Process control safety interlocks

Active. Using process controls, safety interlocks, and emergency shutdown systems to detect potentially hazardous process deviations and to take immediate corrective action. 

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. 

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. 

Active controls use engineering controls, safety interlocks and emergency shutdown systems to detect process deviations and take appropriate corrective or remedial action. Their effectiveness depends on proper selection, installation, testing, and maintenance. 

Implementation of process actions within process control systems is perfectly acceptable. Furthermore, it is also permissible (and probably advisable) for responsible operations personnel to be authorized to bypass or ignore a process action. 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 actions such as verifying that the interlock continues to be functional, such situations must be infrequent and incorporated into the design of the interlock. 

A safety interlock control function must be separate from the BPCS. Its function is not on-spec product but the prevention of a catastrophic event that would result in human injury or death or damage to equipment. Safety interlocks are usually hardwired to make it difficult to bypass or defeat them. This is done because there have been past occurrences of a unit engineer or a process technician jumping to the conclusion that an alarm was faulty and there was no problem. They tried to go around the interlock to shut off the alarm or prevent process interference. Safety interlocks must not be bypassed without written approval. 

BPCS Process control (PIDs), interlocks for safety related variables, monitoring of misc. variables especially corrosion, material build up, etc., and provide safe... 

Lower Monitoring control Process control Process control General process control and monitoring functions, for example, modulating control, on—off control, sequential control, safety interlock... 

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. 

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. 

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

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

There are few chemical plants that are so forgiving that a control system or a safety interlock system is not required. Process engineers provide controls to assure product yield and quality and maintain safe operating conditions. This type of control system is a BPCS. The BPCS acts to alarm and moderate a high or low operating condition specified by the normal operating limits within the never exceed critical limits. The SIS is provided to shut down or otherwise place the process in a safe state if the BPCS fails to maintain safe operating conditions. A BPCS should not be used as the sole source of a process safety shutdown. 

Process control and safety shutdowns must be provided during all modes of operation, not only in the RUN mode. Other modes will require a BPCS configured for the mode operating algorithm and very likely a different set of safety interlocks must provide appropriate protection. Hardwire devices, like timers or software logic, can be used to actuate the SIS pertinent to the operating mode. 

An inherently safe plant1112 relies on chemistry and physics to prevent accidents rather than on control systems, interlocks, redundancy, and special operating procedures to prevent accidents. Inherently safer plants are tolerant of errors and are often the most cost effective. A process that does not require complex safety interlocks and elaborate procedures is simpler, easier to operate, and more reliable. Smaller equipment, operated at less severe temperatures and pressures, has lower capital and operating costs. 

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

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. 

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). 

What will the measurement results be used for Will they be used for process control (closed loop, open loop, feed-forward, feedback) Will they be used as a safety interlock (This puts very strict requirements on the analyzer reliability, and may even require duplicate analyzers.) Will the results be used to accept raw materials or to release product Will they be used to sort or segregate materials How frequently does the measurement need to be made How rapid does one individual measurement need to be How accurate does it have to be How precise How much analyzer downtime is acceptable ... 

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