Control Valve Transient Theory

We can use the methods for known flow or pressure to calculate the conditions at a control valve. AFT xStream flow and pressure control valves allow the control setting to be changed with time. Whether constant or changing, the flow or pressure is known and can be used to solve the equations.

Flow Control Valves

With the known flowrate, one can substitute into the Compatibility Equations and solve for the upstream and downstream pressures, respectively, at the valve. The valve loss solution can then be compared against the user-specified loss range to determine if the valve can control to the setpoint for that time step.

Once the valve loses control, it acts like a regular valve. However, it is possible for the control valve to later regain control if the upstream pressure raises sufficiently.

Pressure Control Valves

Pressure control can be specified on the upstream or downstream side of the valve, or as a differential pressure across the valve. All three control types use identical approaches to find a solution at each time step which satisfies the control setpoint.

An iterative approach is required to find a solution that meets the controlled parameter. A flowrate through the valve is guessed at, the positive characteristic is used to determine the upstream pressure, and the negative characteristic used to determine the downstream pressure. The upstream or downstream pressure is compared against the valve setpoint, and further iterations are used to find a flow solution which meets the setpoint.

If the valve is fully open or fully closed yet cannot meet the control setpoint, the valve will lose control and behave as a regular valve. It is possible for the control valve to later regain control if the upstream pressure raises sufficiently. Note that the Pressure Drop Control Valve will never lose control and open fully, as there must be a control pressure loss by definition.