Modeling a Balancing Valve in Series with a Control Valve


PIPE-FLO's and Flow of Fluid's Control Valve Operations feature can be used to help determine balancing valve settings for hydronic heating and cooling systems. The balancing procedure in these systems typically starts with setting all control valves fully open and setting the balancing valve in each circuit to   achieve the design flow rates. Once the circuits are balanced the control valves are used to vary the flow to achieve the desired heating or cooling. 

This same process can be modeled in PIPE-FLO or Flow of Fluids. Both the control valves and balancing valves can be modeled using the control valve tool, which can be set to a specific flow or position. The program can be used to help size the balancing valves and once the valves are selected, their data can be entered into the model to determine the required valve settings to achieve the design flows. After the balancing valves are set, the control valve set-points can be varied to model different flow scenarios in the balanced system.  

To show how this process can be modeled, the 10 circuit primary-secondary chilled water system is analyzed. 

The total system capacity is 200 gpm with a design flow of 20 gpm through each circuit. The circuits consist of an air handling unit, two-way control valve, and calibrated balancing valve (CBV). The control valve tool is used to simulate both the two-way control valve and balancing valve. Initially, neither the balancing valves or control valves have been selected for the system.  

The system is first set-up with all control valves set to fully open and the balancing valves set to the design flow rate through each coil.

The differential pressure needed to achieve the design flow rate for each balancing valve is calculated. From the results, flow coefficients can be calculated and balancing valves can be selected for the system. The balancing valve data can be manually entered if it is provided by the vendor or the Control Valve Calculator tool can be used to approximate the valve characteristics. Once the balancing valve data is entered into the model the set positions can be calculated. To facilitate this, the Control Valve Operations feature allows you to quickly change all control valves in the system from a set value to the calculated set position.  

The feature is accessed by selecting “Control Valve Operations” from the “System” menu. In the “Control Valve Operations” dialog, select “Set Control Valves to Calculated Percent Open” for the operation and then select which valves the operation will be applied to.  

In the example, all of the CBV valves are selected and are set to their calculated positions. The image shows the setting for each CBV that achieves the design flow rates through each circuit.

Now that the system is balanced with the balancing valves set to fixed positions, the two-way control valves may be throttled to evaluate conditions other than design flow. In doing so a range of operating conditions can be simulated for the control valves to aid in their selection. The Lineups feature can be used to model the different operating scenarios, which may include simulating the variation in cooling loads throughout a typical day or year, or evaluating the system with one or more loads isolated.

The  model can prove particularly valuable throughout the life of the system by using it to evaluate future expansions. When any new loads are added to the system it will need to be rebalanced. The additional loads can be modeled and the same procedure can be used to balance the newly expanded system.