Pressure vessels, abbreviated as PV, are designed to hold or contain process fluids, liquids or gas, at a pressure significantly higher or lower than the ambient pressure, either internally or externially.  Each pressure vessel must be operated within it's design temperature and pressure, which is the vessel's safety limits.  These vessels 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.

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• See Article  -  Pressure Vessel Glossary Pressure Vessel Abbreviations

Pressure Vessel Index

• Pressure Vessel Purpose
• Pressure Vessel Use
• Pressure Vessel Geometry
• Pressure Vessel Orientation
• Pressure Vessel Material Selection
• Pressure Vessel Materials
• Pressure Vessel Walls
• Pressure Vessel Pressure
• Pressure Vessel Temperature
• Pressure Vessel Stress
• Pressure Vessel Components
• Pressure Vessel Heat Transfer Surfaces
• Pressure Vessels in the Oil & Gas Industry
• Pressure Vessel Codes
• Pressure Vessel Standards

Pressure Vessel Purpose

Boiler  -  A closed vessel in which water or other fluid is heated under pressure for generating steam or other hot fluids.  When sizing a boiler you must have enough capacity to handle the maximum hourly load of the system.

Heat Exchanger  -  Heat exchanger is a device used to transer heat from one medium to another at different temperatures.  The heat transfer can be air or a liquid such as water or oil.  A heat exchanger has many functions.  They are critical to the process industries as they used to condense, heat, cool, or chill a fluid.  It also can be used to change the phase of a fluid (liquid to vapor or vapor to liquid) like a boiler.

Storage Pressure Vessel  -  Storage vessels 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 and liquid nitrogen. Examples are storage tanks.

Process Presser Vessel  -  This vessel in which various processes are performed, such as breaking down a process, combining products, removing various elements or aspects of a product and more.  Some examples are separators, drums, reactores, distillation tower fractioning tower, heat exchangers, etc.

Pressure Vessel Use

Pressure vesses are used are used in a variety of applications in both industry and private sector.  They appear in these sectors as industrial compressed air receivers and domestic hot water storage tanks in the pharmacutical, food, and petroleum industry, etc.

Pressure Vessel Geometry

Cylindrical Pressure Vessel  -  These vessels have a cylindrical shell and one or more heads.  The cylindrical shell is the main body of the pressure vessel.  The heads are like end caps that cover the contents of the vessel and may have a flatter or more rounded profile.  They are less expensive to produce than spherical vessels but not as durable.  It then requires cylindrical pressure vessels to have thicker walls to withstand equal amounts of pressure than a spherical vessel.

Spherical Pressure Vessel  -  These vessels are suitable for holding liquids under a lot of pressure.  Spherical vessels will use less material than cylindrical vessels if they are the same size.  Spheres also don’t take up as much space as other shapes.  The pressure is spread out evenly on the sphere, so there are no weak points.  Spherical vessels are also less likely to transfer heat than other shapes due to their smaller surface area.

Pressure Vessel Orientation

Horizontal Pressure Vessel  -  These vessels are used if your well stream has a large liquid to gas ratio and maintains a constant flow.  If your well stream experiences foamy productions where the large liquid surface are available will allow greater gas break out and foam breakdown.  If your production facility has vertical height limitations.  If your well stream requires three-phase liquid separation with difficult to separate liquids, such as a free water knockout.

Vertical Pressure Vessel  -  These vessels are used if your well stream has a large gas to liquid ratio.  If you need a sand scrubber or a separator that can tolerate sand and mud.  If your well stream has slugs or large instantaneous volumes of liquid.  If the area has horizontal space limitations such as off-shore platforms.  If the vessel is being placed downstream of equipment that causes liquid condensation such as coolers.

Pressure Vessel Material Selection

Strength Requirement  -  Can the material meet the strength requirement of a particular application.  Materials must withstand specific internal and external pressures, and structural stresses during the pressure vessel's service life.

Corrosion Resistant  -  This is one of the most important properties of the vessel since it is expected to be reliable in harsh environments.

Ease of Fabrication and Maintenance  -   The sheet metal is formed into a shape, it must have good machinability and weldability.  Vessel internals must be easily installed.

Availability  -  Standard sizes for vessel materials must be readily available.

Return on Investment  -  Cost of materials, fabrication, and maintenance must be considered during the lifecycle of the vessel.

