Safety instrumentation systems principle

Instrumentation, Systems, and Automation Society (ISA) Standard 84, Application of Safety Instrumented Systems for the Process Industries, outlines the principles of high integrity shutdown systems. 

The international standard IEC 61511 [2] gives advice on the design of safety instrumented systems (SIS) and presents a layer concept to achieve reliability of protection systems. These principles can be applied to the protection of chemical reactors [3]. Figure 10.3 represents this layer of protection principles. The first layer is the process itself, meaning that it should be designed in such a way that it cannot give rise to a runaway reaction. Some concepts for achieving this objective are reviewed in Section 10.3. 

The safety instrumented system logic solvers addressed include Electrical (E)/Electronic (E)/ and Programmable Electronic (PE) technology. Where other technologies are used for logic solvers, the basic principles of this standard may also be applied. This standard also addresses the safety instrumented system sensors and final elements regardless of the technology used. This International Standard is process industry specific within the framework of the lEC 61508 series. 

It must be recognized that the Safety Lifecycle and associated Safety Instrumented Systems need to be part of an overall Process Safety Management System (PSM) for the entire plant. PSM can be defined as a program or set of activities involving the application of management principles and analyses to ensure the overall process safety of process plants. PSM, therefore, covers aU aspects of process safety, not just functional safety. 

Sensors in a safety instrumented system measure process variable conditions in order to recognize a potential hazard. Usually these are the same process variables that are used for control. So the first and perhaps most important consideration when selecting sensors for safety applications is that they accurately and reliably measure the process variable. Another key parameter is that any process wetted materials must be compatible with the chemicals of the process. These are two of the key principles required in a "well designed system."... 

The following important principles are to be observed when designing and constructing a PCE safety instrumented system ... 

The following provides a brief discussion of basic design principles that address the relationship of power sources to shutdown actions of safety instrumented systems. Some typical support systems include electrical, pneumatics, and hydraulics. 

S. Nunns, Principles for Proof Testing of Safety Instrumented Systems in the Chemical Industry, Prepared by ABB Ltd. for the Health and Safety Executive Contract research report 428/2002, 2002, http //www.hse.gov.uk/research/crr pdf/2002/crt02428.pdf. 

Further guidance Is In the HSE research report CRR428 Principles for proof testing of safety instrumented systems in the chemical Industry. ... 

This practical exercise is to eonstruct a fault tree diagram using the basic principles introduced in Chapter 3. It uses an example of a simple reactor with automatically controlled feeds that has the potential to cause a serious risk to plant personnel. Once the basic fault tree has been drawn, the model is to be adjusted to incorporate a safety-instrumented system and to demonstrate the resulting risk reduction. 

Introduction to Process Technology Safety, Health, and Environment Process Instrumentation The Principles of Quality PT 1—Equipment PT 2—Systems PT 3—Operations Process Troubleshooting... 

The computerized systems, both hardware and software, that form part of the GLP study should comply with the requirements of the principles of GLP. This relates to the development, validation, operation and maintenance of the system. Validation means that tests have been carried out to demonstrate that the system is fit for its intended purpose. Like any other validation, this will be the use of objective evidence to confirm that the pre-set requirements for the system have been met. There will be a number of different types of computer system, ranging from personal computers and programmable analytical instruments to a laboratory information management system (LIMS). The extent of validation depends on the impact the system has on product quality, safety and record integrity. A risk-based approach can be used to assess the extent of validation required, focusing effort on critical areas. A computerized analytical system in a QC laboratory requires full validation (equipment qualification) with clear boundaries set on its range of operation because this has a high... 

Process control plays an important role in how a plant process upset can be controlled and subsequent emergency actions executed. Without adequate and reliable process controls, an unexpected process occurrence cannot be monitored, controlled and eliminated. Process controls can range from simple manual actions to computer logic controllers, remote from the required action point, with supplemental instrumentation feedback systems. These systems should be designed such as to minimize the need to activate secondary safety devices. The process principles, margins allowed, reliability and the means of process control are mechanisms of inherent safety that will influence the risk level at a facility. 

Modern cryogenics plants are well-instrumented to indicate process temperatures and pressures, flow rates, liquid levels, and contaminant concentrations. These instruments are used for measurement, control, and safety if a particular variable (temperature, pressure, flow rate) falls outside the control range, a corrective action is initiated automatically and, if necessary, an alarm may also be actuated. Complete alarm systems are now available commercially for use with all the common safety devices. Further, many are designed on the failsafe principle and sound an alarm in the event a power failure occurs. 

Instrumentation and safety systems. The trainee will be provided with an explanation of the facility s instrumentation, computer control systems, and emergency shutdown systems. The principles of measurement and control, including proportional, differential, and integral controllers, are explained. 

Criticality. The process plant is designed on the principle of safe geometry under all conditions (e.g., the use of HARP tanks). Continuous monitoring is provided by an approved criticality detection and alarm system. Where additional operational control is required to maintain safe conditions, this is specified in the appropriate nuclear safety assessments and Criticality Clearance Certificate. The latter specifies limits and conditions that need to be complied with during operations for example, in respect of limited tap density, moisture content, isotopic inventory and mass. It identifies any systems or instrumentation that demonstrate that compliance is maintained during operation. In addition key points of the plant are monitored by neutron monitors to give early warning of the unanticipated build-up of solid plutonium which could lead to the development of unsafe conditions. 

Chernobyl, on the contrary, is an example of what can happen if a completely opposite principle is applied, that to do only what is necessary for safety. In RBMK reactors, like the Chernobyl reactor, the safety margins were not stringent enough. For example, the plant had a containment system for the primary circuit but it was only partial the reactor itself, and in particular the fuel channel heads, were not included in it. The designers thought that it was sufficient only to install protective monitoring instrumentation. Figure 3-4 shows the containment for a typical 900 MWt PWR and the Chernobyl reactor containment. 

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