Pressure Vessels

Pressure Vessels

Pressure vessels are able contain a wide variety of substances and can be designed with specific purposes in mind. They are used for various industrial applications within the chemical, pharmaceutical, food and beverage, oil and fuel and plastics industries. Many tanks are required to be registered ASME pressure vessels and adhere to strict safety and quality regulations put forth by the American Society of Mechanical Engineers to ensure the safety of those working with pressure tanks. Certified ASME tanks are important because due to the nature of pressure vessels, even the tiniest leak can cause a major explosion or shrapnel damage. There are a number of different types of pressure vessels. Autoclaves like grease kettles use steam and pressure to cause chemical reactions producing many different substances, including food, lubricants and chemicals, and process tanks are designed to hold and store liquids. High pressure vessels are the strongest type of pressure tank, and are used with the highest psi. They are typically stainless steel vessels, which provide the best resistance to pressure, temperature and corrosion. While many pressure vessels are used in manufacturing facilities to produce substances, others are used in different applications such as: expansion tanks found in every residential closed water heating system, water pressure tanks as part of wells, and vacuum tanks as an integral component in sewage applications. These vessels are mostly used to store substances short term.

The substances contained in a pressure vessel, whether it be gas, liquid or a mixture of materials, determine the design components such as vessel material, size, volume, shape, temperature and pressure level. When a substance is stored under pressure, the potential for rupture and leakage is greater. The risk of damage from a pressure vessel increases when vessel contents are toxic, flammable or gaseous substances. Engineers take precautions when creating a pressure vessel to limit the occurrence of vessel failure. The division of vessel creation into steps, which include design, construction, testing and inspection, keeps safety hazards to a minimum. In the design process, engineers determine the logistics of how pressure vessel manufacturers will create the vessels. Engineers must determine the pressure level, temperature, material components, size and shape. All pressure vessels are measured in gallons, and range anywhere from 20 to several hundred thousand gallons depending on their application. They are often equipped with many different components, such as ladders or stairs, removable or detachable lids, sight glass for observation, heating and cooling systems and propellers or agitation systems for mixing applications. Engineers also consider the corrosion resistance and abrasion potential of the vessel before deciding these factors. Pressure levels are also taken into account before deciding upon a material and shape. Pressure vessel manufacturers fabricate a range of pressure vessels; from a few hundred pounds per square inch (psi) to measuring up to 150,000 psi.

Vessel design and maintenance must be considered carefully as even a small imperfection increases the risk of pressure vessel failure, posing a serious safety hazard. This is the reason for stringent quality and manufacturing standards placed on pressure vessels by the ASME. As pressure vessels have a temperature range that can exceed 750°F and the contents of those vessels are constantly under high pressure, operator safety is of large importance. There are standard regulations and formulas to which the pressure vessel manufacturers' designs adhere in order to avoid potential hazards associated with pressure containment. The American Society of Mechanical Engineers (ASME) provides a Boiler and Pressure Vessel Code on which engineers base pressure vessel design. Although the ASME Code remains the most common standard, engineers also follow other codes, such as that of the American Petroleum Institute (API). Rigorous analyses for complex pressure vessels are created when standard design rules do not apply. In such instances, engineers conduct intensive mathematical and scientific analyses to ensure design and construction methods meet the stringent requirements of pressure vessels: material, size, shape, temperature and pressure level of the pressure vessel, as well as personal preferences.

The materials used to create pressure vessels must be high strength and durable, and able to maintain their shape and properties even under pressure. Common materials can include carbon alloy steel, stainless steel, titanium, zirconium, aluminum, nickel alloys and niobium. Engineers usually create vessels using one of three manufacturing processes-forging, welding and brazing, all of which involve heating pieces of metal and joining them together. Forging refers to the formation of metal parts through the application of heat and pressure. Welding involves melting two similar metals together by heating their edges until molten. Finally, during the brazing process, metals are joined by filling the space between them with a nonferrous metal. Most of the metal has been cold rolled, which creates a stronger metal than hot rolling. They are also often galvanized, quenched or tempered to increases temperature resistance and tensile strength. Testing of the pressure vessel ensures design technique success, proper vessel operation and certification approval. Regular inspection remains necessary to ensure that the vessel continues to meet industrial standards and safety requirements.

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Types of Pressure Vessels

• ASME tanks are pressure vessels that meet the standards set by the American Society of Mechanical Engineers.
• Autoclaves are closed pressure vessels that use steam and high pressure to sterilize instruments.
• Cookers are a type of pressure vessel that are used to bring about a physical change in their contents. Examples of this are digesters, vulcanizers and rendering tanks.
  
• Fired pressure vessels utilize fuel combustion to generate heat. Examples include boilers, furnaces, gas water heaters and autoclaves.
  
• Heat exchangers include a variety of configurations of vessel equipment in which heating or cooling is performed on one side of the vessel and the opposite conversion on the other side. (http://www.iqsdirectory.com/heat-exchangers/)
  
• Kettles are pressure vessels that use steam to heat fluids.
• Pressure tanks are vessels that hold contents at pressure levels greater than atmospheric pressure.
• Rotating pressure vessels usually contain steam, which is then used to dry articles such as paper, fabric or plastics. The materials are passed over the rotating vessel via rollers to come into direct contact with the emitting steam.
  
• Steam jacketed vessels are used to heat liquid to a moderate degree. Steam is distributed between the inner and outer shells of the vessel and is used in the commercial preparation of foods such as candy.
  
