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Pressure Vessel Glossary

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A

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  • Absolute Pressure  -  A pressure at absolute zero can only exist in a total vacuum and any pressure above this is called absolute pressure.
  • Acid Gas (Sour Gas)  -  Any significant amount of hydrogen sulfide (H2S) and carbon dioxide (CO2) contains a natural gas that can cause corrosion.
  • Agitation  -  Can be used in jacketed vessels and with internal coils to improve the homogeneity of the fluid properties.  Agitation is commonly used in batch applications.
  • Atmospheric Pressure  -  The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth.

B

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  • Baffle  -  Reduces disturbance and surges from the inlet fluids in horizontal or vertical.  The most used type is a perforated plate in the shape of a segmented circle.
  • Base Plate  -  A plate attached to the bottom of the support that sits on the foundation.
  • Barlow's Formula  -  The point just before or at when a pipe or tubing fails under pressure.
  • Bladder Tank  -  A steel tank that has a liquid tight rubber bladder inside that contains pressurized air and water separated by a flexible membrane.  As water pressure changes, the volume of air in a bladder tank contracts and expands.  Periodically, the amount of air in the tank should be measured and the tank recharged if the air is too low.
  • Blanket Gas  -  A gas phase in a vessel above a liquid phase.  The purpose may be for protecting the liquid from contamination, for reducing the hazard of detonation, or for pressuring the liquid.  The source of the gas is external to the vessel.
  • Blind Flange  -  A piping component for covering or closing the end of a pipe, valve, vessel or tank.
  • Blowdown  -  The act of emptying or decompressing a vessel.  This may also refer to the discarded material such as blowdown water from a boileror cooler tower.
  • Boiler  -  A closed vessel in which water or other fluid is heated under pressure for generating steam or other hot fluids.
  • Bottoms  -  The liquid or residual matter which is withdrawn from the bottom of a fractionator or other vessel during processing or while in storage, also, the heaviest product remaining in the liquid phase after distillation.
  • Breathing  -  The moment of vapor in or out of a storage tank because of the change of level of the storage liquid, a change in the temperature of the vapor space above the liquid, or of atmospheric pressure.
  • Bubble Point  -  The temperature at a given pressure at the instant the liquid is in equilibrium with the vapor phase in the containing vessel.
  • Buffer Tank  -  It provides additional water storage in the heating system.

C

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  • Carbon Dioxide  -  CO2
  • Carbon Monoxide  -  CO
  • Catalyst  -  A material that aids or promotes a chemical reaction between other substances but does not react itself.
  • Cavitation  -  The creation and collapse of bubbles in a liquid.
  • Clean in Place  -  This system sanitize the interior surfaces of pressure vessels used in the production of food and beverages, medical devices, and pharmaceutical products, among others.  CIP systems do eliminate the need to completely disassemble the equipment in order to clean in between uses.
  • Conical Head  -  Posses more inherent strength than flat heads.
  • Conventional Jacket Vessel  -  Used for small volume applications less than 300 gallons.
  • Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.
  • Corrosion Allowance  -  The amount of material in a pipe or vessel that is available for corrosion without affecting the pressure containing integrity.
  • Corrosion Inhibitor  -  A substance that slows down the chemical reaction rate of corrosion on metal that is exposed to the environment.
  • Crude  -  A mixture of naturally occuring liquid hydrocarbons before refining.
  • Crude Oil  -  See crude
  • Cylinder Hoop Stress  -

D

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  • Design Pressure  -  The pressure the vessel is designed for.
  • Design Temperature  -  The temperature the vessel is designed for.
  • Dimple Jacket Vessel  -  Normally deals with internal vessel pressure around 300 PSI.
  • Dished Head of End  -  The end of a cylindrically shaped vessel.
  • Dish Radius  -  Also known as crown radius.  It is the major radius of the formed head, usually measured from the inside of the head.
  • Distillate  -  Products of distillation formed by condensing vapors.
  • Dry Gas  -  A natural gas with with very little liquid hydrocarbons or water vapor.

E

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  • Elastic Modulus  -  The ratio of the stress applied to a body or substance to the resulting strain within the elastic limits.
  • Eliptical Head  -  The most common type of head.
  • Enthalpy  -  Measures the sum of internal energy changes in heat under constant pressure of the system.
  • Expansion Joint  -  Heat and cold causes piping systems to expand and contract. 
  • Expansion Tank  -  Ensures that constant pressure is maintained within the pipes so there is no damage to the pipes.

F

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  • Field Separator  -  A vessel in the oil or gas field for separating the gas, hydrocarbon, liquid, and water from each other.
  • Flange  -  A bolted connection where two pieces of pipe, equipment, fittings or valves are connected together to form a piping system.
  • Flash Point  -  The temperature at which combustion is initiated.
  • Flat Head  -  Consisting of a toroidal knuckle connecting to a flat plate.
  • Flue  -  A passage for combustable products.
  • Flue Gas  -  A combustable product that flows through a flue.
  • Free Water Knockout  -  A pressure vessel that uses residence time to separate water from the production process.

