Water tanks

Unigb water tanks are pressurised tanks employed to secure the correct water supply by the system

Functioning principle

Water lifting system

The drinkable water distribution service through the public network can sometimes  (widespread demand, inconvinient locations…) not  guarantee the needed pressure to bring adequate flow to the highest or farther away withdrawing points. In such situations, or when the public network is not available, it is necessary to use a water lifting system to increase the supply pressure to the adequate value.

The lifting system is basically consisting of a pump and its control system (pressure switch, flow switch and the like). Since the users' request is discontinuous, it is neccessary to protect the pump from continuous ON-OFF cycles providing a tank which accumulates higher pressure water (Water tanks). The stored pressure provides an adequate flow to every withdrawing points of the system. The users will draw directly from the accumulator tank and the pump will turn on only when the water pressure in the tank drops below the minimum required, thus restoring the storage: the users' requests will always be met and the pump will contain the ON-OFF cycles. Tanks, like in heating sysems, can be either open or closed. The closed tank is installed at groud level and is pressurized with compressed air up to the pressure level required by the system. The compression of air in the tank can be achieved:

1) directly using a compressor or through the precharge of the membrane equipped tank;

2) indirectly using the pumped water itself in the container (mambrane-less tank)

INDIRECT COMPRESSION 

We can highlight three distinct phases in the evolution of operation:

a) p_{atm}, V_a

a) EMPTY WATER TANK: total volume  V_a is filled with air at atmospheric pressure p_{atm}, a small amount of water is present in the lower part

b) p_{2}, V_2

b) WATER TANK AT MINIMUM WORKING PRESSURE: the volume is filled with both air and water in an equilibrium state in which the pressure is the same as the minimum pressure required by the system p_2. The volume filled with air is V_2

c) p_{1}, V_1

c) WATER TANK AT MAXIMUM WORKING PRESSURE: the volume is filled with both air and water in an equilibrium state in which the pressure is the same as the maximum pressure allowed by the system p_1. The volume filled with air is V_1 The water volume is the maximum allowed

In most cases p_1 = p_2 + 1 ÷ 2 bar

Triggered by the pressure switch the pump boosts the pressure p_2 and it switches off once reached the pressure p_1. Pressure p_2 value is equal to the sum of the losses of the circuit , of the additional pressure needed for the taps and utilities at the most remote points of the system (usually the highest point of installation). The value of p_1 (in accordance to the project) defines the characteristics of resistance of the reservoir. The difference between the pressure p_1 and the pressure p_2 is proportional to the volume of the water tank. It can be computed as the minimum pressure p_2 of air present with a water volume that can be expressed as V_a - V_2.

DIRECT COMPRESSION

In such case, the minimum working pressure is reached without the need to pump water inside the tank. There are membraneless tanks, in which a compressor provides directly air at the minimum pressure p_2, to the system, or tanks with membrane, in which an air cushion distinct by the elastic wall of the membrane is tuned at the pre charge pressure p_2.

Using the membrane water tank involves several advantages:
• Energy saving: there is no compressor.
• Avoids corrosion of the tank thanks to the membrane which parts water and tank walls and air, prolonging the operating lifetime of the device.
• Water is protected and confined by the membrane, avoiding unwanted side effects of the mixing with air.
• Easy maintenance, changing of the membrane and inspection of the vessel.
•Membrane is moreover a dumper, so it counteracts the water hammer head effect and absorbs changes of the fluid pressures.
•Minor footprint (volume is reduced by V_a-V_2)

The tank is installed tuning the pre-charge pressure of roughly 0.2 bar less the pump-on pressure p2.
Pre-charge pressure has to be considered of the tank without water: if it were more than the pump-on pressure, system operation would be faulty.

p_2,V_2

p_1,V_1

It has to be remembered that  precharge pressure needs to be tuned with respect to the control device of the pump (usually the pressure switch), at a pressure slightly less than the minimum pressure (we suggest 0.2 bar less). This is requiered to guarantee the correct start of the pump when needed by the system.

In line water tanks: 

They provide supportfree direct install on the piping line.

Colour: BLUE (ral 5015)

Precharge: 2.0 bar

Vertical water tanks:

they allow minimum footprint and assure the correct operation employing the most reliable membrane configuration.

Colour: BLUE (ral 5015)

Precharge: 2.0 bar

Horizontal water tanks:

stocked with anchoring plate, they allow the direct installation of the pump on top of the tank.

Colour: BLUE (ral 5015)

Precharge: 2.0 bar