DESCRIPTION AND APPLICATION

Water deliveries for fire suppression systems can derive from public water networks or private supply tanks.

The first ones are reliable and guarantee a regular flow, but they are normally limited in pressure and flow rate.

The second ones are expressly built to suit the suppression system but can be scarcely reliable when maintenance, as it often happens, is not regularly carried out.

Water supplies are normally of three types:

Elevated water tanks;
Pressurised water tanks;
Water tanks with pump systems.

The elevated (or gravity) water tanks described in the UNI 9490 norm are becoming less and less frequent due to the high cost of construction. There is no doubt about their relialability; no thrust for the water is necessary as thanks to the piezometric column the water pressure reaches 4-5 bars and therefore there are no risks of interruption in the flow rate caused by black outs or failures of the motor pumps.
For this kind of systems the following items are required:

A levelled refill system of 1 lpm for each cubic meter of capacity;
A ventilation gate;
A level indicator visible from the ground and an acoustic signal;
A device signalling an overflow situation.

The pressurised water tanks utilize the autoclave principle. It is the air cushion that provokes the outlet of water from the tank. However, this supply system does not represent the ideal solution when a high water flow is required. In fact the relevant high pressure would necessitate the adoption of plated tanks capable of resisting an 8 bar pressure and the cost of them and their maintenance would be very high.

Therefore the pressurised water tank is valid for the protection of low hazards with small or medium systems.

The most used supply system is the one made by a reserve tank and an automatic starting pump.
The system of reserve tank and pumping station is highly reliable and can be divided into two types according to the water reserve system adopted:

Underground water reserve tanks;
Ground water reserve tanks.

The choice of an underground reserve tank is normally compulsory when limited space or architectural impact occur. The water tank is built in reinforced concrete put directly into place.
In case of water leakage due to scarce maintenance or ageing, it is possible to recover the functionality through the adoption of special PVC or PEAD gaiters at a reasonable cost.
Covering, although not compulsory, is recommended in order to minimize the high cleaning problems for weed accumulation but the choice of the "swimming pool" does not solve the problem of space which is normally the main reason for the underground choice.

Also for this type of system the following accessory items are requested:

A levelled refill system of 1 lpm for each cubic metre of capacity;
Two access manholes, on opposite sides, in order to allow good ventilation when maintenance is done inside
the tank;
A visible level indicator and an optical acoustic signal connected to a manned location;
An overflow indicator .

For ground water tanks there are many solutions.

The best and mostly used also for its reasonable price is the one adopting zinc plated tanks of cylinder shape. The main problems of ground tanks, in addition to the architectural impact, are due to the construction of a connecting network and of a room dedicated to receive the pumps.
Another important aspect is the one linked to the mechanical resistance of the ground where the reinforced concrete structure containing the tank has to be placed.

For the building of a concrete ground tank it is possible to use prefabricated parts which, in addition to their low cost, allow an easy installation. Accessory items are similar to those indicated above.

A fundamental component for water tank systems is the Pumping Station.

It must be placed on a room totally independent and fire resistant for 12 minutes fitted with external access; in case of a hydrant system the norm UNI 10779 allows that the pumping station be placed in a room where other water systems exist at the condition that no significant fire hazard exists. Alternatively, an isolated construction can be built but the characteristics of fire resistance must be preserved.

The pumping station can be built in the following ways:

Pumping station and water tank both at the same ground
level;
Pumping station and water tank both underground;
Pumping station on top to the water tank.

The first solution seems to be the best both technically and economically; it is not subject to floods due to leakage of pipes and can be easily and adequately ventilated.
Since it is at ground level, ordinary centrifugal pumps can be installed.

In the second solution a number of problems must be solved, endangering the facility and the reliability of operation. The problems include the risk of floods and leakage, the difficulty of access and exit in case of emergency and that of an adequate ventilation system.

The third solution is more rationale but vertical pumps must be adopted with immerse rotors and external engine. The UNI 9490 norm allows the installation of immerse enblock pumps and also of horizontal suction pumps not pressurized with adequate suction system.

For a good choice of pumps it is also recommended to take these requirements into account:

The respect of NSPH relevant to the pump chosen;
The rotating speed which should not exceed the 15-20% of maximum reachable speed in order to rely on a
possible increase of capacity;
The specification of the electric start and control panel and of its components;
The availability of spare parts so as to minimise the reparation time.

As for the choice of the pump, electrical or diesel, it is easy to suggest that the first one is the best solution. In fact it is more reliable and the risk of reduction of power is reduced.
The choice of a diesel pump is recommended only when required power is higher than 100 kilowatt and the distance between the electric cabin and the pumping station is more than 100 metres.

When installing an electric pump it is recommended to check the load protections which are typical of an industrial use of engines and are designed to avoid damages to the equipment in case of overload but can produce undesired interruptions in a fire system.

It is rare to see pump stations electrically operated coupled with diesel engines in case of an emergency in the electrical supply. This solution in fact adds the starting uncertainty of pumping stations with the typical ones of diesel engines. A better alternative for systems of limited power can be the use of uninterruptible power supplies which can guarantee the electric input up to a few tens of kilowatts.