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 the two types of demand mode (low demand and high demand). Demand mode is a mode of operation (the other type being continuous). This triggers 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.