Safety Instrumented Systems (SIS) are often regarded as having a very high initial and maintenance costs. However, as late Mr. Trevor Kletz stated “If you think safety is expensive, try an accident". Accidents cost a lot of money, which not only causes damage to plant and personal injury, but also leads to loss of company’s reputation & potential longterm revenue loss. Many research studies and analysis have shown that the benefit-to-cost ratio calculations for implementation of SIS are in its favor i.e. a properly designed, installed, tested, and maintained SIS reduces the likelihood of process risk & prevents the hazardous event from taking place and thereby achieving higher functional safety of the process plant.
Due to many disasters’/accidents which occurred in past like Seveso, Piper Alpha platforms, Chernobyl, Bhopal gas tragedy many standards were framed/designed to improve the inherent safety of the plant. International standards on functional safety for Process Industry are IEC61508 & IEC61511.
The role of EPC, End user, Licensor, SIS integrator for each phase of the IEC61511 safety lifecycle is indicated in Table 1.
The following case study is based on a project whose name cannot be disclosed due to confidentiality. The facility in question, a petrochemical plant, had the SIF designed for powder from degasser.
In case of very low Temperature in Degasser, safety logic shall close the outlet knife gate valve O-port. If the safety logic fails to close the Knife gate valve, then there is a chance that the powder can pass through the valve into the downstream equipment and if it comes in contact with the open atmosphere then there is a high possibility of leading it into a fire & explosion.
Considering the criticality of the SIF, SIF was classified as SIL 2 by licensor in FEED SIL report.
Licensor recommended knife gate valve O-port make and model was not certified for SIL 2 application.
Options proposed by EPC contractor to Licensor & End User
- Option A Licensor to accept recommended make and model of knife gate valve without SIL certificates based on their past Project applications & reliability data.
- Option B Inclusion of new shut off valve which is certified for this application.
- Option C Alternate make and model of Knife gate valve was proposed by EPC to licensor for approval.
Licensor response to EPC options
- Option A Proven in use Concept to be used in absence of SIL certificates.
- Option B Two valves in series was not acceptable due to Powder application.
- Option C Matching performance & SIL certificates of Alternate make & model is EPC responsibility.
Problems posed by licensor’s response
- Option A As per IEC61511 edition 2, Proven in use concept is replaced by Prior Use concept. Prior Use concept requires Operational & maintenance documentary evidence that the device has been used in similar project in similar environmental conditions (i.e. similar operating temperature, pressure, humidity etc.). Licensor/Recommended vendor/End User to provide necessary documentation required for prior use concept based on past operational data of a similar project with similar environmental conditions.
- Option B Option ruled out based on process concern.
- Option C Alternate make and model of Knife gate valve Techno-commercial evaluation by EPC Contractor. EPC Contractor had to get the approval from Licensor before ordering the valves.
Solution taken by EPC contractor
- Option A was dropped due to lack of documentary evidence indicating operational data
- Option B was dropped.
- Option C was evaluated & adopted by EPC contractor.
Impact of Selected Option (i.e. Option C) for EPC Contractor
- Cost Impact: Vendor of the Alternate make provided the SIL certificates but at an additional cost and actually got the knife gate valve model certified in the process under the EPC's cost.
- Schedule Impact: Delay in Ordering of Knife gate valve due to Techno-commercial evaluation of new vendors
Conclusion of Case Study
- During SIL classification study, there shall be awareness for the use of mandated vendor devices & their limitations related to availability of SIL certificates & safety manuals.
Key points to be taken into consideration during project engineering phase as per IEC61511 edition 2
- Hazard and Operability study (HAZOP) makes the process robust by analysing Risk’s.
- SIL study specifies the required reliability for each component of SIF.
- Risks and Demand scenarios shall be properly defined during HAZOP/SIL so that all risks are properly covered & not exaggerated.
- Hardware Fault tolerance (HFT=1) to be met for SIL 3 loops.
- SIL certificates and safety manual to be collected during SIF verification for each component of the SIF.
- Functional Safety Audit (FSA) to be included & carried out as per Safety lifecycle plan document during engineering & O&M phase.