DESCRIPTION AND APPLICATION

The low, medium and high foam suppression systems are deluge systems using the foam mix prepared by special devices to create a compact layer covering the surface of the areas on fire.

The systems have a deluge control valve, an automatic or manual release valve based on fire detectors and a device to generate the right mixture of water and foam.

Their main applications are on the industrial fields using inflammable materials such as tank farms of oil products. Foam can be used to cover in a few minutes the surrounding basins and suppress or render inert the expanding liquids.

The foam mix can be prepared in three ways:

through a suction system with injectors;
through direct injection with pumps;
through liquid shift from bladder tanks.


Suction mixture

It is obtained through injectors called on- line mixers using pressurised water in order to suck a calculated quantity of foam to be mixed with water. The quantity of foam is determined by the diameter of the orifice connected through a branch pipe to a dip tube. A stop cock prevents water going into the foam in case of counter pressure in the bottom line.

These devices operate thanks to the energy generated by pressurised water and must be used at the flow rate at which the branch pipe is set. The nominal flow rate varies from 200 to 2000 litres per minute of water-foam mixture at a feed pressure between 7 and 8 bars.


Liquid shift mixture

They are a variation of the suction systems. These devices are made by a pressurised tank fitted with a bladder of rubber tissue containing foam and a pre-mixer placed outside. The bladder prevents the foam to get in contact with water and with the tank wall thus avoiding the dilution and possible contamination of the foam and the corrosion of the tank. The latter can be of horizontal or vertical type.

The pre-mixer is a pipe of the Venturi type fitted into the water pipe supplying the system. Above the venturi branch-pipe a pipe connects the stub pipe to the water tank and below, before the ejecting cone, another pipe connects through a dip tube the surface of the foam tank. After the Venturi pipe a manual or automatic valve is placed operating when the system is at work. In the same way the flow of the water into the pre-mixer is always open or it is controlled by an automatic valve.

When the system is not in operation the two liquids (water and foam) are at the same pressure whereas when at work the foam liquid is at a lower one. Water therefore pours into the tank, presses the bladder so that the foam comes out from the top and is sucked into the Venturi pipe where mixture with water is made.

The percentage of foam is regulated by a baffle plate with a calibrated hole fitted before the mixer.


Injection mixture

The mixture is made through pumps channelling the foam mixture at a pressure higher than water making it possible its inlet in the water flow feeding the system.

The proportions through injection can be made in various ways. Those most used are:

Proportions through balanced pressure and fixed orifices;
Proportions with feeding variable upon request.

The proportions through balanced pressure and fixed orifices is made with a device keeping the input line of the foam at a pressure higher than the water network. This is normally obtained by recycling the foam mix in excess into the tank. Every collector serves a generator or a group of generators, has a separate injection line where a calibrated orifice is fitted. The input of foam into the water is therefore kept constant even when the pressure of the system varies.

The variable proportion system is made of a Venturi meter and of a valve regulating the pressure of the foam.
The valve is regulated by the water pressure above the Venturi meter and by the foam line. The two pressures operate on the two opposite surfaces of the diaphragm at which the valve stem if fitted. The displacement of the bladder allows a variable foam flow according to the depression created by the Venturi pipe. The foam enters into the water near the branch pipe at a pressure equal to the one of the incoming water.

A variable of the injection mixers are the Hydraulic Proportioners. They are made of a co-axial hydraulic engine and by a volumetric pump.
The hydraulic engine is operated by the water of the system, whereas the volumetric pump takes the foam from a pressurised tank placed near the mixing device and pushes it into the water immediately after the hydraulic engine. The great advantage is that these systems do not require external energy which is supplied by the water feeding the plant.

The foam generators are:

Ejectors using the pressurised foam mixture in order to suck air and blend it with the mixture forming foam
bubbles. In this case foams with expansion ratios from low to medium/high are obtained;
devices which, blowing air into the foam mixture, cause a turbulence forming bubbles of the desired form and
dimension obtained by letting them pass through the meshes of a net. In this case high expansion ratios are
obtained.

Depending on the chemical and physical specifications of the foam mixture used, the foam generators may need an "expansion chamber" allowing a smooth formation of the foam.


Foam generators at low expansion

They are generally used in installations with tank farms of petroleum products at atmospheric pressure.

They are called foam branch-pipes and are, in general, stub pipes with flanges fitted into the pipes of the foam system. A nozzle is placed at the entrance from which the stream of foam is ejected of adequate shape in order to produce the Venturi effect and a good turbulence. On the surface of the stub pipe there are openings to favour the air inlet.


Fixed roof tanks

For their protections three solutions are possible:

Systems with application from above;
Systems with application from the bottom through a floating hose inside the tank;
Systems with application from the bottom using product pipes or other pipes built for the purpose.

The system with application from above is the most used. On the outside face of the wall, immediately under the roof, a foam chamber is located consisting of a vertical cylinder container from the bottom of which the inlet foam pipe is fitted. This container has a side exit connected with a flange to a pipe laying across the tank and that at the other end is connected to a pourer conveying the foam on the side of the tank so that it descends and comes into contact with the burning liquid.

