Fail-safe interlock

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... 

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... 

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. 

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... 

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. 

Control set points, alarm, and interlock switch points (as applicable) Engineering range and signal type/level Operating/calibration tolerances Fail-safe mode... 

Depending on the degree of potential catastrophe, there usually is more than one safety interlock for a potential catastrophic event. Each of these safety interlocks including the sensor/transmitter, control function, and final control element are usually independent of the other safety interlocks for the same event. For maximum protection, each sensor for the same event should be unique to eliminate the potential of a common failure. The safety interlock must be fail-safe. This means that any loss of interlock power—electricity, air, hydraulics, etc.— loss of signal, must produce the same action as the safety interlock produces when it is activated (tripped). 

INTERLOCKS ON - Equipment with internal hazards, such as X-ray diffraction cameras, or areas in which the space is rendered unsafe to enter by the presence of a hazard, are often provided with a fail safe circuit, or interlock, which will turn off the equipment representing the problem if the circuit is broken. The sign provides a warning that the interlock is on to prevent access to the hazard. 

An audible or visible warning device is needed if a required interlock is bypassed or defeated. The warning system should be fail safe, i.e., if it became inoperative the unit should be inoperable. 

Warning labels used where guarding, interlocks, or other fail-safe measures required ... 

Safety interlocks, common to machinery, provide a means either of preventing operator access to a hazardous area until the hazard is removed or of automatically removing the hazardous condition (i.e., electric shock, moving parts) when access is gained. Safety interlocks have special requirements, such as fail-safe design, positive opening, and nonoverridable type. 

Machines safety circuits sometimes require special components such as relays, contactors, interlocks, and E-stops. Common terms associated with these machine components are control reliable, fault tolerant, aaA fail-safe, which means that they fail to a safe condition after a single fault (not multiple faults). 

Tolerate the Hazard. The design needs to be fault tolerant. That means, in the presence of a hardware/software fault, the software still provides continuous correct execution. Consider hazard conditions to software logic created by equipment wear and tear, or unexpected failures. Consider alternate approaches to minimize risk from hazards that cannot be eliminated. Such approaches include interlocks, redundancy, fail-safe design, system protection, and procedures. 

Some powered platforms have controls at the platform or basket. Usually there is a second set of operator controls at ground level for general and emergency use. The platform should lock at the level desired and not be subject to free fall if there is a power failure. Users must leam how to set up and operate elevated work platforms. Lifelines connect fall protection harnesses or safety belts to the platform to prevent workers from falling. Controls should be fail-safe and have features to prevent inadvertent actuation. Some controls interlock to level indicator switches to prevent tipping the equipment over. 

All large gamma-radiation processing plants incorporate amongst their safety systems an interlock between the door of the irradiation chamber and the position of the radioactive source. In order to make this system fail-safe, it is almost always necessary to use gravity as the means of moving the source into its safe store. 

Interlocked guards can be hinged, sliding or removable but the integrity of the design of the interlocking mechanisms is crucial. The mechanisms must be reliable, capable of resisting interference and the system should fail safe. 

Control valve as final element if -proof test and maintenance records demonstrate that it meets the shutoff closing speed needs -the fail safe action is defined correctly -it is not shared with another IPL for the same scenario -the interlock has to work direct on the actuator Acceptable if the valve is not the only IPL which reacts within the process safety time for this scenario (e.g. PSV or alarm but nonsafety related) Acceptable as second final element Acceptable second final element NOTE This architecture is not allowed for new or upgraded installations. 

Both mechanical and electrical interlocking is used in this technique, depending on the type of machine to be guarded. The system must be designed such that should a fault occur, the guard will fail safely, making the machine inoperable. 

The approach is not different from the traditional control system where fail safe and plant alarm/interlocks can be brought into action at a moment s notice so as not to endanger life or plant. 

The economic impact of a spurious or nuisance trip of an ESD system can be disastrous. An ESD system is an important layer of protection to prevent and prevent hazardous situations from occurring. So, it is needless to mention that the ESD system must be extremely reliable and function on demand. During an emergency, it must put the process in a safe state in orderly fashion. Also ESD systems design shall be based on a fail safe independent system, that is, ESD systems are such that during a failure of a component the process reverts to a condition considered safe and not a vulnerable serious hazardous event. Reliability and availability are major parameters for ESD system performance. Reliability is a function of system failure rate (its reciprocal) and mean time between failures. Spurious trip conditions may initiate a so-called fail safe incident that may result in accidental shutdown of equipment or processes. However, undetected process design errors or operations may initiate dangerous incidents that may disable the safety interlock and may even cause accidental process... 

Did the loop testing confirm that the alarm/interlock (safety non-safety) action proved, under all conceivable failure conditions, to be fail-safe ... 

Safety integrity level 1 equates to a simple non-redundant single path designed to fail safe with a typical availability of 0.99. Level 2 involves a partially redundant logic structure, with redundant independent paths for elements with lower availability. Overall availability is in the range of 0.999. Level 3 is composed of a totally redundant logic structure. Redundant independent circuits are used for the total interlock system. Diversity is considered an important factor and is used where appropriate. Fault tolerance is enhanced since a single fault of an ESD system component is unlikely to result in a loss of process protection. 

Direct laser fiber monitoring is a more fail-safe safety interlock approach. This may use a second optical signal such as a fiber-optic communication link on the same optical fiber as is carrying the laser. A number of such systems have been... 

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