Oxygen Flow Control with Diaphragm Valve
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?Ball Valves
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?Check Valves) Do you know the Cv of the valve vs. opening
percentage?Butterfly Valves 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.
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 Gate Valves(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. Globe ValvesMost 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.
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