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Glossary Term: MTTF
Mean Time to Failure - The average amount of time until a system fails or its “expected” failure time. Please note that the MTTF can be assumed to be the inverse of failure rate (lambda) for a series of components, all of which have a constant failure rate for the…
Glossary Term: Modulation
1. The process or the result of the process by which some characteristic of one wave is varied in accordance with some characteristic of another wave (AM, amplitude modulation; PM, phase modulation; FM, frequency modulation). 2. The action of a control valve to regulate fluid flow by varying the position…
Glossary Term: Mode (Low Demand)
(also demand mode per IEC 61511) when demands to activate the safety instrumented function (SIF) are infrequent compared to the test interval of the SIF. The process industry defines this mode when the demands to activate the SIF are less than once every two proof test intervals. The low demand…
Glossary Term: Mode (High Demand)
(also continuous mode per IEC 61511) Similar to continuous mode only there is specific credit taken for automatic diagnostics. The split between high demand and continuous mode is whether the automatic diagnostics are run many times faster than the demand rate on the safety function. If the diagnostics are slower…
Glossary Term: Mode (Continuous)
When demands to activate a safety function (SIF) are frequent compared to the test interval of the SIF. Note that other sectors define a separate high demand mode, based on whether diagnostics can reduce the accident rate. In either case, the continuous mode is where the frequency of an unwanted…
Glossary Term: Markov analysis
A fault propagation method used to analyze failure rate or probability for safety instrumented functions. A diagram is constructed to represent the system under consideration including the logical relationships between its components. In Markov analysis there are a group of circles, each of which represents a system state. The different…
Glossary Term: LOPA
Layer of Protection Analysis. A method of analyzing the likelihood (frequency) of a harmful outcome event based on an initiating event frequency and on the probability of failure of a series of independent layers of protection capable of preventing the harmful outcome.
Glossary Term: Longitudinal redundancy check (LRC)
Error detection scheme that consists of a byte where each bit is calculated on the basis of the parity of all the bits in the block that have the same power of two.
Glossary Term: Loop
A combination of two or more instruments or control or safety functions arranged so that signals pass from one to another for the purpose of measurement and/or control of a process variable or executing a safety function.
Glossary Term: Load cell
A transducer for the measurement of force or weight. Action is based on strain gauges mounted within the cell on a force beam.
Glossary Term: Linear variable reluctance transducer (LVRT)
A position sensor consisting of a centre tapped coil and an opposing moving coil attached to a linear probe; the winding is continuous over the length of the core, instead of being segmented as in an LVDT.
Glossary Term: Linear variable differential transformer (LVDT)
A position sensor consisting of a central primary coil and two secondary coils wound on the same core; a moving iron element linked to a mechanical member induces changes in self induction that are directly proportional to movement of the member.
Glossary Term: Limited variability language (LVL)
This type of language is designed for process sector users, and provides the capability to combine predefined, application specific, library functions to implement the safety requirements specifications. An LVL provides a close functional correspondence with the functions required to achieve the application. Typical examples of LVL are ladder diagram, function…
Glossary Term: Likelihood
The frequency of a harmful event often expressed in events per year or events per million hours. One of the two components used to define a risk. Note that this is different from the traditional English definition that means probability.
Glossary Term: LEL/LFL
Lower explosive (or flammable) limit. See flammability.
Glossary Term: Latent fault
A fault that is present but hidden from regular means of detection. Typically these faults can only be identified as part of an accident or a detailed proof test.
Glossary Term: Laser Doppler flowmeter
An apparatus for determining flow velocity and velocity profile by measuring the Doppler shift in laser radiation scattered from particles in the moving fluid stream
Glossary Term: Ladder diagram
Symbolic representation of a control scheme. The power lines form the two sides of a ladder like structure, with the program elements arranged to form the rungs. The basic program elements are contacts and coils as in electromechanical logic systems. Typically programs of this form fall into the limited variability…
Glossary Term: Jacketed valve
A valve body cast with a double wall or provided with a second wall by welding material around the body so as to form a passage for a heating or cooling medium. Also refers to valves which are enclosed in split metal jackets having internal heat passageways or electric heaters.…
Glossary Term: IPL
Independent protection layer or layers. This refers to various other methods of risk reduction possible for a process. Examples include items such as rupture disks and relief valves which will independently reduce the likelihood of the hazard escalating into a full accident with a harmful outcome. In order to be…
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