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X - Z

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. 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.



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


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.


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

The force exerted by the atmosphere on the earth's surface.  At sea level, atmospheric pressure is 1.013 bar (14.7 psi).  As elevation increases, atmospheric pressure decreases.


Axial flow pump

Refers to a design of a centrifugal pump for high flow and low head.  The impeller shape is similar to a propeller.

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.


Back-to-back double seal

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

Back vane

Located behind the impeller shroud in some impeller designs to lower stuffing box pressure.


Balanced seal

A seal 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.



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.


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.


Best efficiency point (B.E.P.)

It is the point where the power coming out of the pump (water horsepower) is the closest to the power coming into the pump (brake horsepower) 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.


Break horsepower (B.H.P.)

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


Brinell 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.

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.


Capacity (flow rate)

Fluid flow measured in m3/hr, liters/min, gpm, 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.



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".


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.



The body of the pump, which encloses the impeller.  It guides fluid from suction connection to impeller eye, collects energy added fluid from impeller periphery, and discharges fluid through discharge connection.



A chemical additive that decreases the time of a chemical reaction without 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.

Centipoise (cP)

The metric system unit of viscosity.


Centistoke (cSt)

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


Centrifugal force

A force associated with a rotating body.  This force causes the rotation body to move away from its center of rotation.


Centrifugal pump

A machine that moves fluid with centrifugal force.  Centrifugal pumps use an impeller and a volute to create the partial vacuum and discharge pressure necessary to move fluid through the casing.  The impeller and volute form the heart of a pump – their design determines its flow, pressure, and solid handling characteristics.  As the impeller rotates and churns the fluid, it purges air from the casing, creating an area of low pressure, or partial vacuum, at the eye of the impeller.  The weight of the atmosphere on the external body of fluid pushes fluid rapidly through the piping and pump casing toward the eye of the impeller.  Centrifugal force created by the rotating impeller pushes fluid away from the eye, where pressure is lowest, to the vane tips, where pressure is the highest.  The velocity of the rotating vanes pressurizes the fluid, forcing it through the volute and discharging it from the pump.


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.


Change of state

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


Check valve

A device for preventing flow in the reverse direction.



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 oxide

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


Closed impeller

The impeller vanes are sandwiched within a shroud which keeps the fluid in contact with the impeller vanes at all times.  This type of impeller is more efficient than an open type impeller.  The disadvantage is that the fluid passages are narrower and could get plugged if the fluid contains impurities or solids.



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


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 overheated.  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 casing

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


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.


Cooling jacket

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


Corrosion resistant

Corrodes at a rate of less than 0.05 mm (0.002 in) 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.



The narrow space between the impeller and the casing in the discharge area of the casing, directs the pumped liquid to the discharge piping.

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.



The weight of a fluid per unit of volume, measured in kg/m3, g/cm3 or lb/in3.  A better term than specific gravity.


Dial indicator

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


Diaphragm pump

A positive displacement pump using diaphragm to create suction and pumping force, ability to pump a wide variety of viscous, chemically aggressive, abrasive and impure liquids.  



Located in the discharge area of the pump, the diffuser is a set of fixed vanes often an integral part of the casing that reduces turbulence by promoting a more gradual reduction in velocity.



A liquid that thickens (increases its viscosity) with strain or displacement.


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 (0.5 mm or 0.025 in) 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.


Double balanced seal

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


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.


Drooping curve

Similar to the normal profile except at the low flow end where the head rises then drops as it gets to the shut-off head point.


Dry running

Running without fluid at the seal face.

Dual (double) seal

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



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


Duty point

The point (flow rate and total head) at which the pump operates.  It is located at the intersection of the system curve and the performance curve of a pump.  It corresponds to the flow and head required for the process.


Dynamic elastomer

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


Dynamic discharge head

The sum of the static discharge head and the discharge friction loss in the discharge line.  Also referred to as total discharge head.


Dynamic suction head

The sum of the static suction head and the suction friction loss in the suction line.  Also referred to as total suction head.

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.


