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 2.88a 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.