Pressure Vessel Materials

Alluminum  -  Aluminum has a high strength-to density ratio, means it has high strengty and lightweight at the same time. Also having a good corrosion resistance.  However, it is not suitable for high pressure applications since it has less density.

Carbon Steel  -  Carbon steel vessels have a high tensial strength for a minimal wall thickness.  But it can be difficult to bend due to its high strength.

Hastelloy  -  It has excellent corrosion resistance, cracking, and oxidation reducing properties.  It maintains its strength at high temperatures.  It is easily welded, formed, and shaped due to its ductability.

Nickel Alloys  - These alloys offer a good corrosion and weathering resistance, and protection against thermal expansion.  It is more difficult to work with.

Stainless Steel  -  This material is known for its chemical, corrosion, and weather resisytance.  It has high strength for a lower wall thickness.  It is easier to form than carbon steel due to its increased ductility and elasticity.

Titanium  -  Titanium has a high strength-to-density ratio, which means it has high strength and lightweight at the same time, and also has a good corrosion resistance.

Pressure Vessel walls

Thin Vessel  -  The value of the ratio of wall thickness and internal diameter of a pressure vessel is less than or equal to 1/20, the pressure vessel is considered a thin pressure vessel.  Thin pressure vessels are generally employed where the holding pressure of gas or liquid is not very high.

Thick Vessel  -  The value of the ratio of wall thickness and internal diameter of a pressure vessel is greater than 1/20, the pressure vessel consider as a thick pressure vessel.  Thin pressure vessels are generally employed where the holding pressure of gas or liquid is very high.

Pressure Vessel Pressure

The design pressure is derived from the maximum operating pressure, which is the anticipated surges in pressure during upset conditions such as start-ups, emergency shutdowns, and process changes.  The maximum allowable working pressure is the highest permissible preasure measured at the top of the equipment at which the vessel must operate based on it's design temperature.  The design pressure of the pressure vessel is divided into four pressure levels.

• Low Pressure Vessel (code L)  -  0.1 MPa ≤ p < 1.6 MPa
• Medium Pressure Vessel (code M)  -  1.6 MPa ≤ p < 10.0 MPa
• High Pressure Vessel (code H)  -  10.0 MPa ≤ p < 100.0 MPa
• Ultra High Pressure Vessel (code U)  -  p ≥ 100.0 MPa

Pressure Vessel Temperature

The maximum allowable stress is highly dependent on the temperature, as strength decreases with increased temperature and becomes brittle at very low temperatures.  The vessel should not operate at a higher temperature where the maximum allowable pressure is elevated.

Pressure Vessel Stress

The maximum allowable stress is obtained by multiplying a safety factor to the value of maximumstress of the material can withstand.

Pressure Vessel Components

Shell  - 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.

Head  -  A head is one of the end caps on a cylindrically shaped pressure vessel.  The inward pressure of each type of head determines the range of its use.

• Dish Radius  -  Also know as crown radius.  It is the major radius of the formed head, usually measured from the inside.
• Inside Depth of Dish  -  Measurement from the inside center of the head to the tangent line.
• Knuckle Radius  -  Also known as corner radius. It is the formed radius of the head which transitions the dish portion to the straight flange. This process is done on a flanging machine.
• Straight Flange  -  Is the straight portion of the head measured from the tangent line to the edge of the head.  The industry standard is 1 1/2".
• Tangent Line  -  The point on the head where the knuckle radius meets the straight flange.  Refers to the point of contact, tangency, between the cylinder and the knuckle portion of the vessel head.  The distance from the tangent line on one head to the tangent line on the opposite head is known as the straight side or tangent-to-tangent.

Nozzle  -  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.

Slirt and Legs  -  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).

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

Pressure Vessel Heat Transfer Surfaces

Pressure Vessels in the Oil & Gas Industry

Wemco Induced Static Floatation (ISF) VesselFilter 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:

• Free Water Knockout
• Gas Liquid Cylindrical Cyclone Separator
• Heater Treater
• Three Phase Separator
• Two Phase Separator

Pressure Vessel Codes

Pressure vessels are designed to operate safely at a specific pressure and temperature.  A vessel that is inadequately designed to handle pressures beyond its maximum allowable working pressure (MAWP) is a very significant safety and environmental hazard.

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