the head. Its radius varies between the major and minor axis, which is usually 2:1. The ellipsoidal head and its shell have the same wall thickness. This type of head is ideal for containing high-pressure gases due to its height-to-weight ratio. It can handle pressures greater than 15 bars. Ellipsoidal heads are resistant to pressure and have high overall strength, which makes them economical due to their reduced thickness requirement.
    
    

    

    
    

Vessel Orientation

The axis of a cylindrical vessel may be oriented vertically or horizontally.

• Vertical vessel orientation is used:
  • When the floor space is small.
  • When the vessel volume is small.
  • In mixing tanks because it allows efficient mixing since the fluid is distributed at a smaller cross-sectional area.
  • When gas to liquid ratio is high.
  • In liquid-liquid separation for easier removal of components.
• Horizontal vessel orientation is used:
  • In heat exchangers since this orientation allows easier cleaning.
  • In settling tanks and flash drums, where low downward velocity is required. Low velocities have less entrainment.

  

  
  

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Chapter 3: Materials Selection for Pressure Vessels

The criteria for selecting the appropriate material of construction for pressure vessels are:

• Can meet the strength requirements of a particular application. Materials must withstand specific internal and external pressures, and structural stresses during the pressure vessel‘s service life.
• Corrosion resistance. This is one of the most important properties of a pressure vessel since it is expected to be reliable in harsh environments.
• Return of Investment. Costs of materials, fabrication, and maintenance must be considered during the lifecycle of the pressure vessel. Economic analyses are done to determine the best material which yields the least cost. The Return of Investment must be evaluated and assessed if the acquisition of a pressure vessel is profitable.
• Ease of fabrication and maintenance. Since metal sheets are formed into shapes to create the geometry of the pressure vessels, they must have good machinability and weldability. Vessel internals must be easily installed.
• Availability. Standard sizes for pressure vessel materials must be readily available in the region of the manufacturer.

The commonly used materials of construction for pressure vessels are the following:

• Carbon Steel. Carbon steel is a type of steel that has a higher carbon content of up to 2.5%. Carbon steel vessels are known for their high tensile strength for a minimal wall thickness, which is suitable for a wide range of applications. They are to impact and vibration. However, carbon steel is difficult to bend and form into shapes due to its high strength. It is also more susceptible to corrosion and rusting than stainless steel since it does not contain chromium.
  
  

  

  
  
• Stainless Steel. Stainless steel is a type of steel that has a higher chromium content of up to 10.5 – 30% and lower carbon content and trace amounts of nickel. They are known for their excellent chemical, corrosion, and weathering resistance which is attributed to their chromium content. A thin, inert chromium oxide film is formed at the surface to prevent oxygen diffusion to the bulk of the metal. Like carbon steel, it also exhibits high strength for a lower wall thickness. It is easier to form compared to carbon steel due to its increased ductility and elasticity.
  
  

  

  
  
• Hastelloy. Hastelloy is an alloy composed of nickel, chromium, and molybdenum which was the first alloy formulated by Haynes International, Inc. It is a widely used material for reactors, pressure vessels, and heat exchangers in the petrochemical, energy, and oil and gas industry. It can be used as a material for nuclear reactors. It has excellent corrosion resistance, cracking, and oxidizing and reducing agents. It maintains its strength at high temperatures. It is easily welded, and formed, and shaped due to its good ductility. With proper maintenance, its service life can last up to several decades which increases its cost-efficiency.
• Nickel Alloys. Nickel alloys offer good corrosion and weathering resistance, and protection against thermal expansion. The addition of chromium to the nickel alloy further increases its heat resistance. Pressure vessels constructed from nickel alloys are widely used in the oil and gas industry, cryogenic applications, and in other harsh environments. It also has a longer service life. However, it is difficult to work and has a higher fabrication cost. The purity of nickel alloys is important to protect their strength and reliability.
  
  

  

  
  

• Aluminum. Aluminum is known for its high strength-to-density ratio, which means it has high strength and lightweight at the same time. It is cheaper and more fabricated than stainless steel. It also has good corrosion resistance. Aluminum vessels are commonly used in laboratory-scale applications. However, it is not suitable for high-pressure applications since it has less density, which is one-third of stainless steel.

  

  
  
• Titanium. Titanium also offers high strength and rigidity for a minimal wall thickness. It has good corrosion resistance and biocompatibility, and it is also non-toxic. It has a higher melting point than steel and aluminum, hence it is ideal for higher temperature applications. It also has high thermal conductivity and facilitates efficient heat transfer, which is an ideal material for heat exchangers.

Chapter 4: Design of Pressure Vessels

The following are the parameters used in the design calculations of a pressure vessel. Such parameters are critical in evaluating the wall thickness of the shell and heads.

