Glossary index


Glossary A


  • Connects and aligns the power end of an ANSI pump to the wet end.

A.N.S.I. Standard

  • American National Standards Institute. A set of specifications (envelope dimensions) for centrifugal pumps manufactured in the United States

A.P.I. gland

  • A seal gland that incorporates a non sparking disaster bushing along with a vent and drain connection (quench), and a flushing connection.

A.P.I. Specifications

  • American Petroleum Institute Specifications. Usually adopted by oil refineries for petroleum applications. Includes seal gland and piping recommendations.

Absolute pressure

  • Atmospheric pressure added to gauge pressure.

Active metal

  • A corrosion resistant metal that has not been passivated. The protective oxide layer has not formed.

Affinity laws

  • They predict how capacity, head and horsepower are affected by changes in the centrifugal pump impeller diameter or shaft speed.

Air ingestion

  • Air is coming into the stuffing box because of a negative suction pressure.


  • The center line of the pump is perfectly aligned with the centerline of the driver (usually an electric motor).

Alpha sintered

  • A type of silicone carbide that does not contain silica.

Ambient heat/pressure

  • The heat or pressure in the area where the equipment is located.


  • To soften the metal by heating it to a predetermined temperature somewhere below its melting point.


  • A treatment used on aluminum to put a heavy stable film of oxide on the metal surface.

Anti-friction bearing

  • Usually referring to a ball or roller bearing


  • A description of the fluid and operating conditions that we are trying to pump or seal.

Atmospheric pressure

  • At sea level, atmospheric pressure is 14.7 psi.

AVS Standard

  • An old, obsolete standard that has been replaced by the A.N.S.I. standard.

Glossary B

Back plate

  • Used in some centrifugal pumps to position the stuffing box and provide an impeller wear surface.

Back pull out pump

  • A design that allows the wet end of the pump to be left on the piping when the power end and adapter are removed. A.N.S.I. pumps are designed this way.

Back to back double seal

  • The rotating seal faces are facing in opposite directions. The worst possible configuration. In the past this term was used to describe a higher barrier fluid pressure between dual mechanical seals.

Balanced seal

  • A design in which the seal face closing area is reduced to lower the closing force, and reduce the heat generation between the faces.

Balance Ratio

  • A 70/30 balance ratio means that 70% of the seal face closing area is seeing the stuffing box pressure and 30% is not seeing the pressure.

Ball bearing

  • Consists of an inner race, an outer race, and a series of balls between them. Often called a precision or anti friction bearing.


  • Metric term for one atmosphere of pressure.

Barrier fluid

  • The high pressure fluid that is circulated between two mechanical seals. The fluid should enter the bottom and leave the top to prevent air pockets.

Base plate

  • The pump and motor mount on this unit. The pump and motor feet closest to the coupling should be doweled to the base plate.


  • The mechanical seal drive lugs wear into the drive slots and prevent the seal faces from moving forward to compensate for wear.


  • Supports the rotating shaft and allows it to turn with a minimum amount of friction. Could be either sleeve or anti-friction type


  • Can be manufactured from metal or non metallic materials to eliminate flexing, rolling or sliding elastomers in mechanical seal designs.

Bellows plate

  • A thin, stamped disc. Two are welded together to form a convolution.

Bernoulli’s law

  • A moving stream of liquid or gas exerts less sideways pressure than if it were at rest. The result is that things seem to be drawn into the stream, but they are really being pushed in by the higher pressure from outside.


  • The best efficiency point. It is the point where the power coming out of the pump (water horse power) is the closest to the power coming into the pump (brake horse power) from the driver. This is also the point where there is no radial deflection of the shaft cause by unequal hydraulic forces acting on the impeller.


  • Brake horse power. The actual amount of horsepower being consumed by the pump as measured on a pony brake or dynamometer.

Body bound bolts

  • The bolt has an interference fit with the bolt hole.

Brinnell hardness

  • A method of measuring the hardness of metal parts and hard seal faces. Above 350 the standard machining operations of turning, boring, drilling, and tapping become uneconomical.

Buffer fluid

  • The low pressure fluid that is circulated between dual mechanical seals.

