Minimum Flow Valves (MMV Type)
In practice two types of minimum flow control valves have been proven successful.
If the required flow is lower than the minimum flow, the minimum flow valve will open completely.
If the required flow is lower than the minimum flow, the minimum flow valve will open controlled. Required flow and discharged minimum flow are equal to the corresponding necessary minimum flow.
The advantage of the two-step control is based on the simple interruption-free construction. It can be properly used for operation in small and medium sized plants. The permanent control is more profitable because it saves more energy for large-sized plants and for plants being often started and stopped. For speed-governed feed pumps operation the minimum flow can be adjusted to the rpm of the pump.
Design and Function
The basic design of the fittings is shown in the illustrations. The throttling of the feedwater is effected by several stages. The minimum flow valve features a control-cone fitted upstream of the throttling device. Through suitable shaping and corresponding stepping of the total pressure on the individual throttles, noise and cavitation are controlled. Elements prone to severe duty are made of resistant material. Because of a practical design of the valve, wear parts are easily replaceable.