Inside the chamber the pipe ends with a couple of flanges having a glass disc inside possibly engraved in order to facilitate its rupture thanks to the foam coming from the branch-pipe.
At the top of the foam room a watertight lid is placed for periodical inspection and possible replacement of the glass. The function of the glass is to ensure the sealing between the inside of the tank and the foam pipe.

A branch-pipe can supply more than one pourer.
The NFPA standard recommends the sprinkling of water into the external wall of the tank in order to avoid foam to dry after contact with the overheated surface and before it reaches the liquid.

The system with application from the bottom with a floating hose is obtained by placing the hose in a vertical container inside the tank or in an oblique container outside it. The hose is then expelled by the pressurised foam.

The system with application from the bottom through fixed pipes requires special branch-pipes and special foams not to be contaminated by the oil products.

The injection from the bottom can reduce the itinerary of the foam and is a valid alternative to the insertion from above for tanks with a diameter lower than 30 metres. The number of foam inlet points at the bottom depends upon the diameter of the tank and upon the flash point of the product.

Tanks with a floating roof

They have all along the perimeter of the floating roof a system of tight seal in order to avoid evaporation of the liquids stored inside. Fire resistant gaskets are also included. On top of the roof, at a certain distance from the seals, there is a round steel bulkhead apt to receive the foam when fire in the gaskets spreads out .When the roof of the tank is floating, there is no more space between it and the liquid, and therefore fire can only start along the circumference of the roof.

In this case the foam flow coming out from the dispensers collides with a plate shield and from there descends along the internal side of the tank reaching the ring roof and flows down until it reaches the foam produced by the two adjacent dispensers.


Foam generators at medium and high expansion

They can be of the suction type or of the type using blown forced air. The latter are also called fan generators.

Generators at medium expansion are all of the suction type, whereas those at high expansion can either be of the suctioning or the air blown type. For high expansion generators fans can be run by the same system operating the generator or by electric engines.

In high expansion generators of the suction type the air is introduced by the action of a battery of nozzles.

The systems can be of two kinds:

High expansion foam systems with total saturation;
High expansion foam systems with local application.

For total saturation air blown generators are normally used, whereas for local application those of the suction type are preferred.

High expansion foam systems with total saturation

These total flooding systems are used to fill underground rooms, basements, holds, ample sheds used to store goods. For the protection of large sheds, in order to reduce the volumes to be filled, some precautions have to be taken such as the controlled descent of fireproof material (wool glass and similar) fitted at regular intervals on the ceiling. The presence of fixed or provisional walls allows the high expansion foam to accumulate and then roll down toward the burning area. Adequate fire vents must be foreseen in order to avoid that combustion residues are recycled in the air feeding the generators.
Planning of these systems is quite laborious.

The functioning of these systems is fundamental if a satisfactory control and fire suppression has to be achieved. Therefore an automatic fire detection system and an alarm system are essential to start foam delivery.
Optical and acoustic warns must also be installed to inform personnel around.

In the planning of high expansion foam systems with total flooding, these factors have to be taken into account:

Minimum height of foam carpet;
Volume required to cover material to be protected;
Time required to cover the material;
Foam capacity required.

For high expansion foams, the minimum height must not be lower than 110% of the height of the stuff at risk and by no means must be less than 60 cm. above that height.
The volume of submersion for high expansion foams is calculated multiplying the area of the floor to be protected by the minimum height of foam required. From the result obtained, volume of objects normally placed in the premises can be deducted.

The immersion time for high expansion foams depends on the kind of stuff to be protected. Such time can vary between 2 and 8 minutes. Lower time can be necessary when materials to be protected have a flash point lower than 38 °.

The foam capacity can be determined through a formula indicated in the NFPA 11 standard. The foam quantity available must be sufficient for at least 25 minutes of operation or be four times the submersion time.

The submersion volume must be kept in place for at least 60 minutes.

High expansion foam systems with local application

For the operation of these systems, generators of medium-high expansion are generally used. The systems are used both internally and externally and are mainly used to protect loading racks for road and rail trucks of inflammable oil products, pumping stations in oil refineries , basins of oil tanks, decantation tanks etc.

The automatic detection systems generally use cartridge fuses or bulbs. The required capacities for solid and liquid fuels must be able to guarantee within 2 minutes the complete coverage of the protected surface with a foam carpet of at least 60 cm. The whole system must operate for at least 12 minutes.


Fixed monitors

Foam monitors and foam/water monitors have well determined applications. They are used in oil wharfs, on decks of oil tankers, elevated heliports and industrial installations.
The placing of monitors must take into consideration their reduced range in case of adverse winds.

They can be fed directly through a foam branch-pipe operated in the ways illustrated above or be singularly equipped with a mixer supplied with pressurised water and foam.

They can be hand-wheel or lever operated or remotely through hydraulic, pneumatic or electric controls. Foam branch-pipes can be fitted with deflectors (remotely controlled) in order to change the flow rate (full or fixed).

They have capacities variable between 20.000 and 40.000 litres per minute.