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Glossary Term: Derivative control
Change in the output that is proportional to the rate of change of the input. Also called “rate control.”
Glossary Term: Decision table
A table of all contingencies that are to be considered in the description of a problem, together with the actions to be taken. Decision tables can be used in place of flow charts for problem description and documentation.
Glossary Term: Dead time
The interval of time between initiation of an input change or stimulus and the start ofthe resulting response.
Glossary Term: D Diagnostics
Some safety rated logic solvers are designated as having capital D diagnostics. These are different from regular diagnostics in that the unit is able to reconfigure its architecture after a diagnostic has detected a failure. The greatest effect is for 1oo2D systems which can reconfigure to 1oo1 operation upon detecting…
Glossary Term: Dangerous failure
A failure of a component in a safety instrumented function that prevents that function from achieving a safe state when it is required to do so. See failure mode.
Glossary Term: Cross talk
The unwanted energy transferred from one circuit, the disturbing circuit, to another circuit, the disturbed circuit. Typically signals electrically coupled from another circuit.
Glossary Term: Coriolis flow meter
A mass flow meter which measures mass flow of a fluid by determining the torque resulting from radial acceleration of the fluid. The name comes from the Coriolis effect that describes the accelerating force acting on any body moving freely above the earth’s surface, which is caused by the rotation…
Glossary Term: Consequence
The magnitude of harm or measure of the resulting outcome of a harmful event. One of the two components used to define a risk.
Glossary Term: Common mode failure
A random stress that causes two or more components to fail at the same time for the same reason. It is different from a systematic failure in that it is random and probabilistic but does not proceed in a fixed, predictable, cause and effect fashion. See systematic failure.
Glossary Term: CPU
central processing unit: The part of a computing system that contains the arithmetic and logical units, instruction control unit, timing generators, and memory and I/O interfaces. This is typically a very complex element which requires Type B classification for SIL hardware fault tolerance requirements according to IEC 61508.
Glossary Term: CPQRA
(Guidelines for) Chemical Process Quantitative Risk Analysis
Glossary Term: CPT or PTC
Proof test coverage – The percentage failures that are detected during the servicing of equipment. In general it is assumed that when a proof test is performed any errors in the system are detected and corrected (100% proof test coverage).
Glossary Term: Check valve
A flow control device that permits flow in one direction and prevents flow in the opposite direction
Glossary Term: CFSE/CFSP
Certified Functional Safety Expert/Professional Qualifications for safety engineers in either process applications, machine applications, hardware, or software that demonstrates competence in safety lifecycle activities. These qualifications are administered by the non-profit CFSE Governance managed by a global consortium of vendor, user, integrator and consultant companies.
Glossary Term: Cavitation
A two stage phenomenon of liquid flow. The first stage is the formation of voids or cavities within the liquid system; the second stage is the collapse or implosion of these cavities back into an all liquid state. Cavitation can cause excessive wear and damage to devices in regions where…
Glossary Term: Cause and effect diagram
One method commonly used to show the relationship between the sensor inputs to a safety function and the required outputs. Often used as part of a safety requirements specification. The method’s strengths are a low level of effort and clear visual representation while its weaknesses are a rigid format (some…
Glossary Term: Capacitance
The ability of a capacitor to store a charge. The greater the capacitance, the greater the charge that can be stored. Also applied to tanks in process fluid flow systems.
Glossary Term: Calibration curve
A plot of indicated value versus true value used to adjust instrument readings for inherent error; a calibration curve is usually determined for each calibrated instrument in a standard procedure and its validity confirmed or a new calibration curve determined by periodically repeating the procedure.
Glossary Term: Butterfly valve
A valve consisting of a disc inside a valve body which operates by rotating about an axis in the plane of the disc to shut off or regulate flow.
Glossary Term: Bus
A group of wires or conductors, considered as a single entity, which interconnects part of a system.
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