Buna N

  • Some times called Nitrile. A common elastomer used in the sealing of oil or water. Sensitive to Ozone attack and therefore has a short shelf life.


  • A close fitting support device used to restrict flow between two liquids, thermally isolate a hot liquid, support the rotating shaft, break down pressure etc. Commonly made of carbon or Teflon.

Bypass line

  • Used to either re-circulate fluid from the pump discharge to the stuffing box, the stuffing box to the pump suction, or the pump discharge to a lower pressure point in the system.

Glossary C

C frame adapter

  • Used to connect and align the pump to the motor with registered fits. (imperial dimensions. Called the D frame adapter in the metric system)


  • Center line average. A method of measuring surface finish in the metric system. Uses microns as a unit.

Canned pump

  • A non seal pump with the shaft, bearings and rotor contained in a can to prevent product leakage. Limited to pumping clean lubricating liquids.


  • Fluid flow measured in gpm, liters/min, M3/hr. etc.


  • The compound formed when carbon combines with an element. The carbides of metal are very hard and are often used as a mechanical seal face.

Carbon bushing

  • Used as a restrictive bushing in flushing applications, a thermal barrier in high temperature applications, a disaster bushing in an A.P.I. gland and to support a deflecting shaft in many mechanical seal applications.

Carbon/ graphite

  • A common mechanical seal face material chemically inert to most fluids with the exception of oxidizers, bleaches, halogens and a few other fluids.


  • A reduction of hydrocarbons resulting in the formation of carbonaceous residue that will interfere with the movement of a mechanical seal. Often called “coking”.


  • A cancer producing substance.

Cartridge seal

  • A self contained assembly containing the seal, gland, sleeve, and both stationary and rotating seal faces. Usually needs no installation measurement. Must be used if impeller adjustments are made. Cartridge seals are the standard for A.P.I. seal applications.


  • A chemical additive that decreases the time of a chemical reaction with out being affected by the reaction.


  • Cavities or bubbles form in the fluid low pressure area and collapse in a higher pressure area of the pump, causing noise, damage and a loss of capacity.

Center line design

  • The pump is suspended on feet attached to the sides of the volute instead of the bottom. Used in higher temperature (> 100°C) pumping applications.


  • The metric system unit of viscosity.


  • The kinematic unit of viscosity. Viscosity in centipoises divided by the liquid density at the same temperature, gives kinematic viscosity in centistokes.

Centrifugal pump

  • Moves liquid with centrifugal force. Available in circular and volute configurations.

Centrifugal separator

  • Sometimes called a cyclone separator. Uses centrifugal force to throw solids out of the fluid. Does not work very well in slurry seal applications.


  • A hard, chemically inert seal face material that includes products refereed to as silicone carbide.

Change of state

  • When a liquid flashes into a vapor, solidifies, crystallizes, cokes etc.


  • An “elastomer like” material manufactured by Green Tweed of England. Used to seal most solvents and other aggressive fluids.

Chloride stress corrosion

  • Occurs in the 300 series of stainless steel. Caused by a combination of tensile stress, chlorides and heat. No one knows the threshold values.

Chrome carbide

  • Forms when chrome forms with carbon in the heat affected zone during the welding of stainless steel. The use of low carbon stainless steel is recommended in these applications.

Chrome Oxide

  • The passivated layer that forms on the 300 series of stainless steel.

Circular casing

  • Used with centrifugal pumps that circulate fluid rather than build head or pressure.

Clam Shell

  • Used to set the pitch (distance between convolutions) while heat treating a metal bellows core.

Close coupled

  • The pump impeller is mounted directly on the motor shaft. There is no separate bearing case.

Coated Face

  • A hard coating is plated or welded to a softer base material. Presents problems with different thermal expansion rates, the hard coating can “heat check” or crack. Not recommended as a seal face material.


  • A hard black substance that forms when petroleum products are over heated. It interferes with seal movement and will open the lapped faces.


  • When used in the content of mechanical seal faces, it refers to either a non metallic material or a combination of non metallic face inserted into a metal holder.

