Multiple Safety Devices

There should be a provision for more than one safety device (pressure and temperature switch, mpture discs, and safety valves) to ensure safety to persons and plant units even if one of the devices fails. 

VERY HIGH 6AS PRESSURE AT COMPRESSOR OISCHAROE WILL TRIP DRIVE WOTOFL 

LOW COQLINQ WATER PRESSURE AT AMMQhMA CONDENSER R LET WILL TRIP DR(VE MOTOR. 

HIGH LIQUID AMMONIA LEVEL N EVAPORATOR CHILLER), WILL THROTTLE LIQUID INLET CONTROL VALVE, 

4 Ammonia-based chilled water plant with safety interlocks 

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Ammonia-based chilled water plant has provision for multiple safety devices (pressure and temperature switch, and safety valves). These are shown in the Fig. 6.4. 

Fuzes usually incorporate protective features that enable the projectile to reach a safe distance from the weapon before they become active. Safety devices may lock the firing fuze mechanism until acceleration moves the lock backwards in a manner restricted by an escapement. Multiple locks may operate sequentially until the projectile is well under way before the projectile is armed. Proximity fuzes can be locked electronically, although these projectiles may be protected with a system that controls the time at which the detonator is placed in alignment with the main charge. 

One way to promote reliability in the separation of two fluids is redundancy, and many of the arrangements used as utility-process connections include multiple devices. Reliability also increases with quality and with features designed to enhance the action of safety devices. An example of the latter is the use of internally force-loaded check valves (FLCVs) rather than the more conventional design. 

Clause 6.2.2 states, A SIS may have a single safety function or multiple safety functions that have a common logic solver and/or input and output devices. When multiple safety functions share common components, the common components shall satisfy the highest SIL of the shared safety function. Components of the system that are not common must meet the SIL... 

The relatively large, two-story JACADS complex included myriad electrical, mechanical, automated, and robotic equipment (including many monitoring devices) used to destroy or dispose of multiple types of agents and all their varied kinds of containers and weapons systems. Constant attention had to be paid to safety while meeting production goals. 

Decomposition should be averted for both safety and environmental reasons. In case of decomposition, multiple levels of protection are applied in appropriate sequences of procedural controls, instrument controls, interlocks, and relief devices to minimize damage to the plant and impairment of environment. Because the discharge of ethylene and its decomposition products into the air involves considerable risk, precautions for safe venting must also be considered. 

What the PED does say in its ESR related to SRVs is that pressure-limit- PED categories ing devices must prevent the maximum allowable pressure being permanently exceeded. The safety valve (SV) should be set at PS (= MAP) or lower, with exception of momentary pressure surge (accumulation) which is 10% of PS in all cases (multiple valves also) and except for fire, this can be higher if proven safe. This is much more general but definitely differs with ASME as described in detail in Section 3.6. 

Active for example, safety shutdown systems to prevent accidents (e.g., a high level alarm in a tank shuts automatic feed valves) or to mitigate the effects of accidents (e.g., a sprinkler system to extinguish a fire in a building). Active systems require detection of a hazardous condition and some kind of action to prevent or mitigate the accident. Multiple active elements involve typically a sensor (detect hazardous condition), a logic device (decide what to do) and a control element (implement action). 

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