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Old December 21st, 2017, 10:41 PM   #1
huyenthaigia
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Control flowrate filter sand

Hi everyone.
I'm working at water treatment plant. I have some filter sand, every filter sand have a vavle analog use cotrol flow, a transimiter messure level. Help me control flowrate filter sand to level is constant. I think use PID loop,
Thanks so much
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Old December 21st, 2017, 11:18 PM   #2
Peter Nachtwey
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This should be easy but you need to say whether you are controlling the in flow or the out flow.

I would NOT use a full PID. I would start with a proportional gain only. The is NO need to use a derivative gain. The integrator gain is only necessary if the level must be controlled exactly. I can answer better if when I know whether the valve is controlling in flow or out flow.

Usually when I see these question the valve controls the in-flow and there is desire not to stir up the sand with sudden flows of water.
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Old December 21st, 2017, 11:22 PM   #3
huyenthaigia
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Hi Peter Nachtwey,
I control valve for out flow
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Old December 22nd, 2017, 03:44 AM   #4
MR.KIMPHIEN
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help me! i can sofware step 7 v5.3
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Old December 22nd, 2017, 05:07 AM   #5
Peter Nachtwey
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I would use a simple proportional band. There must be a optimal level you want to control to. If you want to keep the level at 1 meter then the valve should be off at 1 meter and fully open at some other level like 1.1 meter. This way the level will stay between 1 and 1.1 meters UNLESS the in-flow is greater than the out-flow of the valve. If the level must be maintained very close to 1 meter then set the integrator time constant do a value like 1 minute but for a more accurate value I would need to know the peak in-flow and the surface area at the set point. However, I don't see when a simple proportional band would not be good enough.
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Old December 22nd, 2017, 10:47 AM   #6
Tom Jenkins
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Peter has identified the feedback loop for controlling your sand filter. I can't see why you would need to control level precisely - sand filters are pretty forgiving.

You are in luck in that most of the time the control changes will be very slow as a result of daily flow fluctuations and gradual plugging of the sand with solids.

You have to make sure your logic includes stopping and restart provisions after backwash cycle.
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Old December 22nd, 2017, 12:54 PM   #7
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I just scale my position command signal from zero to 100% over the permissible level range (i.e. 1 to 1.1m in Peter's example). The rapid gravity sand filters I've worked with have a 1m operating range above the media bed.

On start up or shutdown or after backwash I ramp the valve position more slowly to either fully open or fully closed before switching back to auto mode.
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Old December 22nd, 2017, 02:40 PM   #8
Mike Lamond
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On my last water filtration project, the filter valves had electric motor actuators with Modbus communications. The positioning control on the effluent valve had 1% deadband moving in the same direction as the last move and 3% deadband to move in the opposite direction. I used a three-step control, down/hold/up, which kept the effluent flow close to the setpoint. Cycle time was 40 seconds, slightly longer than the actuator's 100 start per hour rating. Setpoint was calculated to track raw water pump flow, adjusted to maintain settled water level and divided by the number of filters in operation. Plant flow was higher at night when the bigger pumps ran at lower off-peak electric rates.

Backwash flow, with the same style actuator, simply used a series of position presets and time delays to ramp the flow up, hold high flow then ramp down to resettle the sand bed.
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Old December 25th, 2017, 07:55 AM   #9
huyenthaigia
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Quote:
Originally Posted by Mike Lamond View Post
On my last water filtration project, the filter valves had electric motor actuators with Modbus communications. The positioning control on the effluent valve had 1% deadband moving in the same direction as the last move and 3% deadband to move in the opposite direction. I used a three-step control, down/hold/up, which kept the effluent flow close to the setpoint. Cycle time was 40 seconds, slightly longer than the actuator's 100 start per hour rating. Setpoint was calculated to track raw water pump flow, adjusted to maintain settled water level and divided by the number of filters in operation. Plant flow was higher at night when the bigger pumps ran at lower off-peak electric rates.

Backwash flow, with the same style actuator, simply used a series of position presets and time delays to ramp the flow up, hold high flow then ramp down to resettle the sand bed.
Can you post sample code?
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