Measuring flow rates of Two control valves using a single flow meter

[ddeshi2]

Hi all, I'm new to PLC programming and this is my first project. In our project we feed water, Methane and air to the combustion chamber. For feeding air we are using two control valves (Low air and Large air) and measuring flowrate using a single flow meter.Low air valve is opened during initial stages of the process and Large Air is opened after a certain period of time(as required by the process).The specifications of control valves are:
Low Air: Min-2.5 SCFM, Max: 10 SCFM
Large Air: Min-10 SCFM, Max: 100 SCFM
So , if I want a total flow rate of 90 SCFM I'll open the Low Air first till I get 10 SCFM and then open the Large Air to get a total of 90 SCFM..How can I set up PID Loops for this?? should I use two PID loops one for Low Air, one for Large Air? And if I want a total flow rate of 90 SCFM ..Should I input this value through HMI as a set point to the Low Air Valve PID Loop (but the control valve Max is 10 SCFM only)??or should I use 10 SCFM as the setpoint and then open the Large Air valve and ramp up the setpoint to 90 SCFM??? please help..

Thank you,

Ding

 

[Mickey]

Typically this is done with a split range arrangement. One PID loop,the first valve ( low air) is operated between 0 and 50% ( 0 closed 50% full open). The second valve ( large air) operates between 50 and 100% ( 50% full closed 100% full open ).

Another method is called "inching control or trim. I prefer this method. Again only one PID loop.

The first valve (low air) is controlled by a PID. When it's output gets to 80 to 85% you start the second valve ( large Air) and slowly ramp it open. This will cause the first valve’s PID output to go down. At about 55 to 60% stop ramping the second valve and leave it open at whatever position its at. If the first valve’s PID increases again to 80 to 85% ramp open the second valve again. Continue this cycle as needed. If the PID reduces back down to 20 to 25 % then ramp closed the second valve ,stop ramping down when the PID increases to 55 to 60%. Continue this cycle as needed. You decide at what % to shut off the second valve.

 

[ddeshi2]

Thanks Mickey for your reply. I understand what you said in your post but don't know how to implement this using ladder logic. Can you please provide me an example ladder logic program where this was implemented..

Thank you,

Ding.

 

[Mickey]

What PLC are you using complete catalog number if you have it ?

 

[ddeshi2]

I'm using Allen Bradley MicroLogix 1500 PLC. I understand conceptually what your saying, and it actually is straightforward. But how to get one PID to output control to 2 different valves...I have an analog 4-20 output to control each valve.

 

[Mickey]

Which option were you considering?

The split range or the "inching control.

If the split range then you can run the 4-20 to both in series if the load is not to great. Or get a splitter for the 4-20ma.


If the inching control then...
Only one valve is controlled by the PID. The other is controlled by a different output which gets is value from logic that is ramped-up or down using the PID's output value.

If I have a little extra time this afternoon I will try to get you an example.

tell us a little more about your ML1500, what cards you have and what slots they are in.
If it already has a program in it then post that here ( it's.RSS file zip it first)

 

[ddeshi2]

I'm using 1762-IF4,1762-OF4 Analog I/O Modules.Analog Input is located in the 4th slot and Analog output is located in the 5th slot.I don't have the program I'm trying to write one. Air is fed from the compressor to the chamber. The pipeline connecting the compressor to the chamber has a flowmeter and two control valves (the pipe splits into two and two control valves are installed on the two pipes to the chamber).I have to control the flow rate of air. The Low Air is opened first and air is fed to the chamber (the flow rate should be around 2-3 scfm). Then after achieving that flow rate we ignite the flame in combustion chamber . once the flame is detected we open Large Air valve and ramp up the flow rate to say 80 SCFM (if I want a total flow rate of 80 SCFM) keeping the Low Air valve opened. How this is implemented using PID loops? What should be my setpoints?

Thanks,
Ding

 

[Mickey]

The 1762 series modules are for a ML1200 or ML1100. Is that what you have? (Which one)

 

[ddeshi2]

I'm sorry Mickey.. It's not 1762 series ..It's 1769 I/O modules (4-20 ma).

Thank you,
Ding

 

[Mickey]

Ok thanks.

Something to consider, burner management is a whole industry in itself.
You may want to check regulatory/insurance issues before trying to roll your own.

Burner management companies have controls to take care of all this. ( i.e Honeywell)

 

[ddeshi2]

Yes Mickey you are right. Other persons in my company are taking care of that burner issues. My task is to develop the logic as explained in the above posts.How to implement a split-range PID.. please give an example.

Thank you,

Ding

 

[Mickey]

Split range only needs one PID loop. Then take the output to both values. Run the 4-20ma in series if load permits. Or Split the signal with a 4-20ma split splitter.
Does each value have it's own I/P? If so then calibrate one 4-12ma to stroke the valve 0-100% ( low Air). And calibrate the other 12-20ma to stroke the valve 0-100%.

If valves have postioners then the calibration can be done there.

See link below for some info on split range control

Also sample program for PID. Controller action reverse/direct and tuning will have to be adjusted to suit your process.

http://www.google.com/#hl=en&sugexp...=1&bav=on.2,or.r_gc.r_pw.&fp=39e5bf4e97622fe9

 

[TurpoUrpo]

Does the fuel come in constant amount. Is there need of controlling output of the burner power?

Use one PID for smaller valve at startup to get needed flow keep larger valve PID at manual and 0% at this point. When you get ignition and flame is detected set smaller valve PID to manual and 100% (dont know how flame handles this, so might need ramp) and enable larger valve PID.

 

[ddeshi2]

Thank Mickey! Now I'm getting an idea of how split-range control is implemented.. I'm trying to write my own program and will post it once I finished it. Also I was unable to open the TwoControl_split.zip file. Can you please post .pdf version of the program.

Thank you,

Ding

 

[ddeshi2]

No..The burner power is not controlled. And are you talking about the CV% when you said " keep larger valve PID at manual and 0% at this point".

Thank you,
Ding