Standard model for maintenance interval optimization

In many situations the expected cost per time unit may be written on the following form:

C(τ) = C_PM / τ + λ_E(τ) [C_CM + C_EP + C_ES + C_EM]

where

• C_PM is the cost per preventive maintenance activity
• C_CM is the cost of a corrective maintenance activity
• C_EP is the expected ecconomic value of production loss upon a failure
  • C_EP = Pr(P) ⋅ (C_P·MDT +C_T)
  • Pr(P) is the probability that a component failure gives a system failure with production loss
  • C_P value of proudction loss per time unit (or per hour) when the system is down
  • MDT is the mean down time after a failure (typically in hours)
    • MDT = MLD + MRT, where MLD = Mean Logistic Delay, and MRT = Mean (active) Repair Time
  • C_T a fixed cost upon a trip, i.e., when the system goes down independent of the duration of the downtime
• C_ES is the excpected ecconomic value related to safety loss upon a failure, and is often expressed as:
  • C_ES = Pr(S) ⋅ C_S
  • Pr(S) is the probability that a component failure gives a system failure with safety impact
  • C_S is the corresponding cost given that the "safety event" occurs
• C_EM is the expected ecconomical value of material losses upon the component failure