Reliability & Availability
Reliability and availability are very important issues for CHP packages, which must be able to operate continuously for extended periods to achieve the best economic returns. In practice, any engine or gas turbine CHP unit requires inspection and maintenance at least once a year, and this necessitates at least one planned shutdown annually. However, it is the equipment’s susceptibility to additional, unscheduled stoppages that determines its reliability and actual availability over a given period of time.
Provision of a packaged CHP unit is often accompanied by:
- A contract for full maintenance of the equipment.
- Guarantees of plant availability, with penalties or bonuses payable according to actual achievements.
The distinction between reliability and availability is often poorly defined, and suppliers’ guarantees should be carefully scrutinised to ensure a true understanding of what is being provided. The time period used is typically a year of 8,760 hours. The formulae that follow illustrate one method of making the calculations:
- % Reliability = T - (S + U)/ T - S x 100
- % Availability = T - (S + U)/T x 100
- S = scheduled maintenance shutdown (hours/year)
- U = unscheduled shutdown (hours/year)
- T = period when plant is required to be in service or available for service (hours/year).
Typical manufacturers’ guaranteed figures for a packaged CHP unit might be as follows:
- Maximum scheduled outage (S) 438 hours per year (8,760 hours).
- Maximum unscheduled outage (U) 420 hours per year (8,760 hours).
Using the formulae given:
- Guaranteed reliability = 8,760 - (438 + 420)/ 8,760 - 438 x 100 = 94.95%
- Guaranteed availability = 8,760 - (438 + 420)/ 8,760 x 100 = 90.21%
Reliability and availability vary for different sizes and types of CHP package, but the figures given represent typical levels that can be expected. Figures should be calculated for the planned operating regime, e.g. 17 hours per day.
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