GRP Fire Fighting System

Introduction

In a gas facility, several new units were implemented in the existing plant. The fire water system was designed to provide a secure source of firewater storage, make-up, and supply distribution capacity to meet the largest firewater demand for the complete plant area.

Analysis

The objectives of the surge analyses were as follows:

  • To define the maximum and minimum pressures and fire water flow rates during steady state and several upset scenarios;
    • Case 1 – pump startup;
    • Case 2 – hydrant closure.
  • To safeguard the fire water network from excessive pressures or vacuums surpassing the allowable values of the pipes, it is necessary to establish the appropriate measures, such as robust tensile-resistant joints (including surge drums, vacuum breakers, pressure relief devices, etc.).
  • To determine the rate of pressure change/time histories at selected locations.

Results

During the opening of the hydrants and before the start of the main firewater pump, the pressure in the firewater transfer line dropped to -0.84 barg. The firewater lines were fully vacuum-resistant under buried conditions and therefore this low-pressure level was acceptable. The maximum pressure stayed within acceptable levels after the start-up of the main fire water pump.

The analysis demonstrated that the hydrant closure scenario was critical. If the valves were closed too fast, the transient pressure peak would exceed the allowable level.

“One of the most critical cases was the hydrant closure”

Measures were required to distribute the closure of the open hydrants over a time interval and prevent the intermediate time interval between closures of vales from dropping below 0.5 seconds. Alternatively, the closure time for each hydrant could be stretched beyond the 1 second/inch of line size. If closure times were based on 11/3 seconds/inch of line size the resulting peak pressures would also stay within allowable limits.

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