Non-return valves and shuttle valves

Non-return valves and shuttle valves

The non-return valve (NRV), or check valve as it is sometimes known, is a special type of directional control valve. It allows the fluid to flow in one direction only and it blocks the flow in the reverse direction. These valves may be operated directly or by a pilot circuit. Some examples are shown in Figure 2.88.

• Figure 2.88a shows a valve that opens (is free) when the inlet pressure is higher than the outlet pressure (back pressure).
• Figure 2.88b shows a spring-loaded valve that only opens when the inlet pressure can overcome the combined effects of the outlet pressure and the force exerted by the spring.
• Figure 2.88c shows a pilot controlled NRV. It opens only if the inlet pressure is greater than the outlet pressure. However, these pressures can be augmented by the pilot circuit pressure.

(i)      The pilot pressure is applied to the inlet side of the NRV. We now have the    combined pressures of the main (primary) circuit and the pilot circuit acting against the outlet pressure. This enables the valve to open at a lower main            circuit pressure than would normally be possible.

(ii)     The pilot pressure is applied to the outlet side of the NRV This assists the      outlet or back pressure in holding the valve closed. Therefore, it requires a             greater main circuit pressure to open the valve. By adjusting the pilot pressure in these two examples we can control the circumstances under   which the NRV opens.

• Figure 2.88d shows a valve that allows normal full flow in the forward direction, but restricted flow in the reverse direction. The valves previously discussed did not allow any flow in the reverse direction.

• Figure 2.88e shows a simple shuttle valve. As its name implies, the valve is able to shuttle backwards and forwards. There are two inlet ports and one outlet port. Imagine that inlet port A has the higher pressure. This pressure overcomes the inlet pressure at B and moves the shuttle valve to the right. The valve closes inlet port B and connects inlet port A to the outlet port. If the pressure at inlet port B rises, or that at A falls, the shuttle will move back to the left. This will close inlet port A and connect inlet port B to the outlet. Thus, the inlet port with the higher pressure is automatically connected to the outlet port.

Conditioning equipment

The working fluid, be it oil or air, has to operate in a variety of environments and it can become overheated and/or contaminated. As its name implies, conditioning equipment is used to maintain the fluid in its most efficient operating condition. A selection of conditioning equipment symbols is shown in Figure 2.89. Note that all conditioning device symbols are diamond shaped.

Filters and strainers have the same symbol. They are normally identified within the system by their position. The filter element (dashed line) is always positioned at 90° to the fluid path.

Water traps are easily distinguished from filters since they have a drain connection and an indication of trapped water. Water traps are

particularly important in pneumatic systems because of the humidity of the air being compressed.

Lubricators are particularly important in pneumatic systems. Hydraulic systems using oil are self-lubricating. Pneumatic systems use air, which has no lubricating properties so oil, in the form of a mist, has to be added to the compressed air line.

Heat exchangers can be either heaters or coolers. If the hydraulic oil becomes too cool it becomes thicker (more viscous) and the system becomes sluggish. If the oil becomes too hot it will become too thin (less viscous) and not function properly. The direction of the arrows in the symbol indicates whether heat energy is taken from the fluid (cooler)