Institute of Hazard Prevention
HAZOP and Safety Integrity Level (SIL studies encourage adding safeguards such as alarms and trip with little consideration for cost or operability of the plant once it is running. This results in processes that cannot be operated without bypassing a large number of the interlocks, and important alarms are lost in the hundreds or thousands of signals assaulting the operator. Many of these bypasses stay on for longer than needed, or perhaps are never turned off. This results in a situation where, despite the best efforts of the Hazard study team, the plant is no safer than if those alarms or trips were nonexistent.
The Bowtie is a method to effectively use the SIL method while ensuring that a plant is still operable at the completion of the study. The Bowtie visualizes, on one diagram, all equipment failures which lead to similar hazardous events. Safeguards are added to reduce the overall likelihood of the hazardous events occurring. Instead of adding more alarms and trips to protect against each failure, the Bowtie allows the SIL participants to easily see which existing safeguards could be used to reduce hazardous event likelihoods. As well, the Bowtie makes it simpler for participants to visualize how increasing the reliability of one safeguard may be able to reduce the total number of trips and alarms needed for safe operation. Through visualization of the SIL method, the Bowtie emphasizes fewer, more effective safeguards to get the same SIL at a lower cost, while lowering downtime for maintenance and spurious trips.