WTP Filter Outlet Valves PID control

HI.

We are looking at controlling a filter outlet valve based using a PID block in the PLC. the outlet valve will open & close (modulate) to maintain the correct level in the filter. the PID controller linked to a transducer or ultrasonic level transmitter.

Has anyone configured such a system? how can l come to have a fully functional control system?

Design it, program it, build it, install it, test it, tweak it.

It sounds like you're in the initial stages. Are you seeking a contractor?

You don't need PID control. This is simple on/off stuff. When the level is too low close the valve. When the level is too high open the valve. Simple. 2 rungs of code.

Make no mistake, I've been scouring these posts for 4 months now to relearn PLC's and learn 5000, so I am keenly aware that pretty much everyone here is waaaayyy up on the ladder of brains and skill relative to me. Especially Nachtway. I say that with all respect.

Having said that, I used to service 1 to 3 MGD Water Plants for years and for gravity filters fed by a higher reservoir or via raw pumps, I have worked on rate of flow/ water level? maintaining control loops in those applications.

One plant in particular averaged say 1.5 mgd, and the raw water from a reservoir came in through a raw valve, from there to a flash mixer, floculators, then through 3 filters in parallel, before going to the clear well.

So lets call it 1 mgd average 24/7 for this instance. At any given time each filter might be flowing 694/ 3 = say 230 gpm.

This is a rough average as flow changed per demand, with time allowed for backwashing each filter as needed, per loss of head and/ or time in service, depending on raw water quality.... but I digress.

Point is there were rate of flow DP transmitters that plumbed off of orifice plates just in front of the throttling valves on the effluent side below the filters. (It has been almost 10 years since I did this sort of work and I am trying to recall the exact system.)

So for that plant and I must say, others, throttling valves were used to control rate of flow and the level above the filters was rather constant while maintaining substantial flow through the rate of flow control valves at all times.

I will probably roll this around in my head this weekend and recall more. (I was thinking of designing a plant program like this for practice this month from raw valves to filters to high service pumps to tank sites etc...) I am too early in the programming curve to be much help. But I have seen it and, except when backwashing, the valves were always throttling in loop control around the clock.

Good Luck and Regards

theafricana didn't mention anything about maintaining a flow. I also doesn't look like he can throttle valves. It is simply and on-off affair. It should be simple. It is possible to do PWM type of control if one knows the exact level. Then it should be possible to open the valve 90% of the time if the water level is 90% of the way to the upper limit from the lower limit.

Filter effluent valve control is VERY touchy. Opening too fast will cause a spike in turbidity (bad!). It is important that the controller (PID loop) be updating frequently and operating within a small deadband. This causes quite a bit of valve activity during plant flow-rate changes, but it levels-off nicely once the rate is stable.

I use a simple on/off controller for this


very simple to do in ladder, has built in deadband and only need two rungs? Is this what you are after or do you have full analog control of the output, eg a 4-20ma loop with 0-100% ?

if you need surge control (no surging but actual level is not important) I would use an inlet and outlet flow meter and have a PID controller attempting to balance the two flows.

MasterBlaster said:

Filter effluent valve control is VERY touchy. Opening too fast will cause a spike in turbidity (bad!).

Click to expand...

Are the valves slow opening and closing? There are slow opening and closing valves in hydraulics to avoid pressure spikes.

An alternative is to have 2 valves of different sizes. Open the small one first.

Constant level control is one fairly common filter control technique. PID would probably work satisfactorily because the filter restriction as the sand begins to plug is fairly slow, and flow rate changes are also fairly slow for most WTPs. For a fully functional system you also need to coordinate alarms for level and accommodating backwash. I sgest you contact the filter manufacturer for some guidance.

I do agre with Peter that a simple deadband controller with small incremental opening and closing adjustments would provide good process performance and be more stable and simpler for the operators to tune.