FMEDA as you guessed, is an acronym for Failure Modes and Effects Diagnostic Analysis, the main subject of this blog. The FMEA (failure modes & effects analysis) technique was invented by the aerospace industry in the 1960s, but is still being used today.
FMEDA is an improvement over the older FMEA technique because now we can also get diagnostics about the failures due to easy availability of electronics and microprocessor based systems.
In fact the technique has become popular in several industries and sectors, not just aerospace. One of the main uses of this technique has been in estimating reliability of devices, such as industrial valves and actuators or electronic instrumentation such as pressure transmitters. This technique in theory can be used in any industry, but in the process industry or for large complex plants quickly becomes cumbersome and lengthy. Of course we can always use software to speed it up, but because other risk assessment and reliability techniques are avaialble for process industry applications, we do not use it there.
However the devices that are used in the process industry such as solenoid valves, on off valves or actuators, transmitters of various types can be comparatively easier to be analyzed by using this method. The result is a reliability study that shows us the various failure modes and probabilities that the device will fail in a particular way. For example a Solenoid valve can fail by either remaining open even when the coil is de-energized, or the other way round, o be stuck in between. thus there are only three ways that it can fail to operate in the desired way.
We go on analyzing the failure of every component (such as coil, springs, etc) that leads to each failure and this is how we have an FMEA study. We can now estimate what is the likelihood of a spring getting hard and being stuck up (by using data is published online at various sources) and thus for every such failure we estimate how likely is the SOV to fail open or fail close or fail stuck open. Thus we now can estimate the reliability and then use this information to design our safety instrumented system.
Now if this same technique is used on an electronic transmitter, then the failure modes could be failure of a resistor (either open or short), or failure of a transistor (in 6 possible ways- open or short on either the base-emitter, base-collector or collector-emitter junctions). But now because we have some lectronics already on the device we can generate an alarm which indicates to us that the transistor has failed in a particular way. This alarm then is known as a diagnostic alarm and can be configured so that either a backup circuit kicks in or it sends a "shut down" system to the safety instrumented system so that the plant gets shut down. This prevents a possibility that when a dangerous event occurs in the plant, the safety system cannot activate and protect it.
If this seems a bit complex and confusing, well it is, although I have tried to explain it in a simple way. To have an indepth knowledge of how this is used in a safety system, you can take a course.
For an excellent ecourse on Safety Instrumented System, please take a look at what Abhisam has to offer.
