PE Control Systems key terms

How close a measured value is to the true value of the variable, usually stated as a percentage of span or of reading over a stated range.

How closely an instrument gives the same output when the same input is applied again and again under identical conditions, approached the same way each time.

The difference in an instrument's output when the same input value is reached from increasing versus decreasing directions, caused by friction, backlash, or elastic effects in the movement.

The algebraic difference between the upper and lower range values of an instrument; it is the width of measurement range that the output signal represents.

An analog signaling standard where 4 mA represents the zero or live-zero value and 20 mA represents full scale; current signaling resists voltage loss in long wires.

A differential-pressure flow element made of a flat plate with a machined bore clamped between pipe flanges; measured pressure drop relates to flow by a square-root relationship.

A resistance temperature detector, usually platinum, whose electrical resistance rises predictably as temperature increases, giving accurate and stable measurement over a wide span.

Comparing an instrument's indication with a traceable reference standard and adjusting its zero and span so the output correctly matches known input values, with results documented.

A feedback algorithm whose output combines proportional action on current error, integral action on accumulated error, and derivative action on the rate of error change.

The error, expressed as a percentage of measurement span, required to drive the controller output through its full range; it is the reciprocal of proportional gain.

The unwanted buildup of the integral term while a controller output sits saturated at a limit, which causes overshoot and slow recovery once the process returns to range.

A sharp controller output spike produced by derivative action when the setpoint is stepped; it is normally avoided by taking the derivative on measurement rather than error.

The interval between a change in controller output and the first detectable response of the process variable, caused by transport delays or slow analysis.

A structure in which the output of a primary controller becomes the setpoint of a faster secondary controller, so disturbances inside the inner loop are corrected before they upset the main variable.

Control action taken on a measured disturbance before the controlled variable moves, using a process model; it is usually combined with feedback to correct the remaining error.

Dividing a single controller output between two or more final elements so each strokes over its own portion of the signal range, such as separate heating and cooling valves.

The controller gain at which a closed loop cycles continuously with integral and derivative action removed; paired with the ultimate period, it sets Ziegler-Nichols tuning constants.

The final control element that throttles fluid flow in response to a controller output signal, regulating the process variable by changing the open area of the flow path.

A capacity number equal to the gallons per minute of water at 60 degrees Fahrenheit that a valve passes with a one psi pressure drop across it.

A trim characteristic in which equal increments of stem travel produce equal percentage changes in flow, which compensates for pressure drop that falls as flow increases.

The actual flow-versus-travel relationship a valve shows in service, differing from the inherent curve because of piping losses, fittings, and changing pressure drop across the valve.

A device that senses stem position and adjusts air to the actuator so the valve follows the controller signal, overcoming friction and improving response on large or high-pressure valves.

A valve that strokes open as air pressure rises and springs closed when air fails, also called fail-closed; its failure direction is chosen to protect the process.

Formation and collapse of vapor bubbles in liquid passing through a valve's narrowest point, producing noise, erosion, and reduced capacity as pressure recovers downstream.

An independent protective action performed by a safety instrumented system that moves the process to a safe state when a defined hazardous condition is detected.

A discrete rating from one to four assigned to a safety function according to its required risk reduction and allowable probability of failure on demand.

Boiler feedwater control that combines drum level, steam flow, and feedwater flow signals, letting level be held steady while the loop responds quickly to load changes.

Protection that recycles or vents flow to keep a compressor's throughput above its surge limit, preventing flow reversal and damage during low demand or high pressure ratio.

A networked plant platform of controllers and operator stations that share data while keeping individual control loops running locally, even if higher levels fail.

Supervisory control and data acquisition: a system that collects field data and issues supervisory commands over a wide area, often through remote terminal units and telemetry.

A structured review that sets each alarm's limit, priority, and operator response and removes nuisance or duplicate alarms so the board highlights what truly needs action.

A drawing that shows process equipment, piping, instruments, and control loops using standardized symbols and tag numbers, serving design, construction, and maintenance work.

Hazard and operability study: a team review that applies guide words to process parameters at defined nodes to find deviations, their causes, and their consequences.

Layer of protection analysis: a semi-quantitative method comparing the frequency of an initiating event with credited independent protection layers against a tolerable risk target.

A protection method that limits voltage and current in hazardous-area circuits using barriers or galvanic isolators, so sparks and surface temperatures cannot ignite the atmosphere.

The division of plant areas into zones or divisions by how often and how long flammable gas or dust may be present, which controls the equipment approvals allowed there.

A formal process that reviews and approves modifications to equipment, chemicals, or procedures before they are made, confirming hazards and documents stay current.

A chart with trip causes listed as rows and output actions as columns, documenting exactly how the safety system responds to each initiating event.