Using a SLC 5/05 PLC. I need to balance water flow between two pipes that both have magmeters to measure the flow and modulating valves to control the flow. My process requires that the flow be balanced to the most open valve. I'm basically trying to fill two claricones that have water entering in and filling up the claricone and the water exiting is by trough. Each pipe goes to a claricone to fill it. I guess my dumb question is how do I do this. I know it sounds simple but many simple ideas have complicated solutions. If any one has some sample logic for me to take a look at, I'd appreciate it.

Thanks Guys

 

Sounds like some variant of 'pacing control' ala Shinskey may be the ticket.

To paraphrase what you might be trying to do: Maximize flow to 2 claricones while ensuring the flow to each is the same. (If this isn't it, please clarify what you're trying to do).

Set up two simple PI flow controllers, one for each claricone using the existing magmeters and valves. Create another PI controller which uses as input, MAX(valve position 1, valve position 2). The setpoint to this controller will be, say, 90 or 95% of maximum. The output of this controller will be the flow setpoint for both of the flow controllers. This is a "valve position controller" which cascades its output to the flow controllers' setpoint.

Some notes:
1) the valve position controller (VPC) should be detuned (slower) relative to the flow loops. (Tune the flow loops first for desired response, then tune the VPC so everthing behaves gracefully).

2) Flow controllers may have 3 modes: manual (operator has control of valve), auto (operator has control of flow setpoint with PI controller active), cascade (PI flow controller active with setpoint originating from output of VPC controller).

3) Only allow the VPC in automatic if both flow loops are in auto with their setpoints cascaded from the VPC. If either flow loop not having its setpoint cascaded, reset the output of the VPC to the MAX of the two flowrates. (you can apply other restrictions... the idea is to have everything behave nicely).

Good luck

 

Hey there Jim. Thanks for the help. You were right about the flow pacing of the claricones.Just want to clarify what you said about the 3rd PI controller to act as the valve position controller. I'm a little bit foggy by what you meant as far a MAX valve position. Can you please clarify? I understand the part about the output of the VPC acting as the setpoint of the the two PI controllers. Its the input part of the VPS thats a bit confusing.

Thanks

 

The output of each flow controller corresponds to its respective valve position, let's call them Out1 and Out2. The VPC then has as its input, Max(Out1, Out2). The setpoint of VPC is typically something like 95%. this is good for maximizing the flow between the two and / or minimizing the pump costs (when a variable speed drive is used). But based on what follows, I doubt this is what is needed.

I'm assuming you're the one that posted, http://www.plctalk.net/qanda/showthread.php?s=&threadid=7585. If so, it strikes me that you're not sure what the customer needs. A word of experience... before solving any problem, make sure you have the problem adequately specified. In this case, there's a master plant rate that needs to be equally apportioned between two claricones. Find out what determines the master rate; it's either an operator or the result of some inventory (level) controller. Get/make a process and instrumentation diagram (P&ID) and sit down with your colleagues and/or customer and determine what needs to be done.

Maybe call the claricone supplier and ask indirectly what typical control strategies are used for their equipment (be mindful that your conversation can make its way back to the customer, especially if the equipment supplier lost the bid to control it). It's always fair to ask for literature and to inquire about any special precautions to observe when controlling their equipment - in the end, they want their equipment to be successfully applied. They may even have a primer or white paper on their site that will clarify this application.

good luck.