Calculation of Expansion Vessel

Calculation of Expansion Vessel

In water supply systems, an expansion vessel is used to solve various problems, and different methods are used to calculate it for each case. The above algorithm for calculating the expansion vessel allows you to choose tanks to solve the two most popular tasks and determines the initial pressure of the gas space, as well as the turn-on and turn-off pressure of the pump.

The first version of the expansion vessel calculation prioritizes the pump switching frequency. In systems with booster stations, well and booster pumps, an expansion vessel tank is necessary to reduce the frequency of switching on the pump.

Water cannot be compressed like air due to its physical properties. Therefore, even a short-term opening of one water tap in an apartment building can cause the pump to turn on. Frequent activation of the pump leads to its rapid wear and failure.

The switching frequency is related to the electric power of the pump. For example, pumps with a power of more than 8 kW are recommended to be turned on no more than 10 times per hour, pumps with a power of less than 5 kW - no more than 20 times per hour, and pumps whose power falls into the range of 5 to 10 kW a little more than 15 times per hour. This dependence is the basis of the above calculation algorithm.

In addition to the power of the pump, the permissible switching frequency is affected by many other factors. For example, the greater the mass of moving parts, the lower the permissible switching frequency. Therefore, pay attention to the correspondence between the values of the switching frequency obtained as a result of the selection of the expansion vessel and the optimal frequency for the selected pump, and if necessary, repeat the calculation of the tank.

The second option for calculating the expansion vessel prioritizes the stored water volume. It is recommended for systems in which the interruption of water supply is not allowed, but there are interruptions in the supply of electricity or water from centralized water supply systems.

Selection of Expansion Vessel

The volume of the selected expansion vessel must be greater than or equal to the volume obtained as a result of the calculation. There are no negative consequences from overestimating the volume of the hydraulic accumulator beyond the calculated value, regardless of how much it is exceeded.

When choosing an expansion vessel, its temperature and strength characteristics should be taken into account. The maximum pressure of the tank must be greater than or equal to the maximum pressure at the point of its connection. If the installation of the expansion vessel is planned indoors, it should be taken into account that tanks with a diameter of more than 750mm and a height of more than 1.5m may not pass through the door, and mechanization means will be needed to move them. In this case, it is better to prefer not one but several expansion vessels of smaller capacity.

When choosing an expansion vessel, remember that the volume of water stored in it is on average 40-50% of the volume of the tank.

Pressure Setting in Expansion Vessel

Before connecting to the water supply system, it is necessary to set the pressure of the gas space in the expansion vessel (Pg) according to the value given in the calculation results. For this, it may be necessary to depressurize it through a nipple or pump up the tank with a compressor, depending on the factory-set pressure of the gas space. After creating the pressure of the gas space, connect the tank to the water supply system.

The automatic control of the pump should be adjusted so that the start-up pressure and cut-off pressure of the pump correspond to the calculated values.

The maximum water pressure Pmax is taken to be no more than 4.5 bar, in some cases 6 bar. The minimum Pmin is the pressure at which the excess pressure at the upper water distribution valve is the minimum permissible value, usually (0.2-1.0 bar).

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