Safety interlocking, instrumentation/control

Safety Instrumented System (SIS) The instrumentation, controls, and interlocks provided for safe operation of the process. 

Introduction The chemical processing industry relies on many types of instrumented systems, e.g., the basic process control systems (BPCSs) and safety instrumented system (SIS). The BPCS controls the process on a continuous basis to maintain it within prescribed control limits. Operators supervise the process and, when necessary, take action on the process through the BPCS or other independent operator interface. The SIS detects the existence of unacceptable process conditions and takes action on the process to bring it to a safe state. In the past, these systems have also been called emergency shutdown systems, safety interlock systems, and safety critical systems. 

Decomposition should be averted for both safety and environmental reasons. In case of decomposition, multiple levels of protection are applied in appropriate sequences of procedural controls, instrument controls, interlocks, and relief devices to minimize damage to the plant and impairment of environment. Because the discharge of ethylene and its decomposition products into the air involves considerable risk, precautions for safe venting must also be considered. 

Under normal operating conditions, the National Instruments controller monitors for process faults and takes corrective action upon identification. The system will override operator commands issued at the HMI during a safety interlock. The unsafe conditions that result in a process interlock are ... 

Most systems, especially for use in industry, include an autosampler that allows unattended, overnight operation of the instrument. Computer software in commercial instruments controls the autosampler and the instrument, collects the data, performs the calculations, prints out the results, and shuts down the instrument when the analysis is completed. Most instruments have computer-controlled safety interlocks that shut down the plasma or the instrument and autosampler in the event a problem is detected. 

The instrumentation required to perform FCCE is relatively simple. The power supply can be any commercially available HVDC power source, but an ideal power supply would have dual polarity, 0-30 kV at several 100 elA, with safety interlocks and computer control capabilities. The maximum voltage needed is determined by the desired electric field strength and length of the capillary. In order to allow for some margin of error in control of the counter flow, the minimum length of capillary should be at least three to four times the physical width of the desired separation window. These margins allow for the analyte bands to be maintained within the inner half of the capillary so that they are not pushed out on either side of the capillary, with detection occurring in the center of the capillary. 

Cortes, R, Krishman, S. M., Lee, I., Goldman, M., Improving the Safety of Patient Controlled Analgesia Infusions with Safety Interlocks and Closed-Loop Control, Biomedical Instrumentation Technology, Vol. 37, No. 8, 2004, pp. 169-175. 

The level and sophistication of instruments and process control systems is largely determined by local preference although in order to satisfy the basic requirement, as stated above, there is a minimum requirement for instrumentation, control and safety interlocks which are common to all sulphonation plants. 

Safety instrumented system (SIS) SIS is meant to prevent, control, or mitigate hazardous events and take the process to a safe state when predetermined conditions are violated. An SIS can be one or more SIFs, which is composed of a combination of sensors, logic solvers, and final elements. Other common terms for SISs are safety interlock systems, emergency shutdown (ESD) systems, and safety shutdown systems (SSDs). So, SIS is used as a protection layer between the hazards of the process and the public. SIS or SIF is extremely important when there is no other non-instrumented way of adequately eliminating or mitigating process risks. As per recommendations of standards lEC 61511 2003 (or ANSI/ ISA-84.00.01-2004), a multi-disciplinary team approach following the safety life cycle, conducts hazard analysis, develops layers of protections, and implements an SIS when hazardous events cannot be controlled, prevented, or mitigated adequately by non-instrumented means. 

Extrinsically safe A term applied when safety is built in by adding instrumentation, controls, alarms, interlocks, equipment redundancy, safety procedures, and the like during engineering, design, constmction, or operation of a component, system, or facility. Contrasted with inherently safe. 

Where loss of control could lead to severe consequences, the integrity of the basic process control system and the protective safeguards must be designed, operated and maintained to a high standard. Industry standards such as ANSI/ISA-S84.01 (1996) and IEC 61508 (2000) address the issues of how to design, operate and maintain safety instrumented systems such as high temperature interlocks to achieve the necessary level of functional safety. The scope of these standards includes hardware, software, human factors and management (HSE 2000). 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

Also, the design practice includes P ID documentation, database specification and verification of purchased equipment, control design and performance analysis, software configuration, real-time simulation for DCS system checkout and operator training, reliability studies, interlock classification and risk assessment of safety instrumented systems (SIS), and hazard and operability (HAZOP) studies. 

Energizing power systems, operational testing of plant equipment, calibration of instrumentation, testing of the control systems, and verification of the operation of all interlocks and other safety devices, without yet introducing process materials. These activities are usually described as cold commissioning . In parallel, it is usually necessary to commission the plant utilities, such as cooling water and compressed air systems, in order to enable equipment operation. 

Process safety refers to the application of engineering, science, and human factors to the design and operation of chemical processes and systems. The primary purpose of process safety is to prevent injuries, fatalities, fires, explosions, and unexpected releases of hazardous materials. Process safety focuses on the individual chemical processes and operational procedures associated with these systems. A process safety analysis is used to establish safe operating parameters, instrument interlocks, alarms, process design, and start-up, shutdown, and emergency procedures. Process safety programs cannot completely eliminate risk they can only control or reduce those risks. 

Out of all the team members, the team leader and scribe (secretary) are required to possess experience in HAZOP and excellent communication skills. Generally, for plant HAZOP analysis the team is formed from people with a technical background. In most of the plants dealing with hazardous materials, and/or any other hazardous situation, a person from the HSE department is made a team member. In some countries it is mandatory to keep one person from HSE. Since in most plants, control instrumentation plays a great role in ensuring plant safety through interlock and protection, it is better to keep one person from process, operation/production... 

Critical controls, interlocks (both safety non-safety), alarms and instruments ... 

Ill-103. Reactivity control system, reactor shutdown system The function of the mechanical and electrical design shall be described. The description shall include the materials and dimensions and shall be supported by drawings. The reactivity control mechanisms and their instrumentation, such as their position or status (coupled/decoupled), should be presented, together with their insertion time and interlocks. The effects of corrosion, fatigue, neutron doses, etc., on the lifetime of the mechanical and electrical components shall also be discussed. The safety related design parameters should be presented, such as ... 

To insure that no instrument can be made inoperative during the operation of the reactor by holding its reset button down, all reset buttons are included in the interlock circuit. Pushing one of these buttons physically breaks the continuity interlock circuit and causes a scram condition to exist. In addition, all cable connectors to the control and safety rods have two pins, which have been wired into the interlock circuit. Any connector which is not connected properly will therefore open the interlock circuit. If any of these circuits are interrupted, the "interlock open" light on the scram chassis panel will be lighted. 

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