Fail-safe principles

Much railway equipment is based on fail-safe principles dedicated to the concept that gravity is a force that will always be there. As a result, semaphores, signal switches, and the lights to which they are connected are weighted devices. In the event of a power or system failure, a heavy arm is allowed to drop and activate the fail-safe warning signal. In the processing plants, if instrument air is lost control valves go to the more safe position— either fail open or fail closed. 

Use as many Fail Safe Principles [Kritzinger (2006) Chapter 7 para 3] as possible to help provide substantiation of qualitative probability declarations. 

Reactivity control a) Reactor regulatmg system b) Shutoff rods (28) c) Poison mjecnon d) Loss of DjO moderator Active Active/Passive Passive Passive power changes-set back and trip I Fail safe principle, redundancy Fail-safe prmciple, redundancy Loss of D20 moderator leads to subcrihcality ... 

A new type of hydraulic drive mechanism is used to drive the control rods in HR-200. In the drive system the reactor coolant (water) is the actual medium. The water is pumped into step-cylinders of which the movable parts contain the neutron absorber. A pulsed flow, generated by a controlling magnetic valve in the control unit moves the movable part of the step-cylinder step by step. The drive system is very simple in its stmcture and is designed on the "fail-safe" principle, i.e. all control rods will drop into the reactor core by gravity under loss of electric power, depressurization, postulated breaks in its piping systems and pump shut down events. 

The central control building (switch gear building) is designed to withstand earthquake only. In the case of its destruction the reactor is safely shut down due to the fail safe principle. The reactor then is monitored from the emergency control room inside the protected reactor building. 

The main steam and feedwater lines is equipped with three system-fluid-operated isolation valves, one of every three being of a different design from that of the other two valves. Closure of the main steam valves to isolate the containment is based on the fail-safe principle employing solenoid pilot valves or through passive actuation. 

Answer There are four general principles considered in the reactor control design. These are (l the principle of duality> (2) the fail-safe principle, (3) the sequential principle, f nd (4) the principle of reliability. 

A veto is implemented by the opening of a contact in series in the electric power supply chain of an IIS, following the fail-safe principle. A veto on an IIS Beam is a... 

Redundancy experience has shown that no structure is immune to failure. Hence the increasing use of redundancy in the form of duplicated systems and multiple load paths. For more information, see the fail-safe principles discussed in Chapter 7. 

The principles of operation of a hydraulic actuator are like those of the pneumatic actuator. Each uses some motive force to overcome spring force to move the valve. Also, hydraulic actuators can be designed to fail-open or fail-closed to provide a fail-safe feature. 

Lee developed a set of principles of a fail-safe medication use system striving for a sysfem thaf is pafienf cenfered, based on respect for others, and requires an acceptance of responsibility and a collaboration of interests. ... 

These principles sef the tone for characteristics of a fail-safe medication use system that ... 

The opposite approach to inducing proliferation is to induce cell death, using conditional alleles of Fas or caspases. A Fas death switch has been used to eliminate engineered T cells infused into animals [44], as a model for depleting the T cells that cause graft-versus-host disease following bone marrow transplantation [45]. More potent caspase-based switches can also be used [46] and, in principle, could be installed into any therapeutic cell to provide a fail-safe mechanism for cell destruction should adverse events ensue. 

The most important design factor about pressure-relieving devices is the underlying principle of intrinsic safety. They must fail safe or not at all. 

Fail Safe design principles (see Kritzinger (2006) Chapter 7) will go a long way in preventing CMA vulnerabilities. More specifically, the following strategies should keep common failure modes to a minimum ... 

Figure 26.13 Limit switch with cover removed showing the principle of fail-safe operation. (Courtesy Dewhurst Partner p.I.c.)...

The principle of fail-safe design shall be considered and shall be adopted in the design of systems and components important to safety, as appropriate systems at research reactor facilities shall be designed to pass into a safe state, with no necessity for any action to be initiated, if a system or component fails. 

In Brazil, creating machineries and tools must satisfy the principle of fail-safe (BRASIL, 2011). Thus, in case of emergency or any failure, the machine does not necessarily need to stop, but it must operate without risk, ensuring the integrity of employees. Thereby, the machine does not must offer risk in any situation, even if it shows failure in its normal running. 

The fail-safe design principle will be applied in the new system to the extent possible. The possibility for undetectable failures will be significantly reduced. 

The fail-safe criterion is not fully implemented for the reactor trip system but will be implemented in the design of the new system. The ESFAS system is designed as fail-safe to the extent possible the same principle will be applied to... 

Following these principles in a more specific way for polymerization reactions, three levels of priority can be defined in decreasing order, the first priority is the reduction of severity by design. As a second priority, technical measures for control of the reaction to avoid runaway should be considered. The aim is to obtain a fail safe process by reduction of the probability of occurrence of an incident. As last resort only, emergency measures should be taken in order to mitigate the consequences of runaway. In any case, the basic principle remains Avoid runaway rather than mitigate its consequences. ... 

There are various fault flow charts in standards or other rules of technology, covering the specific design requirements of fail-safe behaviour for the safety device in question. The principle of all fault flow charts is always the same. The chart begins with the "ist failure" (e.g. emitter-collector of any transistor short circuit). It is to be verified that after each "ist failure" no dangerous situation may occur. If so, one has to ask what else happens after the "ist failure". There are 4 answers to this question ... 

Fail-safe design ensures that the product either ceases to function or alerts the user when certain performance limits are violated. It prevents the user from making mistakes that cost time and effort. For example, in some car designs audio alert sounds if the headlights are left on. The same principle can be applied to sustainability. Some Singapore taxis are equipped with a bell that chimes when a certain speed is exceeded, alerting the drivers to slow down and save fuel. 

This section describes a simplified safety concept for the fail-safe wind turbine control and protection using well tried safety principles and solutions (common practice). The loo2 (D) dual channel architecture shown in Figure 3 is based on two independent processors, two shared diverse input sources (rotation speed) and two output relays connected in serial to the safety chain. 

The field connections to the safety sensors and outputs are established through dedicated safety certified input/output sub systems that carry the essential diagnostics and an independent means of shutdown in aecordance with the principles we have seen. Profisafe is a certified protocol with diagnostics and fail-safe responses. The system is designed to maintain functional independence for all safety functions in hardware and software. The standard Profisafe DP protocol can then share the same network to communicate with standard controllers and standard peripheral devices such as I/O sub systems or variable speed drives. 

A particular designed dynamic logic principle that achieves the fail-safe property, from the principle itself and not from additional components or test circuits. 

Designing in such a way that failures result in known failure modes is one method of accommodating expected failures of systans or components. Failures should produce not only predictable failure modes but also failure modes that place the system in a safe state. The Requirements for Design require that the principle of fail-safe design be considered and incorporated as appropriate into the design of plant systems and components important to safety (Ref. [1], para. 5.40). 

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