Pneumatic control valve

Hello experts,

There is a control loop with PID controller and pneumatic actuator that controls the position of the valve. Pneumatic actuator has positioner and is controlled with 4 to 20 mA.

There is one thing I don't understand and need your help and it is not that much related to controls or PLC.
On the nameplate of the pneumatic actuator is written max 6 bar, and in the installation of instrumentation air there is about 5.5 bar. However in the datasheet of the pneumatic actuator, there is so called "control pressure range" and "control force".
I don't understand what this actually means. For example, I have specific actuator with 60 mm full stroke (marked in the attached picture), and it's control range pressure is: from 0.2 up to 1.6 bar.

I don't understand why there is a range in the column "from" and there is a range in the column "to". From: 0.2-1.0 bar and To: 0.8-1.6.
I have Sipart PS 2 positioner, and I'm not sure if I need to order filter pressure reducing valve.

What is confusing to me is the fact that actuator is suitable for max 6 bar, but it's control range is only about 1.5 bar. I'm not sure how to interpret this.

As far as I know, most instrumentation air installations are 6 bar, and SIPART supports up to 7 bar, but pressure range on pneumatic actuator is only 1.5 bar.

I hope there are experts here who can clarify this to me.

Thank you!

I suspect this is your situation: There are two parts to your valve actuator. There is a cylinder that is able to exert a large force to overcome valve friction, hydrodynamic forces, etc. while moving the main valve disc or globe. That large force uses normal shop air at around 5.5 bar (80 psi) to move the cylinder's piston.

The air to the cylinder is controlled by a pneumatic positioner. It uses 0.2 to 1.4 (3-15 psi) and feedback from the cylinder to provide analog positioning. The cylinder moves until the position feedback matches the pneumatic signal, which in turn is regulated by the 4-20 mA I/P signal conditioner.

If you are supplying 6 bar (90 psi) air to the positioner you will need a pressure regulator to drop the pressure.

Hello Tom and thank you for the reply. Yes, there is a pneumatic actuator that needs to develop a large force to move the valve. Actuator is air to close, spring to open. Therefore air is supplied to the actuator only on one side. This pressure will develop force to overcome spring force, and all friction and other forces including the resistant force of the fluid (water). What confuses me is the fact that 0.2 bar and 1.6 bar are mentioned as control range of the actuator. This data is not in positioner's manual. I think of 0.2 bar as pressure needed to overcome spring force in order to start moving the acruator. In that way 1.6 bar is enough for actuator to extend full stroke. I think it is only to overcome spring. The pressure in application must be greater than that because there are many other opposing forces.

I believe your functional description is correct.

I suspect that the pressures stated on the actuator spec/label are for bench operation, the pressures needed to create the force needed to fully close against the spring pressure.

The PS2 positioner measures position, not pressure, so it allows its internal valve at the Y1 port to stay open as long as necessary to allow supply air to continually flow, chargin the actuator's volume above the diaphragm with more and more air, which creates a higher and higher pressure on the diaphragm until the valve stem moves to a position that matches the 4-20mA demand signal position, at which point the Y1 output valve closes.

The positioner needs a higher supply pressure than the nominal working pressure in order to overcome flow forces and stiction in the valve.

No need to diminish the supply pressure; the position will only use air as needed to create the pressure needed to position the valve stem.

danw said:

I believe your functional description is correct.


No need to diminish the supply pressure; the position will only use air as needed to create the pressure needed to position the valve stem.

Click to expand...

Thank you. This is the information I was looking for. It hasn't been planned to buy additional pressure reduce valve for the air supply.

I have found on spirax sarco web page additional explanation. These control pressure ranges are actually adjustable values needed to overcome the spring tension. In operation, there is an additional force that needs to be overcome too.