Oxygen Flow Control with Diaphragm Valve

[cateng]

We use oxygen injection in our metal melting process. The oxygen flow rate is set by adjusting the air pressure (using a pressure regulator)on a pneumatically controlled diaphragm valve which moves a sliding gate in the oxygen supply line.

If you set the pressure in the air to say 40kPa you get a different oxygen flow rate each time. Is this the nature of diaphragm valves - that they open differently for different pressures?

 

[btrueblood]

With only 6 psi of differential, yes, you may have trouble controlling the oxygen flow due to several factors.

What is the liquid head developed at the injection lance (how deep is the end of the pipe in the liquid metal pool, and how much pressure is then required for the oxygen to flow)? If the depth of the tube varies, your flow and pressure will/must vary too.

How are you controlling the opening of the valve (or do you just modulate between full closed and full open?) Do you know the Cv of the valve vs. opening percentage? If yes, then you need to control or measure the differential pressure across the valve as well as the upstream pressure. You may have better luck with a fixed orifice in the line, a shut-off valve downstream of that, and a pressure regulator feeding the orifice.

 

[ApC2Kp]

cateng,
The diaphragm valve would be OK for shutoff, and for fine throttling if the diaphragm was mechanically moved. You are probably right that the diaphragm opens differently each time, since the oxygen line pressures can be different causing different diaphragm positions for flow (balance of air pressure and line pressure on diaphragm). I also think the friction in the sliding gate is causing hysteresis that contributes to the lack of repeatability in your system.
A pressure regulator supplies air to a diaphragm actuator, not a diaphragm valve, in typical control valve. If the accuracy and repeatibilty needs to be improved, then a positioner that compares the sliding gate valve stem position to the input (air) signal and compensates output to the actuator would accomplish more accurate control. If the gate actuator were a piston actuator, then there is additional source of friction. One brand of positioner is PMV, and control valve vendors have many types of positioners with air or electronic input signal and outputs to pneumatic, hydraulic, or electric actuator.
The sliding gate valve is not the best pattern for throttling. The gate wedge or knife blade usually has some loose fit that allows vibration and turbulent flow from partially open gate. Most small to medium size throttling applications are done with globe pattern valves. Larger sizes greater than NPS 8" to 12" use butterfly pattern valves for throttling. There are some specialized tthrottling applications that use V-pattern ball, plug, or knife gate valves for throttling. Is there a special reason for using a sliding gate valve in your flow control case?