The sequence valve is basically just a relief valve that relieves from one part of a pressurized circuit to the next instead of relieving to tank.
This is a useful function where no flow should be provided to a sub-circuit until pressure rises to a specified level in the main circuit.
The symbol for a sequence valve is basically the same as it is for a relief valve, except that a drain to tank (3rd hose) will be shown if the valve is pilot operated.
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