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Glossary Term: SIL - Safety Integrity Level
Safety Integrity Level - A quantitative target for measuring the level of performance needed for safety function to achieve a tolerable risk for a process hazard. Defining a target SIL level for the process should be based on the assessment of the likelihood that an incident will occur and the…
Glossary Term: Signal common
1. The signal common shall refer to a point in the signal loop which may be connected to the corresponding points of other signal loops. It may or may not be connected to earth ground [S50.1]. 2. The reference point for all voltage signals in a system. Current flow into…
Glossary Term: Signal isolation
Signal isolation refers to the absence of a connection between the signal loop and all other terminals and earth ground.
Glossary Term: SIF - Safety Instrumented Function
Safety Instrumented Function – A set of equipment intended to reduce the risk due to a specific hazard (a safety loop). Its purpose is to 1. Automatically taking an industrial process to a safe state when specified conditions are violated; 2. Permit a process to move forward in a safe…
Glossary Term: Set pressure
The inlet pressure at which a safety relief valve opens; usually a pressure established by specification or code.
Glossary Term: SFF
Safe Failure Fraction - The fraction of the overall failure rate of a device that results in either a safe fault or a diagnosed (detected) unsafe fault. The safe failure fraction includes the detectable dangerous failures when those failures are annunciated and procedures for repair or shutdown are in place.
Glossary Term: Set point
1. An input variable which sets the desired value of the controlled variable It is expressed in the same units as the controlled variable.
Glossary Term: Sensor group
For complex safety functions, there may be more than one property which is measured to determine if a shut down is required.
Glossary Term: Sensor
device or combination of devices that measure the process condition (e.g., transmitters, transducers, process switches, position switches, etc.)
Glossary Term: Segmented ball
A closure piece in a valve that is a segment of a spherical surface which may have one edge contoured to yield a desired flow characteristic.
Glossary Term: Seal leg
The piping from the instrument to the top elevation of the seal fluid in the impulse line. seal on disk A seal ring located in a groove in the disk circumference. The body is unlined in this case [S75.05].
Glossary Term: Seal chambers
Enlarged pipe sections in measurement impulse lines to provide a) a high area to volume displacement ratio to minimize error from hydrostatic head difference when using large volume displacement measuring elements, and b) to prevent loss of seal fluid by displacement into the process. Also known as Seal Pots
Glossary Term: SCADA
Supervisory control and data acquisition: Operator interface and monitoring of (usually remote) control devices by computer.
Glossary Term: Saturation
A situation when a further change in the input signal produces no significant additional change in the output.
Glossary Term: SAT
Site acceptance test. Involves shipment of the system〈s〉 to site, installation and start-up activities. Tests then validate that the installed safety instrumented system and its associated safety instrumented functions achieve the requirements as stated in the Safety Requirement Specification. Note: Full loop checking may come at a later stage.
Glossary Term: Sampling rate
For a given measurement, the number of times that it is sampled per second in a time division multiplexed system. Typically, it is at least five times the highest data frequency of the measurement.
Glossary Term: Sample interval
The rate at which a controller samples the process variable, and calculates a new output. Ideally, the sample interval should be set between 4 and 10 times faster than the process dead time.
Glossary Term: Safety Requirement Specification (SRS)
Specification containing all the requirements of the safety functions that have to be performed by the safety-related system. It includes both what the functions must do and also how well they must do it. It is often a contractual document between companies and is one of the most important documents…
Glossary Term: Safety manual
Document required for equipment certified in accordance with IEC 61508 that describes the conditions of use for that equipment in safety applications. It typically includes usage requirements/restrictions, environmental limits, optional settings, failure rate data, useful life data, common cause beta estimate, inspection and test procedures. The “safety manual” may be…
Glossary Term: Safety lifecycle
The procedures to first analyze the situation and document the safety requirements (Analysis Phases). Then, translate these requirements into a documented safety system design, using appropriate software and hardware subsystems and design methodology (Realization Phases). Next, evaluate the system against the required integrity and reliability specifications and modify it as…
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