• Storage vessels include air tanks, hot water tanks, propane or other gas tanks, which contain contents under pressure when needed.
  
• Thick walled pressure vessels are the least common. They are any cylinder [shell] ratio that is 10% or more the ratio of the thickness to the inside diameter.
  
• Thin walled pressure vessels are one of the most common of the vessels. They are any cylinder [shell] ratio which is 10% or less of the ratio of the thickness to the diameter, or a pressure vessel is thinned walled if the diameter is 10-times or more of the thickness.
  
• Transportable vessels are in contrast to those that are stationary. Examples of such are those are road or rail tankers; propane and gas tanks are considered to be in this category.
  
• Unfired pressure vessels are not exposed to direct heat. Generated heat, if any, is produced through electric heat or steam, and sometimes through the chemical reactions of vessel contents.
  
• Water pressure tanks hold water at levels exceeding atmospheric pressure.

Pressure Vessel Terms

Agitator - Device used for agitation of the product or substance found inside a pressure vessel.
 
Atmospheric Pressure - The amount of force the atmosphere exerts upon the earth's surface, measuring 14.7 psi at sea level.
 
Baffle - Primarily used in pressure vessels with agitators. Commonly used to increase the amount of agitation or mixing.
 
Boiler - Pressure vessel that heats water and creates and heats using combustible fuels or energy.
 
Brittle Fracture - Fracture of steel associated with exposure to very low temperatures often in circumstances in which stress levels have not exceeded yield strength.
 
Corrosion Allowance - An increase in pressure vessel shell thickness designed to compensate for the corrosion and abrasion of certain pressure vessels; protective coatings and linings are also added to pressure vessels to prevent corrosion.
 
Creep - Permanent deformation of steel caused by strength reduction resulting from exposure to high temperatures.
 
Cryogenic Temperatures - Extremely low temperatures, ranging from -250° C to -100° C
(-418° F to -148° F).
 
Design Pressure - The maximum internal and external pressure limits that a pressure vessel can withstand, usually equivalent to 110% of pressure vessel operating pressure.
 
Design Temperature - The temperature of the pressure vessel metal when the vessel is subjected to the design pressure.
 
Dewar - Pressure vessel created to sustain cryogenic temperatures.
 
Ferrule - A nozzle made for sanitary and low pressure applications. A ferrule contains an inset gasket surface and is designed to work with clamps instead of bolting.
 
Fluid - Any substance, whether liquid, gas or vapor, in which the particles move freely, resulting in no permanent shape and low resistance to pressure.
 
Fusible Plugs - Pressure relief mechanisms initiated with rapid pressure increases resulting from a fire.
 
Heat Transfer Surface (HTS) - Broad terminology for providing a pressure vessel or pressure tank with a means of temperature control.
 
Hydropneumatic Tanks - Pressure vessels in which both liquids and gases are collected.
 
Hydrostatic Test - Test in which pressure vessels are subjected to 150% of the design pressure.
 
ID (Inside Diameter) - Diameter of the pressure vessels measured from the interior of the pressure vessels surfaces. This dimension does not include the material thickness.
 
Lifting Lugs - Lugs attached to the outside of the pressure vessels specifically placed to help aid lifting of the pressure vessels.
 
Manway - Access port to the internal region of the pressure vessels.
 
Megapascal - Unit of measurement equal to 1,000,000 pascals, used to express tensile strength and yield strength.
 
National Board Inspection Code - Worldwide standard for pressure vessel inspection, alteration and repair.
 
Newton - Unit of force producing an acceleration rate of one meter per second per second on a one-kilogram mass.
 
Nozzle - The spout through which fluid is released from or introduced into pressure vessels.
 
Operating Pressure - The pressure at which pressure vessels operate while in use.
 
Pascal - Unit of pressure measurement equal to a Newton per square meter.
 
Psi (Pounds per Square Inch) - Unit that measures the amount of pressure applied to an object.
 
Relief Valve - Pressure relief mechanism that automatically releases liquids from pressure vessels when vessel pressure exceeds set pressure, and closes when the pressure level returns to normal.
 
Rupture Disks - Pressure relief mechanism designed for single usage that instantly relieves pressure in a pressure vessel. These can be used in conjunction with other pressure relief mechanisms.
 
Safety Relief Valve - Pressure relief mechanism that automatically releases liquid and vapor streams from pressure vessels when vessel pressure exceeds set pressure, and closes when the pressure level returns to normal.
 
Safety Valve - Pressure relief mechanism that automatically releases gases and vapors from pressure vessels when vessel pressure exceeds set pressure, and closes when the pressure level returns to normal.
 
Set Pressure - The predetermined pressure at which a pressure relief device begins to discharge fluid.
 
Straight Flange - Small, cylindrical pressure vessel component consisting of a bolted rim connected to the body of the pressure vessel.
 
Strain - The change in an object's dimensions, resulting from the application of force.
 
Stress - The force responsible for causing a change in an object's dimensions.
 
Stress Corrosion Cracking - Fracture resulting from the combination of corrosion and tensile stress.  
Tensile Strength - The maximum stress level exerted upon a test specimen before the specimen fractures.
 
Tensile Stress - The force applied to a test specimen to produce strain.
 
Vessel Heads - The end of a pressure vessel, including flanged and dished, ellipsoidal, flat, conical and hemispherical.
 
Vessel Shell - The pressure vessel body.
 
Yield Strength - Amount of stress producing an inelastic strain in a pressure vessel. Exceeding the yield strength can result in permanent deformation of the vessel.