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Glossary Term: Nozzle
1. A short flanged or welded neck connection on a drum or shell for the outlet or inlet of fluids; also a projecting spout through which a fluid flows. 2. A streamlined device for accelerating and directing fluid flow into a region of lower fluid pressure. 3. A particular type…
Glossary Term: Noise
1. In process instrumentation, an unwanted component of signal or. See “interference, electromagnetic”. 2. Any spurious variation in the electrical output not present in the input. 3. An unwanted component of a signal or variable which obscures the information content. 4. Random variations of one or more characteristics of any…
Glossary Term: NC / (NO)
Normally Closed (Normally Open) 1. A switch position where the usual arrangement of contacts permits (prevents) the flow of electricity in the circuit. 2. In a solenoid valve, an arrangement whereby the disk or plug is seated (open) when the solenoid is de-energized. 3. A field contact that is closed…
Glossary Term: NIOSH
(US) National Institute of Occupational Safety and Health
Glossary Term: NEMA standard
Consensus standards for electrical equipment approved by the majority of the members of the US National Electrical Manufacturers Association.
Glossary Term: Needle valve
Its essential design feature is a slender tapered rodlike control element which fits into a circular or conoidal seat. Operating the valve causes the rod to move into or out of the seat, gradually changing the effective cross sectional area of the gap between the rod and its seat. Typically…
Glossary Term: MWP
maximum working pressure. See Pressure, maximum working
Glossary Term: Multiplexing
The transmission of a number of different messages simultaneously over a single circuit.
Glossary Term: MTTFS
Mean Time to Fail Spurious - The mean time until a failure of the system causes a spurious process trip.
Glossary Term: MTTR
Mean Time to Repair – The average time between the occurrence of a failure and the completion of the repair of that failure. This includes the time needed to detect the failure, initiate the repair and fully complete the repair.
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.
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