Operation Principle of Pressure Sequence Valve
Pressure sequence valves sense a change in pressure in a system and transmit a hydraulic signal when the set pressure has been reached. The valve may be normally open or normally closed, changing state when the system reaches the set pressure. They may be used to assure priority hydraulic pressure in one system before another can operate.
An important feature of all pressure sequence valves is a separate drain connection from the spring chamber. This is because, unlike a conventional relief valve, a high pressure can occur in the output port during the normal course of operation. Should it be internally drained, any pressure in the output port will be reflected back into the spring chamber causing a malfunction. In fact a sequence valve may be used as a relief valve in any circuit here excessive back pressures are encountered in the return line. The independently drained pilot makes sequence valves insensitive to downstream back pressure.
A normally closed sequence valve with integral reverse-flow check valve is shown in Figure 1 together with an established application which is to sense that a component has been clamped before initiating the next stage in a "sequence" of operations. When the component is undamped, the pressure falls and the sequence valve closes. The check valve prevents the signal being trapped and allows it to decay back past the sequence valve poppet.
Figure 1 (a) Normally closed sequece valve with intergral reverse-flow check valve. (b) Clamping application.
Two-stage sequence valves suitable for high flow rates are available and one specialized form known as a 'Circuit Breaker' or 'Kickdown' sequence valve is Figure 2. The valve is normally closed until the pressure setting of the control section is reached, when the main spool opens fully with very little resistance to flow. It remains open even if conditions in the downstream circuit cause the circuit pressure to fall below the control setting.
Figure 2 ‘Circuit breaker’ or ‘kickdown’ sequence valve.
The function of the valve is similar to that of the two-stage relief valve in Figure 3, except that once the main spool has lifted, the 'kickdown’ jet is connected to the output port. In this condition, the input pressure necessary to hold the valve fully open has only to overcome the resistance caused by the secondary circuit pressure and the light spring situated behind the main spool. It remains open even when the secondary circuit pressure is less than the valve set pressure only resetting at a very low value.
Figure 3 Pilot-operated relief valve.
Direct-acting sequence valves are employed in low flow applications such as providing signals to operate directional control valves or to positively release a brake before a machine can function. Where the output flow is used to drive cylinders directly, two-stage valves are usually more appropriate.
As the name might imply, the sequencing of cylinder movements is a common application. In Figure 4 when the directional control valve is switched to the 'tramline' condition, cylinder A will extend followed by cylinder B. The flow to cylinder B is through sequence valve S1 which will open when the pressure at the full bore end of cylinder A has attained a certain value, probably owing to it having been stopped by some external object or a: the extremity of its stroke. With the control valve in its 'crossover' condition, cylinder B will retract before cylinder A, with change-over initiated by S2.
Figure 4 Cylinder sequence circuit.
In circuits where pressure-sensing is used to control cylinder movements, it must be borne in mind that sequence valves operate when a specific pressure has been achieved and do not guarantee that the cylinders have completed or reached a particular point in their strokes.