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Glossary Term: Pressure relief device
A mechanism that vents fluid from an internally pressurized system to counteract system overpressure; the mechanism may release all pressure and shut the system down (as does a rupture disc) or it may merely reduce the pressure in a controlled manner to return the system to a safe operating pressure…
Glossary Term: Pressure, maximum working
The maximum total pressure permissible in a device under any circumstances during operation, at a specified temperature. It is the highest pressure to which it will be subjected in the process. It is a designed safe limit for regular use. NOTE: MWP can be arrived at by two methods: a)…
Glossary Term: Pressure, design
The pressure used in the design of a vessel or device for the purpose of determining the minimum permissible thickness or physical characteristics of the parts for a given maximum working pressure (MWP) at a given temperature.
Glossary Term: Predictive control
1. A type of automatic control in which the current state of a process is evaluated in terms of a model of the process and controller actions modified to anticipate and avoid undesired excursions. 2. Self tuning. 3. Artificial intelligence.
Glossary Term: PLC
Programmable Logic Controller. These computers replace relay logic and often have PID (proportional integral and derivative) controllers built into them. PLCs are very fast at processing discrete signals (like a switch condition). They can be designed for either regular or SIL rated applications.
Glossary Term: Positioner
A position controller, which is mechanically connected to a moving part of a final control element or its actuator, and automatically adjusts its output pressure to the actuator in order to maintain a desired position that bears a predetermined relationship to the input signal. The positioner can be used to…
Glossary Term: Plug valve
1. A valve with a closing element that may be cylindrical, conical or a spherical segment in shape that is opened or closed with rotary motion. 2. A type of shutoff valve consisting of a tapered rod with a lateral hole through it. As the rod is rotated 90° about…
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…
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