Gate Valve CFD: Flow Coefficient (Cv) of a Large Double-Reduced-Bore Valve

A valve manufacturer needed the full-open flow coefficient of a large, high-pressure-class gate valve for its datasheet. The valve has a double reduction in the bore from the end connection to the seat, and the company makes the same design in other sizes, so it wanted a number it could trust before taking the range further.

Shirsh Technosolutions ran gate valve CFD in ANSYS Fluent on the full 3D flow path and worked out Cv and Kv at a fixed pressure difference. Before the large valve, the client's R&D team gave Shirsh a smaller sample valve to see how the method and the reporting would look.

A sample valve first. The R&D team sent a small gate valve as a trial. Shirsh modelled it, estimated Cv and Kv and sent a short report. The client then asked for the inlet pipe to start two pipe diameters upstream and the outlet to run six diameters downstream, which is what its flow-test standard calls for. Shirsh rebuilt the domain to that layout, re-ran the case and issued a revised report the same day, and the client's R&D head accepted it.

The large valve with the double reduction. For the main job the client supplied the 3D model of the large valve. Shirsh checked the geometry before meshing and pointed out that the double step could not be seen in the first file. The client sent a corrected model, and that one was used.

Fluid domain and mesh. The wetted volume of the valve was extracted with the gate fully open and joined to pipe runs of two diameters upstream and six downstream. A poly-hexcore mesh was built in Fluent Meshing, finer through the reduced bore and the seat, and the cell quality histogram was checked before solving.

Solver set-up. The case was solved as steady, incompressible, turbulent water flow with the pressure-based coupled solver, the k-omega SST turbulence model and second-order schemes for pressure and momentum. A pressure inlet and a pressure outlet held a constant pressure difference across the valve and pipe, so the flow rate came out of the solution.

From flow to Cv and Kv. The volume flow at that pressure difference was converted to Cv with the standard liquid sizing relation, using the specific gravity of water, and Kv was taken from Cv. Velocity and pressure were plotted on the central plane and on five cross-sections along the flow, and the residual history was used to confirm convergence.

What the contours showed. Velocity rises sharply through the stepped bore and peaks at the seat, the gate cavity stays almost stagnant, and a low-velocity zone forms along the bottom of the pipe just after the seat before the flow settles. The pressure contours show the sharpest drop across the stepped section and the seat, with part of it recovered in the pipe downstream.

Common questions

How is the Cv of a gate valve calculated with CFD? The full-open valve is modelled with straight pipe on both sides and a fixed pressure drop is applied between inlet and outlet. CFD gives the volume flow at that drop, and Cv follows from Q x sqrt(SG / dP) in US gallons per minute and psi. Kv is the metric equivalent.

Why do the 2D upstream and 6D downstream pipe lengths matter in valve CFD? Flow coefficients are measured with pressure taps a set number of pipe diameters before and after the valve, and the jet leaving the seat needs some pipe length to recover. Matching the same lengths in the CFD model makes the simulated Cv comparable with a rig test and with catalogue data.