A–B–C–D–E–F–G–H–I–J–K–L–M–N–O–P–Q–R–S–T–U–V–W–X–Y
Safety Factor
Is a factor applied to the nominal operating torque or thrust given by the valve manufacturer. Can be between 1.2 and 2.0 depending on application.
Safety Relief Valve
A quick-acting spring-loaded valve that opens (relieves) when the pressure exceeds the spring setting. It specifically opens proportionally with pressure increasing. Often installed on the body cavity of ball and gate valves to relieve thermal overpressure in liquid services.
SAW (Submerged Arc Welding)
Submerged Arc Welding is a common arc welding process. It requires a continuously fed consumable solid or tubular (flux cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being submerged under a blanket of granular fusible flux. When molten the flux becomes conductive and provides a current path between the electrode and the work.
SCC (Stress Corrosion Cracking)
Cracking of metals involving anodic processes of localized corrosion and tensile stress (residual and/or applied) in the presence of wet H2S.
Schedule
A system for indicating the wall thickness of pipe. The higher the schedule number the thicker the wall for a given pipe size.
Scoring
A severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding.
Scotch-Yoke Mechanism
The mechanism within a quarter-turn actuator for converting linear movement to 90° rotary output. Several yoke configurations (symmetrical, canted & demi-canted) allow selection of the actuator output torque to best suit the valve torque characteristics.
Scotch-Yoke Operator
A quarter turn operator based on a scotch-yoke mechanism rather than gears. The scotch-yoke mechanism produces a torque output at the beginning and ending of its stroke that is about twice the magnitude of the torque output at the middle of its stroke.
Scragging
The process of operating a new spring several times prior to installation into a spring cartridge. Assures optimum and consistent spring performance from the first operation of the actuator.
Scratching
The mechanical removal or displacement (or both) of material from a surface by the action of abrasive particles or protuberances sliding across the surfaces.
Screwed Ends
Internally threaded end connections supplied on some (small size) valves. Usually tapered pipe threads (NPT).
Seal
A device used to prevent the passage of fluid. Types include ‘O’ ring, lip seal, gasket, etc.
Seal Dynamic
A sealing element used between parts that have relative motion, i.e. stem seals and seat seal.
Seal Static
A sealing element used as a gasket between two non-moving parts, i.e. valve bonnet seal and flange gasket.
Seat
That part of a valve against which the closure element effects a tight shut-off. In ball and gate valves, it is a floating member containing either a soft (plastic) or a metallic seating element.
Seating Surfaces
Contact surfaces of the obturator and seat which ensure valve sealing.
Self Relieving
The process whereby excessive internal body pressure is automatically relieved either into the upstream or downstream line by forcing the seats off the closure element.
Shore Hardness
Shore Hardness using either the Shore A or Shore D scale is the preferred method for measuring rubbers/elastomers hardness. It is also commonly used for “softer” plastics. The Shore A scale is used for soft to medium-hard rubbers (Shore D less than 20). The Shore D scale is more accurate on samples harder than 90 Shore A. Shore Durometer is the instrument used to gauge corresponding hardness. The ASTM test number is ASTM D2240, while the analogous ISO test method is ISO 868.
Short Gate Valve
A gate valve whose seat rings contact the gate only in the closed position. Such valve is not through-conduit as the gate is completely withdrawn from the flow area in the open position.
Short Pattern Valve
A valve whose face-to-face dimension is smaller than that in API 6D / ISO 14313 standard.
Shrinkage
After vulcanization, dimensional loss in a molded rubber product that occurs after it has been removed from the mold and allowed to cool. In seal service, a decrease in seal volume due to extraction of soluble components from the rubber compound by environmental fluids.
Shut Off Valve
A valve designed for on/off service only not a throttling valve. Sometimes referred to as a block valve.
Shuttle Valve
3-port device, 2 inlets and 1 outlet. Output is switched when the preset cracking pressure is exceeded to change position.
Side Entry Ball Valve
One of ball valve designs that allowed the removal of the valve from the pipeline for disassembly and maintenance on site or at the manufacturer’s plant.
Side-Mounted Handwheel
Handwheel operators and electric actuators can be side mounted on a valve and driven through a bevel gearbox to allow easier operator access.
