Power Operated Valves

Besides manually power operated valves, an important part is pPower Valveslayed 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, power operated 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.
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