Fail-safe defined

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. 

For ESD isolation valves (i.e., EIVs) a fail safe mode is normally defined as fail closed in order to prevent the continued flow of fuel to the incident. Blowdown or depressurization valves would be specified as fail open to allow inventories to be disposed of during an incident. Special circumstances may require the use of a foil steady valve for operational or performance reasons. These applications are usually at isolation valves at components, i.e., individual vessels, pumps, etc., where a backup EIV is provided at the battery limits that is specified as fail closed. The fail safe mode can be defined by the action that is taken when the ESD system is activated. Since the function of the ESD system is to place the facility in its safest mode, by definition the ESD activation mode is the foil safe mode. 

Actual failures of instruments can be classified as "fail-safe," "fail-danger," or another failure mode. Such failure modes will be defined in this chapter in the context of an individual instrument. Note that sometimes the application must be understood before these classifications can be made. It must be remembered that the safety instrumented function may or may not fail when one instrument has failed. A redundant architecture may compensate for instrument failures. 

Most practitioners define "Fail-Safe" for an instrument as a failure that causes a "false or spurious" trip of a safety instrumented function unless that trip is prevented by the architecture of the safety instrumented function. Many formal definitions have been attempted that include "a failure which causes the system to go to a safe state or increases the probability of going to a safe state." This definition is useful at the system level and includes many cases where redundant architectures are used. 

An annunciation failure is therefore defined as a failure that prevents automatic diagnostics from detecting or annunciating that a failure has occurred inside the equipment. Note that the failure may be within the equipment that fails or inside an external piece of equipment designed for the purpose of automatic diagnostics. These failures would be classified as "Fail-Safe" in the definition provided in lEC 61508. 

With reference to Fig. 1.3, Step 3 is all about validating that the proposed system architecture will indeed meet the safety requirements and to consider which actions are needed to define more robust and/or fail safe system architecture. 

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. 

In this paper the term security is defined as computer security for information technical systems. In [11] security is specified as property of a fail-safe system, which can only enter system states, which can cause no unauthorised manipulation or retrieval of information. 

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. ... 

Fail-safe behaviour. Where the failure of components are as defined in the criteria, and the safe behaviour of the system must be demonstrated in the presence of those failure modes. 

Safety failure can be defined as the inability of a system to perform a required function which leads to serious or dangerous consequences 4. Since the Interpretation of "serious consequences" is left to the designer, user, etc. he may define a failure to be critical even though it is not considered as such. How can a designer overcome a critical failure There is no unique answer to this but rather a variety of solutions, depending on specific event and requirement. Most safety criteria require a system to be just fail-safe However in some cases self-recovery or prevention is required. Hence the final solution is dependent upon the requirements of the individual system. 

The "fail-safe" and "fail-danger" modes of the system are well defined. 

Example 2.11 describes an FSM which shows fail-safe behavior. The others clause ensures that if the FSM were to go to the state defined by encoding "ll , then it is reset to the state SO. 

Define safe state - Fail Safe State for initiator is HH - Isolation valve closed... 

A systematic project activity to ensure that a space payload intended for flight has sufficient structural integrity as to present no critical or catastrophic hazard. This activity also ensures quality of performance in the structural area for any payload. Central to the program is fracture control analysis, which includes the concepts of fail-safe and safe-life, defined as follows ... 

The four basic integrity levels defined in the lEC document range from the highest integrity (Class 4) to the lowest integrity (Class 1). At the lowest defined level (Class 1) simple fail safe techniques are all that is required, these imply a level of self test and monitoring and provided a separate control system is in place, a simplex safety system pic would be adequate. 

Title Railway Applications - Safety Related Electronic Systems for Signaling Description ENV 50129 has been produced as a European standardization document defining requirements for the acceptance and approval of safety related electronic systems in the railway signaling field. The requirements for safety related hardware and for the overall system are defined in this standard. It is primarily intended to apply to fail-safe and high integrity systems such as main line signaling. 

The high levels of functional safety needed from essential systems are usually achieved by some form of fail-safe design. The fail-safe design concept considers the effects of failures and combinations of failure in defining a safe design. The application of the fail-safe concept is probably the most important discipline involved in the design of systems and operations. It has evolved over many years. The definition first appeared in the dictionary in the mid-1950s after the final reports on the Comet disasters were published. 

The calculation of the fire s outcome in the third step includes the distribution of heat, smoke, and toxic gases throughout the building of concern. It allows the introduction of people into that building and monitors their movement in response to the fire. They may escape safely or fail to escape due to heat or the inhalation of toxic smoke. The benefits of changing some component of the defined fire problem is observed in the change in the number of deaths predicted, rather than by direct comparison of the toxic potencies of the different smokes. This mirrors the complexity of real-life fires. 

Finally, if the registrant can prove that all risks are under control and the substance can be safely manufactured and used, the corresponding initial exposure scenario is defined as the final exposure scenario. In the end, the final exposure scenario is communicated within the framework of extended safety data sheets in order to ensure the safe use of the substance down the supply chain (Caveat The legal text of REACH usually refers to the term exposure scenario while in reality speaking of the final exposure scenario.) By contrast, if the registrant fails to lower the risk characterization ratio below 1, despite the aforementioned refinements and modifications, he must prevent the use of the substance under circumstances where the risks are not controlled. 

If this EU Directive on the protection of personal data were to be taken literally, most clinical trials would cease, particularly where clinical data were transferred outside the EU. Of the non-EU countries, only Switzerland and Hungary apparently meet the requirements of the Directive at the present time Other countries such as the US, Ganada, Australia and New Zealand apparently fail to meet the requirements of the Directive when handling personal data and therefore, in theory, European data cannot be processed in these countries. Fortunately, the so-called Safe Harbor scheme allows clinical data to be sent to defined organisations in the US that comply with the Directive s principles. It is anticipated that similar agreements will be set up to overcome these legislative hurdles so that global studies driven by European pharmaceutical companies and institutions can be undertaken without problems. 

As chemicals are inspected, there needs to be a definition by the facility as to what constitutes an unacceptable risk (or unsafe). If unsafe is not defined, then one cannot determine when a container fails inspection. If needed, review commercial industry practices for reported safe storage times before chemical disposition occurs. DOE sites could also determine what other DOE sites are doing. 

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