Control Valve junctions are always internal to the system, with two connecting pipes. This junction type allows modeling of valves that offer special pressure or flow control characteristics at a location in the pipe system.
The Control Valve Properties window follows the first of the two basic Properties window formats, displaying the connecting pipes in a fixed format. The Control Valve junction does not have an explicit flow direction, but adopts a flow direction from the connecting pipes.
Control Valve Types
You can model four types of control valves: Pressure Reducing Valves (PRV’s), Pressure Sustaining Valves (PSV’s), Flow Control Valves (FCV’s), and Pressure Drop Control Valves (PDCV’s). Loss information for a control valve is not required, because control valves are dynamic devices that change their geometry in response to the pipe system behavior. The loss that results is that required to maintain the control parameter. You can, however, specify the full open loss. This is the loss that will occur should the valve fail to a full open state.
A PRV is a device that controls the pressure in a pipe system. The PRV maintains a constant control pressure downstream of the junction as long as the upstream pressure exceeds the control pressure. If the upstream pressure is lower than the control pressure, the ability to control pressure is lost.
A PSV is similar to a PRV in that it controls pressure in a pipe system. While the PRV maintains a constant downstream pressure, the PSV maintains a constant upstream pressure. If the downstream pressure rises higher than the control pressure, the ability to control pressure is lost.
An FCV is a device that maintains a constant flow rate in a pipe system. By setting the junction to an FCV type and entering a flow rate, the junction will limit the flow through the connecting pipes to be equal to the control flow rate. The FCV can lose its ability to control flow when the pressure drop across it becomes zero or backward flow begins.
A PDCV is a device that maintains a constant (stagnation) pressure drop. For this option, the valve is never allowed to not meet its setpoint. An indicator that an unrealistic pressure drop has been demanded is a failure to obtain a converged solution.
PRV/PSV Static vs. Stagnation Pressure
The control pressure for a PRV or PSV can be either static or stagnation. The default selection is static, as this is the most frequent application in industry.
Action If Setpoint Not Achievable
Always Control (Never Fail) - It is possible that a control valve cannot achieve a given set point. By default this option is on - to change this, uncheck the box and click Set as Default. For additional information, see Control Valve Setpoint Not Achievable.
Use Default Actions - The behavior of the control valve when it cannot meet its setpoint can be modified by unchecking this option. The default behaviors represent the normal behaviors for the standard valve types.
Control valves have two special conditions. To open fully and not control, or to close.
Open Percentage Table
The Optional tab allows data to be entered for special control valve characteristics. Specifically, the valve Cv/Kv, xt, and Flow Area can be specified vs. the Open Percentage of the valve. This data does not affect AFT Arrow’s flow solution, but will affect the reported Cv/Kv. The Valve Summary (in the Output window) always displays the valve Cv, and if data is specified for open percentage and flow area it will also display open percentage and flow area at the operating point.
If open percentage data is not specified, then a Cv/Kv value will be estimated for the control valve.