Pressure Vessel

pressure vessel banner 1Pressure vessels, abbreviated as PV, serve many functions.  They can used for storage of high pressure gas, gas scrubbers, two and three phase separators and other functions.  They can have various internals depending on the process.  Two phase separators may have a wave breaker, vortex breaker, mist eliminator and splash plate.  Three phase separators will typically contain a  the same components with the addition of a weir to help ensure separation.

Science Branches

Applied Science
Mechanical Engineering
Manufacturing Engineering

Article Links

Association Links

General Links

Mathematics Links

Tag Links

Nomenclature & Symbols



Pressure Vessel Glossary


Pressure Vessel Types

Autoclaves  -  Large vessels that are pressurized and brought to high temperatures.

Bladder Tank  -  A tank that has a rubber bladder inside to be filled with air or water, whichever is approperate.

Boiler  -  A closed vessel in which water or other fluid is heated under pressure for generating steam or other hot fluids.

Buffer Tank  -  Provides additional water storage in the heating system.

Expansion Tank  -  Ensures that constant pressure is maintained within the pipes so there is no damage to the pipes.

Heat Exchanger  -  A device used to transer heat from one medium to another at different temperatures.

High-pressure Vessel  -  This vessel is durable, works under heavy loads, and has the best resistance to corrosion.  Usally made of stainless steel.

Process Vessel  -  Designed to hold and store liquids.

Solar Tank  -  Stores the heated water from the sun or comercial or residential use.

Surge Tank  -  It is a reserve storage resivoir.

Vacuum Tank  -  Uses high-pressure hoses to clean out liquids and sludges without any damage to the object being cleaned.

Water Pressure Tank  -  Stores the heated water from gas or oil firing for comercial or residential use.


Pressure Vessel Components

The shell in most vessels is what provides the longitudinal length of the vessel.  Or all vessels, the thickness of the shell is based on the pressure the shell will see (both internal and external), plus any additional loads created by the supports, nozzles or other attachments.  Most shells are circular in shape.  This is based partially on economics of forming the shell but mostly is due to the equal distribution of forces on the circular shape.

Nozzles are inserted into the shell or head and are the first step of what connects to the process piping.  Nozzles can connect to flanges or threaded fittings depending on the design.  Nozzles must be designed depending on the metallurgy of the shell & nozzle, internal and external projections of the nozzle, orientation of the nozzle and  internal and external forces on the nozzle, such as pressure and piping forces.  

The saddle is another designed component which carries the weight of the vessel.  A saddle is a plate that supports a pressure vessel and connects to the foundation.  The design of this component takes the physical dimensions of the vessel (weight, length and design) and external forces (Seismic Zones and Wind  Forces, for example).

Pressure vessels usually have internals that are specific to the process.  Click on for more reading on Vessel Internals


Pressure Vessels in the Oil & Gas Industry

DSC06581Wemco Induced Static Floatation (ISF) VesselDSC06623Filter Vessel










In the upstream oil and gas industry, pressure vessels are used from air accumulators (to help control the frequency an air compressor turns on and off while the system is under heavy load) to three phase separators (which separate oil, water and gas from the production stream).  Some examples of pressure vessels and their usage are shown below.  Each is linked to a page with more information about the individual piece of equipment:


Pressure Vessel Codes

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 pressures beyond its maximum allowable working pressure (MAWP) is a very significant safety & environmental 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, Canadian Standards Association (CSA) in Canada, the Pressure Equipment Directive of the European Union (PED) or Japanese Industrial Standard (JIS).


Pressure Vessel standards

API Standards

  • API 510 - Pressure Vessel Inspection Code: In-service Inspection, Rating, Repair, and Alteration

ASME Standards

  • ASME B1.1 - Unified Inch Screw Threads (UN and URN Thread Forms)
  • ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard
  • ASNE B18.2.2 - Nuts for General Applications: Machine Screw Nuts, Hex, Hex Flange, and Coupling Nuts (Inch Series)
  • ASME B1.20.1 - Pipe Threads, General Purpose, Inch
  • ASME B31.1 - Power Piping
  • ASME B31.2 - Fuel Gas Piping
  • ASME B31.3 - Process Piping
  • ASTM B36.10M - Welded and Seamless Wrought Steel Pipe
  • ASTM B36.19M - Stainless Steel Pipe
  • ASME PCC-1 - Guidelines for Pressure Boundary Bolting Flange Joint Assembly
  • ASME PCC-2 - Repair of Pressure Equipment and Piping
  • ASME PTC-25 - Pressure Relief Devices
  • ASME QAI -1 - Qualifications for Authorized Inspection


Pressure Vessel Glossary


  • 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.
  • Atmospheric Pressure  -  The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth.