SIL (Safety Integrity Level)
SIL is defined as a relative level of risk-reduction provided by a safety function or to specify a target level of risk reduction. The SIL requirements for hardware safety integrity are based on a probabilistic analysis of the device. To achieve a given SIL, the device must have less than the specified probability of dangerous failure and have greater than the specified safe failure fraction. These failure probabilities are calculated by performing a Failure Modes and Effects Analysis (FMEA). The actual targets required vary depending on the likelihood of a demand, the complexity of the device(s) and types of redundancy used. Four SIL levels are defined, with SIL4 being the most dependable and SIL1 being the least. IEC 61508 and IEC 61511 standards define the functional safety requirements for the process industry.
Silencer
A device to reduce the gas flow noise by tuned resonant control of gas expansion.
Silicone rubber
Silicon-oxygen backbone elastomer with excellent high temperature and low temperature properties.
Simple Apparatus
Passive or defined low energy devices generating less than 1.5V, 100mA, 25mW.
Single-Acting Actuator
See Spring-Return Actuator.
Sizing Pressure
The pressure stated as the fluid pressure available to size the actuator to operate the valve.
Slab Gate
A valve gate having flat finely finished parallel faces, as opposed to a wedge gate. Such a closure element slides across the seats and does not depend on stem force (mechanical seal) to achieve tight shut-off.
Slam Retarder
A device designed to prevent the clapper of a check valve from slamming as it closes upon flow reversal. Hydraulic damping cylinders, rotary vanes and torsion springs are all used for this purpose.
Slurry Service
An application involving a slurry fluid. Coal slurry consisting of about equal parts of coal and water is transported by pipeline from coal mines to power plants where the coal is de-watered and burnt.
Smart Actuator
An actuator or positioner which includes a microprocessor for communicating with a central control system and to make local control decisions, distributing part of the control function normally performed in the central DCS.
SMAW (Shielded Metal Arc Welding)
A manual arc welding process that uses a consumable electrode coated in flux to lay the weld. An electric current in the form of either alternating current or direct current from a welding power supply is used to form an electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintegrates giving off vapours that serve as a shielding gas and providing a layer of slag both of which protect the weld area from atmospheric contamination. Because of the versatility of the process and the simplicity of its equipment and operation, the shielded metal arc welding is one of the world’s most popular welding processes.
SNG (Synthetic Natural Gas)
A substitute natural gas made from the by-products of chemical plants and refineries.
Socket End
An end connection in which a pipe or tube is inserted into a counter bored hole and then brazed or fillet-welded.
Soft Seated Valve
This term has been cited in this valve handbook under “SEAT”. A picture of a soft seat arrangement has also been included.
SOHIC (Stress Oriented Hydrogen Induced Cracking)
Staggered small cracks formed approximately perpendicular to the principal stress (residual and/or applied) resulting in a “ladder-like” crack array linking pre-existing HIC cracks.
Solenoid
A device that turns an electrical signal sent to a coil into a mechanical movement inside a solenoid valve, thus allowing it to control the flow of liquid and gases. It is often used in the control piping of powered hydraulic or pneumatic cylinder operators.
Sour Gas
Natural gas containing significant amounts of hydrogen sulfide (H2S). The presence of hydrogen sulfide can result in a number of mechanisms of cracking of metallic materials such as stress cracking (SSC), stress corrosion cracking (SCC), hydrogen induced cracking (HIC), stepwise cracking (SWC), stressoriented hydrogen-induced cracking (SOHIC), soft zone cracking (SZC) etc. NACE MR0175 / ISO 15156 presents requirements and recommendations for material qualification and selection of metallic materials for service in equipment used in oil and gas production and in natural gas sweetening plants in H2S containing environments.
Sour Service
Exposure to oilfield environments that contain H2S and can cause cracking of materials by the mechanism specifically addressed by NACE MR0175 / ISO 15156.
Spalling
The separation of macroscopic particles from a surface in the form of flakes or chips, but also resulting from impacts events.
Specific Gravity
The ratio of the weight of a given volume of fluid to the weight of an equal volume of water (if the fluid is a liquid) or to the weight of an equal volume of air (if the fluid is a gas).
Speed Control Valve
A 2-port valve, usually a needle valve, for restricting the flow into or out of an actuator to adjust the operating time.
Spiral Wound Gasket (SPWD)
Identifies a flat gasket used between two RF flanges (without ring grooves). SPWD gaskets contain a spiral wound metallic filler for reinforcement.
Spring Cartridge
The welded enclosure containing the compressed spring used in spring-return (single-acting) actuator types.
Spring-Return Actuator
An actuator which requires power in one direction with a spring operating in the other direction. Also referred to as a single-acting actuator.
Spur Gear
The simplest type of gear. In a gear set, the input spur gear and output spur gear are aligned on parallel shafts. An idle gear may be used so that the direction of rotation on the two shafts is in the same direction.
