Is the sequence valve obsolete?

By Josh Cosford, Contributing Editor

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We don’t hear much anymore regarding sequence valves, and probably for good reason. A sequence valve is like a self-regulating inline relay that opens when work pressure overrides the spring setting. It’s a form of hydraulic automation that provides sequential operation for two or more functions using one fluid supply source. However, with the proliferation of inexpensive automation and control systems, you can literally program a PLC on your phone to operate two directional valves quite easily. This begs the question: are sequence valves obsolete?

In the example hydraulic circuit, you can see I’ve placed a sequence valve just after the pump and branched to the primary work line with the relief valve and the cylinder subcircuit. When the pump starts, the path of least resistance is through the open center directional valve and straight to tank. When the directional valve actuates to power the cylinder, hydraulic pressure rises to load pressure, which, for the example to work, must be less than the 2,000 psi setting of the sequence valve.

This particular circuit is an example of a clamp and drill function, so the cylinder will bottom out on the load before it arrives at the end of the stroke. When the cylinder stops, pressure rises until at 2,000 psi, the sequence valve opens. Like many pressure valves, the sequence valve operates like a relief valve. Indeed, a sequence valve is a relief valve with a spring chamber drain that allows pressure to bleed when both work ports are simultaneously exposed to pressure. Without such a drain (or vent), the spring chamber could fill with pressurized oil, subsequently increasing the spring force or locking it altogether.

The location of this sequence valve negates the need for a reverse flow check valve. The check valve allows free flow in the opposing direction, which is beneficial when placed in the work ports of the actuator, as there is no backward flow path through the valve. Sequence valves are handy for creating a secondary function on existing systems, such as a tilt function to a bin lift or a locking pin on a tailgate lift.

As mentioned, sequence valves require a drain port, so you’ll see three ports on every inline sequence valve body. For integrated circuits (custom manifold blocks), the cartridge valves have three ports. Both spools and poppets may be used for sequence valves, although poppets tend to suit higher pressure applications better. Some valves used on low-duty applications may employ a vent rather than a full fledged drain port.

High-flow applications, like many large valves, are pilot-operated. When a sequence pilot valve operates a slip-in cartridge valve, hundreds of gallons per minute are possible. Most sequence valves offer a balanced piston, which provides stability despite the effects of flow forces or high-pressure differential.

Some of the “big guys” in the cartridge valve industry offer myriad designs and options of sequence valves. Despite the growth of microelectronic control systems or smart PLCs, nobody is obsoleting sequence valves any time soon.

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How sequence valves work

Sequence valve design and operation is very similar to relief and reducing valves.

Sequence valve use mostly spool elements, with some poppets, which sense pressure over a set area to move the spool against an adjustable spring. When the pressure reaches a pre-set value it opens a second port to divert the flow.

Sequence valves are available as:

Direct or Indirect Operation Direct operation is when the pressure is applied directly onto the main spool opening area. Indirect is also known as pilot operated or two stage valve, and is when the pressure is applied onto a smaller pilot valve that opens first, and it's output then opens the larger main stage.

NO (Normally Open) or NC (Normally Closed) designs NO valves are open i.e. allow free flow in their normal, inactivated condition. NC valves don't allow free flow in their normal, inactivated condition but will once they are activated.

2 or 3 way options A 2 way valve will have a single inlet and outlet line. It will almost certainly require additional pilot feed or drain lines as well. A 3 way valve will have 1 inlet and 2 outlet lines, they may also have additional pilot feeds or drain lines.

Internal or External Pilots and Drain Lines An internal pilot is when the pressure signal to switch the valves is taken from the supply or load lines of the valve. An external pilot is when the pressure feed come from a separate part of the circuit via a separate connection. Drain lines may also be internal or external. An external drain will almost certainly be required if you don't have one line that returns back to tank. Without a constant, zero, reference feed the set pressures will fluctuate up and down depending on whatever line pressure they are referenced to.

Each of the above features will provide different performance characteristics

Kick-down valves are sequence valves but have a low bypass pressure drop when open.

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