Understanding Differential Pressure Type Valves.

Ever installed a new 2 port valve and it did not operate when you wanted it to? Spent ages trying to figure out why? The problem might have been the valve was a differential pressure type. How clear was the catalogue or instructions? If your unfamiliar with the term differential pressure, chances are not very. My article will describe the operation principle to help make this easier to understand.

Common uses for pneumatic directional control valves would include applications such as industrial robots. They would be specified for use with compressed air. Other industrial applications might use steam, water, coolant, oil or another fluid as the motive force, or require these fluids to be transported around a system (for example, coolant moved around a laser application). The directional control valves I have mentioned above would not be suited to these types of fluids, but another type of valve would be.

These are commonly known as Process Valves. In most cases these are either two or three port, two position valves (commonly referred to as 2/2 or 3/2. 2 port 2 position or 3 port 2 position. The two positions being closed or open). I will describe two common types.

Poppet type

In this example is a normally closed, pilot operated, poppet type process valve. Some inlet fluid pressure is siphoned via an internal passage (pilot) into the armature and acts upon the upper surface of the poppet. This pressure, combined with spring force, keeps the poppet down on the orifice, keeping the valve closed.

When the solenoid is energized, the core will attract the armature. In zero differential valves, this would be enough to open the valve, but the type shown in my diagram is a differential type (I will elaborate on pressure differential later on in this article). Fluid pressure is acting on top of the poppet, so the magnetic flux from the solenoid assembly alone is not enough to overcome the combination of fluid force and the spring force to lift the armature.

There needs to be enough pressure acting on the bottom surface of this poppet to enable the armature to be lifted, opening the valve. There has to be some back pressure from the outlet port acting on the underside of the poppet for this valve to open.

Diaphragm type

The basic operating principle of the diaphragm type process valve is the same as the poppet type, but the construction is different.

Some of the inlet fluid pressure flows through an internal orifice (sometimes just a hole in the diaphragm) and fills the chamber on top of the diaphragm. This pressure, along with the spring force, this keeps the diaphragm closed. The spring in the armature keeps the pilot closed.

When the solenoid is energized, the armature is pulled toward the core and opens the pilot valve. Supply fluid pressure then flows through this into the chamber below the diaphragm.

This has the effect of reducing the pressure in the top chamber. The pressure now in the bottom chamber pushes the diaphragm up, overcoming the spring force and opening the valve and allowing fluid to flow through to the outlet port.

Differential pressure

This topic can be a minefield for an engineer or technician not familiar with the terminology, and has been the cause of much confusion and lost time troubleshooting when your shiny new replacement valve does not operate when you install it (if his has happened to you, don’t worry. Believe me you are not the first and you will not be the last!).

Simply put, this is the difference in pressure between the primary and secondary ports, or pressure at the inlet port minus the pressure at the outlet port. For example, if 0.5 MPa is present at the supply port, and 0.2 MPa is at the outlet port, differential pressure is 0.3 MPa.

This will fall into two categories which is important for engineers and technicians to know. Maximum operating differential pressure and minimum operating differential pressure. Some valves will be subject to one or both of these categories. The product catalogue should detail these to enable the viewer to select the correct product for their application.

Lets look at Maximum operating differential pressure first. This is the maximum pressure differential which is allowable for valve operation (either opening or closing). If we take my poppet type valve I mentioned earlier as an example, the poppet is held down by inlet pressure as well as spring force, so some pressure must be present at the outlet to enable the solenoid to lift the armature. Lets assume you have a valve with a specified maximum operating differential of 0.3 MPa. If your supply fluid pressure is 0.5 MPa, then there needs to be at least 0.2 MPa present at the outlet port for the valve to open when the solenoid is energized.

Please view the schematic below for an example of where it is important to understand max operating differential pressure. This shows a nozzle being controlled by a process valve. Lets use a max operating differential of 0.3 MPa again as an example, with a supply pressure of 0.5 MPa. As there is no pressure (close to 0 MPa) on the outlet side of the valve, the valve will not open when the solenoid is energized, as the differential seen across the poppet is 0.5 MPa, which is greater than the max allowable by the valves specifications. In this case, using a zero differential process valve should be considered.

The minimum operating differential pressure is the minimum allowed to keep the valve open (rather than to just open it).

For example, consider a valve with a specified minimum operating differential of 0.05 MPa. If the pressure entering the supply port is 0.3 MPa and the outlet is 0.25 MPa, the valve will stay open as long as the solenoid is energized. If the supply drops to 0.27 and outlet stays at 0.25, the valve will close, even if the solenoid is still energized.

Both scenarios can seem counter intuitive, why would you want back pressure on a valve in order for it to operate or stay open? Below is a basic example of where a designer might consider this.

A tank of fluid is supplying a nozzle. The designer wants to ensure the tank does not empty completely, even if the supply to the tank stops for any reason. As the fluid empties, the pressure at the supply port of the valve will reduce. Eventually, this pressure will start to equalize with the pressure at the outlet port. As this happens, the differential between supply and outlet will fall below 0.05 MPa. When this happens, the valve will close, ensuring the tank does not empty completely.

Back pressure

Another consideration for using these types of valves is the effect back pressure from the outlet port can have on the valves operating mechanism. If back pressure is too high, this can lift the poppet or diaphragm, causing backflow though the valve. If there is potential for this to happen in an application other types of valves should be considered instead (again, if this has happened to you, you are not the first and will not be the last).