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Glossary of pump terms: S


Sag boring: A machining techniques that bores components with a slight eccentricity to compensate for the natural sag of a rotor due to gravity. Usually used on casing wear rings and center rings to prevent the rotating parts from rubbing with the casing stationary wear parts during start-up or coast down.

Screw impeller: An impeller that is the shape of a screw. Used to pump liquids with high percentage of solids. Originally developed to pump liquid containing fish.

Seal faces: The metallic faces (lapped so they are very flat – to within a few hundred nanometers across their entire surface) in a mechanical seal that form the seal.

Sealless pump: A specially designed pump that is used to pump dangerous liquids. It prevents them escaping into the atmosphere because the internal rotor is assembled with the impeller and does not extend outside the casing.

Seal life: The life expectancy of a mechanical seal. Seals should last until the sacrificial (usually carbon graphite) is worn away.

Seal only pump: A pump that contains only seals and no soft packing, due to the fact that it has no conventional stuffing box.

Self align: Some seals are designed to align with the shaft exactly by built in mechanisms.

Self-priming pump: A pump that contains a reserve amount of process fluid that helps to create an initial vacuum and lift fluid from the source.

Self sintered silicone carbide:. One form of Silicon Carbide. To bond SiC powder into ceramics, the powder is sintered. At temperatures above 1700 °C, the alpha form of SiC is formed.

Self-venting pump: A pump in which the suction and discharge control valves can be opened to flush trapped gas during or prior to the start up sequence. API Standard 610 considers a pump to be self-venting "if the nozzle arrangement and casing configuration permit adequate venting of gases from the first-stage impeller and volute area to prevent loss of prime during the starting sequence."

Semi-open impeller: An impeller with no front (suction) shroud. Used typically when pumping particle containing fluids.

Series operation: As in electricity, this is the case when two pumps are connected such that the discharge of the first pump feeds into the suction of the second pump. Care must be taken to match the discharge of pump 1 with the intake of pump 2, otherwise overheating or cavitation can occur.

Shaft: The component of the pump that transmits the radial force from the motor to the impeller.

Shaft flexibility factor (SFF): A dimensionless number, characterizing the relative stiffness of a shaft or rotor. SFF is calculated using the formula SSF = (L3 ) / (D4). Where L is the length of the shaft and D is the diameter. Can be used to compare shafts.

Shaft packing: Seals are expensive, so manufacturers supply their pumps with soft packing to seal the shaft. Mechanical seals then replace the shaft packing.

Shaft sleeve: A thin cylindrical, metal sleeve placed around the shaft to prevent wear and tear.

Shelf life: Refers to the length of time a component will perform as intended by the manufacturer. Some elastomers (notably Buna N) have short shelf lives due to their breakdown when exposed to air and ozone.

Shore "A": A scale used to measure hardness of materials. Defined by Albert Shore in the 1920s.

Shut-off head: The shut off head is the head (pressure) delivered by the pump with the discharge valve closed (shut off).

SiC: Silicon carbide. Very hard and durable, therefore commonly found in seal faces.

Side channel pump: Is a specially designed pump that provides high head at low flows. Ideal for liquefied gases, as the pump can pump liquids with up to 50% gas.

Single plane balancing: Also known as static balancing. The process of balancing an impeller in a single plane only.

Single stage pump: A pump with a single impeller. Compare to two-stage and multistage pumps.

Single suction pump: A pump with a single suction impeller, either the first impeller in a multi-stage pump or the only impeller in a single stage pump. See double suction pump.

Single suction impeller: An impeller with a single inlet. The liquid enters from the side only. Mixed flow and axial flow impellers are always single suction type.

Sintered material: Manufactured from a powder rather than melted and poured into a mold, or formed.

Siphon: A pipe or pipe system that allows the liquid to flow uphill. Powered by the fact that the discharge point is lower than the surface of the inlet reservoir.

Skidding: This term refers to the manner in which an impeller skids along debris or solids along the bottom of a pump.

Sleeve bearing: A type of non-precision bearing. Usually a tube that fits around the shaft composed of metal or teflon.

Slip stick: Mechanical seal faces that alternately slip, then stick. Usually caused by poor quality lubricant.

Sludge pump: A pump design that prevents solids in a liquid from settling within the pump.

Slurry: A mixture of liquid and solid.

Slurry pump: A heavy duty pump designed to withstand the corrosive or abrasive effects of particles in the process fluid. Achieved by lining the pump with extra material that can withstand the assault.

Soft foot: Centrifugal pumps that are manufactured such that one of the feet on the pump has a tendency to rise when not bolted down. Makes it difficult to balance and align the system.

Soluble: When one compound (liquid or solid or gas) is dissolved into a second liquid compound, the first is said to be soluble.

Spalling: Damage to bearings caused by improper lubrication, mechanical damage, defect or fatigue.

