Definition of the dangerous failure modes by safety analysis is intrinsic to the proper determination of the failure rate. SIL is for electrical controls only and does not relate directly to the caT architecture in EN It appears to be a precursor to PL ratings that are now the new requirements which encompass hydraulic and pneumatic valves. A device or system must meet the requirements for both categories to achieve a given SIL. The SIL requirements for hardware safety integrity are based on a probabilistic analysis of the device.
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In order to achieve a given SIL, the device must meet targets for the maximum probability of dangerous failure and a minimum safe failure fraction. The concept of 'dangerous failure' must be rigorously defined for the system in question, normally in the form of requirement constraints whose integrity is verified throughout system development.
The actual targets required vary depending on the likelihood of a demand, the complexity of the device s , and types of redundancy used. For continuous operation, these change to the following. Probability of dangerous failure per hour. Hazards of a control system must be identified then analysed through risk analysis. Mitigation of these risks continues until their overall contribution to the hazard are considered acceptable.
The tolerable level of these risks is specified as a safety requirement in the form of a target 'probability of a dangerous failure' in a given period of time, stated as a discrete SIL. Certification schemes are used to establish whether a device meets a particular SIL.
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Electric and electronic devices can be certified for use in functional safety applications according to IEC , providing application developers the evidence required to demonstrate that the application including the device is also compliant. This standard is used in the petrochemical and hazardous chemical industries, among others.
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Thomas, E. Medoff, R. From Wikipedia, the free encyclopedia. This article's tone or style may not reflect the encyclopedic tone used on Wikipedia. See Wikipedia's guide to writing better articles for suggestions. April Learn how and when to remove this template message. Hiten A. A: All systems fail; it's just a matter of when.
Redundant systems fail less often as it takes two simultaneous failures. Whether it's worth the expense or not depends on your down time costs.
If you lose a million dollars due to an unplanned shutdown e. If a failure has little impact, then it's often not worth the trouble or expense. A: It entirely depends on your facility. If your process can stop and be down for a couple of hours while repairs are made, and then start back up with little or no financial consequence, redundancy is not cost-effective. On the other hand, if you have a process that takes a couple of weeks to reach steady state and produces hundreds of thousands of dollars per day of revenue, then it is most likely to be cost-effective.
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This question should be resolved by a cost-benefit analysis, where the cost of a nuisance shutdown is balanced against the cost of redundancy, using a year mean time to failure MTTF for a typical, non-redundant industrial PLC. Ed Marszal edward. A: Yes. In my view and experience, especially in continuous process. In batch process, where one can shut down and change cards, it may not be required.
Gambhir Harvindar. Gambhir ril. A: One needs to understand the difference between redundancy and contingency. The application of double- or triple-redundancy applies to the space shuttle.
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Once airborne, no Apollo 13 issues can take place. No room for error. This, of course, depends on the cost associated with the downtime scheduled or not , and the switch-over is immediate and automatic.
Contingency, on the other hand, allows the process or machine to recover quickly, but with manual intervention. Processor redundancy is easy. Most vendors do it for you. Power supplies can be redundant as well. Most systems don't design for failure, since the consequences aren't that important. The system shuts down, and it gets fixed.
But if you can't do that, then the result of full system redundancy is priceless.
crowdfundteferra.dev3.develag.com/the-impossible-advantage-winning-the-competitive-game-by.php Be aware that most failures come from external devices like valves, drives and sensors. PLC hardware has proven to be very robust. You can go too far! Jeremy Pollard, CET jpollard tsuonline. A: Brian, that depends on the use to which the PLC is being put. If you're working in a critical control area or a safety instrumented system, you probably want all the redundancy you can get.
Batch processing in the food, pharma and biopharma industries are examples of critical control, where having a fail-over redundant system might save hundreds of thousands of dollars or more. Walt Boyes wboyes putman. I know the usual instruments will never work in such a harsh environment, but if I have to satisfy my customer, what options do I have? Is there any manufacturer who can supply such a measuring device? It has to be a non-contact type, but I am worried about the temperature and radiation part. A: I once used Foxboro Target flowmeter with remote mounted electronics on molten salt in a melamine plant.
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I would expect they may have a high-temperature version. A: The only one that can be used at this high temperature is Flexim's non-contact www.