Glossary S

Seal faces

  • The lapped faces that 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.

Seal only pump

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

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.

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.

Shaft packing

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

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.


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

Sintered material

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


  • 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.

Soft foot

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


  • When one liquid dissolves or mixes with another liquid.


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

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.

Specific heat

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

Specific speed

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

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

Spring force

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

Stainless steel

  • Alloy steels containing a high percentage of chromium.

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.

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

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

Stuffing box pressure

  • Between suction and discharge pressure but closer to suction pressure.

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.

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

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.

System curve

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

System head

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


  • On February 09, 2018