Sag boring: A machining technique used to bore components with a slight eccentricity in order to compensate for the natural sag of a rotor caused by gravity. This method is typically applied to casing wear rings and center rings to prevent contact between rotating parts and stationary wear parts during start-up or coast down.

Screw impeller: An impeller that is shaped like a screw and is specifically designed for pumping liquids with a high concentration of solids. It was originally developed for pumping liquids containing fish.

Seal faces: The flat metallic faces in a mechanical seal that come into contact with each other to create a seal. These faces are lapped to ensure a very smooth surface finish.

Sealless pump: A specialized pump design used for pumping hazardous liquids. It prevents the escape of these liquids into the atmosphere by incorporating an internal rotor that is assembled with the impeller and does not extend outside the casing.

Seal life: The expected lifespan of a mechanical seal. The seals are designed to last until the sacrificial material, usually carbon graphite, is worn away.

Seal only pump: A pump that exclusively utilizes mechanical seals for sealing and does not employ traditional soft packing due to the absence of a conventional stuffing box.

Self-align: Refers to seals that are designed to automatically align with the shaft through built-in mechanisms, ensuring precise alignment without additional adjustment.

Self-priming pump: A pump that contains a reserve amount of process fluid, which assists in creating an initial vacuum and lifting fluid from the source, enabling self-priming operation.

Self-sintered silicon carbide: A form of silicon carbide produced by sintering SiC powder at temperatures above 1700 °C, resulting in the formation of the alpha form of SiC. This material is commonly used in various applications due to its hardness and durability.

Self-venting pump: A pump that incorporates suction and discharge control valves that can be opened to flush trapped gas during or prior to the startup sequence. The venting capability ensures the prevention of loss of prime by allowing gases to be vented from the first-stage impeller and volute area, as defined by API Standard 610.

Semi-open impeller: An impeller design that lacks a front or suction shroud, typically employed when pumping fluids containing particles or solids.

Series operation: Refers to the connection of two pumps in which the discharge of the first pump is connected to the suction of the second pump. Care must be taken to match the discharge of the first pump with the intake of the second pump to avoid overheating or cavitation.

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

Shaft flexibility factor (SFF): A dimensionless number used to characterize the relative stiffness of a shaft or rotor. The SFF is calculated using the formula SFF = (L3) / (D4), where L represents the shaft length and D represents the shaft diameter. This factor is useful for comparing different shaft designs.

Shaft packing: Soft packing material used to seal the shaft in pumps. It is often provided by manufacturers as a cost-effective alternative to mechanical seals.

Shaft sleeve: A thin cylindrical metal sleeve that is installed around the shaft to protect it from wear and tear caused by contact with other components.

Shelf life: The length of time during which a component is expected to perform as intended by the manufacturer. Some elastomers, such as Buna N, have relatively short shelf lives due to their tendency to degrade when exposed to air and ozone.

Shore “A”: A hardness scale developed by Albert Shore in the 1920s to measure the hardness of materials.

Shut-off head: The head, or pressure, delivered by the pump when the discharge valve is closed (shut off). It represents the maximum pressure capability of the pump.

SiC: Abbreviation for silicon carbide, a hard and durable material commonly used in seal faces due to its excellent properties.

Side channel pump: A specialized pump design that provides high head at low flow rates. It is particularly suitable for pumping liquefied gases and can handle liquids with up to 50% gas content.

Single plane balancing: Also known as static balancing, this is the process of balancing an impeller in a single plane to ensure smooth rotation.

Single stage pump: A pump that contains a single impeller. This is in contrast to two-stage or multistage pumps, which feature multiple impellers.

Single suction pump: A pump that incorporates a single suction impeller, either as the first impeller in a multistage pump or as the only impeller in a single stage pump. The single suction impeller allows liquid to enter from one side only. Contrast with a double suction pump.

Sintered material: Material manufactured by compacting and heating a powdered substance without melting or pouring it into a mold.

Siphon: A pipe or pipe system that allows liquid to flow uphill, powered by the difference in fluid levels between the discharge point and the surface of the inlet reservoir.

Skidding: The term used to describe the movement of an impeller along debris or solids settled at the bottom of a pump.

Sleeve bearing: A type of non-precision bearing that typically consists of a metal or Teflon tube surrounding the shaft to provide support and reduce friction.

Slip stick: A phenomenon observed in mechanical seal faces where they alternately slip and stick during operation. This behavior is often attributed to the use of poor-quality lubricants.

Sludge pump: A pump specifically designed to handle liquids containing a high concentration of solids, preventing the settling of these solids within the pump.

Slurry: A mixture of liquid and solid particles.

Slurry pump: A robust pump specifically designed to handle process fluids containing corrosive or abrasive particles. These pumps are equipped with additional lining materials that can withstand the harsh effects of the particles.

Soft foot: A condition found in centrifugal pumps where one of the pump’s feet tends to rise when not securely bolted down. This makes it challenging to balance and align the pump system properly.

Soluble: The property of a compound, whether it is a liquid, solid, or gas, to dissolve into another liquid compound.

Spalling: The damage that occurs to bearings due to factors such as inadequate lubrication, mechanical stress, defects, or fatigue.