End suction pump

A type of pump construction where the suction nozzle located at the end of the pump shaft.


Endurance limit

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


Equivalent length

A method used to establish the friction loss of fittings.


Ethylene propylene (EPDM)

The 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.



Also known as repeller.  A second impeller used to lower the stuffing box pressure.

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 (0.3 microns or 0.0000116 in).


Face lubrication

The fluid or vapor that 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 particles than a strainer.


Finite element analysis (F.E.A.)

A computer generated method of predicting element distortion under stress.



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


Flat curve

Head decreases very slowly as flow increases.


Flexibility factor

Same as L3/D4 used 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.


Float switch

A device used to start and stop a pump based on preset water levels.



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


Fluid type

Whether the fluid being pumped is clean or dirty, contains any solids or abrasives, or is a hazardous material.



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.


Foot valve

A check valve that is put on the end of the pump suction pipe, often accompanied with an integrated strainer.



Created whenever pressure works on an area.  The units are newton or 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.


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 mechanical seals and bearing grease or lip seals.



The force produced as reaction to movement.  All fluids are subject to friction when they are in motion.  The higher the fluid viscosity, the higher the friction force for the same flow rate.


Friction head

Head required to overcome the resistance to flow in the pipe and fittings.

Galvanic series

A list of metals with those on the top of the list being attacked by those lower down in the list.  The further apart on the list, the faster the attack.



Used between two static surfaces to provide a seal.  Made from a variety of deformable materials.

Gear pump

A positive displacement pump using rotation of the gears to pump liquid.  They are used for high pressure industrial transfer and metering applications on clean, filtered, lubricating fluids.



The part that controls the pumped liquid leakage rate through packing and is attached to the stuffing box.


Grease seal

A spring loaded elastomer seal commonly used to seal bearings.  Sometimes called a "lip seal".


Elements that attack carbon.  Fluorine, bromine, chlorine, astatine and iodine.


Hard face

A seal face either rotating or stationary.  The most common materials are silicone carbide, ceramic, tungsten carbide, stellite, Ni-resist.  The hard face must be the wider seal face.


Harmonic vibration

Vibrating in harmony with something near by.  This can be a big problem for bearings in stationary or non running equipment.


Hastelloy C

A nickel rich, corrosion resistant metal used for pump components and mechanical seal springs and metal bellows because it is not sensitive to chloride stress corrosion.



The equivalent height of the liquid.  20°C water is used as the standard where 10 m (33.9 ft) of water equals one atmosphere (1 bar or 14.7 psi).  The term head is used instead of pressure in the centrifugal pump business.


Helium light band

A method of measuring seal face flatness.  One helium light band equals 0.3 microns (0.0000116 in).  Seal faces are normally lapped to within three helium light bands of flatness.



A common method of measuring power.  1 horsepower equals 76 kgf.m/s (33,000 lbf.ft/min).  


Hydraulic balance

A method of reducing mechanical seal face loading by reducing the seal face closing area.


Hydraulic force

Occurs when liquid pressure acts on a seal face area.


Hydraulic gradient

All the energy terms of the system (for example velocity head and piping and fitting friction loss) are converted to head and graphed above an elevation drawing of the installation.



A petroleum product consisting of hydrogen and carbon.


Inside diameter.



International standards organization.  Sets pump and seal standards for the metric community.



A rotating disk with a set of curved vanes coupled to the drive shaft that produces centrifugal force within the pump casing of a centrifugal pump.  Available in fully-open, semi-open and closed designs.


Impeller clearance setting

Open impellers require a clearance between the volute or the pump back plate depending upon design.  This clearance must be set when the pump is at operating temperature and must be reset to compensate for wear.

Impeller eye

The center of the impeller or the point where fluid enters the impeller.


Impeller shroud

The plates located on one or both sides of the impeller vanes.  Prevents solids from penetrating behind the vanes.


Impeller vane

Located between the eye and the discharge side of the impeller.  Imparts movement and pressure to the fluid and directs the flow of the liquid to the outside diameter of the impeller.



