Reliability and Trip Systems Flashcards
What is reliability?
The probability that an item will perform a characteristic function under stated conditions for a stated operating period, t.
What is the one limitation for t in the reliability?
t must be similar to the expected life of the item.
What are two useful measures of the reliability?
- failure rate
- mean time between failures
What is the assumption of the failure rate?
It is assumed to be random.
What is the relationship between the failure rate and the mean time between failures?
The mean time is the inverse of the failure rate
Describe the bathtub curve for the failure rate?
There is high failure initially due to burn in. There is then a horizontal line for steady random failure and then high failure rate towards the end of the lifetime due to wear out.
How is the burn-in period controlled?
This should be controlled through the selection of the equipment and good installation.
How is the wear out period controlled?
Equipment should be replaced before the wear out period.
How is it ensured that a pump will work reliably?
It wouldn’t be bought from a cheap source unless there was quality assurance. Good installation, inspection and training can assure that when the pump is started it works reliably.
What is the relationship between R, lambda and t in the constant failure rate period?
R = exp (- lambda * t)
What is the probability that an item will fail, F, given by?
F = 1 - R
Is this equation used for systems with high or low failure rates?
Low failure rates
What is the reliability for N items running in series?
Rs = R1 x R2 x R3 x … x Rn
What is the failure for N items running in parallel?
Fs = F1 x F2 x F3 x … x Fn = 1 - Rs
So what can be said about the failure rate, if the reliability is high?
The failure rate is low
Why is it significant in series for the reliabilities to be multiplied?
Series systems fail much more frequently. If one component fails then the whole system shuts down so you multiply reliabilities. No matter if the reliability is high then the reliability goes down once the product has been taken.
Why is it significant in parallel for the failures to be multiplied?
The system fails only if all the components have failed. The reliability is high.
Is the reliability higher for series or parallel systems?
Parallel systems
What are the two types of device for trip systems?
- Active devices
- Passive devices
What is an active device?
They are doing something observable/ active.
What is a passive device?
Devices only operating as required. You don’t know that they work until they are tested.
What is the definition of a failure?
Something is unable to perform its normal function
What are the two types of failure?
- Active failures
- Passive failures
What is an active failure?
Active failures are revealed. Something was functioning and stops.
What is a passive failure?
Passive failures are unrevealed. The item will not perform its function when it is required sometime after the failure. You don’t know about the failure until the device fails to respond to a demand.
What is the definition of the fractional dead time?
It is the proportion of all time for which the device will not respond correctly to a demand. This includes the time that is needed for repair as the device is not operating.
What are trip systems?
Trips are control devices which shut down all or part of the plant when control limits are passed.