Controlling sequential VFD pump and control valve system

[drbitboy]

Wanting to set between 20 - 50Hz, its in NZ hence not ~60. But to give some deadband was thinking to start at ~22.5Hz

Its a small plant, doing between 10 - 20 m3/h. If I turned on the pump at 20Hz and with the valve fully open, it'll do ~22m3/h. So thereabouts on the top end of my range, but double on the bottom.

I'm expecting the control valve would need to be around 45% open with the pump at minimum speed to bring it to 10m3/h (~75% BEP).

No sure what the highlighted section means (maybe "double" was supposed to be "doable?").

Anyway, if the pump running at minimum (20Hz) can exceed the maximum expected inflow (> 20m³/h), then there is no reason to control flowrate with the VFD speed reference, as the least wasteful flow control method when pumping would be to continue to use the valve. I.e. the way to minimize energy usage would be to run the pump at the minimum speed (20-22.5Hz), as

• any higher pump speed would require throttling the valve down further, which would use more energy.
• and any lower pump speed would be below the minimum.

 

[MaxK]

Below I will give, it seems to me, a completely feasible and workable way to solve the problem (if I understood it correctly)
In the example, fictitious process parameters are used, because we don't know the real ones
ONE PI controller is used (Upi)
Valve (Uv) working logic
Uv = Upi * 2
if Upi <0 then Uv=0
if Upi>50% then Uv=100%
Pump (Up) working logic
Up = (Upi- 16.7%) * 1.2
if Upi <33% then Up=0
if Upi >50% and Upi <51% then Up =40%
if Upi >33% and Upi <=50% then Up = Up

 

[rahxephon]

Hi all,

Thanks for inputs all, looking back I think may have explained it in a way that led to some confusion. May be clearer using pressures rather than flows.

The system in piped order goes:
Tank - pump - flow control valve - flow meter - clarifier

Simplifying some details:
- Tank supply pressure (from its water level) varies between 0 - 10m slowly (<1m/hr, ~sinusoidal)
- Delivery is to be a constant 5m pressure (from clarifier water operating level)
- Pump and flow control valve work to remove supply pressure fluctuations
- Brief deviations (e.g <2mins) in delivery pressure are fine due to attenuation in clarifier

Gary, that sounds about inline with what I had been thinking initially, and Max, that has given me some ideas as well.

For using a single controller and same notation
output = Upi
valve position = Uv
Uv = 2.5 * Upi
if Upi > 40% then Uv = 100%
pump speed = Up
Up = Upi / 2
if Upi > 35% or Upi <= 40% then Up = 20 (min speed)
if Upi < 20% then Up = 0

Last two are to give bands for startup and shutdown of the pump, exact figures would need to be sorted during commissioning.

 

[drbitboy]

By "Delivery is to be a constant 5m pressure," do you mean the pressure downstream of the control valve?

If yes, then when the upstream tank does have enough head (level) to make the pressure downstream of the control valve be 5m without the pump, then the control valve controls that downstream pressure without the pump running?

And when the upstream tank does not have enough head to make the pressure downstream of the control valve be 5m with the valve wide open, then the pump comes on at 20Hz (40%) and the control valve controls that downstream pressure with the pump running?

So the way to control the 5m pressure is by using the flow meter as a proxy for that pressure i.e. the PI controller's PV is the mean flow rate with a fixed setpoint, and when there is a constant flow through the flow meter and a constant level in the clarifier, the pressure will be 5m?

And the PI controller's SP is the mean flow into the tank, SP = mean_flow i.e. the flow with the sinusoidal variation removed, i.e. instantaneous_flow(time) = mean_flow + flow_variation_amplitude * sin(time/T), where mean_flow, flow_variation_amplitude, and T are all more or less constant