The opposite of explode.  Bubbles implode in the higher pressure areas of the pump making noise and causing damage to the metal parts.  This is normally called cavitation.



A small axial flow vane that attaches to the impeller of a centrifugal pump to increase the N.P.S.H. available.


Induction motor

The most common type motor used in industry.  Has a slippage of 2 to 5 percent compared to synchronous motors.


Inline pump

Mounted in the piping.  No base plate or alignment required.


Internal recirculation

A loss of efficiency caused by liquid flowing through wear rings or the impeller to volute clearances.


Intergranular corrosion

A corrosion of the grain boundaries in the body of the material.


Usually refers to the heating/cooling jacket surrounding the casing or stuffing box on some pumps.


Jet pump

A special effect pump using venturi to create low pressure providing a vacuum at the suction and allowing the pump to lift fluids.


A metric unit for the measurement of energy.  Defined as the energy required to move one newton over one meter.


An "elastomer like" material manufactured by E.I. Dupont that is used to seal most solvents and other aggressive fluids.  It is available in several different grades.


Kelvin scale

Used to measure low temperatures in the Centigrade scale.  Similar to Renkin which is used to measure low temperatures in the Fahrenheit system.


Kilopascal (kPa)

A metric unit for pressure.  100 kPa equals 1 atmosphere.


Kilowatt (kW)

A metric unit for power.  1 kW equals 1000 W.

L3 / D4

A guide line for determining pump shaft stiffness where the length of the shaft is compared to its diameter.  This number should be below 2.0 in metric units and 60 in imperial units.


Labyrinth seal

A non contacting seal utilizing a tortured path for the escape of the fluid.  Utilizes a series of pressure drops to reduce the leakage.



A distinct flow regime that occurs at low Reynolds number (Re < 2000).  It is characterized by fluid particles in layers moving past one another without mixing.


Lantern ring

A device used to supply lubricant to packing.  Usually located in the middle of the packing ring set.


Line bearings

Position the rotor or shaft radially.  Normally of the sleeve type.


Line bored

When the drilling or boring is done on a jig that insures every component is in a straight line.


Lip seal

See grease seal.


Lobe pump

A positive displacement pump where by meshing of the lobes forces liquid through the outlet port under pressure. Primarily used in food applications because they handle solids without damaging them.


Low flow

A condition that can cause excessive heat inside the pump volute.  A temperature rise of 10°C (18°F) across the operating pump is considered excessive.  Usually caused by throttling a pump discharge valve.


Low N.P.S.H. pump

A pump designed for application with a low N.P.S.H. available, usually has an inducer.



Any fluid that will maintain a film thickness of one micron or more at its operating temperature and load.

Mach number

The relationship between a moving body and the speed of sound in that locality which can vary with temperature, altitude and therefore pressure.


Magnetic drive

A type of seal less pump that is pretty much limited to pumping clean lubricating liquids.  Similar in concept to a canned pump.


Magnetic seal

Uses magnetic materials rather than springs to keep the lapped seal faces together.  Limited to non corrosive fluids because of the magnets.


Mating ring

Another name for the hard face in a mechanical seal.  It can be either rotating or stationary.


Maximum suction lift

The height (approx. 7.5 m or 25 ft) that water can be lifted by a centrifugal pump in actual conditions, taking into consideration altitude, friction loss, temperature, suspended particles, and the inability to create a perfect vacuum. The 7.5 m suction lift is attainable for cold water (15°C) at sea level.  Suction lift diminishes as elevation increases, due to the reduction in atmospheric pressure.  In addition, suction lift decreases as the water temperature increases since warm water contains more entrained air.


Mean diameter

The middle diameter.  Usually refers to the center of the seal face.  The term is commonly used with metal bellows seals to describe the middle diameter of the bellows plate.


Mechanical seal

A positive sealing device used to seal all fluids (liquids and gases).  The primary seal is a set of lapped seal faces that are installed perpendicular to the shaft.


