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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.
Glossary Term: SIL - Safety Integrity Level
Safety Integrity Level - A quantitative target for measuring the level of performance needed for safety function to achieve a tolerable risk for a process hazard. Defining a target SIL level for the process should be based on the assessment of the likelihood that an incident will occur and the…
Glossary Term: Signal common
1. The signal common shall refer to a point in the signal loop which may be connected to the corresponding points of other signal loops. It may or may not be connected to earth ground [S50.1]. 2. The reference point for all voltage signals in a system. Current flow into…
Glossary Term: Signal isolation
Signal isolation refers to the absence of a connection between the signal loop and all other terminals and earth ground.
Glossary Term: SIF - Safety Instrumented Function
Safety Instrumented Function – A set of equipment intended to reduce the risk due to a specific hazard (a safety loop). Its purpose is to 1. Automatically taking an industrial process to a safe state when specified conditions are violated; 2. Permit a process to move forward in a safe…
Glossary Term: Set pressure
The inlet pressure at which a safety relief valve opens; usually a pressure established by specification or code.
Glossary Term: SFF
Safe Failure Fraction - The fraction of the overall failure rate of a device that results in either a safe fault or a diagnosed (detected) unsafe fault. The safe failure fraction includes the detectable dangerous failures when those failures are annunciated and procedures for repair or shutdown are in place.
Glossary Term: Set point
1. An input variable which sets the desired value of the controlled variable It is expressed in the same units as the controlled variable.
Glossary Term: Sensor group
For complex safety functions, there may be more than one property which is measured to determine if a shut down is required.
Glossary Term: Sensor
device or combination of devices that measure the process condition (e.g., transmitters, transducers, process switches, position switches, etc.)
Glossary Term: Segmented ball
A closure piece in a valve that is a segment of a spherical surface which may have one edge contoured to yield a desired flow characteristic.
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