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Power Operated Valves

Besides manually operated valves, an important part is played by valves and similar device actuated by some form of auxiliary power such as electricity, compressed air or hydraulic pressure.

A solenoid valve is a combination of a valve with an electromagnet, which provides the power to operate it. A rod to the core of the magnet connects the valve disc. Functionally there are three main types of solenoid valve. In the first type (Fig. 1) the core of the magnet and the valve disc are pulled upwards against the force of a spring when the magnet is energized. When the current is switched off and the magnet thus de-energized, the spring thrusts the disc against the valve seat, thereby closing the valve.

In the valve shown in Fig.2, the pressure of the fluid is utilized to control the valve. When the magnet is energized, the valve disc is lifted clear of the primary control passage, and the space above the differential piston is brought into communication with the valve outlet. The pressure over the differential piston is thus reduced. Since the amount of fluid that can flow through the narrow compensating passage is smaller than the amount that flows through the primary control passage, a difference in pressure is developed, causing the differential piston to be lifted off the valve seat. On the removal of the pressure, the primary control passage is first closed. Pressure now builds up again above the differential piston, so that this piston is thrust downwards. The valve closes.

To function properly, valves of this kind require a certain minimum pressure difference between valve inlet and outlet. In the third kind of solenoid valve (Fig.3) a magnetically operated three-way valve and a piston valve form a unit. Control is affected with the aid of pressure supplied by an auxiliary source of power. While the valve is in the closed position the magnet is de-energized and the bypass passage is communication with the outlet. There is then no pressure in the space under the piston. When the magnet is energized, the auxiliary pressure is admitted under the piston, so that the latter rises, causing the valve to open (left-hand diagram in Fig.3).

A diaphragm valve (Fig.4) is controlled by the action of a diaphragm, which is actuated by liquid or pneumatic pressure. A magnetically operated three-way valve or changeover valve is connected to the space over the diaphragm. While the magnet is de-energized, access to this space is closed, the diaphragm being held in the closed position by the compression spring. When current flows through the magnet, compressed air is admitted to the space over the diaphragm and develops the force needed to thrust the diaphragm downwards (against the pressure of the spring and the pressure of the fluid acting against the valve disc,) thereby causing the valve to open.

In long pipelines with high rates of flow is undesirable to effect valve closure abruptly in a single stage, as this will cause sudden pressure build up which may harm the pipeline or the fittings and measuring devices installed in it. This can be avoided by employing two valves, one in the pipeline itself and the other in a bypass pipe of smaller bore. On closure of the main valve there remains a flow of 10-20% in the bypass, which can then be closed as a final stage of the closing operation. Alternatively, a double-diaphragm valve (Fig.5) may be used, which closes in two stages.

It may be equipped with quick-action air-relief valves (as shown in Fig.5) or with magnetically operated changeover valves. Release of the air pressure may be affected either manually or automatically by closing the control pipelines. Valves of this kind are widely used in industrial processes, especially automatic process with centralized control: e.g., for the delivery of predetermined quantities of fluid.

Besides the disc-type valves described in the foregoing, other types of valve, such as sluice valves and flap valves can likewise be power-operated.

The sluice valve illustrated in Fig.1 is provided with an electric motor for raising and lowering the gate. Alternatively, a hydraulic drive system may be used to do this. The hydraulic actuating cylinder is mounted directly over the valve and is connected to the valve gate by means of a rod. The hydraulic fluid (oil, for instance) is admitted into the cylinder either over the piston (to close the valve) or under it (to open the valve).

There exist many types of valve for a variety of specific purposes -e.g., in pipelines, in refineries, for water turbines, for hot gases, etc. An important type is annular measuring valve (Fig.2). The rate of flow through the valve is measured in terms of the difference in pressure between the two points where the two small side pipes are connected to the valve casing, these pipes being connected to manometer. The inlet casing, the upstream end of the gate, and diffuser are so shaped that a reliable flow-rate measurement is obtained over almost the entire range of valve-gate movement from closed to fully open. The gate may be actuated manually or with the aid of electric or hydraulic power and is held in the desired position by means of a self-locking warm drive. Such valves, with or without flow-measuring facilities, are used in water engineering as valves for pipelines, pumping stations etc.

In such circumstances they often have to perform the additional function of a nonreturn valve in the event of pump failure or as a check valve to protect the pump itself from reverse flow. For this purpose, a special quick-action closing mechanism may be employed (Fig.3). It is electrically connected to the pump-drive motor and functions as follows. The electromagnet is normally energized, the clutch is engaged, and the self-locking work drive holds the valve gate in the position to which it has been set. In the event of a power failure, the magnet becomes de-energized and the clutch is automatically disconnected by spring action.

The connection to the worm drive is broken, the drop weight descends and -through the agency of a crank mechanism-moves the gate to the closed position. To slow down the final stage of closure and thus avoid too sudden a pressure buildup, an oil-breaking cylinder (dashpot) is provided, whose braking action can be controlled by means of valves in the oil-bypass pipes.

Another important class of valves is formed by the flap valves, of which the automatically acting checks valve (Fig.4) is a particular type. It comprises a disc, or gate, which is pivoted at its upper end and is held open by the flow; but if the flow reverses, the weight of the disc and the movement of the fluid force the disc on to the seat. In the butterfly valve (Fig.5) the closing element is a circular disc pivoted along a diameter. Closure is effected manually, electrically or hudraulically. Such valves are used, for example, in the penstocks to the turbines of hydroelectric power stations. They may be equipped with quick-action closing devices of the type shown in Fig.3.