• Design Pressure. The design pressure is a value in which the vessel specifications are calculated. It is derived from the maximum operating pressure, which is the anticipated surge in pressure during upset conditions such as start-ups, emergency shutdown, and process abnormalities. It is always higher than the maximum operating pressure. The pressure relief system of a vessel is also based on this parameter to minimize the risk of explosions. According to Towler, the design pressure should be overdesigned by 5-10% from the maximum operating pressure.
  For vessels that potentially can experience vacuum pressure, the design pressure must be set to resist one full vacuum (-14.7 psig).
• Maximum Allowable Working Pressure (MAWP). The MAWP is the highest permissible pressure measured at the top of the equipment at which the vessel must operate based on its design temperature. It is the highest pressure that the weakest part of the vessel can handle at its design temperature. MAWP value is designated by the American Society of Mechanical Engineers (ASME) and is used by industries to ensure that the vessel will not operate beyond this value to establish safety protocols and prevent explosions.
  MAWP is different from the design pressure. MAWP is an extensive property that is based on the physical limitations of the material. Corrosion and wear lower the MAWP of the material. The design pressure, on the other hand, is based on the operating condition of the process, and it may be lower than or equal to the MAWP.

• Design Temperature. The maximum allowable stress is highly dependent on the temperature, as strength decreases with increasing temperature and becomes brittle at very low temperatures. The pressure vessel should not operate at a higher temperature where the maximum allowable pressure is evaluated. The design temperature is always greater than the maximum operating temperature and lesser than the minimum temperature.

  There are several rules of thumb in evaluating the design temperature. Towler suggests that the design temperature must be 50°F from the maximum operating temperature and -25°F from the minimum operating temperature. For Turton, a maximum allowance of 25°C must be given for vessels that will be operating between -30 to 345°C. The disturbances that have a drastic influence on the temperature of the pressure vessel must be considered by the designer.

  

  
  
• Maximum Allowable Stress. The maximum allowable stress is obtained by multiplying a safety factor to the value of maximum stress the material can withstand. The safety factor accounts for possible deviations from the ideal construction and operation of the pressure vessel.
• Joint Efficiency. The ASME Boiler and Pressure Vessel (BPV) Code has four categories of welded joints:

  Category A	Longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks or nozzles.
  Category B	Circumferential welds in the main shell, necks or nozzles or connecting a formed head other than hemispherical.
  Category C	Welds connecting flanges, tubesheets or flat heads to the main shell, a formed head, neck or nozzle.
  Category D	Welds connecting the communicating chambers or nozzles to the main shell, to heads or to necks.

  The joint efficiency is the ratio of the strength of the welded plate to the strength of the unwelded virgin plate. Generally, the strength is lower at the welded joint. Welded joints without further inspection and radiographic testing are assumed to be weaker due to defects such as porosity are potentially present. Joint efficiencies allowed under ASME BPV Code Sec. VIII D.1 is summarized in the table below:

  Joint Description	Joint Category	Joint Efficiency (Based on degree of radiographic examination)
  		Full	Spot	None
  Double-welded butt joint or equivalent	A, B, C, D	1.0	0.85	0.70
  Single-welded butt joint with backing strip	A, B, C, D	0.9	0.8	0.65
  Single-welded butt joint without backing strip	A, B, C	NA	NA	0.60
  Double full fillet lap joint	A, B, C	NA	NA	0.55
  Single full fillet lap joint with plug welds	B, C	NA	NA	0.50
  Single full fillet lap joint without plug welds	A, B	NA	NA	0.45

  

  
  
• Corrosion Allowance. There are several rules of thumb in estimating the corrosion allowance and maybe arbitrary to the manufacturer. Generally, corrosion allowance should range from 1.5 – 5 mm. According to Peters, Timmerhaus, and West, corrosion allowance should be 0.25 – 0.38 mm annually or 3 mm for 10 years. Meanwhile, Turton suggests that the corrosion allowance should be 8.9 mm for corrosive conditions, 3.8 mm for non-corrosive streams, and 1.5 mm for stream drums and air receivers. In heat exchanger equipment, corrosion allowance must be small because wall thickness affects the rate of heat transfer.

Fabrication of Pressure Vessels

The shell of the vessel and its heads are constructed by forging, rolling, and welding the metal sheet. The thickness of the metal sheet is the wall thickness which is obtained by thorough calculation, considering the above-mentioned factors. For the pressure vessel to serve its purpose, auxiliary equipment and devices and accessories are installed:

• Nozzles allow the introduction and discharge of feed, products, and utilities. They are usually welded perpendicularly on the shell or head and away from the weld lines.
• Pressure relief valves as a safety feature during its operation
• Heating or cooling jacket for stirred reactors
• Support such as saddles, skirts, or legs that allow thermal expansion of the material in operation.

