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Dorot

High-expansion foam plant UNI EN 13565 Case Study by FSE PROGETTI

Italy, Italy

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About the project

The aim of the project is to develop a high-expansion foam suppression system to be used in an intensive storage site for flammable liquids.
The building consists of reinforced concrete pre-cast elements and was constructed in the early 60s. It is isolated and protected by external and internal fire hydrants. It is also equipped with an automatic smoke and heat detection system.
As per the high-expansion foam suppression system, the areas to be protected have been divided into four zones where liquids are stocked according to their level of flammability and toxicity.
The high-expansion foam suppression system has been designed in compliance with the European regulation UNI EN 3565-2 2009, Fixed firefighting systems - Foam systems - Part 2: Design, construction and maintenance.

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Use of Dorot Solutions

WATER HAMMER AND STRESSES


The term “water hammer” indicates the hydraulic phenomenon which occurs in a pipeline when a moving fluid, which is considered as incompressible, is forced to stop suddenly due to the quick closure of a valve. This determines an overpressure caused by fluid inertia, which presses against the valve. The main parameters affecting this phenomenon are the fluid speed and the geometric features of the pipes, such as their diameter and length. However, the essential parameter to take into account is the closure time of the valve.
The considered plant, which is a high expansion foam deluge system, needs to fill very big spaces in few minutes. In order to do that, it is necessary to introduce quickly large amounts of water, duly mixed with the high-pressure foaming agent (pressure needed for the optimum operation of the foam producing devices and to reach the correct coverage), thus generating considerable stresses on the pipes, which can break if not conveniently sized. If they are not correctly fastened, they generate movements which can determine the breaking of the pipes.
In the considered plant, the time needed for the valves to close completely, according to the manufacturer's estimations, amounts to 15 seconds. During this time, the pumping group has to close the delivery line in order to avoid introducing water in a closed system.

In order to solve this kind of issues, a section with two valves has been installed on the manifold of the foam mixing station: the first one is an automated safety valve, controlled by a diaphragm, with a resilient disk membrane activated by a diaphragm. When the pressure on the valve exceeds a pre-set value, the valve opens, modulating the flow rate automatically in order to generate a pressure drop, which is necessary to keep the required inlet pressure constant and, therefore, to reduce the pressure level in the plant. If the pressure level on site reaches a lower value than the pre-set one, the valve closes in a sealed state.
A second combined pneumatic valve (usually defined “release valve”) is arranged in parallel. It allows the air to enter the plant when internal pressure is lower than the atmospheric one, and it allows the air to be released from the pressurized pipe filled with water, thus reducing overpressure or vacuum conditions which may damage the pipelines.

 

A very important factor to be taken into account when considering this plant is the dynamic stress created even during the activation phase when, at valve opening, the pressurized flow enters the empty pipes at high speed. Powerful stresses are created when the flow direction changes, similar to seismic stresses.



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