Tài liệu TROUBLESHOOTING A TECHNICIAN''''S GUIDE pdf

[...]... to a safe state Fail-safe failures are generally self-revealing A fail-dangerous instrument fails in a manner that moves toward a dangerous state In a continuous system this generally happens immediately; in a demand system this might be a latent failure that, when subjected to the demand, makes the system fail to function and a dangerous situation occur A fail-dangerous latent example might be a plugged... Systematic failures are A B C D 5 systematic failures not random decrease over time all of the above Mean-time-between-failures (MTBF) is A B C D 4 infant mortality failures wear-out failure common-cause failure systematic failure the same as common-cause failures failures in the useful life of an instrument due to human error the same as functional failures Common-cause failures are due to A human errors... eventually, and finding the cause of failure is a big part of troubleshooting Understanding failure mechanisms is important when the cause of the failure is not readily apparent Failures can take different forms, including hardware and software failures A failure can be functional, due to misapplication or abuse Systematic failures result from human error Failures from a single cause can affect multiple... can classify failures according to four general types: • Hardware failures • Software failures • Systematic failures • Functional failures The troubleshooter’s primary purpose in an operating plant is to find what has failed so that it can be repaired and be made available again Keeping the process running properly is the primary concern At its heart, this means identifying the root cause of a failure... “Instrumentation Symbols and Identification.” SUMMARY Instrument failures can be classified in a number of different ways Instruments can fail safely, fail dangerously, in a known state, or in an “I don’t care” state The failure can be self-revealing or overt, or it can be latent or covert The failed state in which you find an instrument is not always the actual failure It may be in that state because it was... failure occurred For example, the fact that a valve has failed closed does not imply that it is strictly a valve failure If the valve is a failclose valve, the valve may have lost its motive power or its signal may have gone to zero Information about final control elements and failure modes should appear on the instrument’s loop drawing and on the piping and instrument diagram (P&ID), and must be taken... equation: MTTF Availability = -MTTF + MTTR An availability of 0.99 would mean that an instrument is available 99% of the time To have a high mean-time-to-failure (i.e., a low failure rate) select a well-designed, sturdy instrument and apply it properly Selecting an instrument designed and properly installed for maintainability is essential to having a low MTTR Unfortunately, other factors... or channels and lead to longer and more complex cause-and-effect chains Mostia2005.book Page 17 Wednesday, October 12, 2005 1:25 PM Troubleshooting 17 QUIZ 1 Failures that occur early in an electronic instrument’s life are A B C D 2 Software failures are A B C D 3 the same as mean-time-to-failure a measure of reliability of a repairable instrument how long an instrument will last none of the above... information and skills Through experience, you get information about classes of instruments and about individual instruments or systems, such as how a particular control valve works and how control valves work in general It is particularly important to be able to generalize about classes of instruments All control valves, for example, have components in common (such as an actuator, a stem, and a trim),... covert, or latent failures Such failures often appear after troubleshooting another failure, after a demand is placed on the system, or during routine testing Testing is the most common way that latent failures are found and defeated Latent failures can be confusing when they are combined with another failure: A failure that has nothing to do with the problem you are troubleshooting can lead you down . home-study courses are also available commercially. Look for ads in technical and trade magazines. Many companies allow their technicians to attend trade shows troubleshooter’s primary purpose in an operating plant is to find what has failed so that it can be repaired and be made available again. Keeping the process