Mercury (Hg)

A metal that remains liquid at room temperature.  This property makes it useful when used in a thin vertical glass tube since small changes in pressure can be measured as changes in the mercury column height.  The inch of mercury is often used as a unit for measuring vacuum level or pressures below atmospheric pressure.


Metal bellows

Used in mechanical seal designs to eliminate the need for a dynamic elastomer.


Metal fatigue

A breakage of the metal caused by the bending and flexing of a metal part beyond its endurance limit.


Micro organisms

Used in a variety of chemical processes.  Can cause corrosion of stainless steel if they penetrate the passivated layer and attack the carbon content of the stainless steel.


Minimum flow rate

A minimum flow rate that a pump can operate without causing excessive heat inside the pump volute.  A temperature rise of 10°C (18°F) across the operating pump is considered excessive.  


Minimum N.P.S.H.A.

The margin of safety or minimum N.P.S.H.A. that should be available depends in part on the amount of suction energy of the pump.  



When one liquid mixes or blend with another liquid.  Same as soluble.


Mixed flow pump

Refers to the design of a centrifugal pump for medium head and medium flow, in between axial flow and radial flow pump.


Modulus of elasticity

Refers to the stiffness of the material.  The higher the modulus the stiffer the material.  Also called Young's modulus.  The relationship of stress to strain.  If material have a high modulus they are less likely to distort under stress.


Moment of inertia

Referring to rotation about an axis.  In the pump business it refers to a formula that describes the shape of the shaft.  A solid shaft would have a different moment of inertia than a hollow shaft.


Monochromatic light

A single color light used with an optical flat to read seal face flatness.


Moody diagram

A graphical representation of the laminar and turbulent flow equations.

Negative pressure

Any pressure less than atmospheric pressure.


Net positive suction head available (N.P.S.H.A.)

The net positive suction head available in the system to prevent cavitation of the pump.  It is defined as: atmospheric pressure + gage pressure + static pressure - vapor pressure - friction loss in the suction piping.


Net positive suction head required (N.P.S.H.R.)

The net positive suction head required by the pump to operate without cavitation.  This number is given to you by the pump manufacturer.  Since the number was generated by testing with cold fresh water, it can be increased in some cases if you are pumping hot water or some hydrocarbons.


Newton (N)

A metric unit for force.  Kilogram x gravity.


Newtonian fluid

A fluid whose viscosity is constant and independent of the rate of shear.


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.


Outside diameter.


Oil analysis

A sensible preventative maintenance technique to check the oil for solids, water etc.


Oil life

Lubricating oil has a useful life of about thirty years at thirty degrees centigrade if it is not contaminated.


Oil mist

A lubrication system where oil and air are mixed and sent into the pump bearing housing.  Not friendly to the environment.


Open impeller

The impeller vanes are open and the edges are not constrained by a shroud.  This type of impeller is less efficient than a closed type impeller.  The disadvantage is mainly the loss of efficiency as compared to the closed type of impeller and the advantage is the increased clearance available which will help any impurities or solids get through the pump and prevent plugging.


Operating length

Measured after the seal has been compressed the proper amount.  


Operating point

See duty point.  


Optical flat

A high quality glass lapped flat on one side and used with a monochromatic light to read seal face flatness.


O-ring groove

The space into which an O-ring is inserted.  Dynamic O-ring grooves use a different dimension than static O-ring grooves.


Overhung impeller

Impeller is mounted on the end of a shaft that overhangs its bearings.  Typical arrangement for end suction pump.  



Combines with carbon to form carbon monoxide or carbon dioxide.  The oxidizers attack all forms of carbon including the seal face and any black O-rings in the system.



Created by oxygen atoms combining with oxygen molecules in a high energy atmosphere.  Will prematurely age Buna N rubber.  Ozone forms in the shop as a result of the sparking of electric motor brushes.

P.V. factor

An attempt to correlate the relationship between the pressure and the velocity at the mechanical seal faces.



The soft rings to control the leakage of the liquid pumped where the shaft passes through the casing.  Packing must leak because it works on the theory of a series of pressure drops to reduce the stuffing box pressure to the point where the leakage is acceptable.


