Interlocks, safety

These safety interlocks are suggested to minimize consequences arising from human errors and damage to key/costly equipment. 

These are to be provided for automatic tripping of the plant equipment/systems to protect the boilers, to prevent atmospheric pollution, to prevent damage to main air blower, etc. The detailed design of the interlocks should be made as per site conditions, actnal plant design, etc. 

The working of these safety interlocks shonld be checked at least once every week and recorded. Any non-functioning link/item should be immediately rectified and working of the interlock re-checked again to ensure plant and environmental safety. 

Note The technology to produce sulfuric acid, oleum, etc. using multi-stage catalytic converters has evolved over six decades and has been perfected for thermally and mechanically stable plants. It is therefore possible to consider digitally controlled systems (without human intervention) to run the plant. With this in view, the following safety interlocks may be considered as a step in that direction. 

Safety interlocks for sulfuric acid, oleum, and liquid SO3 plants  

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The entire x-ray exposure cabinet including it s extension into the lower electrical cabinet is shielded with a minimum of one-inch of steel clad, lead plate with fiilly welded (fused lead) joints. The safety interlock switch on the cam-lock inter-face ... 

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. 

By attempting to maintain process conditions at or near their design values, the process controls so attempt to prevent abnormal conditions from developing within the process. Although process controls can be viewed as a protective layer, this is really a by-product and not the primaiy func tion. Where the objective of a function is specifically to reduce risk, the implementation is normally not within the process controls. Instead, the implementation is within a separate system specifically provided to reduce risk. This system is generally referred to as the safety interlock system. 

Where hazardous conditions can develop within a process, a protective system of some type must be provided. Sometimes these are in the form of process hardware such as pressure rehef devices. However, sometimes logic must be provided for the specific purpose of taking the process to a state where the hazardous condition cannot exist. The term safety interlock. system is normally used to designate such logic. 

The purpose of the logic within the safety interlock system is veiy different from the logic within the process controls. Fortunately, the logic within the safety interlock system is normally much simpler than the logic within the process controls. This simplicity means that a hardwired implementation of the safety interlock system is usually an option. Should a programmable implementation be chosen, this simplicity means that latent defects in the software are less likely to be present. Most safety systems only have to do simple things, but they must do them very, very well. 

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. 

Modifications to the process controls are more frequent than modifications to the safety interlock system. Therefore, physically separating the safety interlock system from the process controls provides the following benefits ... 

The possibility of a change to the process controls leading to an unintentional change to the safety interlock system is eliminated. 

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. 

Safety interlocks. These are designed to protect the pubhc, the plant personnel, and possibly the plant equipment from process hazards. 

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. 

Safety interlocks are assigned to categories that reflect the severity of the consequences should the interlock fail to perform as intended. The specific categories used within a company is completely at the discretion of the company. However, most companies use categories that distinguish among the following ... 

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. 

A process hazards analysis is intended to identify the safety interlocks required for a process and to provide the following for each ... 

The logic for the safety interlock, including inputs from measurement devices and outputs to ac tuators. 

Diversity is recognized as a useful approach to reduce the number of defects. The team that conducts the process hazards analysis does not implement the safety interlocks but provides the specifications for the safety interlocks to another organization for implementation. This organization reviews the specifications for each safety interlock, seeking clarifications as necessary from the process hazards analysis team and bringing any perceived deficiencies to the attention of the process hazards analysis team. 

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. 

As part of the detailed design of each safety interlock, written test procedures must be developed for the following purposes ... 

The concentration of fuel in air in a process should be maintained at or below 25 percent of the LFL, with automatic instrumentation and safety interlocks however, up to 60 percent of LFL is permitted by the NFPA—except for ovens or furnaces. (Ovens and furnaces are covered in NFPA 86.)... 

The gas turbine control systems are fully automated, and ensure the save and proper startup of the gas turbine. The gas turbine control system is complex and has a number of safety interlocks to ensure the safe startup of the turbine. 

Active—Using controls, safety interlocks, and emergency shutdown systems to detect and correct process deviations e.g., a pump that is shut off by a high level switch in the downstream tank when the tank is 90% full. These systems are commonly referred to as engineering controls. 

A pressure sensor giving a continuous indication which is displayed on the control panel and can be observed by the operator. The sensor has a high pressure safety interlock set at a predetermined pressure that activates an emergency shutdown system. 

Although many engineers provide only the minimum adequate vessel design to minimize costs, it is inherently safer to minimize the use of safety interlocks and administrative controls by designing robust equipment. Passive hardware devices can be substituted for active control systems. For example, if the design pressure of the vessel system is higher than the maximum expected pressure, an interlock to trip the system on high pressure or temperatures may be unnecessary. 

Basic Process Control System (BPCS) and Safety Interlock System (SIS)... 

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