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McNally - pump and seal terms Category: General technical and industrial > Pump and Seal terms Date & country: 24/09/2008, USA Words: 312

U.S.C.S.
United States Customary System. Al dimensions are in inch units.

V.D.M.A.
A German standard for mechanical seals.

T.D.H.
Total discharge head. A combination of the suction head and the head being produced by the pump.

P.D. Pump
Positive displacement pump. It can pump a high pressure or head, but at a low volume.

P.V. Factor
An attempt to correlate the relationship between the pressure and the velocity at the mechanical seal faces. Unfortunately carbon graphite is sensitive to P, but not to V so the correlation does not work too well.

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.

O.E.M.
Original equipment manufacturer. The pump or seal company, not the distributor of the products.

N.P.S.H.R.
Net positive suction head required to stop a pump from cavitating. This number is given to you by the pump manufacturer. Since the number was generated by testing with cold fresh water, it can be lowered in some cases if you are pumping hot water or some hydrocarbons.

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 60 in imperial units and 2.0 in metric units.

N.P.S.H.A.
The net positive suction head available to prevent cavitation of the pump. It is defined as: Atmospheric pressure + gage pressure + static pressure - vapor pressure - friction loss in the suction piping.

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

F.P.M. (fpm.)
Feet per minute. When used in the context of seals it is measured at the center of the seal face.

I.D.
Inside diameter.

D.I.N. standard
The German standard for industrial products.

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

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

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

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

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.

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

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

Wave spring
A disc washer used when axial space is at a premium. Does not allow very much axial travel of the seal.

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.

Watt
A measure of power. 746 watts equals one horsepower.

Water Horse Power (W.H.P.)
The calculated horse power coming out of the pump using the formula WHP = head x gpm/3960

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.

Vortexing liquid
Creating a 'whirlpool affect' that can draw air into the suction of the pump.

Volute casing
Derives is name from a spiral shaped casing surrounding the pump impeller. It converts velocity energy to pressure energy.

Viton
An E.I. Dupont Dow manufactured elastomer widely used in the sealing industry. The generic name is fluorocarbon. Many of these compounds are attacked by water and steam.

Viscosity
Resistance to pouring. Higher viscosities can restrict seal movement. Centrifugal pumps can handle a maximum viscosity similar to 30 weight oil at room temperature. Above this viscosity a positive displacement pump should be used.

Vent
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.

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.

Velocity head
Part of the total head calculation. Derived from the formula h = V²/2g

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

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.

Vapor pressure
Below this pressure, the liquid at this temperature will vaporize.

Vane passing syndrome
A type of cavitation caused by the impeller/ cutwater clearance being too small.

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

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.

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

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

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

Turbulence
Disturbed fluid. Can cause cavitation problems in a centrifugal pump. Often caused by an elbow located too close to the pump suction inlet.

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.

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

Total head
The amount of head produced by the pump. Discharge head minus suction head. If suction head is a negative number it is added to the discharge head.

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

Thrust
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.

Thrust bearing
Locates the rotor or shaft axially. Normally located clse to the coupling.

Thixotrophic fluid
The viscosity of the fluid decreases with agitation. Non drip paint is an example of such a fluid.

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

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

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

Tensile strength
The strength measured when the part is being pulled axially.

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.

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

System head
The head caused by friction in the piping. valves and fittings.

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

Surface speed
A measure of the feet per minute the seal face or some other component is moving.

Synchronous motor
Runs with out slippage. Used in elevators and compressors frequently, but seldom used in pumping. Pumps mostly use induction or 'squirrel cage' motors.

Suction specific speed
A formula that will predict one of the types of cavitation. Pumps should be purchased with a number below 8500 (10,000 metric)

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 head
The head on the suction side of the pump. You subtract it from the discharge head to determine the head being produced by the pump. It is a sum of the static, pressure and friction heads.

Stuffing box pressure
Between suction and discharge pressure but closer to suction pressure.

Stuffing box
That portion of the pump that held the packing and now holds the mechanical seal.

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.

Stiffness ratio
Another name for L³/D⁴

Stress
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.

Static head
The maximum height the liquid is being pumped as long as you take into consideration the siphon affect in some piping systems.

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 5000 fpm. or 25 meters per second.

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

Stainless steel
Alloy steels containing a high percentage of chromium.

Speed of sound
In air it is 1090 feet per second, in water it is 4800 feet per second.

Spool piece
Usually refers to a seal and bearing combination that is supplied for some mixer applications. It depends upon the application, but this is not a good idea if there are already two anti-friction bearings already installed on the shaft

Specific speed
A formula that describes the shape of a pump impeller. The higher the specific speed the less N.P.S.H. required.

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

Specific Gravity
A measure of the weight of a liquid. Fresh water at 4°C (39°F) is given a value of one. If the liquid you are questioning will float on water the specific gravity is less than one. If it sinks, it is higher than one. Density is a better term.

Soluble
When one liquid dissolves or mixes with another liquid.

Span
The width of the metal bellows. The distance from the inside diameter to the outside diameter.

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

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

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.

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

Skidding
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.

SiC
Silicon carbide. A common mechanical seal face material. A type of ceramic.

Shelf life
Usually refers to the seal elastomer. Buna 'N' is the biggest problem because of Ozone attack.

Shore "A"
A scale used to measure elastomer hardness.

Shut off head
The maximum head that the pump can generate with a given impeller outside diameter and horsepower driver.

Shaft packing
The soft packing supplied by pump manufacturers. Most of these leaking packings are being replaced by mechanical seals.

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.

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

Self sintered silicone carbide
Same as Alpha sintered. No free silica present. Can be used in caustic and higher pH liquids.

Seal only pump
The pump does not have a conventional stuffing box so it could not be supplied with soft packing.

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.

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