One example of where control valves might be in series is in the HVAC industry. The first valve could be a circuit setter, automatic control valve or pressure independent control valve, while the second valve could be flow control valve. When two valves are placed in series in PIPE-FLO, the Overcontrolled message will appear if each valve is set to a fixed flow rate. This article explains the process on modeling control valves in series and alleviating the Overcontrolled message.
Following are two examples from different models that are not modeled correctly.
Example 1, two valves in series with fixed flow rates: In this screen shot you can see two Sizing Valves and both are set to a fixed 25 gpm flow rate. This will produce an Overcontrolled message.
Example 2, multiple control valves in series with fixed flow rates:
In this screen shot you can see a control valves being used on the main supply line and in each loop to control the heating load. This will produce an Overcontrolled message if all the control valves are set to a fixed flow rate.
One method to alleviate the Overcontrolled message, no matter what type of valve it is, is to add the flow coefficient data to one of the valves and set the valve operation to a manual position. An example is shown below. This screen shot show control valve manually set to 100% open.
The second method is to set one valve to “Fully Open”. You will get a flow rate through the pipeline based on the Conservation of Mass. For a “Fully Open” Sizing Valve, the software will not calculate a pressure drop because it does not know the flow coefficient. Following is a example.
This screen shots shows one sizing valve having a fixed flow rate and the second one set “Fully Open”. The flow rate through both valves will be 25 gpm.
For branching systems, the Conservation of Mass still applies. If you set the main header control to the total flow rate needed and leave one loop with an control valve fully open, then that “Fully Open” loop will get a flow rate that is the difference between the total flow rate and the summation of the flow through the individual loops. Following is an example. This screen shot is showing the main header ACV set to 100 gpm, the Heat Load 1 ACV set to 50 gpm and Heat Load 2 loop ACV set to “Fully Open”. When the model is calculated, the flow rate in the “Fully Open” loop will be 100 - 50 = 50 gpm, due to the Conservation of Mass.
It is recommended that all valves be model as Control Valves and the manufacture’s flow coefficient data entered. Manually setting the valve to a percentage open will eliminate the Overcontrolled message and the pressure drop through the valve will accurately be accounted for.