In feedwater pump operation fixed minimum flow must be obtained. Valve capacity below this fixed minimum flow causes superheating of the feedwater; consequently generation of steam may result in cavitation damage. If the boiler feed is lower than the minimum flow, the differential flow must be discharged through bypass-line which is accomplished by the minimum flow valve.
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.
The basic construction of a minimum flow plant can be seen on the above illustration. The volumetric delivery is obtained via an orifice plate or a nozzle. The switching equipment signals a contact-impulse to the two-step control via a pressure-drop variator. The limit contacts are operated directly or indirectly for opening and closing of the minimum flow valve. Similarly the valves are operated by permanent control.
For example, the switching equipment is replaced by a permanent controller that can also be locked into the corresponding variable-command of the applied rpm-controlled pumps. The minimum flow valve is controlled according to the minimum flow of the required throughout, so that the total necessary minimum flow does not fall below the set limit.
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.