Fail-safes

FIG. 8-70 Spring and diaphragm actuator w ith an up fail-safe mode. Spring adjuster allow s slight alteration of bench set, (Cmiiiesij Fislwr-Biismnoiinl.)... 

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

Installation of internal excess flow or fail-safe remotely operated valves for tanks at locations where piping connects... 

Ensure that control systems are fail safe both individually and collectively... 

Eail-Safe Design features which provide for the maintenance of safe operating conditions in the event of a malfunction of control devices or an interruption of an energy source (e.g., direction of failure of a control valve on loss of signal). A system is fail-safe if failure of a component, signal, or utility that would create a hazard initiates an action that maintains the system in a safe condition. 

The eddy current brake controls for the power recovery string are designed for fail-safe operation of the eddy current brake. These controls provide all of the logic required to make the eddy current brake recovery string a viable contender for recovering power from previously wasted energy. 

Figure 8-11 shows the effleieney variation with the tip speed ratio. This eurve also shows the runaway speed. Runaway speed is aehieved when turbine torque falls to zero at blade speeds higher than the design speed. If failure oeeurs above the tip speed, the rotor ean be defined as a fail-safe rotor design. 

More difficult to provide for positive fail-safe operation. 

Design not fail-safe (i.e. no indication of component failure)... 

Does the process fail safe in respect of heat, pressure, fire and explosion ... 

Skins with bonded doublers have been used successfully on a large number of civil aircraft and are still used on new designs. The only widespread in-service problem with bonded doubler assemblies has been delamination caused by unstable surface preparation. Early fuselage skins with bonded doublers and inadequate surface preparation experienced severe delamination and subsequent corrosion. A majority of these delaminated doublers were the fail-safe tear straps (Fig. 27). Although the tear strap bond does not cany structural load, the bond... 

Fail Safe-An instrument that on loss of power (pneumatic, electric, etc.) will go to a position that cannot create a safety hazard. 

Fail-safe Design features that provide for the maintenanee of safe operating eonditions in the event of a malfunetioning eontrol deviee or an interruption of an energy souree (e.g., direetion of failure of a motor-operated valve on loss of motor power). 

The AP600 passive safety systems use natural forces gravity, natural circulation, and compressed gas. Simple, mostly fail-safe, s alves align the passive safety systems upon... 

Technician fails to follow procedures Should have no consequences Low Critical Assembly is designeii to fail safe. 

It generally is recommended, and often required, that gas dcicciiuii systems be installed in a fail-safe manner. That is, if power is disconnected or otherwise interrupted, alarm and/or process equipment shutdown (or other corrective action) should occur. All specific systems should be carefully reviewed, however, to ensure that non-anticipated equipment shutdowns would not result in a more hazardous condition tlian the lack of shutdown of the equipment. If a more hazardous situation would occur with shutdown, only a warning should be provided. As an example, a more hazardous situation might occur if blowout preventers were automatically actuated during drilling operations upon detection of low levels of gas concentrations than if drilling personnel were only warned. 

Generally, electrical control systems are designed Fail-Safe. If power is temporarily lost, unnecessary shutdown of the process may occur. Thus, most safety systems such as fire and gas detectors, Nav-Aids, communications, and emergency lighting require standby D.C. power. 

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. 

All instruments and controls must be of the fail-safe" type... 

It is virtually impossible to design a fail-safe operation of a chemical process. However, many companies have attempted to minimize liazardous conditions by developing a systematic approach to process design. Implemention of these acdons hoped to aeliieve maximmn protection to personnel, equipment, and die public. 

If there is a pow cr failure what fail-safe procedure(s) docs tlie process contain ... 

Figure 12-31. Capacity control suction valve unloaders are available in either of two designs with pneumatic operators (A) direct-acting (air to unload) (B) reverse-acting or fail safe (air to load), which automatically unloads the compressor in the event of control air failure (C) an innovation in manually operated unloaders. Here, the lever cam arrangement provides positive loading or unloading, eliminating the requirement to turn a handwheel completely in or out. (Used by permission Bui. 9-201B, 1991. Cooper-Cameron Corporation.)...

Design of the product to meet the anticipated environmental stresses as well as the functional requirements. The product must have a designed-in means for reducing the possibility of failure plus either a fail-safe mode or an indication of incipient failure that can be monitored. 

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