Safely interlocks

The difference in the nature of process controls and safely interlock systems leads to the conclusion tnat these two shonld be physically separated (see Fig. 8-92). That is, safety interlocks should not be piggybacked onto a process control system. Instead, the safety interlocks shonld be provided by equipment, either hardwired 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 safely interlock system. Therefore, physically separating the safely interlock system from the process controls provides the followiiig benefits ... 

Provide fail-safe interlocks on equipment, doors, valves. 

Fail-safe interlock—An interlock where the failure of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode. 

A formal written system of operation and a reliable software control system where appropriate is essential for the maintenance of plant integrity. SOPs written with the design limitations of the plant in mind are an essential element in maintaining a secure containment system. Fully automatic operation with fail-safe interlocks (to prevent, for example, release of effluent before treatment is complete) can greatly assist in... 

After all, failure of a standby instrument loop, such as an alarm or safely interlock, will not become evident until a potential hazard is detected. Potential defects developing in these loops must be discovered by periodic prooftesting. 

Fahrenheit the temperature scale commonly nsed in the United States, where the freezing point of water is 32 degrees Fahrenheit and the boiling point is 212 degrees Fahrenheit at sea level Fail-safe interlock an interlock where the failnre of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode... 

This paper integrates interlock and design practices into the project safety management system so that design groups can implement cost effective, consistent, and safe interlocking schemes. The objective is to ensure that plants are safe and meet or exceed the safety requirements of the U.S.A-Occupational Safety and Health Administration. 

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. 

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

Gas pressurized Monitor tank level and provide interlock for feed feed shut-off overpressurizes, alternate fluid delivery system (e.g., pump) centrifuge system when feed vessel delivery gas pressure to maximum safe empties working pressure of downstream system (e.g., pressure regulation) Restrict feed flow rate to be consistent with vent capacity Ensure adequate vent capacity for maximum possible gas flow ... 

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

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. 

From Joe s and Mary s viewpoints, each may be correct. Joe s diverse interlock system is indeed inherently safer as a layer of protection than the alternative using identical sensors, but it is still part of a process which is inherently less safe than alternatives which... 

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. 

The view is therefore growing that we should try to design plants so that they are safe even if there is a fault in the software. This can be done by adding on independent safety systems, such as relief valves and hardwired trips and interlocks, or by designing inherently safer plants that remove the hazards instead of controlling them (see Chapter 21). 

Heaters and furnaces should also be designed in accordance with standards and codes. Boilers and heating units must be inspected periodically in accordance with codes, insurance requirements and state regulations. Proper controls, interlocks and fail-safe instnunentation must be provided. The heaters should also be provided with sight glasses for flame observation, monitoring devices for flame-out detection, and temperature alarms. 

Inspecting and maintaining the interlock system is a vital function. It is required to ensure general safety and to avoid risks of boiler explosion. Interlocks provide for the safe sequence of boiler startup and shutdown procedures. 

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. 

Further study requirec Does not apply 4-Completed 4- 14.Special interlocks needed for safe 4... 

The filling method (See Figure 2) is essentially a fail-safe system in that controls are designed to prevent double-cycling. The filling valve and the reservoir valve are electrically interlocked so... 

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. 

Failures can either be fail-safe or fail dangerously. Fail safe incidents may be initiated by spurious trips that may result in accidental shutdown of equipment or processes. Fail dangerously incidents are initiated by undetected process design errors or operations, which disable the safety interlock. The fail dangerously activation may also result in accidental process liquid or gas releases, equipment damage, or fire and explosions. 

Safety mnst be the first consideration of any process analytical installation. Electrical and weather enclosures and safe instrnment-process interfaces are expected for any process spectroscopy installation. The presence of a powerfnl laser, however, is nniqne to process Raman instruments and mnst be addressed due to its potential to injnre someone. Eye and skin injnries are the most common resnlts of improper laser exposure. Fortunately, being safe also is easy. Becanse so many people have seen pictnres of large industrial cutting lasers in operation, this is often what operations personnel erroneonsly first envision when a laser installation is discnssed. However, modem instmments nse small footprint, comparatively low power lasers, safely isolated in a variety of enclosures and armed with various interlocks to prevent accidental exposnre to the beam. 

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