G

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  • Gas Liquid Cylindrical Cyclone Separator  -  The design is a very simple two phase metering unit designed to measure two phase flow.  It consists of a vertical cylinder with an inclined, tangential inlet.
  • Gate Valve  -  One of the most frequently used valve in piping systems and is classified as either "rising-stem" or "nonrising-stem" valves.
  • Globe Valve  -  A type of valve used for regulating fluid flow, both on/off and throttling, it is a control valve.
  • Gusset Plate  -  A thick steel plate used to strengthen joints on new and retrofitted structures.

H

I

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  • Inside Diameter  -
  • Insulation  -  A material with low thremal conductivity used to reduce heat loss.

J

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  • Jacket Vessel  -  An external cavity around the vessel, controlling the temperature of the contents of the vessel through a cooling or heating fluid.

K

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  • Knuckle Radius  -  Also known as inside corner radius.  It is the formed radius of the head which transitions the dish portion of the head to the straight flange.

L

M

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  • Manway  -  An opening on a tank or vessel designed as an entryway for personal access and equipment.
  • Mass Flow Rate  -  The average velocity of a mass that passes by a point.
  • Maximum Operating Pressure  -  The maximum temperature permissible that provides sufficient flexability during normal operation.
  • Modulus of Elasticity  - See Elastic Modulus

N

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  • Nameplate  -  On all vessels. Gives the manufacturer, year manufactured, pressure, temperature, and any other indormation needed about the vessel.
  • Natural Gas  -  A naturally occuring hydrocarbon gas consisting primarly of methane.
  • Nozzle  -  A nozzle, abbreviated as NOZ, is a An opening in a tank or vessel for a flanged connection.

O

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  • Oil Burner  -  A burner for firing oil.
  • Operating Pressure  -  The operating pressure of equipment occurs when the plant or process is operating at steady state conditions.
  • Outlets and Drains  -  Pressure vessels typically contain a drain or outlet at or near the lowest point of the tank.  The outlet and joint must be able to withstand the pressure held in the tank to avoid leaks or blowouts.  Drains for containers in sanitary services must be placed carefully and at minimal lengths to ensure that the vessel will drain completely.
  • Outside Diameter  -
  • Over-all Height  -  This is the total height of the head and is measured by adding inside depth of dish, straight flange height, and the thickness together.
  • Oxidation  -  The loss of electrons in a chemical reaction in which an element combines with oxygen.  Oxidation and reduction always occur at the same time in equal amounts.
  • Oxidation Atmosphere  -  An atmosphere which tends to promote the oxidation of immersed materials.

P

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  • Pad Mounted Vessel  -  A pressure vessel will sometimes be mounted on a dedicated pad of poured concrete.  This pad may be sloped or leveled as needed for drainage.  But on its own, a pad-mounted vessel does not allow access to the underside of the container.
  • Petroleum  -  See crude
  • PH  -  A scale that indicates the degree of acidity or alkalinity of a solution.
  • Pitting  -  A non-uniform corrosion of a metal, not in the form of cracks, whereby a number of cavities, are formed in the surface.
  • Portible Vessel  -  Pressure vessels may be constructed with supports that provide easier portability, including legs and casters.  They may also be built onto carts and trolleys with axles and wheels to facilitate easy transport.
  • Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.
  • Pressure Differential  -  The pressure difference between two points of a system.
  • Pressure Loading of Thin-walled Spherical Vessel  -
  • Pressure Loss  -  The difference in upstream and downstream pressure.
  • Pressure Safety Valve  -  Used to protect vessels and tanks from overpressure. It is designed to protect critical system components such as pressure vessels, tanks and flange ratings.
  • Pressure Vessel Head Shapes  -
  • Programmable Logic Controller  -  The types of signals are analog output (AO), analog input (AI), discrete output (DO), and discrete Input (DI).
  • Pulsation  -  Rapid changs in pressure.