Parallel operation

The pumps are discharging to a common header.  It is important that the impeller speed and outside diameters be the same or one of the pumps will throttle the other.


Pascal (Pa)

A metric unit for pressure.  100000 Pa equals 1 atmosphere.



The protective oxide layer has formed on the metal surface.


Peripheral pump

Also known as regenerative or regenerative turbine pump.  These are low capacity, high head pumps.  The impeller has short vanes at the periphery to add energy to the fluid particles.  Peripheral pumps are more efficient at these low flow high head conditions than centrifugal pumps; they also require much less NPSHA than an equivalent centrifugal pump.  They can also handle liquids with up to 20% entrained gases.



A measure of the acidity or the alkalinity of a fluid.  The scale ranges from 0 (acid) to 14 (alkali) with 7 considered neutral.


Pipe roughness

A measurement of the average height of peaks producing roughness on the internal surface of pipes.  It normally used to calculate friction loss across the pipe.


Pipe strain

The strain on the pump volute caused by the piping.  It will cause excessive pump components movement and can cause contact between rotating and stationary pump and seal components.



Surface voids caused by corrosion, erosion or cavitation.


Plastic range

If you stress metal enough, it stretches through the elastic range, its yield point and ends up in the plastic range where it loses its memory and will not return to its original shape.


Positive displacement pump

Called a PD pump.  Gear, sliding vane, progressive cavity, lobe, diaphragm, piston etc.  The energy is periodically added to fluid by application of force and direct increase in pressure to move the fluid.  The capacity determined by the pump speed while the maximum head is determined by the horsepower available and the casing strength.


Power end

The end of the pump that attaches to the power source and does not get wet from the pumpage.  The bearings are in this part.


Power factor

A measure of how the voltage leads or lags the amperage.


Press fit

The best way to insert a carbon/graphite face into a metal holder.  The carbon will shear to conform to the holder's irregular shape.  Usually done with an arbor press.



The application of a force to a body producing more or less compression within the liquid.  In a static fluid pressure varies with height.


Pressure drop

Referring to the loss of pressure from the outside to the inside of the mechanical seal faces or across the individual rings of packing.


Pressure gradient

The pressure drop between the seal faces.  Usually illustrated by a wedge.


Pressure head

The pump head exerted by atmospheric pressure or any additional pressure that might be in the vessel.



The creation of a partial vacuum inside the pump casing, which allows water to flow into the pump.


Progressive cavity pump

A positive displacement pump consists of only one driven metal rotor rotating within an elastomer lined (elastic) stator.  These pumps are ideal for fluids that are just too tough for other pumps to handle. e.g. – pastes, greases, sludge etc.



The property of a fluid whose viscosity increases slowly with rate of shear.



A machine to lift, transfer, or increase the pressure of a fluid (liquid or gas) or to create a vacuum in an enclosed space by the removal of a gas.


Pump performance curve

A diagram supplied by the pump manufacture to describe the relationship between the head and the capacity of a particular pump using various impeller sizes or pump running speeds.  The curve also includes information about efficiency, horse power consumption, N.P.S.H.R., etc.


Pump out vane

See back vane.


Pumping ring

Used with a convection system to get circulation between two mechanical seals.  Absolutely necessary if oil is used as a barrier fluid because of oil's poor specific heat.


The introduction of a fluid outside the seal to cool the product, or dilute any leakage across the seal faces.


Quill shaft

A hollow shaft with another shaft inside it.


90 degrees to the centerline of the shaft.


Radial bearing

In an end suction centrifugal pump it is the bearing located closest to the stuffing box.  This bearing handles most of the radial loads put on the impeller.


Radial flow pump

Refers to the design of a centrifugal pump for high head and low flow.



At low flow and high flow compared to the flow at the B.E.P. the fluid will start to recirculate or move in a reverse direction at the suction and at the discharge.


Renkin scale

Used to measure low temperatures in the Fahrenheit scale.  Similar to Kelvin which is used to measure low temperatures in the Centigrade system.



See expeller.


