Today we will be concentrated about Pressure sequence valve or sometime also called hydraulic sequence valve.

We will see few quite important points in this post
1. Why we need hydraulic sequence valve
2. How hydraulic sequence valve works
3. Characteristic of hydraulic sequence valve
Hydraulic sequence valve is also type of pressure control valve. Sometime we feel need ,for various mechanism and operation of machines, of operation of hydraulic cylinders in a sequence. Hydraulic sequence valves will be used when we need to provide the sequence in operation of two or more than two hydraulic cylinder.

Now let us consider one example where we will see the requirement of sequence in operation of hydraulic cylinders.

Let us see clamping and punching unit in a manufacturing sector , there will be two cylinders in above unit. First cylinder will be responsible for clamping of workpiece and second cylinder will be responsible for punching the workpiece.

Before second cylinder start to operate, we must be sure that workpiece is properly clamped as if workpiece is not clamped properly then it will create the possibilities of accident and breakdown of clamping and punching unit also.

This assurance will be secured by the application of hydraulic sequence valve.

we need to first clamp the workpiece and once workpiece will be clamped successfully second cylinder will come in operation for punching. Until unless , workpiece is not clamped second hydraulic cylinder will not come in operation.

Let us see the hydraulic symbol used for sequence valve, we must be careful during the study of hydraulic circuit as symbols and internal works of sequence valve , pressure relief valve, pressure reducing valve and counterbalance valve are so similar with each other. One valve will be slightly different with other each above mentioned valves but their function will be completely different with each other.
As we can see in above symbol of hydraulic sequence valve, there will be one check valve for free flow in reverse direction.

### Let us see few points about hydraulic sequence valve

1. Hydraulic sequence valve will normally be in closed position and it will only come in line once pressure increases and reaches the setting point of sequence valve.
2. Hydraulic sequence valve will have one pilot line to sense the inlet pressure.
3.  Hydraulic sequence valve will have one check valve to allow free flow in reverse direction.
4. Hydraulic sequence valve will be externally drained as the outlet of such valve will not be connected with reservoir so such valve could not be internally drained.
5. Hydraulic sequence valve will have a separate drain line directly from valve to reservoir.
6. Hydraulic sequence valve setting pressure will be higher than the operating pressure of first actuator.
7. Suitable sizing of actuator will reduce the energy loss in sequence valve circuit.
Let us see one circuit diagram to understand the operation of hydraulic sequence valve, we will see the hydraulic circuit diagram for above clamping and punching unit.
Let the sequence valve pressure setting in given circuit diagram is 500 psi, it indicates that sequence valve will come in action only, once pressure reached to 500 psi at inlet of sequence valve.Hydraulic oil will flow, from direction control valve, towards first cylinder inlet port because sequence valve will be in closed condition and hence first cylinder will be actuated to clamp the workpiece.

Once workpiece will be clamped successfully, pressure will be increased and will reach to setting pressure of sequence valve and hence sequence valve will come in operation and second cylinder will be extended to perform the punching work.

So what we needed ? We were requiring that first workpiece must be clamped and after that punching operation should occur and above requirement has been successfully completed by the application of sequence valve.

When direction control valve will direct the fluid for reverse direction , in that case oil will flow through check valve in free flow condition.

If you need the retraction of actuators also in sequence , such as if we want that punching cylinder retracts first and then clamping cylinder then this requirement could also be done by using one more sequence valve.