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Gas Turbine reliability calculation and Maintenance factor?
Let assume failure of gas turbine follow exponential distribution. Reliability with start-stop condition, R(t,T), shown higher result compared to reliability of work without stop, R(T), at certain total time, T = 8000 and for t = 4000,
R(T) < R(T,t)
R(8000) < R(8000,4000)
e^-λT < e^-λt
e^-8000λ < e^-4000t
Although R(T,t) has higher reliability for entire of time, but Maintenance factor tend to increase, thus inspection and maintenance must be done more rigorous.
In my opinion with higher reliability, gas turbine should receive less maintenance, because it has lower probability to fail. But this not allowed by maintenance factor, which must increase as start-stop becomes more often, although this condition conform with dynamic loading criteria.
How to interpret reliability and maintenance factor? Are they related or not ?
Thanks in advance.
*) e^-8000λ < e^-4000λ, failure rate, λ, constant.
Combustion Inspection, CI, done every 8000 h (1 year) with maintenance factor, MF = 1 (Full service no stop).
If there are scheduled shut down (no maintenance), let assume 10 times at 8000 h. MF will increase, let say MF = 3, thus CI must be done faster than 8000 h, perhaps 5000 h.
However, according to reliability calculation, second case has higher reliability or less probability to fail. So in my opinon for second case, we do not need lowering CI to 5000 h, because tendency to fail is low and better than full service.
Well, I know that MF is practical criteria given by manufacturer. If calculation of reliabilty indicate high probability of failure, then MF is more relevant to be higher. But it shows that between reliability calculation and maintenance factor are opposite.
Any idea?
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There are not directly related. Load factor and actual service are more likely links between the other two factors.
In the case of some air craft derivative turbines, they can have high reliability numbers but also have high maintenance requirements but if lightly loaded they may have high reliability and lot maintenance requirements.
Constant on/off operation can also be a big factor in both factors.