Reverse balance

A common type of hydraulic balance used on outside mounted mechanical seals.


Reynolds number (Re)

The Reynolds number is proportional to the ratio of velocity and viscosity.  The higher the number (higher than 4000 for turbulent flow) the more turbulent the flow and the less viscosity has an effect, where the pipe roughness becomes the controlling factor for friction loss.  The lower the Reynolds number (less then 2000 for laminar flow) the more the viscosity of the fluid is relevant



The property of a fluid whose viscosity increases with time.


Rigid shaft

A shaft with a rotating speed lower than its first critical speed.



Root mean square.  A measure of surface finish or smoothness.


Rockwell "C"

The scale most often used to measure hardness of a material.


Rotating seal

When the spring loaded or moveable portion of the seal rotates with the shaft.


Run out

Twice the distance that the center of the shaft is displaced from the axis of rotation.

Seal faces

The lapped faces that provide the primary sealing in a mechanical seal.


Seal life

Seals should run leak free until the sacrificial face (usually carbon/graphite) is worn away.  More than 85% of the mechanical seals in use today fail prematurely.


Self align

A method of keeping both mechanical seal faces square to the rotating shaft.


Self-priming pump

A pump that able to purge air from its casing and suction hose, and does not require priming or an initial filling with liquid.  The pump casing carries a reserve of water that helps create a vacuum that will lift the fluid from a low source.


Series operation

The pumps are connected with the discharge of the first pump discharging to the suction of the other.  The speed and impeller widths must be the same or the difference in capacities could cause cavitation or over heating problems.



A rotating rod that transmit power from driver to impeller.


Shelf life

Usually refers to the seal elastomer.  Buna N is the biggest problem because of ozone attack.


Shut off head

The total head corresponding to zero flow on the pump performance curve.



Silicon carbide.  A common mechanical seal hard face material.


Sintered material

Formed from a powder as opposed to being melted and poured into a mold.



A system of piping or tubing where the exit point is lower than the entry point and where some part of the piping is above the free surface of the fluid source.



A term used in describing how an impeller slides on the solids in the bottom of a mixer.


Sleeve bearing

A non precision bearing.  Usually manufactured from carbon, teflon, brass, etc.  Allows too much axial and radial movement for most mechanical seal applications.


Slenderness ratio

Another name for L3/D4.


Slip stick

An alternating slipping and sticking of the seal faces caused by a poor lubricant between the faces.  Will cause vibration problems at the seal face unless the vibration is dampened.



Solids in liquid.  It is impossible to define when the quantity and size of the particles becomes too much for the mechanical seal.

Slurry pump

A heavy duty pump intended for aggressive or abrasive slurry solutions with particles of various sizes.


Soft foot

The tendency for one of the pumps feet to rise when the hold down bolt is loosened.  Very bad for alignment.


Solid concentration

Ratio of solids to liquid in the overall volume of the material being pumped, which is helpful in determining the proper pump for the application.


Solid size

Average diameter of individual particles in the material being pumped, which is important to know when specifying a pump.  Large solids can be filtered with strainers or rock guards.



When one liquid dissolves or mixes with another liquid.


Specific gravity (SG)

The ratio of the density of a fluid to the density of water at standard conditions.  Fresh water at 4°C (39°F) is given a value of 1.  If the liquid you are questioning will float on water the specific gravity is less than 1.  If it sinks, it is higher than 1.  Density is a better term.


Specific heat

Refers to the amount of heat energy or BTUs required to raise a quantity of a liquid one degree.


Specific speed

A formula that describes the shape of a pump impeller and indicates a pump type.  Radial flow pump will have low specific speed where axial flow pump will have high specific speed.


Spring force

The force on the seal faces caused by the spring compression.


Stainless steel

Alloy steels containing a high percentage of chromium.


Static discharge head

The difference in elevation between the discharge level or point of free discharge and the centerline of the pump.  This head also includes any additional pressure head that may be present at the discharge tank fluid surface, for example as in the case of a pressurized discharge tank.


