Information - Pressure tanks - hydrophore tanks
Sizing a Pressure tank, determining volume
Sizing a Pressure tank, determining volume
Author: Akvedukts
Proper selection, sizing of the volume of the pressure tank (bladder vessel, hydrophore) will provide a smoother operation of the water supply system and the water pump and, therefore, significantly increase the service life. Read more about pressure tanks, their different names and types in this article. There you will also read why the volume of pressure tank is not equal to the amount of water this tank can hold and what volume of pressure tank to choose in simple situations.
In cases where the volume of the pressure tank needs to be determined by calculation, several methods can be used. The methods differ in that different inputs are used to determine the volume. Also, the data obtained by different methods for similar situations are slightly different, so the choice of method is left to the professional making the calculation. Below are three different methods. For the first method, the parameters used are pump flowrate, pump power, pump start and cut-off pressure, and pre-charge pressure of air in the pressure tank. The second method determines the volume of the pressure tank based on the maximum pump flowrate, start and cut-off pressure, and the planned duration of the pump work per cycle. The data from the third method is the maximum pump flowrate, the pump start and cut-off pressure, the pre-charge pressure, and the planned number of pump start/stop cycles per hour.
Determining of the volume of the pressure tank- the first method
where
Vt the volume of the pressure tank, liters,
Qmax maximum pump flow rate, l/min,
Pmax pump cut-off pressure, bar,
Pmin pump start pressure, bar,
Pg tank pre-charge pressure, bar Pg = Pmin - 0.2 bar,
K coefficient, which is selected according to the pump power:
| Pump power, kW | <1,5 | 2,2 - 3,0 | 4,0 - 5,5 | 7,5 - 9,0 |
| Coefficient | 0,25 | 0,375 | 0,625 | 0,875 |
Determining of the volume of the pressure tank - the second method
where
Vt the volume of the pressure tank, liters,
Qmax maximum pump flow rate, l/min,
TR the duration of operation of the pump during one start, min,
Pmax pump cut-off pressure, bar,
Pmin pump start pressure, bar.
Determining of the volume of the pressure tank - the third method
where
Vt the volume of the pressure tank, liters,
Qmax maximum pump flow rate, l/min,
A amount of cycles of pump switching on/off per hour,
Pmax pump cut-off pressure, bar,
Pmin pump start pressure, bar,
Pg tank pre-charge pressure, bar Pg = Pmin - 0.2 bar.
Sizing examples for a pressure tank
Maximum flowrate of the pump 25 l/min, 2.0 bar start pressure, 3.0 bar cut-offt pressure, 1.8 bar pre-charge pressure (0.2 bar below pump start pressure). Pump motor power 1.1 kW.
According to the first method: the pump power factor K is 0.25 (since the pump power is <1.5 kW)
By the second method: assume that the pump will run for 15 seconds (0.25 min)
According to the third method: Let's take the number of on/off cycles as 60
For the pressure unit atm (atmosphere) can also be used instead of bar as the values are approximately the same. It is important to remember that the pre-charge pressure of the air in the pressure tank is 0.2 bar lower than the pump start pressure (the lowest pressure of water system).