ISA CAP key terms

A feedback control strategy that computes an output from proportional, integral, and derivative actions on the error between a setpoint and a measured process variable. It is widely used to regulate continuous processes.

The measured value of the process condition being controlled, such as temperature, pressure, flow, or level. It is compared with the setpoint to determine control error.

The desired target value for a controlled process variable. The controller adjusts its output to drive the measured variable toward this value.

A condition where the integral term of a controller accumulates excess error while the final control element is saturated. It can cause overshoot when the process returns within range.

The delay between a change in controller output and the first measurable response of the process variable. It complicates tuning because the controller acts before seeing the effect.

A control arrangement where the output of a primary controller becomes the setpoint of a secondary controller. The inner loop rejects fast disturbances, improving overall response.

A control strategy that measures a disturbance and adjusts the manipulated variable before the process variable deviates. It reacts predictively rather than waiting for feedback error.

An analog signaling standard where 4 mA represents the minimum value and 20 mA the maximum. Current signals resist voltage drops and noise over long field wiring.

A digital communication protocol superimposed on a 4-20 mA analog signal. It lets instruments transmit configuration, diagnostics, and process data without interrupting the analog measurement.

A final control element that modulates fluid flow in response to a controller signal. Its position, trim, and actuator determine how precisely the process can be regulated.

The process of comparing an instrument's indication with a known reference and adjusting it to reduce error. It establishes traceability and confirms measurement accuracy.

A piping and instrumentation diagram uses standard symbols to show process equipment, piping, instruments, and control loops. It is a key document for design, construction, and maintenance.

A safety instrumented system uses sensors, logic solvers, and final elements to bring a process to a safe state when predetermined conditions are violated. It is separate from basic process control.

A safety instrumented function is a specific protective action implemented by a safety instrumented system. It has a defined safety integrity level and response to a hazardous event.

A safety integrity level is a discrete rating of the required risk reduction performance of a safety instrumented function. It is determined through risk assessment and verified by design.

The overall process of managing a safety instrumented system from concept through design, operation, maintenance, and decommissioning. It provides structured phases, activities, and documentation.

A hazard and operability study is a structured team review that examines deviations from design intent in a process. It identifies potential hazards and operability problems using guide words.

Layer of protection analysis is a semi-quantitative method that estimates risk by comparing initiating event frequency with independent protection layers. It helps determine whether additional safeguards are needed.

The average probability of failure on demand is the likelihood that a safety instrumented function fails to perform its protective action when required. It is a key measure for low-demand SIS applications.

A periodic test that reveals undetected dangerous failures in a safety instrumented system. It restores the system to a known good state and supports the claimed safety integrity.

Hardware fault tolerance is the ability of a system to continue performing its safety function in the presence of one or more hardware faults. It is expressed as the number of faults tolerated.

A failure of multiple components due to a shared cause, such as design error, environmental stress, or maintenance mistake. It can defeat redundancy and must be considered in SIS design.

A safety requirements specification defines the functional and integrity requirements for a safety instrumented system. It guides design, verification, and validation throughout the safety lifecycle.

A work breakdown structure decomposes a project into manageable deliverables and work packages. It provides the basis for estimating, scheduling, and controlling project scope.

A factory acceptance test verifies that a system meets specified requirements before shipment. It is conducted at the vendor's facility using documented test procedures.

A site acceptance test verifies that an installed system works correctly in its actual operating environment. It follows installation and often precedes commissioning and startup.

The process of testing and verifying installed equipment and systems to confirm they operate safely and meet design requirements. It bridges construction and startup.

Management of change is a formal process for reviewing and approving modifications to processes, equipment, or systems. It ensures that risks are assessed before changes are implemented.

A document that links each project requirement to its design, implementation, and test evidence. It helps confirm that all requirements are met and changes are controlled.

A seven-layer reference model that describes how data moves from an application through network layers to physical transmission. It helps engineers understand and troubleshoot network communication.

A suite of communication protocols that provides reliable, addressed data transport across interconnected networks. It is the foundation of most modern industrial Ethernet networks.

A simple industrial communication protocol that exchanges register and coil data between devices. It is widely used for PLC, RTU, and instrument integration but lacks built-in security.

An open, platform-independent communication standard for industrial data exchange. It provides built-in security, information modeling, and interoperability across vendors and systems.

A network security device that controls traffic between networks based on rules. It helps protect industrial control systems from unauthorized access and cyber threats.

A demilitarized zone is a buffer network placed between trusted internal systems and untrusted external networks. It limits direct access to control system assets.

A layered cybersecurity strategy that uses multiple independent controls to protect assets. If one layer fails, others continue to reduce risk.

A software system that collects, stores, and retrieves time-stamped process data from control systems. It supports trending, reporting, and performance analysis.

The property of a network or control system that guarantees messages or tasks complete within a predictable time. It is critical for real-time control and safety functions.