Span: Distance across something. The distance from an inside radius to an outside radius, the distance from the shaft to the volute, etc.

Specific gravity (SG): The ratio of the density of a substance compared to the density of a reference (usually water at 4 °C).

Specific heat: The heat capacity per unit mass of a given material. The heat required to change the temperature of a specific mass of the material (e.g. 1g) by a given amount (e.g. 1 °C).

Specific speed: A dimensionless number used to characterize turbomachinery. Normalises impellers to a speed in revolutions per minute to that of a geometrically similar impeller if it were to deliver 1 gallon per minute against 1 foot of hydraulic head.

Speed of sound: How fast sound travels. Differs in different media. In air at sea level it is 340.3 m/s.

Speed-torque curve: A graph of a pump’s speed versus its torque.

Spool piece: A prefabricated section of piping including the pipe, fittings and flanges.

Spring force: The force supplied by the spring on the seal faces.

Standard volute pump close coupled: In this type of pump, the impeller is connected directly to the drive shaft.

Standard volute pump separately coupled: In contrast to a close coupled situation, a separately coupled pump is one in which the impeller is connected to the drive shaft through an intermediate shaft with couplings.

Stainless steel: An alloy of steel that contains a high percentage of chromium. Reduces oxidation.

Static head: The maximum height (pressure) that a pump can deliver.

Stationary seal: Refers to a mechanical seal where the spring loaded portion of the seal does not rotate with the shaft.

Strainer: A metal screen installed at the inlet of a pump to prevent foreign bodies from entering the pump.

Stuffing box: An assembly of mechanical components that houses a gland seal. Prevents leakage.

Stuffing box pressure: Pressure in the stuffing box. Varies widely depending on pump design.

Standby service: A service where a piece of equipment is in a state from which it can be immediately used for operation, either replacing a faulty component of sharing the load.

Static balancing: Also known as single plane balancing. The process of balancing an impeller in a single plane only.

Stiffness ratio: Another name for L3/D4, the ratio of the cube of the length of a shaft to the 4th power of it’s diameter. An indication of the stiffness of a shaft.

Strain: The deformation of a material caused by an applied stress.

Stress: The measure of the forces that particles are exerting on one another in a material.

Stress relief: To remove the residual stress from a component. Important in lapped seal faces.

Stress corrosion cracking: The process of metal undergoing fracturing when it is under tensile stress. Other examples include chloride attack of austenitic stainless steel and caustic embrittlement of cast irons and carbons.

String test: A test of the entire pumping package supplied by a vendor. Usually defined in the purchase contract. Includes all components of the system.

Submersible pump: A pump that operates when it is completely submerged. Must have waterproof electrical circuits and is usually liquid cooled.

Submersion: Submersion refers to the height difference between the surface of the intake reservoir and the opening of the pump intake pipe.

Suction bell: A bell shaped tubular section at the inlet of a vertical line shaft pump. Used for directing the flow of liquid into the impeller inlet.

Suction case: A bell shaped tubular section at the inlet of a horizontal pump. Used for directing the flow of liquid into the impeller inlet.

Suction flow splitter: A piece of metal across the intake of certain pumps to force the liquid into a laminar flow.

Suction guide: An extra mechanical device that helps to produce laminar flow in situations where the piping just before a pump contains a 90° bend.

Suction head: Suction head is the term for the head due to the fact that the source of the pump is above its centre line.

Suction lift: Opposite of suction head. Occurs when the source for the pump is below the pump’s centre line.

Suction line: The suction line of a pump system is piping which transports fluid material from its source to the pump itself.

Suction recirculation: A system to reduce pressure in the stuffing box by recirculating some of the discharge back into the stuffing box.

Suction specific speed: A number that will help to determine what type of geometry to use for maximum efficiency, but prevent cavitation.

Suction static head: The height difference between the surface of the inlet reservoir and the centre line of the pump. If the tank is pressurised, this pressure is also included.

Suction static lift: Also known as suctions static head. Only occurs when the pump is above the inlet reservoir.

Surface speed: A measure in linear terms (feet per minute) of how fast a component that is moving radially is moving.

Swamee-Jain equation: An equation that can be used as a substitute for the Colebrook equation for calculating the friction factor f.

Synchronous motor: Seldom used in pumps, but often in elevators or compressors. Pumps typically use “squirrel cage” motors.

System: The total set of components is referred to as the system. In a pumping system, this would include piping and mechanical equipment from the inlet point to the discharge. as in pump system.

System curve: A graph depicting the pump’s total head vs flow. It is used to estimate how the pump will perform at different flow rates. The Total head includes the static head (constant) and the friction head and velocity head difference – and these depend on the flow rate. The intersection of the system curve with the pump characteristic curve defines the operating point of the pump.

System head: The head (pressure) cause by friction along the pipe walls and due to the joints in the system..

System requirements: The set of conditions (velocity, elevation difference, pressure, etc.) that define or determine the total head.

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