Engineering context

Control valve sizing is a pressure-balance problem, not simply picking a Cv. In a pipe network the valve is the balancing element: it takes up whatever pressure drop remains after the fixed terms (the boundary-to-boundary differential, static head, and any fixed equipment ΔP) and the flow-dependent variable losses (pipe and fitting friction, velocity head losses, equipment) are accounted for.

Preliminary sizing in FluidFlow produces the process data a manufacturer needs — the preliminary Cv or Kv and the process conditions at the valve inlet and outlet — which the vendor then uses for detailed sizing and to propose a suitable valve model from their catalog. Sizing centres on the maximum-flow case, where the available valve pressure drop is at its lowest and the required Cv is at its highest, while the minimum-flow case sets the minimum Cv the manufacturer also needs for selection.

Engineering workflow

1. Establish the maximum-flow case — build the model so that maximum flow is achieved, whether driven by the highest pressure difference between two boundaries or, for pumped systems, the highest flow across the pump and/or control valve.
2. Build and connect the network — place and define the boundary conditions (the fluid is selected from the database, which carries its properties, and applied at the boundary), then add pipes, fittings, elevations, and equipment and connect them.
3. Represent the control valve according to the data available — it can be modelled with any of the control valve types. Take particular care during simultaneous control valve and pump sizing to avoid over-constraining the model.
4. When simultaneous pump and valve sizing is required, model the control valve as an element that generates a set pressure drop at the maximum flow — commonly 10–15 psi as a starting point.
5. For systems using pressure boundaries at inlet and outlet, the flow control valve can be used directly — the model is not over-constrained and FluidFlow resolves the valve pressure drop on its own.
6. Solve the model — let FluidFlow calculate the preliminary control valve Cv and the corresponding process conditions at the valve inlet and outlet.
7. Determine the minimum-Cv case — set the flow control valve to the expected minimum flow (pressure boundary systems) or apply a flat-curve pump assumption (pumped systems).
8. Hand off the process data — provide the preliminary Cv (maximum and minimum cases) and inlet/outlet conditions to the manufacturer for detailed sizing and valve selection.

Why the full system matters

A control valve sized independently of the connected system will see a different pressure drop in service. The valve pressure drop is not a fixed input — it is what remains after all other system terms are accounted for, and it changes with flow, pump curve shape, boundary conditions, and operating case. Sizing the valve inside the connected model ensures the preliminary Cv and process conditions reflect real operating conditions.

How FluidFlow helps

FluidFlow sizes control valves inside a steady-state pipe network using the ISA/IEC 60534 method, returning the preliminary Cv/Kv and the valve inlet and outlet conditions against the connected system — pump or boundary driving pressure, pipe and fitting losses, equipment, and the overall pressure balance. Final valve and trim selection remains with the manufacturer.

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