Post weld heat treatment is done to relieve stress caused by joining and forming.

The fabrication of pressure vessels may be field-erected or shop-erected. Field-erected pressure vessels are too large to assemble inside a shop facility and be transported to the site. Thus, these vessels have their individual parts fabricated in a shop. These parts are delivered to the site wherein the pressure vessel is planned to be located. Welding, finishing, and installation of the accessories are performed on the site. On the other hand, shop-erected pressure vessels are much smaller in size that their components can be put together in a manufacturing facility. These vessels can fit inside a building or an enclosed facility. They are only delivered to the site after their assembly. The major phases of the fabrication are performed in the shop, and the installation of the vessel piping and minor adjustments are only made after the pressure vessel arrives on the site.

Chapter 5: Quality Testing and Inspection of Pressure Vessels

The following are testing methods employed to ensure the reliability of the pressure vessel.

• Visual Testing is a critical part of the maintenance of pressure vessels. The frequency of inspection must be once every five years, and before it is put into service after being installed or repaired. A trained inspector checks the interior and exterior of the vessel structure. The inspector looks for cracks, deformation, blistering, leakage of fluids, corrosion, and other flaws in the entire vessel structure.
• Ultrasonic Testing utilizes high-frequency sound waves to detect surface and subsurface flaws and to measure the wall thickness of the pressure vessel. The ultrasonic sound waves are absorbed by the material and are reflected back into an electrical signal by means of a transducer. The reflected waves are disturbed if flaws are present.
• Radiographic Testing utilizes x-rays or gamma rays to produce an image of a pressure vessel‘s surface and subsurface. The reflected rays will be distorted once it passes any discontinuities, holes, and difference in density and will be exposed in the film. Radiographic testing is highly reproducible and requires minimal surface preparation. However, it is more expensive and requires a highly skilled operator to handle ionizing radiation.
  
  

  

  
  
• Magnetic Particle Testing uses magnetic current to detect discontinuities on the surface in ferromagnetic materials. The inspector runs a magnetic current through the pressure vessel between two probes. If the material is defect-free, the magnetic flux flows through the material without any interruption. However, if cracks or any other imperfections are present, the magnetic flux leaks out of the material. The imperfection will be more visible once ferromagnetic particles, either in a liquid suspension or powdered form, are applied to the vessel.
  
  

  

  
  
• Liquid Penetrant Testing is commonly used on welded seams and plates. The inspector applies a small amount of liquid, called the penetrant, to an area with a possible flaw. The penetrant is allowed to settle after spraying and then wiped to clean the excess penetrant on the surface. The developer is then applied to reveal the penetrant that has seeped into the cracks.
  
  

  

  
  
• Pressure Testing is required by the ASME BPV Code to test for strength and leaks. There are two methods of pressure tests: hydrostatic pressure testing uses water as a medium, while pneumatic pressure testing uses air or nitrogen. The latter is more preferred for safety purposes since compressed liquid contains less energy than compressed gas. It works by removing the air from the vessel, and the unit is filled with the test fluid until the internal pressure is 1.5 times the design pressure for hydrostatic testing and 1.2 – 1.5 times for pneumatic testing. The fluid is then continuously held for a minimum of 10 minutes. The inspector then looks for cracks and leaks in the system. Fluorescent dyes or tracers are used to determine where the cracks are originating. Pressure testing is usually done during a shutdown, or as a validation test after the vessel is repaired from damage.

Conclusion

• Pressure vessels are enclosed containers that hold and store liquids, vapors, and gases at a pressure significantly higher or lower than the ambient pressure.
• Design, construction, repair, and testing of pressure vessels are governed by some regulations such as ASME BPVC and API 510. Such regulations are made to ensure safety during the pressure vessel‘s operation.
• The types of pressure vessels according to their function are storage tanks, boilers, heat exchangers, and process vessels. A pressure vessel may be spherical or cylindrical. Cylindrical vessels are more common, and their heads may be hemispherical, ellipsoidal, or torispherical. The axis of a pressure vessel may be vertically or horizontally oriented.
• The criteria for material selection for a pressure vessel are: can meet the strength requirements of a certain application, corrosion resistance, return of investment, ease of fabrication and maintenance, and availability.
• The critical design parameters for calculating the specification of a pressure vessel are design pressure, maximum allowable working pressure, design temperature, maximum allowable stress, joint efficiency, and corrosion allowance.
• The pressure vessel is fabricated through forging, rolling, and welding the metal sheets. Auxiliary equipment and accessories are installed for the vessel to fully serve its purpose.
• The methods employed to test the reliability of pressure vessels are visual testing, ultrasonic testing, radiographic testing, magnetic particle testing, liquid penetrant testing, and pressure testing.

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