Definition:
Low Demand Mode describes a system that is expected to have fewer than one demand per year on average.
Low demand mode is one of three types of demand mode: low, high, and continuous. Confusingly, two of these are grouped together, low and high, to make up demand mode. These modes trigger various requirements throughout IEC 61511-1.
Key Points:
- Allows use of PFDavg rather than PFH for SIL calculations.
- Common for emergency shutdown systems.
- Most SIFs in the oil, gas, and chemical industries are designed for low demand mode.
Example:
An emergency flare system with a demand once every 5 years operates in Low Demand Mode. Thus, using PFDavg is appropriate, and not PFH.
See Also: mode of operation, demand mode, high demand mode, continuous mode
Cited Source:
- IEC 61511-1:2016, Clause 3.2.46
- Control Engineering – Measuring Safety Instrumented Systems