VELOSI Services for Managing Safety Critical
Operation & Monitoring
SILS versus reliability While the main focus of the SIL ratings is the
interpretation of a process’ inherent safety, an important by-product of the
statistics used in calculating SIL ratings is the statement of a product’s
To determine if a product can be used in a given SIL environment, it must be
shown to ‘BE AVAILABLE’ to perform its designated task at some predetermined
rate. In other words, how likely is it that the device in question will be up
and functioning when needed to perform its assigned task?
Reliability numbers: What do they mean?
It seems that every organization has its own special way of characterizing
reliability. However, there are a few standards in the world of reliability
datum, as indicated below:
Probability of Failure on Demand (PFD) The Probability to Fail on Demand is a
statistical measurement of how likely it is that a process, system, or device
will be operating and ready to serve the function for which it is intended.
Failure Fraction (SFF) The maximum rated SIL environment in which the device(s)
can be used.
statistical representation of the likelihood of a component, device, or system
to fail. The value is expressed as a period of time (i.e. 14.7 years).
Mean Time To Repair is the average time to repair a system, or component, which
has failed. This value is highly dependent upon the circumstances of the
Software tool used for SIL study
VELOSI can provide customized RCM training technology transfer for both
management and nominated RCM team. Training sessions include:
VAIL-SIL is a VELOSI in-house developed Safety Integrity Level calculation tool
with a unique and versatile identity. This software product was developed by
using the Standard ANSI/ISA S84.01, IEC - 615011, IEC – 61508 as reference
documents. Various methodologies are available for assessment of target SILs.
VELOSI has implemented three different methodologies:
of Protection Analysis (LOPA)
Graph VAIL-SIL is not limited to calculating or finding out target SILs only -
it also includes:
Dynamic SIL reporting
Comparison summary of SILs with respect to all three above mentioned
Testing interval calculation(s)
Time Between Failure (MTBF)
Time To Repair (MTTR)
Study Session Team’s Record Management