How to Size Water Valves

To choose the optimal valve to the hydraulic circuit you need to know the valve mounting principle (2-port, mixing valve or diverting valve) and the pressure loss of the hydraulic circuit where the mass flow varies.

In ideal conditions the pressure loss over the valve when fully open should be bigger than pressure loss over the valve + the pressure loss part of the circuit where the mass flow is controlled. This pressure balance is called as valve authority. The diagrams below illustrate different types of circuits and the principle how the valve authority is calculated.

By choosing a smaller valve the pressure loss over the valve is increased, and the valve authority is improved. However, it is important to note that the smaller valve has bigger pressure loss and hence the pump must have sufficient output to provide the required design flow.

The following are the typical steps in choosing the right control valve:-
  1. Calculate the required maximum water flow (m3/h) - this is normally provided by the system designer. Enter this to the flow rate field.
  2. Calculate the pressure loss over the part of the circuit where the mass flow varies. E.g. in diverting circuits the element that causess the biggest pressure loss is typically a heating coil - consult the system designer for the pressure loss. Enter this pressure loss to the pressure loss (circuit) field.
  3. Choose the valve that closest matches the required mass flow and the pressure loss - this is where SyxthSense valve sizing program will automatically provide list and recommendations.
Note: With smaller valve the pressure loss is higher than a bigger valve. Although this gives better control, the pump must have enough head to provide the required design flow. If the pump does not have enough head the water flow through a heating element is smaller than designed and may lead to heat shortages.
     
DIVERTING VALVE CIRCUIT
 
2-PORT VALVE CIRCUIT
 

MIXING VALVE CIRCUIT