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Glossary Term: TI
Test Interval This acronym is typically used in risk analysis equations to represent the proof test interval described above. Temperature Indicator This acronym is used in piping and Instrumentation Diagrams (P&IDs) to designate a device with measures and displays the temperature.
Glossary Term: Thermocouple
Two dissimilar wires joined together that generate a voltage proportional to temperature when their junction is heated relative to a reference junction. See thermojunction.
Glossary Term: Thermistor
A temperature transducer constructed from semiconductor material and for which the temperature is converted into a resistance, usually with negative slope and highly nonlinear.
Glossary Term: Thermal type flow meter
An apparatus where heat is injected into a flowing fluid stream and flow rate is determined from the rate of heat dissipation; either the rise in temperature or some point downstream of the heater or the amount of thermal or electrical energy required to maintain the heater at a constant…
Glossary Term: Target flow meter
A device for measuring fluid flow rates through the drag force exerted on a sharp edged disk centered in a circular flow path due to differential pressure created by fluid flowing through the annulus. Usually, the disk is mounted on a bar whose axis coincides with the tube axis, and…
Glossary Term: Systematic failure
A failure that happens in a deterministic (non random) predictable fashion from a certain cause, which can only be eliminated by a modification of the design or of the manufacturing process, operational procedures, documentation, or other relevant factors. Since these are not mathematically predictable, the safety lifecycle includes a large…
Glossary Term: Suppressed range
A suppressed range is an instrument range which does not include zero. The degree of suppression is expressed by the ratio of the value at the lower end of the scale to the span.
Glossary Term: Supervisory control
A term used to imply that a controller output or computer program output is used as an input to other controllers. See SCADA.
Glossary Term: Stress corrosion cracking
Deep cracking in a metal part due to the combination of tensile stress and a corrosive environment, causing failure in less time than could be predicted by simply adding the separate effects of stress and the corrosive environment.
Glossary Term: Stochastic
Pertaining to direct solution by trial and error, usually without a step by step approach, and involving analysis and evaluation of progress made, as in a heuristic approach to trial and error methods. In a stochastic approach to a problem solution, intuitive conjecture or speculation is used to select a…
Glossary Term: Static pressure
1. The pressure of a fluid that is independent of the kinetic energy of the fluid. 2. Pressure exerted by a gas at rest, or pressure measured when the relative velocity between a moving stream and a pressure measuring device is zero.
Glossary Term: Static head liquid level meter
A pressure sensing device, such as a gauge, connected in the piping system so that any dynamic pressures in the system cancel each other and only the pressure difference due to liquid head above the gauge position is registered.
Glossary Term: Standard condition
1. A temperature of 0°C and a pressure of 1 atmospheres (760 torr). Also known as “normal temperature and pressure (NTP)”; “standard temperature and pressure (STP).” 2. According to the American Gas Association (AGA), a temperature of 60°F (1 5-5/9°C) and a pressure of 30 inches of mercury (762 mm).…
Glossary Term: Solenoid valve
A shutoff valve whose position is determined by whether or not electric current is flowing through a coil surrounding a moving iron valve stem.
Glossary Term: Solenoid
A type of electromechanical operator in which back and forth axial motion of a ferromagnetic core within an electromagnetic coil performs some mechanical function; common applications include opening or closing valves or electrical contacts.
Glossary Term: Snubber
1. A device which is used to damp the motion of the valve stem. This is usually accomplished by an oil filled cylinder/piston assembly. The valve stem is attached to the piston and the flow of hydraulic fluid from one side of the piston to the other is restricted. 2.…
Glossary Term: SIT - Site integration test
Site integration test. Once site acceptance testing is completed, the basic process control system and the safety instrumented system 〈SIS〉 communications and any hard-wired links are integrated and tested as a complete system to ensure that the system as a whole functions correctly. SIS signals, diagnostics, bypasses and alarms displayed…
Glossary Term: SIS - Safety Instrumented System
Safety Instrumented System – Implementation of one or more Safety Instrumented Functions. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s). A SIS is usually has a number of safety functions with different safety integrity levels (SIL) so it is best avoid describing it by…
Glossary Term: SIL verification
The process of calculating the average probability of failure on demand (or the probability of failure per hour) and architectural constraints for a safety function design to see if it meets the required SIL.
Glossary Term: SIL selection
The process of defining tolerable risk, confirming existing risk (both likelihood and consequence) and assigning a SIL rated safety function as needed to achieve a tolerable level of risk.
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