Question 1

What is SIL verification and when is it needed?

Accepted Answer

SIL verification confirms that a safety instrumented function achieves the required Safety Integrity Level by calculating its probability of failure on demand (PFDavg). It is required by IEC 61511 / AS 61511 for all safety instrumented systems in the process industry.

Question 2

What is PFDavg and how does it relate to SIL?

Accepted Answer

PFDavg is the average probability of failure on demand over a proof test interval. SIL 1 requires PFDavg between 0.01 and 0.1, SIL 2 between 0.001 and 0.01, and SIL 3 between 0.0001 and 0.001. Lower PFDavg means higher integrity.

Question 3

What voting architectures does this calculator support?

Accepted Answer

This calculator supports 1oo1, 1oo2, 2oo2, and 2oo3 architectures with common cause failure (beta) factors. It calculates PFDavg for the sensor, logic solver, and final element subsystems, then sums them for the overall SIF.

Question 4

What is PFDavg in safety instrumented systems?

Accepted Answer

PFDavg (average Probability of Failure on Demand) is the average likelihood that a safety instrumented function will fail to respond when a dangerous condition occurs, calculated over the proof test interval. For SIL 1 the PFDavg must be between 0.01 and 0.1 (1 in 10 to 1 in 100 demands), for SIL 2 between 0.001 and 0.01, and for SIL 3 between 0.0001 and 0.001. AS 61511 requires PFDavg to be calculated for each subsystem and summed for the overall SIF. This calculator computes PFDavg from your failure rate, proof test interval, and architecture inputs.

Question 5

How does voting architecture affect the achievable SIL level?

Accepted Answer

Redundant voting architectures dramatically lower PFDavg and increase the achievable SIL. A 1oo1 (single channel) architecture can typically only achieve SIL 1. A 1oo2 (one-out-of-two) architecture, where either channel can trip the system, can achieve SIL 2 or SIL 3 depending on failure rates. A 2oo3 (two-out-of-three) architecture provides high availability with SIL 3 capability. AS 61511 Table 6 defines architectural constraints that limit the maximum SIL based on hardware fault tolerance. Use this calculator to compare how different architectures affect your PFDavg.

Question 6

What proof test interval is required for SIL 2?

Accepted Answer

There is no single fixed proof test interval for SIL 2 — it depends on the PFDavg calculation for your specific equipment failure rates and architecture. Typical proof test intervals for SIL 2 range from 1 to 5 years. Doubling the proof test interval roughly doubles the PFDavg, so a sensor with a dangerous failure rate of 2.0E-6 per hour in a 1oo2 architecture might achieve SIL 2 with a 2-year test interval but fail with a 5-year interval. AS 61511 requires the proof test interval to be documented in the safety requirements specification. Use this calculator to find the maximum proof test interval that keeps your SIF within the SIL 2 PFDavg target of less than 0.01.

SIL Verification Calculator

Target SIL

SIF Elements

Sensor

Logic Solver

Final Element

Test & Common Cause

About This SIL Verification Calculator