Definition:
Maximum Permitted Repair Time (MPRT) is the maximum permitted time to repair a fault after detection. The IEC 61511 definition is not specific if this includes time to put back in service, but we should assume it does. Again, here are the four time variables
- a – time to detect the failure
- b – time spent before starting the repair
- c – effective time to repair
- d – time after repair to put back in service.
MTTR = a + b + c + d –> from the failure itself to the restoration
MRT = b + c + d –> meaning MRT does not include time to detect.
MTTR = a + MRT
MPRT < MRT is commonly the case, though IEC 61511-1 3.2.37.3 allows either direction depending on whether the goal is to tighten or relax the risk margin.
Note that MPRT is permitted in the sense that it is a requirement per site procedures. Meaning it is a target for the facility, but the math in the PFDavg calculation is more conservative and uses the higher MRT.
Key Points:
- Critical input for system availability analysis.
- Affects operational downtime.
- Can be easily confused with the other terms.
See Also: MTTR, MRT, MTTR Blog Post, bypass
Example:
A plant has a SIF with 2oo3 instruments with diagnostics. They have inventory in stock, technicians are always on call, and the repair is straightforward. The MRT is thus set at 12 hours or a 1.5 shifts. The MPRT is 8 hours as a procedural goal.
Cited Source:
- IEC 61511-1:2016, Clause 3.2.37.3
- Limble article – Guide to Understanding Failure Metrics