Q

R 

S

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  • Saddle  -  Used to support horizontal and vertical designs in locations to support the concentrated loads at on each end of the vessel.
  • Sample Box  -  A sample box is a series of pipe, valves and fittings located on a pressure vessel or tank that is used to manually determine the height of the liquid or where the interface between oil and water resides.
  • Saturated Steam  -  The point (temperature and pressure) when steam is in contact with the liquid water (boiling) it came from.
  • Sensible Heat  -  The heat added to a substance which increases its temperature but not the phase is called sensible heat.
  • Shell  -  The cylinder portion of the vessel.
  • Shell Coupling  -  A threaded coupling for connecting gauges, instrumentation, or even sample box valving.
  • Sidewall Support  -  In the event that the vessel cannot be supported from the bottom on a flat surface, it can be bolted to an external frame attached to the sidewall.
  • Sight Glass  -  Can be a liquid level gauge, sight flow indicator, circular sight window, or fuse sight glass.
  • Skid Supported Vessel  -  Similar to the sidewall support, a pressure vessel can be supported by the installation of a cylindrical or conical steel skirt attached to the sidewall and mounted on a concrete pad.  This can both support the container and protect the underside of the vessel.  In addition, it can be used to protect any attachments that exist on the bottom of the container.
  • Slip-on Flange  -  Designed to slip over the outside of pipe, long-tangent elbows, reducers, and swages.
  • Solar Tank  -  Stores the heated water from the sun or comercial or residential use.
  • Sour Gas  -  See acid gas
  • Specific Gravity  -  The density or ratio of any substance to another substance.
  • Spherical Hoop Stress Thick-walled Section (External Pressure Only)  -
  • Spherical Hoop Stress Thick-walled Section (Internal Pressure Only)  -
  • Spherical Hoop Stress Thick-walled Section (Internal & External Pressure)  -
  • Spherical Hoop Stress Thin-wall Section  -
  • Stack  -  A vertical conduit.
  • Stack Gas  -  Anything that comes out of a burner stack in gaseous form.
  • Steam  -  The invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.
  • Stean in Place  -  This system rellies on steam to sterilize equipment.
  • Straight Flange  -  The straight flange refers to the straight potion of the head measured from the tangent line to the edge of the head.  The industry standard is 1 1/2” for ASME F&D’s and 2.0” for ellipticals, but most head manufactures can change this to suit your needs.
  • Stress  -  The force per unit area of cross-section.
  • Stub End  -  Allows a flange to swivel in order to mate with another flanges bolt holes that may not align perfectly.
  • Surge  -  The sudden displacement or movement of liquid in a closed vessel or drum.
  • Surge Tank  -  These tanks are installed on large pipelines to relieve excess pressure caused by water hammer and to provide a supply of water to reduce negative pressure if a valve is suddenly opened.
  • Sweet Crude  -  Crude oil with a sulfur content less than 0.5% by weight, no sulfur smell.

T

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  • Tail Gas  -  The lightest hydrocarbon gas released from a refining process.
  • Tangent Line  -  The tangent line describes the point on the head where the knuckle radius meets the straight flange.
  • Temperature  -  The amount of heat or cold, but it is neither heat or cold.
  • Tension Strength  -  The capacity of a material to resist a force tending to stretch it.
  • Three-phase Separator  -  Most, if not all, production meters require conditioning of the process stream prior to measurement.  The conditioning process allows for solids to drop out of the stream, entrained gas to break out of the stream or oil/water/gas separation.
  • Torque  -  A rotational moment. 
  • Two-phase Separator  -  A two phase separator is used to separate fluid into gas and liquid.
  • Thermal Efficiency  -  The fraction of heat that is converted to work or desired output divided by required input.
  • Thermal Expansion  -  The increase in length, area or volume due to the increase (in some cased decrease) in temperature.
  • Thermal Shocking  -  Introduces alternating low and high temperatures for rapid cooling and / or heating applications.
  • Thermowell  - Used in temperature measurement and provide isolation from the temperature sensor and the process fluid.
  • Torispherical Head  -  Sometimes referred to as flanged and dished heads after the manufacturing process.

U

V

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  • Vacuum Breaker  -  A vent on top of the vessel that allows air to be pulled into the tank.
  • Vacuum Tank  -  A system that filters air or fluids through suction or other techniques.  Not all vessels are able to withstand the difference in internal and external pressure that results from a vacuum.  For this reason, when designing a system that will utilize variations in internal and external pressure, it is critical to consider if a vacuum tank will be necessary to ensure the system’s success as well the safety of the operator.
  • Vapor  -  The gas state of a liquid or solid.  Both vapor and gas are basically the same thing.
  • Vaporization  -  A physical change of a substance from a liquid phase to a gas phase. 
  • Vessel Internals  -  Vessels have internals to help them effectively process the fluids go through it.
  • Viscosity  -  The measure of the internal friction/resistance to the flow of a liquid.
  • Vortex Breaker  -  When the flow exits a nozzle it will create a vortex.  A cross of plates on the discharge nozzle will prevent the discharge from swirling.

W

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  • Water  -  Can exist in three of the four phases of matter: gas, liquid, or solid.
  • Water Hammer  -  A valve is suddenly opened or closed.
  • Water Pressure Tank  -  This tank works by keeping the water store in a constant state of pressure.  When water leaves the tank, the pressure level drops.  When the pressure reaches a pre-set threshold, the well pump activates and replaces the water in the tank.  As the water replenishes, the pressure then rises.
  • Welding  -  The fabrication process that fuses like materials togeather by heating them to a suitable temperatures, this can be acomplished by brazing, soldering or welding.
  • Wet Gas  -  A gas containing a relatively high portion of hydrocarbons that are recoverable as liquids.
  • Wetted Surface  -  The area of contact between the liquid and the wall of the tank often used to determine the emergency venting capacity required for the tank.
  • Working Pressure  -  The normal pressure that a system operates at.

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