Solenoid Pilot Air Valves

A solenoid pilot valve actuator is a device that is used to operate larger compressed air valves.

Earlier, I talked about direct acting solenoid valve actuators. In those pages I discussed how it is impractical to use a direct acting solenoid to actuate larger compressed air valves, due to the relative low valve-mechanism shifting force of a direct acting coil. With a small distance (10th of an inch or so) a direct acting coil generates sufficient force. Longer than that, there may not be enough shifting force to work the air valve.

If longer distances are required, as they are in the operation of larger air valves, the direct acting coil, for sure, does not have enough power.

Electricity Operating Big Valves

How do we use an electrical coil to shift the internal mechanism of larger compressed air valves, then?

That is where the solenoid pilot valve actuator comes in.

We do accept that for small valves with low shifting distances the direct acting solenoid valve actuator does work well. With reasonable quality compressed air, direct acting solenoids (and the small valves they actuate) can be relied on for millions of cycles with little or no problem. They are the right product for the right job.

What is it that the small, direct acting solenoid air valve controls? Why, compressed air, of course.

And why do we use compressed air to do work? Because, compressed air is able to exert significant force.

What do we need to shift the internal workings of larger air valves?

Higher force. In fact, enough force to ensure that the valve poppet or spool will shift reliably, to control the flow of the compressed air when it is required to do so.

Why not harness compressed air to move the spool in larger air valves?

Use Compressed Air

Then, why not use a small, relatively (for its size) powerful and reliable small direct acting solenoid valve to solenoid pilot-operate a larger valve?

And of course, the industry does.

Compressed Air Valve

In the drawing above, I show a small, direct acting solenoid valve, installed on the end of a much larger spool valve, that being item #2.

Item #3 shows the supply line of compressed air to that small, direct acting solenoid valve. Note that it is taken internally from the main valves compressed air supply (item #2).

In the drawing, the small direct acting valve is de-energized, and as a result, the main, larger valve is at rest. The spool is to the left, and the internal main valve actuator, the spring, is extended.

For simplicity, I chose not to try to show the flow paths in the main valve, or those in the small direct acting valve.

Inside the larger air valve will be a spool or a poppet. The end of that spool or poppet will be as big as the valve body will allow or as large as the design engineer decided to make it.

When we are talking about power, remember; "Force=Pressure x Area" with compressed air force measured in PSI.

Compressed Air Valve

In the drawing just above, item #2, the direct acting solenoid pilot valve, has now been energized.

The pilot air (item #3) which had been flowing to the direct acting solenoid pilot valve is now flowing through it (item #1) and filling the area in front of the bigger valve spool.

This pilot air is pushing on the end of the spool (picture a piston in an air cylinder), and since the air is trapped there, it exerts force on the end of the spool. If that force is great enough, the spool will be shifted over against the internal spring, and the flow paths through the main valve will change.

Item #4 shows that in this drawing the air is now flowing out the other cylinder port to the application, and the other port is now flowing to exhaust.

Miniature Air Cylinder

What we have here is essentially a miniature air cylinder inside the main valve body.

If the end of the poppet or spool inside the larger valve body has a surface area of 1/2 a square inch for example, and the air pressure flowing through the lines is 100 PSI, the compressed air pilot signal will be able to generate 50 lbs. of pushing force on the end of that poppet or spool. This is a huge force available to shift a valve poppet or spool.

Solenoid Pilot Operation

The solenoid air piloted valve will typically operate as follows.
  • the solenoid gets an electrical signal
  • electricity will magnetize the coil inside the solenoid
  • the magnetized coil will cause a pole piece (attached to the internal air control mechanism of the small valve) to shift
  • air will flow through the internal air paths of the small direct acting solenoid valve
  • and compressed air flow into and will be used to shift the bigger valve spool

Very neat, and very effective!

Benefits Of Solenoid Piloted Air Valves

A small amount of compressed air bled off the main supply line of the large power valve can shift massive valves using solenoids with low electrical demand (low wattage) and very small footprint coils.

The larger the spool inside the main valve, the larger the spool or poppet end-surface area can be, and the greater the shifting force in that valve when air flows into the spool / poppet area from the solenoid pilot valve.

The greater the shifting force available from the solenoid pilot, the larger the spool or poppet return spring can be, giving designers the ability to select high-flowing air valves, with a high level of reliability of operation.

Possible Problems

The solenoid pilot operated air valve will usually have a minimum operating pressure. Unless you can plumb an air supply line with high enough pressure to the direct acting solenoid pilot, it may not be able to be used to control low pressure compressed air or for vacuum.

Typically, solenoid air piloted valves have a small port on the direct acting solenoid valve so that higher compressed air pressure can be brought to the valve, to provide the shifting force, while the larger valve is controlling the flow of low pressure compressed air or vacuum.