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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…
Glossary Term: Safety ground
1. A connection between metal structures, cabinets, cases, etc. which is required to prevent electrical shock hazard to personnel. 2. Safety ground is not a signal reference point.
Glossary Term: Safe state
The state of the process after acting to remove the hazard resulting in no significant harm.
Glossary Term: Safe area
1. Nonhazardous (unclassified) location. 2. An area in which explosive gas/air mixture are not expected to be present so that special precautions for the construction and use of electrical apparatus are not required.
Glossary Term: Safe failure
Failure that does not have the potential to put the safety instrumented system in a dangerous or fail-to-function state. The situation when a safety related system or component fails to perform properly in such a way that it calls for the system to be shut down or the safety instrumented…
Glossary Term: Rupture disc
A diaphragm designed to burst at a predetermined pressure differential. Symbol rupture disc device A nonreclosing pressure relief device that relieves excessive static inlet pressure via a rupture disc.
Glossary Term: RRF
Risk Reduction Factor - The inverse of PFDavg
Glossary Term: RMP
Risk management plan. Part of the US requirement under the Occupational Safety and Health Administration (OSHA) guidelines for managing risk when dealing with large quantities of certain materials.
Glossary Term: Risk integral
A summation of risk as expressed by the product of consequence and frequency. The integral is summed over all of the potential unwanted events that can occur. If calculating the risk integral for loss of life, the consequence of concern and thus the units of the integral are fatalities. It…
Glossary Term: Risk graph
A qualitative and category-based method of safety integrity level (SIL) assignment. Risk graph analysis uses four parameters to make a SIL selection: consequence, occupancy, probability of avoiding the hazard, and demand rate. Each of these parameters is assigned a category and a SIL is associated with each combination of categories.…
Glossary Term: Risk (Unmitigated)
The level of risk that is present in a process before any safety instrumented systems are considered. This level helps identify how much risk reduction is required to be provided by any safety instrumented system installed as part of a process. This unmitigated risk level must be defined in terms…
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