Squeeze
Compression of an O-ring’s cross-section between mating surfaces, noted as both a decimal measurement (in inches and/or millimetres) and as a percentage of the original cross-section (width). Radial compression occurs on the outside diameter (OD) and inside diameter (ID). Axial compression occurs on the top and bottom surfaces.
SSC (Sulphide Stress Cracking)
It is a form of hydrogen induced cracking (HIC) and involves embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface when exposed to water and hydrogen sulphide. The atomic hydrogen can diffuse into the metal, reduce ductility and increase susceptibility to cracking. High strength metallic materials and hard weld zones are prone to SSC. NACE MR0103 specifically defines materials requirements for resistance to SSC in sour refinery process environments, i.e. environments that contain wet hydrogen sulfide.
SSIV (Subsea Isolation Valve)
A valve used underwater generally in a manifold that will close and isolate a pipeline or process in an emergency.
SSV (Surface-Safety Valve)
Automatic wellhead valve assembly which closes upon loss of power supply.
Stem
A rod or shaft used to transmit motion from an operator to the closure element of a valve.
Stem Extension Assembly
Assembly consisting of the stem extension and the stem extension housing.
Stem Indicator
A position indicating rod supplied with gate valves. It extends from the top of the valve stem and serves to indicate the relative position of the gate. Also termed VPI.
Stem Nut
A one or two-piece nut which engages the stem threads of a valve and transmits torque from an operator to the valve stem.
Stiction
A combination of friction and adhesion (sticking) that must be overcome to move a device from rest.
Stop Collar
The collar on a ball valve which restricts the ball to quarter rotation from the fully open to the fully closed position. See Key Stop.
Stopple (Stop Off)
A procedure used in the repair of a pipeline to isolate a section of line in the absence of a shut-off valve. After welding a flanged saddle to the pipe, the line is “hot tapped” and an expanding resilient plug is inserted into the pipe bore. When repair is completed, the plug is withdrawn and a valve installed on the saddle flange is closed.
Strain Gauge
A device used to measure deformation (strain) of an object. The most common type consists of a metallic foil pattern attached to the object by a suitable means. As the object is deformed, the foil deforms causing its electrical resistance to change. Strain is thus measured by relating it to the change of electrical resistance.
Stress
Defined as force per unit area, stress is a measure of the intensity of internal forces acting within a body.
Stretch
Measured as a percentage increase in the inside diameter (ID) of an O-ring, stretch results in a reduction and flattening of the seal’s cross-section. There are two types of stretch; installation stretch (as the seal is being placed in the groove) and assembly stretch (once the seal is seated).
Stud
A bolt threaded on both ends used in bolting together two members, one of which often has blind tapped bolt holes.
Studded Flange Connection
Flanged end or outlet connection in which thread-anchored studs screwed into tapped holes replace the holes for bolt studs.
Stuffing Box
The annular chamber provided around a valve stem in a sealing system into which deformable packing is introduced. See Packing Gland.
Support Ribs of Legs
Metal structure which provides a stable footing when the valve is set on a fixed base.
Surface Finish
Average value of exterior roughness often expressed in micro inches i.e. RMS (Root Mean Square) or Ra (Roughness Arithmetic Average).
Surge
To rise suddenly to an excessive or abnormal value or a transient sudden rise of pressure in a pipeline. Pipeline surges can be positive or negative and are caused most frequently by the sudden closure of a block valve or emergency shutdown of a pump. Surge pressures in excess of the rated capacity of a pipeline can cause ruptures of the piping system.
Surge Reliever
A valve designed to relieve pressure surges in liquid pipelines thus preventing line rupture due to transient pressures exceeding design limits of the pipe.
Survival Temperature
A media temperature that the valve is only required to survive in typically for a duration of time. The valve is not meant to be operated at this temperature. It is meant to survive at this temperature in either the open or closed position.
Swage
A tool for bending or forming cold metal to a required shape.
SWC (Stepwise Cracking)
Cracking that connects hydrogen-induced cracks on adjacent planes in a steel.
Sweet Gas
Natural gas that does not contain hydrogen sulfide (H2S) or significant quantities of carbon dioxide (CO2).
Swell
Volumetric increase of an elastomeric material when in contact with a fluid.
Swing Check Valve
A check valve in which the closure element is a hinged clapper which swings or rotates about a supporting shaft.
SZC (Soft Zone Cracking)
Form of SSC that may occur when a steel contains a local “soft zone” of low yield strength material.