Safety Instrumented Systems (SIS) are designed to prevent or mitigate hazardous events to ensure human safety, prevent damage to facilities, and protect the environment. It is important to understand the associated cost with Safety Instrumented Systems and the requirements for maintaining the appropriate functional safety level. Level instruments, both switches and transmitters (Figure 1), are certified to a Safety Integrity Level (SIL) based on their reliability, testing and certifications. Safety Integrity Levels range from SIL1 (low risk) to SIL4 (very high risk). For level instruments, one major safety function is the ability to detect an overfill condition in a tank or vessel. If a level instrument fails, a tank may overflow, allowing hazardous fluids into the environment.
ENDRESS HAUSER (2013) Proof Testing Level Instruments