empirical procedure used to tune and control a process using * PID control with optimum controller settings. Developed by J. G. Ziegler (1898–1973) and N. B. Nichols (1903–79) in 1942, it is based on using results from open-loop tests. The settings from the tuned controller result in an underdamped transient response with a * decay ratio of one-quarter. process safety A comprehensive management system that focuses on the management and control of potential major hazards that arise from process operations. It aims at re- ducing risk to a level that is as low as is reasonably practicable by the prevention of fires, explosions, and accidental or unintended chemical releases that can cause harm to human life and to the environment. It includes the prevention of leaks, spills, equipment failure, over- and under-pressurization, over-temperatures, corrosion, and metal fatigue. It covers a range of tools and techniques required to ensure safe operation of plant and machinery to ensure the safety of personnel, the environment, and others, through detailed design and engineering of facilities, maintenance of equipment, use of effective alarms and control points, procedures, and training. It also includes risk assessments, * layers of protection analysis , and use of * permit to work authorizations. process simulation The use of computers to model and predict the operational and thermodynamic behaviour of a process. Sophisticated commercial software packages are used to simulate and model batch, continuous, steady-state, and dynamic processes. They require combined material and energy balances, the properties of the materials being processed, and sometimes combine the use of experimental data with mathematical de- scriptions of the process being simulated. Most software packages feature optimization capabilities involving the use of complex cost models and detailed process equipment size models. process synthesis The conceptual design of a process that identifies the best process * flowsheet structure, such as the conversion of raw materials into a product. This requires the consideration of many alternative designs. Due to the complexity of most processes, the flowsheet is divided into smaller parts and each considered in turn, then choices and deci- sions made. Various techniques are used to arrive at the best flowsheet such as those based on total cost, which needs to be minimized, the use of graphical methods, the use of * heu- ristics , and various other forms of minimization such as the use of * process integration . process upset A sudden, gradual, or unintended change in the operational behaviour of a process. It may be due to process equipment failure or malfunction, operator interven- tion, a surge or fall in pressure, flow, level, concentration, etc. process validation The documented evidence that a procedure, process, or change has been fully evaluated before its implementation that it, which can provide a high degree of assurance of meeting pre-determined specifications and quality. First used by the Food and Drugs Administration in the US in the 1970s to improve the quality of pharmaceu- ticals, it is widely used in many industries and, in particular, the pharmaceutical and al- lied medical industries. It is used alongside other regulatory requirements such as Good Manufacturing Practice. Before the validation process begins, it is necessary to ensure that the system is properly qualified, which includes a design, installation, operational, perfor- mance, and component qualification. These define the function, operation, and specifica- tion of the equipment, process and product. process variable A dynamic feature of a process or system that is required to be con- trolled to ensure that it operates according to design requirements and does not deviate so p