Static suction head

The difference in elevation between the liquid level of the fluid source and the centerline of the pump.  This head also includes any additional pressure head that may be present at the suction tank fluid surface, for example as in the case of a pressurized suction tank.


Static suction lift

The same definition as the static suction head.  This term is only used when the pump centerline is above the suction tank fluid surface.


Stationary seal

The spring loaded or moveable portion of the seal does not rotate with the shaft.  Must be used when the seal surface speed exceeds 25 m/s or 5000 ft/min.



A fitting at the end of a suction hose that prevents solids larger than its solids handling capability from entering the pump.


Stiffness ratio

Another name for L3/D4.


A measure of the amount of deformation produced in a substance when it is stressed.



A measure of the intensity of the load applied to a material.  Stress is expressed as the load divided by the cross-sectional area over which it is applied.


Stress relieve

To take residual stress out of an object.  This is very important with lapped seal faces, especially those that have been inserted into a metal holder.


Stuffing box

A part of the pump that held the packing or mechanical seal.


Stuffing box pressure

Pressure contained inside the stuffing box.  Normally between suction and discharge pressure but closer to suction pressure.


Submersible pump

A centrifugal pump designed to operate within the water source being pumped, thereby eliminating the suction lift limitations which is common to other types.



The height between the free surface of a suction tank and the pump intake pipe.  Minimum submersion is required to avoid vortex formation.


Suction guide

A device that helps straighten the flow ahead of a pump.


Suction recirculation

Piping from the bottom of the stuffing box back to the suction side of the pump.  Used to lower pressure and circulate liquid in the stuffing box.


Suction specific speed

A number that indicates whether the suction conditions are sufficient to prevent cavitation.  According to the Hydraulic Institute the suction specific speed should be less than 8500.


Synchronous motor

Runs without slippage.  Used in elevators and compressors frequently, but seldom used in pumping.  Pumps mostly use induction or squirrel cage motors.



As in pump system.  The system includes all the piping, including the equipment, starting at the inlet point and ending at the outlet point.


System curve

A description of what the pump is required to perform.  The pump will pump where the system curve intersects the pump performance curve.


System head

The sum of the dynamic suction head and the dynamic discharge head.  Also referred to as total head.

Tandem seals

The seals are facing in the same direction with a low pressure barrier fluid circulating between them.


Tensile strength

The strength measured when the part is being pulled axially.


Theoretical suction lift

The maximum height (10 m or 33.9 ft) that water can be lifted inside a tube under perfect vacuum at sea level.


Thermal conductivity

A measure of the material's ability to conduct heat.  This is a very important factor in the selection of mechanical seal faces.


Thermal imaging

A troubleshooting piece of equipment that will let you see hot spots in the equipment.



A plastic material that can be softened or melted repeatedly without change of properties.  Injection molded parts are manufactured with thermoplastic materials.



A plastic material that can be softened and molded but cannot be re-used or reverted to its original state.



The property of a fluid whose viscosity decreases with time.



In a centrifugal pump it refers to the axial movement of the shaft.  The thrust can be towards the wet or power end of the pump and at start up it thrusts in both directions.


Thrust bearing

Designed to take the axial thrust in pump applications.  It is usually located next to the coupling and is often supplied in a "double row" configuration.



One millimeter of mercury.  Used in both the imperial and metric systems as a unit of measurement in vacuum service.


Total discharge head

See dynamic discharge head.


Total dynamic head (T.D.H.)

Identical to total head.  This term is no longer used and has been replaced by the shorter total head.


Total head

See system head.


Total static head

The difference between the static discharge and suction head including the difference between the surface pressure of the discharge and suction tanks if the tanks are pressurized.


Total suction head

See dynamic suction head.



Attached to the pump and used to send a vibration signal to a meter where it can be read.

Tungsten carbide

A common hard face seal material available in several grades depending upon hardness and corrosion resistance. Cobalt and nickel are the two most common types.



The behavior of fluid particles within a flow stream characterized by the rapid movement of particles in many directions as well as the general direction of the overall fluid flow, that occurs at high Reynolds number (Re < 4000).  Can cause cavitation problems in a centrifugal pump.  Often caused by an elbow located too close to the pump suction inlet.