Span: The distance measured across a component or system, such as the distance from an inside radius to an outside radius or the distance from the shaft to the volute.

Specific gravity (SG): A measurement that compares the density of a substance to the density of a reference substance, typically water at a specific temperature.

Specific heat: The amount of heat energy required to raise the temperature of a specific amount of a material by a given increment.

Specific speed: A dimensionless number used to characterize turbomachinery, particularly impellers. It normalizes the impeller’s speed and hydraulic performance to a standard reference point.

Speed of sound: The velocity at which sound waves travel through a particular medium, such as air. It varies depending on the properties of the medium.

Speed-torque curve: A graphical representation of the relationship between the pump’s speed and the torque it generates.

Spool piece: A pre-fabricated section of piping that includes the pipe, fittings, and flanges, often used for easy installation and connection.

Spring force: The force exerted by a spring on the seal faces of a mechanical seal.

Standard volute pump close coupled: A pump configuration in which the impeller is directly connected to the drive shaft.

Standard volute pump separately coupled: A pump configuration where the impeller is connected to the drive shaft through an intermediate shaft and couplings.

Stainless steel: A corrosion-resistant alloy of steel that contains a high percentage of chromium.

Static head: The maximum vertical height or pressure that a pump can deliver.

Stationary seal: A type of mechanical seal in which the spring-loaded component remains stationary while the shaft rotates.

Strainer: A metal screen or filter installed at the inlet of a pump to prevent the entry of foreign particles or debris.

Stuffing box: A chamber that houses a gland seal in a pump to prevent leakage.

Stuffing box pressure: The pressure present within the stuffing box of a pump. This pressure can vary depending on the pump’s design.

Standby service: A state in which a piece of equipment is ready for immediate operation, either as a backup or to share the load with another component.

Static balancing: The process of balancing an impeller or rotor in a single plane to eliminate unbalance.

Stiffness ratio: A parameter calculated by taking the cube of the shaft length and dividing it by the fourth power of its diameter. It provides an indication of the shaft’s stiffness.

Strain: The deformation experienced by a material due to the application of stress.

Stress: The force per unit area acting on a material, which can cause deformation or change in shape.

Stress relief: A process to eliminate or reduce residual stress in a component, which is particularly important for lapped seal faces.

Stress corrosion cracking: The fracturing of a metal material under tensile stress caused by the corrosive environment in which it operates.

String test: A comprehensive test performed on an entire pumping package supplied by a vendor, typically specified in the purchase contract. It includes testing all the components of the system.

Submersible pump: A pump designed to operate while completely submerged in the fluid being pumped. It requires waterproof electrical circuits and is usually cooled by the pumped liquid.

Submersion: The difference in height between the surface of the fluid in the intake reservoir and the opening of the pump’s intake pipe.

Suction bell: A bell-shaped tubular section located at the inlet of a vertical line shaft pump, designed to direct the flow of liquid into the impeller.

Suction case: A bell-shaped tubular section located at the inlet of a horizontal pump, designed to guide the flow of liquid into the impeller.

Suction flow splitter: A component placed across the intake of certain pumps to induce laminar flow and control the direction of liquid entering the impeller.

Suction guide: An additional mechanical device used to promote laminar flow when a pump’s suction piping contains a 90° bend.

Suction head: The pressure or head resulting from the fact that the source of the fluid being pumped is positioned above the pump’s centerline.

Suction lift: The opposite of suction head, occurring when the fluid source for the pump is located below the pump’s centerline.

Suction line: The piping system that transports the fluid material from its source to the pump’s inlet.

Suction recirculation: A system that recirculates a portion of the pump’s discharge back into the stuffing box to reduce pressure in that area.

Suction specific speed: A dimensionless number used to determine the optimal impeller geometry for maximum efficiency while avoiding cavitation.

Suction static head: The vertical height difference between the surface of the fluid in the inlet reservoir and the centerline of the pump. It may also include any additional pressure from a pressurized tank.

Suction static lift: Also known as suction static head, it occurs when the pump is positioned above the level of the fluid source, resulting in a lift to draw the fluid into the pump.

Surface speed: A measurement of the linear velocity at the outer surface of a component, typically expressed in feet per minute.

Swamee-Jain equation: An equation that can be used as an alternative to the Colebrook equation for calculating the friction factor in fluid flow.

Synchronous motor: A type of motor commonly used in applications such as elevators or compressors, but not typically used in pumps. Pumps usually employ “squirrel cage” motors.

System: The complete set of components, including piping and mechanical equipment, involved in a pumping system, from the inlet point to the discharge.

System curve: A graph that illustrates the relationship between the pump’s total head and the flow rate. It is used to estimate the pump’s performance at different flow rates. The system curve is derived from the sum of static head, friction head, and velocity head changes with flow rate. The intersection of the system curve with the pump characteristic curve determines the operating point of the pump.

System head: The total head, including static head, friction head, and velocity head, required by a pumping system to overcome the resistance and deliver the desired flow rate.

System requirements: The collection of criteria, such as velocity, elevation difference, pressure, and more, that establish and dictate the total head within a pumping system. These requirements outline the necessary conditions for achieving the desired flow rate and efficient operation of the system.

Related Links

• Pump Glossary
• Pump info