Study on Effectiveness of F&g System Based on Process Safety Time Effect Model

Study on Effectiveness of F&G System
based on

Process Safety
Time Effect Model

WANG Hai-qing[1],
LIU Jun-fang¹, HUANG Jing-rong²

（1.Dept.
Safety Science & Engineering, China University of Petroleum, Qingdao 266580,
China）

（2.CNPC
Southwest Gas field Company, Zigong Branch, Zigong 643000, China）

Abstract

In terms of the framework for FGS
(Fire &Gas System) proposed in ISA84.00.07, a new method is proposed to
describe the mitigation function(one of the three factors for the overall
effectiveness of FGS) quantitatively using the model of process safety time in
the study. More importantly, the overall effectiveness of FGS can be get using
the new model and the framework proposed in ISA84.00.07.

So far the criterion for the
design of FGS is partly prescription based method and partly risk based method,
however, accidents showed that it?s not much appropriate to measure FGS with
such models, so the performance based model is developed to measure the
effectiveness of FGS, such as ISA84.00.07, which declared that the overall
effectiveness of FGS depends on three parts: The coverage of detector, the availability
of FGS and the effectiveness of mitigation for the accidents. So the calculation
model for the effectiveness of FGS is proposed by the author:

E_FGS=D_C´A_FG´M_{E
             Â} (1)

As the current standard and
research mainly involves the former two factors and rarely the mitigation
function factor, and it?s difficult to model the mitigation function after the
FGS started, in most cases the value of M_E is considered as 1 which
means the third factor, the mitigation function of FGS, is perfectly valid
which seems not appropriate. So the problem of getting the value of E_FGS
now comes out to be how to get the value of M_E. Since the other two
factors can be get by currently existing methods or standards.

This paper points out the Process
Safety Time (T_S) is the time from the alarm beginning to the limit
of the next severity level (without the mitigation system starting), the
Mitigation Response Time (T_M) is the time from the mitigation system
starting to the limit of the next severity level. Apparently M_E is
concerned with T_S and T_M, the larger of the T_M,
the better of mitigation function, only when T_M goes to infinity can
the value of ME be and if the value of T_M is near to T_S,
indicating the mitigation function has little effect on mitigating the accident.
The proposed method to calculate the value of M_E is as following:

                          (2)

Note: T_d is the time
delayed by operator to start the mitigation system.

Here how to get T_S and
T_M is another important question. Process variables are needed to
solve the problem, the variables can be pressure, temperature, flame, or
density of the smog or flammable gas, etc. and each of these limits should be
defined. For a specific scenario, to define the boundary of the model, firstly
single out the variable which is most likely to be the first one getting to the
limitation, and then choosing a right software or model to get T_S
and T_M through calculation.

To verify the usability of the
new method, a case study about a LNG terminal in China is developed in the
paper. Fire in the LNG receiving terminal is simulated by FDS, by the comparison
of T_M and T_S, the result shows the proposed method performing
perfectly for the chosen terminal.

Finally, the new model which can calculate
the overall effectiveness covering the mitigation function is built in the
study, the whole process is shown briefly in Fig1.

Fig1. The process of calculating Effectiveness
of F&G System based on

Process Safety Time Effect Model

Keywords: Fire and Gas system; Mitigation
Effectiveness; Process Safety Time; Safety Instrumented System