Harold Kestenholz
You have a complex problem. If you will eliminate controller harmonics by closing the valve slowly with a screw instead of the pneumatics, you can concentrate on the valve shape. If turning the valve closed with a screw does not produce the problem, then the noise is due the controller.
Pneumatic/spring control is problematic in that powerful fluid dynamics have to be overcome with resonant chambers and harmonic lengths of metal.
Double-diaphragms with fluid trapped between and the fluid passing through a motion-dampening orifice gives greater control than a harmonic spring-resisted air chamber. Zone valves here using wax cartridges in metal pistons have shown durability without resonance.
It is interesting to know the application of this valve. Fifty liters per minute provides a heavy column of water with strong inertia. Providing 120,000 btuh or more with one valve is unusual for a residence. That translates to a 4800 square-foot zone in a larger house, unless operating a 3 or 4-way valve.
