Service
Functional Safety & Safety Instrumented Systems
Safety lifecycle engineering for instrumented safeguards under IEC 61511 / ISA 61511 — from SIL targets to proof testing.
What functional safety is
Functional safety is the part of overall safety that depends on a system operating correctly in response to its inputs. In the process industries, that means safety instrumented systems (SIS): sensors, logic solvers and final elements that detect a dangerous condition and automatically bring the process to a safe state.
The governing standard is IEC 61511, adopted in the United States as ANSI/ISA-61511. It applies a lifecycle approach: the required performance of each safety instrumented function (SIF) is derived from a hazard and risk assessment, specified in writing, designed and verified to meet a target safety integrity level (SIL), validated before startup and then maintained through proof testing and management of change for the life of the facility. IEC 61508 sets requirements for manufacturers of the devices used in these systems.
An SIS is often the last automatic layer before a relief device or a release. A function that is specified incorrectly, never proof-tested, bypassed without control, or built from devices with unknown failure rates may provide far less protection than its SIL label suggests.
The IEC 61511 safety lifecycle
- Hazard & risk assessmentIdentify scenarios needing instrumented protection.
- Allocation & SILAllocate risk reduction to layers; set SIL targets.
- Safety requirements specificationDefine each SIF's function and integrity requirements.
- Design & verificationArchitecture, devices, PFD calculations, systematic capability.
- Install, commission & validateProve the SIS meets the SRS before hazards are present.
- Operate & maintainProof testing, bypass control, demand and failure tracking.
- Modify or decommissionManage every change through the lifecycle.
Running alongside every phase are management of functional safety, verification activities and functional safety assessments (FSA) at defined stages — including an assessment before hazards are introduced.
Our functional safety services
| Service | What it involves |
|---|---|
| SIL determination | LOPA or other recognized methods to establish required risk reduction and SIL targets for each SIF. |
| Safety requirements specification | SRS documents covering trip points, process safety time, response time, safe state, bypass and reset requirements, spurious-trip considerations and test intervals. |
| SIL verification | Calculation of average PFD and spurious trip rate, and checks of hardware fault tolerance and systematic capability against the SRS. |
| Design reviews | Independent reviews of SIS architecture, cause-and-effect matrices, logic and field device selection. |
| Validation support | Site acceptance and validation test procedures demonstrating each SIF performs as specified. |
| Proof test procedures | Written, step-by-step procedures with defined coverage, plus test-interval optimization. |
| Functional safety management | Lifecycle procedures, competency requirements, bypass and override control, and performance monitoring. |
Where an independent functional safety assessment is required, we will confirm assessor independence and competency requirements with you before the work begins.
Alarm management and fire & gas systems
Alarms are only a valid safeguard if operators can detect, diagnose and respond to them in time. Following ANSI/ISA-18.2 (IEC 62682), we support alarm philosophy development, rationalization of safety-critical alarms and performance benchmarking against floods, chattering and standing alarms.
Fire and gas systems detect releases and fires and initiate alarms, isolation or suppression. Their effectiveness depends heavily on detector selection and placement. We support performance-based design concepts described in ISA technical guidance, including detector coverage assessment and setting fire and gas performance targets.
Existing and legacy systems
Many plants operate instrumented safeguards designed before current standards. IEC 61511 allows existing systems designed to earlier practice to continue in service if the owner determines they are designed, maintained, inspected, tested and operated in a safe manner. We help document that determination — and identify where upgrades are warranted — with a prioritized, practical plan.
Typical deliverables
- SIF list and SIL target register
- Safety requirements specifications
- SIL verification calculations and reports
- Proof test procedures and test interval basis
- Functional safety management plan
- Alarm philosophy and rationalization records
- Gap assessment for legacy instrumented safeguards
Frequently asked questions
Is IEC 61511 / ISA 61511 legally required?
The standard is a consensus standard, not a regulation. However, OSHA and EPA expect equipment covered by PSM and RMP to follow recognized and generally accepted good engineering practices, and ANSI/ISA-61511 is widely regarded as RAGAGEP for safety instrumented systems in the process industries.
What is the difference between SIL determination and SIL verification?
SIL determination sets how much risk reduction a function must provide. SIL verification confirms that the designed function — its architecture, devices and test intervals — actually achieves it.
Can a SIS share hardware with the control system?
Generally, a SIF must be independent of the initiating cause and of other protection layers credited for the same scenario. Sharing sensors, logic or final elements with the control system can invalidate the credit. The details should be evaluated case by case.
How often should a SIF be proof tested?
The proof test interval is a design parameter: it is chosen during SIL verification and recorded in the SRS. Longer intervals raise the average probability of failure on demand, so changing an interval in the field is a change that must be evaluated against the SIL target.
Put your SIS on a sound lifecycle footing
From SIL targets to proof test procedures, we can support one phase or the full lifecycle.