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What Is a High-High Level Trip? | Process Engineering Glossary
What Is a High-High Level Trip?
A high-high level trip is an independent, safety-critical level switch or alarm set above the normal high-level alarm on a storage tank or vessel, configured to automatically trigger a protective shutdown action, such as stopping a fill pump or closing an inlet valve, to prevent overfill. High-high level trips are typically implemented as part of a Safety Instrumented System, designed and tested to a defined reliability standard appropriate to the consequences of failure.
The “high-high” naming reflects its position in a typical layered alarm strategy: a normal high-level alarm alerts operators to take corrective action, while the high-high level trip acts as a final, automatic safeguard if the operator response doesn’t prevent the level from continuing to rise.
Applications of High-High Level Trip
Overfill Protection Systems
High-high level trips are the core automated safety function implementing overfill protection on most storage tanks, providing the final automatic barrier against tank overflow.
Safety Instrumented System Design
High-high level trips are commonly designed and verified as part of a formal Safety Instrumented System, with a defined safety integrity level appropriate to the risk being addressed.
Tank Filling Operations
Tank filling operations, whether from pipeline, marine, or truck delivery, rely on high-high level trips to provide automated protection during a period when fill rates can be high and continuous operator attention isn’t guaranteed.
Benefits of Knowing High-High Level Trip
Provides automated protection independent of operator response. A properly designed high-high level trip protects against overfill even if an operator misses or doesn’t respond to an earlier alarm.
Forms a defined layer within an overall safety strategy. Understanding where the high-high trip sits within a layered alarm and protection strategy clarifies its specific role relative to normal alarms and other protection layers.
Supports formal safety integrity verification. Designing a high-high level trip as part of a Safety Instrumented System allows its reliability to be formally verified against a defined target, giving confidence in its expected performance.
Limitations to Consider
Requires genuine independence to function as intended. A high-high level trip that shares sensors or logic with the normal level control system doesn’t provide the independent protection layer the design intends.
Periodic testing is essential to maintain reliability. Like any safety instrumented function, a high-high level trip’s real-world reliability depends on regular proof testing to catch any undetected failures.
Final layer, not the only layer. A high-high level trip is typically the last automated layer of protection, and relying on it as the sole overfill prevention measure, without effective normal alarms and operating procedures upstream, isn’t a robust overall design.
High-High Level Trip FAQ
How does a high-high level trip relate to overfill protection and API 2350?
A high-high level trip is the specific automated function that implements overfill protection, and standards like API 2350 define independence and reliability requirements this trip needs to meet based on tank category, often within a broader consequence analysis.
How does a high-high level trip differ from a low-low level trip?
A high-high level trip protects against overfill at the top of a tank’s safe range, while a low-low level trip protects against the opposite condition, running a tank dry, at the bottom of its range.
How does automatic tank gauging support a high-high level trip function?
Automatic tank gauging instrumentation can feed a high-high level trip, provided it meets the independence requirements needed for the safety function, integrated into the facility’s broader closed-loop control and storage tank management strategy alongside a process hazard analysis confirming its adequacy.
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