  • 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 tank that has a rubber bladder inside to be filled with air or water, whichever is approperate.
  • Blind Flange  -  A piping component for covering or closing the end of a pipe, valve, vessel or tank.
  • Boiler  -  A closed vessel in which water or other fluid is heated under pressure for generating steam or other hot fluids.
  • Buffer Tank  -  It provides additional water storage in the heating system.


  • 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.
  • 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


  • 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.
  • Distillate  -  Products of distillation formed by condensing vapors.
  • Dry Gas  -  A natural gas with with very little liquid hydrocarbons or water vapor.


  • Elastic Modulus  -  The ratio of the stress applied to a body or substance to the resulting strain within the elastic limits.
  • 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. 


  • 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.


  • 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.


  • Head  -  The end closure of the vessel.
  • Heat  -  A form of energy that causes physical change in what is being heated.
  • Heat Exchanger  -  A device used to transer heat from one medium to another at different temperatures.
  • Heat Transfer  -  The exertion of power that is created by heat, or the increase in temperature.
  • Heavy Crude  -  Crude oil with a high proportion of heavy hydrocarbons and metallic content.
  • Holiday  -  A discontinuity in painted or coated surfaces.
  • Hoop Stress  -  The circumferential and perpendicular stress to the axis imposed on a cylinder wall when exposed to an internal pressure load.
  • Hot Water Tank  -  Stores the heated water from gas or oil firing for comercial or residential use.


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


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



  • Latent Heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.
  • Level Switch  -  Provides a discrete input to the PLC.  The level switch is typically used as a method of control in tanks and vessels.
  • Light Crude  -  Crude oil with a light proportion of light hydrocarbons fractions and low metallic compounds.
  • Liquid  -  A specific volume and can hold any shape it is contained within.
  • Liquefied Natural Gas  -  Natural gas cooled to a liquid state.
  • Lug  -  A projection used for grasping or lifting.


  • 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.


  • 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.


  • 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.
  • 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.


  • 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.
  • Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.
  • 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.
  • 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.



  • Raw Water  -  Untreated water.
  • Relative Humidity  -  A measure of the amount of moisture in the air with respect to the temperature.


  • 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.
  • Sight Glass  -  Can be a liquid level gauge, sight flow indicator, circular sight window, or fuse sight glass.
  • 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.
  • 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.
  • 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  -  It is reserve storage resivoir.
  • Sweet Crude  -  Crude oil with a sulfur content less than 0.5% by weight, no sulfur smell.


  • Tail Gas  -  The lightest hydrocarbon gas released from a refining process.
  • 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.
  • 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.



  • Vacuum Breaker  -  A vent on top of the vessel that allows air to be pulled into the tank.
  • Vacuum Tank  -  Uses high-pressure hoses to clean out liquids and sludges without any damage to the object being cleaned.
  • 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.


  • Water  -  Can exist in three of the four phases of matter: gas, liquid, or solid.
  • 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.



  • Yield Strength  -  The minimum stress that leads to permanent deformation of the material.



Display #
Free Water Knockout
Gas Liquid Cylindrical Cyclone Separator
Heater Treater
Hoop Stress
Hoop Stress Buckling Load (External Pressure or Vacuum)
Hoop Stress Thick-walled Section (External Pressure Only)
Hoop Stress Thick-walled Section (Internal & External Pressure)
Hoop Stress Thick-walled Section (Internal Pressure Only)
Hoop Stress Thin-wall Section
Horizontal Vessel Calculator
Longitudinal Stress Thick-walled Section (External Pressure Only)
Longitudinal Stress Thick-walled Section (Internal & External Pressure)
Longitudinal Stress Thick-walled Section (Internal Pressure Only)
Longitudinal Stress Thin-wall Section
Pressure Loading of Thin-walled Spherical Vessel
Pressure Vessel Head Shapes
Radial Stress Thick-walled Section (External Pressure Only)
Radial Stress Thick-walled Section (Internal & External Pressure)
Radial Stress Thick-walled Section (Internal Pressure Only)
Radial Stress Thin-wall Section
Sample Box
Spherical Hoop Stress Thick-walled Section (External Pressure Only)
Spherical Hoop Stress Thick-walled Section (Internal & External Pressure)
Spherical Hoop Stress Thick-walled Section (Internal Pressure Only)
Spherical Hoop Stress Thin-wall Section
Three Phase Separator
Two Phase Separator
Vessel Internals

Tags: Storage Tanks and Prseeure Vessels