Two way balance

A method of balancing a mechanical seal in two directions.  A very important consideration in dual seal applications.

Unbalanced seal

Not hydraulically balanced to generate low heat at the seal faces.  Typical of original equipment designs.


Unfilled carbon

Containing carbon/graphite and nothing else.  Filled carbons contain inorganic that will be sensitive to some chemicals and temperature extremes.  Unfilled carbons are the preferred seal faces.

Variable speed motor

Used to control flow in a system by varying the frequency of the motor.  Recommended for circulating systems and any other system where the main head is friction losses in the piping system.



Any pressure less than atmospheric pressure.  Can present a problem for the elastomer in many seal applications.


Vane passing syndrome

A type of cavitation caused by the impeller and cutwater clearance being too small.


Vane pump

A positive displacement pump using vanes or blades to force fluid in and out the pumping chamber.  Because of the vane strength and the absence of metal-to-metal contact, vane pumps are ideally suited for low-viscosity, non lubricating liquids up to 2.200 cSt (10,000 SSU).  Such liquids include LPG, ammonia, solvents, alcohol, fuel oils, gasoline, and refrigerants.


Vapor pressure

The pressure at which a liquid boils for a specific temperature.  Below this pressure, the liquid at this temperature will vaporize.



The fluid passes from a liquid to a gaseous state.  If this happens at the seal faces the seal faces will be blown open. 



A measurement of the speed of the liquid in the system.  Measured in meters or feet per second.  The pump is a constant velocity device.


Velocity head

Part of the total head calculation.  Derived from the formula h = v2/2g



To remove air or gas from the system.  It is important to vent the stuffing box in vertical pumps to prevent the seal faces from running dry.



A venturi is a pipe that has a gradual restriction that opens up into a gradual enlargement.  The area of the restriction will have a lower pressure and higher velocity than the enlarged area ahead of it.


Vibration damping

Important in metal bellows seal designs.  The elastomer acts as a vibration damper in the other seals.  The vibration can chip carbon faces, destroy anti-rotation drive lugs, and open the lapped seal faces.



The resistance of a liquid to flow at a given temperature.  Centrifugal pumps are affected by viscosity and for fluids with a viscosity higher than 10 cSt, the performance of the pump must be corrected.  Centrifugal pumps can handle a maximum viscosity of 800 cSt.  Above this viscosity a positive displacement pump should be used.


Viscous drag pump

A pump whose impeller has no vanes but relies on fluid contact with a flat rotating plate turning at high speed to move the liquid.



An E.I. Dupont Dow manufactured elastomer widely used in the sealing industry.  The generic name is fluorocarbon.


Volute casing

Derives it’s name from a spiral shaped casing surrounding the pump impeller.  It converts velocity energy to pressure energy.


Vortex formation

Creating a "whirlpool affect" that can draw air into the suction of the pump.


Vortex pump

A type of pump used for excessive solids.  The impeller is recessed into the volute.  A very low efficiency design, but practical in many applications.

Water hammer

Cause by pressure fluctuations in systems with long discharge lines, e.g. in industrial and municipal water supply systems, in refineries and power stations, when the pumped fluid is accelerated or decelerated resulting changes in velocity.


Water horsepower (W.H.P.)

The calculated horsepower coming out of the pump using the formula W.H.P. = density x gravity x capacity x head.


Watt (W)

A metric unit for power.  746 W equals one horsepower.

Wear ring

Used with closed impeller pumps to restrict leakage from the high pressure side of the pump to the low pressure side.  Should be replaced when the recommended clearance is doubled.


Welded metal bellows

A seal design used to eliminate the use of elastomers.  Excellent for cryogenic and hot applications.  Not as effective for hot petroleum applications because of "coking" problems.


Wet end

The part of the pump that gets wet from the pumping fluid.  Includes the volute, impeller, stuffing box, impeller wear rings, and shaft or sleeve.

Yield point

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

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