Sequence valves can improve the operation, efficiency, performance, and safety of fluid power circuits in which they are used. Sequence valves are normally closed and usually allow bidirectional flow when equipped with a bypass check valve. Sequence valves always have an external drain connected directly to tank.
Sequence valve basics
When two or more cylinders operating in a parallel circuit must move in sequence, the only positive way to do this is with separate directional control valves and limit switches or limit valves. This setup ensures the first cylinder reaches a positive location before the second one begins to move. When safety or product quality will not be compromized if the first cylinder does not complete its cycle before the second one starts, a sequence valve can be a simple way of controlling cylinder actuation.
Figure 1 shows a cutaway of a typical sequence valve and its schematic symbol. Most hydraulic sequence valves must be used in series with a directional control valve, and a preset pressure must be reached before the valve allows fluid to pass or change flow paths. The sequence valve shown is a direct- acting, internally piloted design offered by many manufacturers. This valve can be changed in the field to externally piloted when required. When this is done, the valve’s part number should be changed to indicate the conversion. Several manufacturers also offer pilot-operated sequence valves. Pilot-operated sequence valves stay closed to within 50 psi or less of set pressure. Direct-acting sequence valves may partially open at 100-150 psi below set pressure, which could allow premature cylinder creep.
A sequence of events
Fluid at the sequence valve’s inlet is blocked by a balanced spool, which is held in place by adjustable force from a spring. When force from fluid pressure at the inlet exceeds the spring force setting, pressure pushes the spool up, routing fluid to the outlet to keep pressure from increasing. Pressure at the inlet never drops below set pressure when there is flow to the outlet. When outlet pressure exceeds set pressure, the valve opens fully and pressure at both ports equalizes. A sequence valve must always have an external drain, and the drain port must be at no pressure or constant pressure because any pressure in this line adds to the spring setting.
A bypass check valve allows reverse flow when the valve is used in a line with bidirectional flow. In some applications a sequence valve may be externally piloted from another operation.
Be aware that a sequence valve shifts when pressure builds up and can start a second operation prematurely when an actuator stalls or is stopped for any reason. If personnel safety or product damage can occur due to incomplete strokes, use limit switches, limit valves, or electronic controls and directional control valves for each operation sequence.
When flow control valves are required, they must be meter-in type. The signal to the sequence valve should come from the line after the flow control because pressure at this point will be whatever it takes to move the actuator and its load.
Sequence valves at work
The hydraulic circuit in Figure 2 is typical for a machine that must clamp and hold a work piece while a second operation is taking place. Sequence valve 2 is set at 550 psi, so pressure at Cylinder 1 must be at least 550 psi before Cylinder 2 can extend. When Cylinder 2 is extending, pressure in the circuit never drops below 550 psi. If Cylinder 2 requires more than 550 psi, pressure in the whole circuit can increase to the relief valve setting, 1000 psi, in this case.
Sequence Valve 1, set at 450 psi, blocks Cylinder 1 from getting a retract signal until Cylinder 2 has retracted, allowing pressure to increase. Force from Cylinder 1 is maintained by a pilot-operated check valve while Cylinder 2 retracts. The signal to open the pilot-operated check valve comes from the line between Sequence Valve 1 and Cylinder 1, so no signal occurs until Cylinder 2 fully retracts.
This circuit is not safe if pressure buildup comes from a source other than Cylinder 1 contacting a load or reaches the end of its stroke and the Cylinder 2 operates prematurely. Also, sequence valves can generate substantial heat because the first cylinder requires higher pressure to move than subsequent ones. This means there is usually a high pressure drop across a sequence valve that results in wasted energy. A kick-down sequence valve can be used to overcome this condition. Figure 3 shows a cutaway view of a typical kick-down sequence valve, with a schematic representation below it. Pressurized fluid at the inlet flows up to a main poppet, through a control orifice, and to an adjustable poppet, which is held closed by spring force. Pressure tends to push the main poppet up (open), but equal pressure and a light spring on the opposite side holds it shut. When pressure builds enough to open the adjustable poppet, flow starts passing through it faster than it goes through the control orifice. This creates a pressure imbalance, so the main poppet rises. When the main poppet rises high enough to route trapped fluid through a bypass orifice, hydraulic pressure above the main poppet drops because the bypass orifice is larger than the control orifice. At this point the only force holding the main poppet shut is spring force and backpressure at the outlet port. When flow stops, spring force closes the main poppet because pressure has equalized.
The circuit in Figure 4 is the same as in Figure 2, except it is uses kickdown sequence valves instead of standard sequence valves. Cylinder 2 will not move forward in this circuit until pressure on Cylinder 1 has reached 750 psi. When a kick-down sequence valve opens at its pressure setting, it allows fluid to pass at 50 psi plus whatever it takes to overcome resistance after it.
This means the whole circuit from the pump to all actuators is 50 psi plus Cylinder 2 resistance. The pilotoperated check valve at Cylinder 1’s cap-end port keeps it pressurized and at near full force while Cylinder 2 extends at low force. Energy waste is low, so heat buildup is minimal.
This article is an exceprt from Chapter 14 of Fluid Power Basics, an e-book that can be viewed and printed for free from our website. Other sequence valve circuits can be found in Fluid Power Circuits Explained, a second e-book by Bud Trinkel and also available for free at our website. Both complete books can also be purchased in print from the author, who can be reached via e-mail at email@example.com.