People often confuse reliability and availability

People often confuse reliability and availability. Simply put availability is a measure of the % of time the equipment is in an operable state while reliability is a measure of how long the item performs its intended function. We can refine these definitions by considering the desired performance standards.

Availability is an Operations parameter as, presumably, if the equipment is available 85% of the time, we are producing at 85% of the equipment’s technical limit. This usually equates to the financial performance of the asset. Of course quality and machine speed need to be considered in order to have a proper representation of how close we are to this technical limit. This is called OEE. Availability can be measured as: Uptime / Total time (Up time + Downtime).

Reliability is a measure of the probability that an item will perform its intended function for a specified interval under stated conditions. There are two commonly used measures of reliability:

  • Mean Time Between Failure (MTBF), which is defined as: total time in service / number of failures
  • Failure Rate (?),  which is defined as: number of failures / total time in service.

A piece of equipment can be available but not reliable. For example the machine is down 6 minutes every hour. This translates into an availability of 90% but a reliability of less than 1 hour. That may be okay in some circumstances but what if this is a paper machine? It will take at least 30 minutes of run time to get to the point that we are producing good paper.

Generally speaking a reliable machine has high availability but an available machine may or may not be very reliable!