Process interlocks

Interlocks Some of these are provided for safety and are properly called safety interlocks. However, others are provided to avoid mistakes in processing the batch and are properly called process interlocks. 

In continuous processes, most process control applications rely on continuous measurements. In batch processes, many of the process control applications will utihze discrete as well as continuous measurements. In both types of processes, the safety interlocks and process interlocks rely largely on discrete measurements. 

Being excellent at discrete logic, PLCs are a potential candidate for implementing interlocks. Process interlocks are clearlv acceptable for implementation within a PLC. Implementation of safety interlocks in programmable electronic systems (such as a PLC) is not universally accepted. Many organizations continue to require that all safety interlocks be hard-wired, but implementing safety interlocks in a PLC that is dedicated to safety functions is accepted by some as being equivalent to the hard-wired approach. 

Process interlocks. These are designed to prevent process conditions that would unduly stress equipment (perhaps leading to minor damage), lead to off-specification produc t, and so on. 

Basically, the process interlocks address hazards whose consequences essentially lead to a mouetaiy loss, possibly even a short plant shut-... 

Implementation of process interlocks within process control systems is perfectly acceptable. Furthermore, it is also permissible (and probably advisable) that responsible operations personnel be authorized to bypass or ignore a process. Safety interlocks must be implemented within the separate safety interlock system. Bypassing or ignoring safety interlocks by operations personnel is simply not permitted. When this is necessary for ac tions such as verifying that the interlock continues to be func tional, such situations must be infrequent and incorporated into the design of the interlock. 

Hazards that could re.sult in major equipment damage and consequently lengthy plant downtime. No redundancy is normally required for these, although redundancy is always an option. Situations that result in minor equipment damage that can be quickly repaired do not generally require a safety interlock however, a process interlock might be appropriate. 

Although safety interlocks can inappropriately initiate shutdowns, the process interlocks are usually the major source of problems. It is possible to configure so many process interlocks that it is not possible to operate the plant. 

All interlocks checked for correct setting and operation. These will include thermostats, pressurestats, limit switches, pressure switches, process interlocks, etc. ... 

The Sabine River Works of the DuPont Company Plant in Orange, Texas, shared their process interlock classification and test program in 1988. [26] At that time, this large DuPont chemical complex employed about 2,500 individuals and consists of large singletrain processing units. Most of the units are continuous processes for the manufacture of olefins, polyolefins, and chemical intermediates for nylon. 

Gregory McMillan, formerly an engineering fellow in process control for the Monsanto Chemical Company, approached the subject differently. His very entertaining presentation entitled, Can You Say Process Interlocks at an AIChE Loss Prevention Symposium was written as a parody of a popular youngster s TV show. [27]... 

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 ... 

Critical safety systems are usually considered to be those that are necessary for safe plant operation during processing and also those required for the safe shutdown of the plant in the event of an unanticipated malfunction. Within an incineration plant, examples of critical safety systems inclnde agent monitoring systems, testing or inspection of pressure rehef valves, ventilation flow and control systems, fire protection systems, emergency alarm and shutdown systems, process interlocks, and furnace temperature control systems. 

Distinguish between a safety interlock system and a process interlock... 

Interlocks are commonly used safety devices. The function of an interlock is to prevent the occurrence of an event in the piesraice of certain conditions. Some interlocks prevent action or motion, others send signals to other devices that prevent the action or motion. They automatically reconfigure or interrupt final control devices if monitored variables deviate significantly from specifications. Typical process variables monitored are flow, pressure, level, and temperature. Typical machine variables monitored are coolant level and temperature, lubricant level and temperature, vibration, speed, etc. Interlocks allow equipment to start and operate only when monitored variables are within designed specifications. Interlocks inhibit unanticipated actuation of equipment and ensure correct startup/shutdown sequences are followed. A permissive interlock will not allow a process or equipment to startup unless certain conditions are met. There ate two types of interlocks—safety and process interlocks. Each serves a different function. 

Process interlocks make up an automatic system that detects an abnormal condition and either halts process action or takes corrective action to return the process to normal. Process interlock systems may be part of the Basic Process Control System (BPCS). The BPCS is a system of measurements and controls including alarms and interlocks that function to keep the process within acceptable operating limits. The BPCS is usually associated with producing good quality or in-spec finished product. 

Because a process technician or engineer cannot tell the difference from a process interlock tuid a safety interlock, there must be a list available to employees of the interlocks and their type. This list is usually part of the process safety information required by OSHA. Safety interlocks must be tested and the test documented. Often, this is on a yearly basis and done by an outside contractor. 

Cause and effect charts Process interlocks and alarms list... 

Check that the high alarm set-point corresponds to the high-high tank level Check that the tank inlet valves are closed, and the divert valves are open Check that the tank feed water supply pumps/systems are not operating (should turn off automatically due to a process interlock)... 

Process interlocks with the rectifier and process tripping of the rectifier should be verified. They should be proven through all stages of signal processing by actual reproduction of the process conditions, if possible, or at least by simulation and verification of the interlock or trip functionality. 

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