The IEC standards require Functional Safety Assessments (FSA) as part of the SIS Safety Lifecycle. There are 5 stages in the SIS Safety Lifecycle in which FSA are considered, see the picture below. As a minimum an FS Assessment at stage 3 must be carried out prior to plant start-up. The IEC standards refer to this as “before the hazards are introduced”.
This figure quoted from figure 7 of IEC 61511-1 Ed.2 (2016) Lifecycle Assessment stage
The required level of independence of the assessor depends on the (highest) required SIL level of the system. The IEC standards have the following table for this.
| Minimum level of independence | Safety Integrity level / Systematic Capability | |||
|---|---|---|---|---|
| 1 | 2 | 3 | 4 | |
| Independent person | X | X1 | Y | Y |
| Independent department | - | X2 | X1 | Y |
| Independent organization | - | - | X2 | X |
|
Legend:
Y = insufficient X = minimum required X1,X2 = X2 is more appropriate, depends on organization and factors |
||||
This table quoted from table 5 of IEC 61508 Ed. 2 (2010) Minimum levels of independence
From the above table, a safety system containing:
- SIFs with a SIL 1 requirement to be assessed by an independent person.
- SIFs with a SIL 2 requirement can be assessed by an independent person, but preferably to be done by an independent department.
- SIFs with a SIL 3 requirement can be assessed by an independent department, but preferably to be done by an independent organization.
- SIFs with a SIL4 can only be assessed by an independent organization.
Customer Challenges
Ultimately the end user is responsible for all SIS safety lifecycle phases. The end user may delegate or subcontract these but will always keep this final responsibility. End users may not be aware of this. Therefore, the first customer challenge is to be aware of the functional safety responsibilities. The second customer challenge is to arrange that an independent Functional Safety Assessment is at least carried out prior to plant start-up.
Our Solution
Regardless of the make and type of the safety instrumented system in place, Yokogawa Safety Specialists can carry out the independent Functional Safety Assessments for all 5 stages of the SIS Safety Lifecycle.
The functional safety assessment verifies at least:
- That all verifications have been executed.
- That every person involved was competent.
- The SIS meets the SRS.
- All safety related open items have been solved.
- Deliverables are complete and up to date.
- And produces a record of the results.
Customer Benefits
Yokogawa is able to conduct Functional Safety Assessments in any phase of the Safety Lifecycle, see figure 3. Yokogawa Functional Safety Specialists provide an independent assessment to review the processes and preparations leading up to plant start-up. This is regardless of the brand the client has installed or selected. Gaps are identified and improvements are recommended and addressed towards a satisfactory resolution. The FS assessment report is finalized with a compliance statement if all FSM safety objectives have been met. Yokogawa has a global network of certified functional safety experts and specialists to ensure FSM compliance.
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