Compression set

  • The elastomer changes shape when it has been exposed to too much heat. Round O-rings come out square.

Concentric dual seal

  • One seal is located inside the other, with a common hard face shared by both of them. Because of its large radial space requirement the seal is usually limited to mixer type applications.


  • When the parts share the same center line they are concentric to each other.


  • Condensed steam.


  • A natural circulation of fluid. The hot fluid (lighter) rises and the cool fluid (heavier) sinks.

Convection tank

  • Used to contain fluid between two mechanical seals. An enclosed heater or cooler can be used to control the barrier or buffer fluid temperature. Pressure or level gages can indicate which seal has failed.


  • Two metal bellows plates welded or formed together. To count the number of convolutions in a metal bellows you count the spaces between the bellows plates.

Cooling jacket

  • Surrounds the stuffing box of the pump to control the temperature of the fluid in the stuffing box. Usually molded into the back plate.

Corrosion resistant

  • Corrodes at a rate of less than 0.002 inches (0.05 mm) per year.


  • Used to connect the pump to the driver. It transmits torque and compensates for axial growth, but not for radial misalignment.

Critical speed

  • Any object made of an elastic material has a natural period of vibration. When a pump rotor or shaft rotates at any speed corresponding to its natural frequency, minor unbalances will be magnified. These speeds are called the critical speeds.


  • Very cold temperatures.


  • Directs the pumped liquid to the discharge piping.

Cyclone separator

  • A device used in some seal applications to separate solids from liquid by utilizing centrifugal force. Not very reliable.

Glossary D

D frame adapter

  • Used to connect and align the pump and motor (manufactured in metric dimensions). The Imperial version is called a “C” frame adapter.

D-Gun process

  • A metal spray process used to put a hard surface on a softer metal. This is the normal method of applying nickel base tungsten carbide. It is preferred over the popular Plasma Process if you arre manufacturing a mechanical seal face.

D.I.N. standard

  • The German standard for industrial products.


  • The physical touching of a component to arrest vibration.

Dead ending

  • Isolating the stuffing box. No recirculation or flushing lines in or out.


  • Movement or displacement of the shaft in a radial direction.


  • Measured in gm/cmor lb/in 2 A measure of the weight of the fluid. A better term than specific gravity.

Dial indicator

  • A tool used to measure concentricity or displacement of a shaft.


  • A liquid that thickens (increases its viscosity) with agitation.

Disaster bushing

  • Used in A.P.I. glands to support the shaft in the event of a bearing failure, or to prevent product from rushing to atmosphere after a seal failure. The close clearance (0.025 inch or 0.5 mm.) directs most of the leakage through a drain connection in the seal gland to an appropriate container.

Discharge recirculation

  • Connecting a line from the discharge side of the pump to the stuffing box. Should be used with a close fitting bushing in the end of the stuffing box to increase the stuffing box pressure. A common application when pumping a fluid close to its vapor point.

DN factor

  • Do not use precision bearings if the bearing bore (millimeters) x rpm. is 300,000 or greater

Double balanced seal

  • Hydraulically balanced in both directions. A desirable feature, but seldom provided by seal manufacturers.

Double seal

  • An out dated term describing two seals in a pump. The latest terminology is “dual seals”. In the past the term was used to describe a higher pressure barrier fluid between dual seals.

Double suction pump

  • The rotor is suspended between two bearings with the fluid entering on either side of the impeller. Used at higher capacities.

Double volute

  • A centrifugal pump design that incorporates two cut waters to prevent shaft deflection when the pump is operating off of the B.E.P. Lowers the efficiency of the pump and therefore seldom used on smaller size impellers.

Drive lugs

  • These lugs or pins transmit the torque from the set screws to the seal face.

Dry running

  • Running without fluid at the seal face.

Dual Seal

  • Two seals running in various configurations: back to back, tandem, face to face, or concentric.


  • The property of a metal that lets you give it a great deal of mechanical deformation without cracking.

Dynamic elastomer

  • The rubber part that has to move to move or flex to compensate for seal face wear or shaft movement.

Dynamic head (system head)

  • The pump head created by friction in the piping system.

