(109c) Time to Consequence Simulation in LNG Facility

Process safety has always been a key issue during the design and operation of a LNG facility. Safety Instrumented Function (SIF) with a specified Safety Integrity Level (SIL) is necessary to achieve functional safety (IEC Standard 61511).

Safety Instrumented Function is the action that a safety system takes to make the overall process safe during emergency and abnormal operations. Safety Instrumented Function may include only a portion of a Safety Instrumented System (SIS) or it may include multiple SIS’s. Safety Instrumented System is a set of hardware and software that is designed to perform set of actions to confine the process within safe limits.

Time to Consequence (TTC) or Process Safety Time (PST) is the period of time, during a process deviation event, between the time that the measured parameter reaches the value at which the Safety Instrumented System is initiated and the time at which the safety hazard is realized assuming that the Safety Instrumented System somehow fails to execute the final actions required by the Safety Instrumented Function.

According to the CCPS Guidelines for Safe and Reliable Instrumented Protective Systems Process Safety Time (PST) is “the time period between a failure occurring in the process or its control system and the occurrence of the hazardous event.”

API 556 second edition, 2011: “the interval between the initiating event leading to an unacceptable process deviation and the hazardous event”.

Per IEC 61511 US Version: ANSI ISA 84.00.01: Given the degree of uncertainty in the process safety time, the Safety Instrumented System should be capable of completing its action within one-half of its allocated process safety time. TTC ≥ 2 x IRT. IRT is the Instrument Response Time (Sensors, filters, scan time, Final Control Element).

A Fail Safe analysis was done for LNG Facility for all the interlocks to determine if the response of the Safety Instrumented System is quick and fast enough to protect the equipment or for containment. As a result of this, process engineers had to come up with Time to Consequence for different interlocks.

Certain Time to Consequence calculations require dynamic analysis of the system protected by Safety Instrumented Function. Dynamic simulation utilizing rigorous mathematical models has become an influential tool in the process design, design validation, control system verification, startup support and troubleshooting. Many recent developments in the simulation software and technology has led to the development of large scale dynamic models and their use in the entire project life.

In this paper, how an existing dynamic simulation of the integrated LNG facility was used to determine the process safety time or TTC of important interlocks, the steps involved, the methodology and how insufficient TTC has been addressed will be discussed.