Control Valve

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), Pressure Sustaining Valves (PSV), Flow Control Valves (FCV), and Pressure Drop Control Valves (PDCV). 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.

  • A 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.

Special Conditions

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 and Flow Area can be specified vs. the Open Percentage of the valve. This data does not affect AFT Fathom’s flow solution. However, engineers frequently desire to know the valve’s open percentage during operation to ensure it meets design requirements. 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. Tabulated data can be pasted or imported to the table by using the Edit Table menu to the right of the table. A graph of the open percentage, Cv, and flow area data can be displayed by selecting Show Graph to the right of the Open Percentage Data table.

A Cv vs Open Percentage table can be generated quickly for common Cv vs Open Percent profile types by choosing the Create Cv/Open %... button next to the table. This then opens a window in which a curve can be defined using one of the following options:

  • Transferred - If a table has already been defined on the Optional tab of the Properties window, it can be copied over

  • User Defined - Data can be entered directly into the table, or imported from existing data

  • Linear - A linear curve is generated based on the user defined full open Cv and percent increment

  • Equal Percentage - Generated as an equal percentage curve

  • Pre-Defined - Pre-defined curves for several common valve Cv vs Open Percent profiles can be chosen and adjusted for a defined full open Cv value based on data from Val-Matic 2009Val-Matic Valve and Manufacturing Corp., "Surge Control in Pumping Systems," Val-Matic Valve and Manufacturing Corp., Elmhurst, IL, 2009..

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