What Is an Automated Overfill Prevention System
Automated Overfill Prevention System (AOPS) is an independent safety protection layer used in tank farms to prevent liquid overflow. It automatically triggers alarms and executes interlock shutdown when the liquid level reaches a preset high-high level, avoiding tank overflow, fire, and environmental pollution accidents.
This system is usually independent of the Basic Process Control System (BPCS) and is part of the Safety Instrumented System (SIS). Its design must simultaneously meet the requirements of API 2350 and IEC 61511.
Core Requirements of the API 2350 Standard
API 2350 is an industry standard for overfill prevention management of petroleum storage tanks, emphasizing full lifecycle management of "overfill prevention."
Hierarchical Management Concept
- Level 1: Basic level measurement and manual inspection, suitable for low-risk scenarios.
- Level 2: Independent high-level alarm, requiring manual response.
- Level 3: Automated overfill prevention system, automatically triggering interlock without manual intervention.
Key Design Requirements
- Install an independent high-high (HH) level switch, separate from routine measurement.
- Provide alarm response time calculation to ensure operators have enough time to respond.
- Conduct regular functional testing and documentation to ensure system availability.
Functional Safety Requirements of IEC 61511
IEC 61511 addresses safety instrumented systems in the process industry and specifies the functional safety process from design to operation and maintenance.
Safety Lifecycle
- Hazard and risk assessment (HAZOP).
- Determine the Safety Integrity Level (SIL).
- Design the Safety Instrumented Function (SIF).
- Verification and validation.
- Operation, maintenance, and management of change.
SIL Classification and Verification
Overfill prevention interlocks usually require SIL 2 or SIL 3. PFDavg must be calculated through failure modes and diagnostic coverage to verify whether the target risk reduction requirements are met.
Comparison of API 2350 and IEC 61511
| Comparison Item | API 2350 | IEC 61511 |
|---|---|---|
| Scope of Application | Petroleum storage tank overfill prevention | Process industry safety instrumented systems |
| Core Objective | Prevent storage tank overfill | Functional safety and risk reduction |
| Key Indicators | Alarm response time, hierarchical management | SIL level, PFDavg |
| Testing Requirements | Regular functional testing | Periodic verification and audit |
Implementation Steps of an Automated Overfill Prevention System
- Conduct HAZOP analysis to identify overfill risks.
- Determine the SIL level and Safety Requirements Specification (SRS).
- Select independent level switches and safety logic controllers.
- Design interlock logic to ensure separation from the BPCS.
- Carry out installation, commissioning, and functional testing.
- Establish a regular testing and maintenance plan.
Summary and Selection Recommendations
A compliant automated overfill prevention system must simultaneously meet the hierarchical management requirements of API 2350 and the functional safety requirements of IEC 61511. When selecting equipment, engineering technical personnel and procurement should focus on SIL certification, independent measurement, and diagnostic coverage.
It is recommended to prioritize level switches and safety controllers with SIL 2/3 certification, and to retain complete test records in order to pass audits and acceptance.