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Glossary Term: PLL
Probable loss of life. A numerical expression for the magnitude of a consequence in terms of the most likely number of lives that will be lost in a given event or over a given time interval. The value need not be a whole number.
Glossary Term: Pitot tube
1. An instrument for measuring stagnation pressure of a flowing liquid; it consists of an open tube pointing upstream, into the flow of fluid, and connected to a pressure indicator or recorder. 2. An instrument which will register total pressure and static pressure in a gas stream, used to determine…
Glossary Term: P&ID
Piping and instrumentation drawing. Shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on Instrument Society of America (ISA)…
Glossary Term: Pigtail
A 270° or 360° loop in pipe or tubing to form a trap for vapor condensate. Used to prevent high temperature vapors from reaching the instrument. Used almost exclusively in static pressure measurement.
Glossary Term: PID control
Proportional-plus-integral-plus-derivative control, used in processes where the controlled variable is affected by long lag times.
Glossary Term: Physical relief device
Mechanical equipment that performs an action to relieve pressure when the normal operating range of temperature or pressure has been exceeded. Physical relief devices include pressure relief valves, thermal relief valves, rupture disks, rupture pins, and high temperature fusible plugs.
Glossary Term: PHA
Process hazards analysis. Required by both PSM and the safety lifecycle. Identifying the hazards of a process for all reasonably foreseeable circumstances, determining the sequence of events leading to harm, and estimating the likelihood (frequency) and consequence magnitude of the potential harm. Various hazard identification methods include Checklist, What if?,…
Glossary Term: pH meter
An instrument for electronically measuring electrode potential of an aqueous chemical solution and directly converting the reading to pH value. pH is the symbol for the measurement of acidity or alkalinity. Solutions with a pH reading of less than 7 are acid; solutions with a pH reading of more than…
Glossary Term: PFDavg
Probability of Failure on Demand average- This is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFD can be determined as an average probability or maximum probability over a time period. IEC 61508/61511 and ISA 84.01 use PFDavg as…
Glossary Term: Parity
A check that tests whether the number of ones or zeroes in an array of binary digits is odd or even used to verify data storage and transmission. This is usually done by calculating the sum of the ” 1 ” bits in a data unit and determining if it…
Glossary Term: Pareto chart
A display of the number of failures of components by part number in descending order of failure rate or number of failures observed. Data may also be shown taking into account the total cost of each failure.
Glossary Term: Overshoot
1. The amount of output measured beyond the final steady output value, in response to a step change in the input. NOTE: Expressed in percent of the equivalent step change in output. 2. A transient response to a step change in an input signal which exceeds the normal or expected…
Glossary Term: Override control
1. Generally, two control loops connected to a common final control element-one control loop being nominally in control with the second being switched in by some logic element when an abnormal condition occurs so that constant control is maintained. 2. A technique in which more than one controller manipulates a…
Glossary Term: Overrange limit
The maximum input that can be applied to a device without causing damage or permanent change in performance.
Glossary Term: Overrange
In process instrumentation, of a system or element, any excess value of the input signal above its upper range value or below its lower range value
Glossary Term: OSI
Open system interconnection. A seven layered model of communications networks defined by ISO. The seven layers are: Layer 7 Application: provides the interface for application to access the OSI environment. Layer 6 Presentation: provides for data conversion to preserve the meaning of the data. Layer 5 Session: provides user to…
Glossary Term: Orifice meter
A plate with a calibrated sharp edged hole in it. The plate is positioned across the flow stream in a pipe for measuring fluid flow rates. It typically has differential pressure taps positioned near the orifice and a calibrated calculation element to convert the measured pressure difference into a flow…
Glossary Term: On-off control
A simple form of control whereby the control variable is switched fully on or fully off in response to the process variable rising above the set point or falling below the set point respectively. Cycling always occurs with this form of control.
Glossary Term: Offset
1. A sustained deviation of the controlled variable from set point. This characteristic is inherent in proportional controllers that do not incorporate reset action. 2. Offset is caused by load changes. 3. The steady state deviation when the set point is fixed. NOTE: The offset resulting from a no load…
Glossary Term: Occupancy
A measure of the probability that the effect zone of an accident will contain one or more personnel receptors of the effect. This probability should be determined using plant-specific staffing philosophy and practice. See effect zone.
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