Glossary E

Effective diameter

  • In metal bellows terminology it is the calculated diameter where the pressure penetrates between the metal plates. This number is used to determine the hydraulic balance diameter of the seal face.


  • Power out of the equipment divided by power in.

Elastic range

  • The stressed part retains its memory and returns to its original shape.


  • A rubber like material that, when compressed and then released will return to 90% of its original shape in less than five seconds.


  • A process involving chemical change caused by the passage of an electric current through a liquid.

Endurance limit.

  • Beyond this point the metal will fatigue without increasing the stress.


  • Environmental protection agency. A government agency with a mandate to protect the environment.


  • Ethylene propylene rubber. The most common elastomer used in the sealing of water based and higher pH materials. Cannot be used in petroleum products.


  • Permanent displacement of a portion of the O-ring into a gap, under the action of fluid pressure.

Eye of the impeller

  • The center of the impeller, where the fluid enters.

Glossary F

Face combination

  • The materials chosen for the lapped seal faces. Usually a grade of carbon graphite running on a hard face material.

Face to face seals

  • Two seals running against a common seal face. The barrier fluid pressure is always lower than stuffing box pressure.

Face flatness

  • Measured by an optical flat and a monochromatic light. The measurement is read in helium light bands (.0000116 inches or 0,3 microns).

Face lubrication

  • The fluid or vapor that sometimes exists between lapped mechanical seal faces.

Face pressure

  • The sum of all the loads on the seal face including the spring load, hydraulic load and shaft axial thrust, divided by the area of the seal face. This face load is reduced by friction between the sliding elastomer and the shaft or sleeve.

Filled carbon

  • Contains organic or inorganic materials that might be sensitive to temperature, or be attacked by the fluid you are sealing. Usually a low cost carbon.


  • A devise used to remove solid particles from liquid. It removes smaller paricles than a strainer.

Finite element analysis

  • A computer generated method of predicting seal face distortion.


  • A rapid change in liquid state from a liquid to a gas.


  • Measured by Helium light bands (0.0000116″ or 0,3 microns) as opposed to surface finish that is measured by R.M.S. or C.L.A.

Flexibility factor

  • Same as L3/DUsed to predict shaft bending problems.

Flexible member

  • The portion of the seal containing the springs or bellows.

Flexible shaft

  • A shaft with an operating speed higher than its first critical speed


  • The material assumes the shape of its container. It could be either a liquid or a gas.


  • Genetic term for the elastomer called Viton®. Viton is a Dupont Dow elastomer product.


  • Putting an outside liquid into the stuffing box of the pump at a pressure higher than stuffing box pressure. All of this liquid mixes with and dilutes the pumped fluid.


  • Supports the wet and power end of the pump and attaches it to the base plate.


  • Created whenever pressure works on an area. The units are pounds. (F = P x A)

Formed metal bellows

  • Manufactured by stretching and compressing the metal bellows material. Not usually used in mechanical seals because of its high spring rate.

F.P.M. (fpm.)

  • Feet per minute. When used in the context of seals it is measured at the center of the seal face.

Francis vane impeller

  • The most popular impeller shape with a specific speed between 1500 and 4000.

Free length

  • The uncompressed axial length of a seal.

Frett or fretting

  • Damage or grooving caused by the removal of the protective oxide that is formed on most corrosion resistant metals. It happens when a softer material (rubber) rubs against a hard shaft or sleeve. A common problem with low cost O.E.M. mechanical seals and bearing grease or lip seals.

Fugitive emission

  • The government has designated certain chemicals as hazardous to the environment. If any of these chemicals is released to the atmosphere they are called fugitive emissions.

Glossary N


  • A metric unit of force. Kilogram x gravity.

Newtonian fluid

  • A fluid that does not change viscosity as it is agitated.

Non lubricant

  • The fluid that will not maintain a film thickness of at least one micron at its operating temperature and load. A concern with mechanical sealing.

Glossary U


  • United States Customary System. All dimensions are in inch units.

Glossary X-Z

Yield point

  • Where the metal material passes from the elastic to the plastic range.

Learn all about mechanics and how to practically apply it.

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