Hard-wired interlocks

Confirm power supply voltages and quality are adequate Confirm earthing meets manufacturer s requirements Confirm correct versions of software have been supphed and installed Confirm hard-wired interlocks have been correctly installed... 

The oleum collection vessel should be fitted with a means of determining the level (sight glass) and a high-level alarm to warn the operator of the need to empty the vessel and to give an indication, from records, of excessive oleum production. The discharge of oleum from the vessel is normally a manual operation, although in the case where oleum is pumped to the SO2 scrubber, hard-wired interlocks must be used to prevent the transfer of oleum when the scrubber is not in operation. 

Hard-wired interlocks should be provided to prevent opening of the SOj/air isolation valve until the reactor emergency system is primed. The system should be activated by a hard-wired interlock to an organic flow sensor. 

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. 

The difference in the nature of process controls and safety interlock systems leads to the conclusion that these two should be physically separated (see Fig. 8-89). That is, safety interlocks should not be piggy-backed onto a process-control system. Instead, the safety interlocks should be provided by equipment, either hard-wired or programmable, that is dedicated to the safety functions. As the process controls become more complex, faults are more likely. Separation means that faults within the process controls have no consequences in the safety interlock system. 

Although the traditional point of reference for safety interlock systems is a hard-wired implementation, a programmed implementation is an alternative. The potential for latent defects in software implementation is a definite concern. Another concern is that solid-state components are not guaranteed to fail to the safe state. The former is addressed by extensive testing the latter is addressed by manufacturer-supplied and/or user-supplied diagnostics that are routinely executed by the processor within the safety interlock system. Although issues must be addressed in programmable implementations, the hard-wired implementations are not perfect either. 

The potential that the logic within the interlock could contain a defect or bug is a strong incentive to keep it simple. Within process plants, most interlocks are implemented with discrete logic, which means either hard-wired elec tromechauical devices or programmable logic controllers. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

Sequence control of process operations, and recipe/batch management and tracking. Alarm and device interlocking (often in addition to separate hard-wired systems). Event and alarm recording, and historical trend recording of process variables. 

EaciUties to allow external hard-wired circuits, such as interlocks, to be monitored and used by the IMCS. 

At least one hard-wired Emergency Stop function shall be generated to create an emergency shutdown independent of the PLC and shall function even if a component of the PLC fails. The Emergency Stop function shall be interlocked in to the PLC software. The E-Stop hardwiring shall open appropriate power circuits to PLC outputs. 

Another common interlock configuration is to locate a solenoid switch between a controller and a control valve. When an alarm is actuated, the solenoid trips and causes the air pressure in the pneumatic control valve to be vented consequently, the control valve reverts to either its fail-open or fail-close position. Interlocks have traditionally been implemented as hard-wired systems that are independent of the control hardware. But, for most applications, software implementation of the interlock logic via a digital computer or a programmable logic controller is a viable alternative. Programmable logic controllers (PLCs) used for batch processes are considered in Chapter 22 and Appendix A. 

These protective interlocks enable on-line configuration changes in the protective logic units and are hard-wired into both safety display and command racks and control rod controllers to constrain movement sequences. 

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