Progressive Casecade Split Range Valve Sequence

If a large valve is suddenly opened from a high pressure to a low pressure section of piping, there can be problems with surge or liquid hammer. To prevent this, a small valve and a larger valve can be installed in parallel. Opening the small one first allows the piping to equalize gently in pressure; then the larger valve can be opened to provide full flow. This is variously described as soft, staged, or cascaded valve operation. It is common in steam and refrigeration systems, among others.

Similarly, large modulating or regulating valves tend to overshoot (hunt) when controlling low flows. For better control at low flow, a smaller regulating valve can be paralleled with the larger one. When the larger valve is nearly closed, control is passed to the smaller valve.