Frequently Asked Question
We have a number of applications providing dual logic solver outputs, via 2 separate solenoid operated valves & 2 separate pneumatic booster relays, to a single air actuated valves. How do we model this?
Last Updated 5 years ago
Modeling redundancy within a single leg is a bit tricky, What you will need to do is to take the solenoids/booster relays as a black box feeding the single actuator/valve. In your final element configuration, choose a user defined device for your final element interface. Then give this device the equivalent failure rates of the redundant solenoid/booster.
To get the equivalent failure rate, model the redundant solenoid/booster configuration in a separate SIF. For this SIF set the mission time to the proof test interval and set the demand mode to high. Model your final element with just the solenoid and boosters in the appropriate voting. The calculated PFH is the equivalent dangerous undetected failure rate of the configuration (use this as you DU failure rate for your black box). The calculated MTTFS can be used to determine the safe undetected failure rate, LambdaSU = 1/MTTFS. Note that the MTTFS is in years and LambdaSU should be in hours, so use: LambdaSU = 1/(MTTFS*8760).
To get the equivalent failure rate, model the redundant solenoid/booster configuration in a separate SIF. For this SIF set the mission time to the proof test interval and set the demand mode to high. Model your final element with just the solenoid and boosters in the appropriate voting. The calculated PFH is the equivalent dangerous undetected failure rate of the configuration (use this as you DU failure rate for your black box). The calculated MTTFS can be used to determine the safe undetected failure rate, LambdaSU = 1/MTTFS. Note that the MTTFS is in years and LambdaSU should be in hours, so use: LambdaSU = 1/(MTTFS*8760).