help on valves

[shreeram]

Dear all,
I am pursuing a project on PLC. i would be using seimens s7 and i would be only simulating the ladder program but i want a help related to valves. My project is chemical dosing in pre water treatment plant.. So the pH meters and turbidty meters take the respective values and the ladder logic brings the pH to 7 while turbidity <2 ntu... So i wanted to know which valves will give me the desired result of openin the valves according to the pH or turbidity will open in steps so that lime or alum (for pH) and polyelectrolyte(for turbidty ) is added .. And how do i program these valves

 

[Jeff23spl]

In here we like to use dosing pump for liquid stuff, it works with a small piston or membrane resistant from corrosive that give a stroke each turn and the motor speed is controlled with a 4-20mA output...

A quick search bring me here:
http://www.directindustry.fr/prod/prominent/pompes-doseuses-a-membrane-6048-490080.html

http://www.techniquesfluides.fr/jesco.html

Usually we read and track the water flow and give a proportional output according to it. Double flow = double chemical needed. Usually the water source pre-level remain the same so once adjusted, the proportional ratio works good.
But i like to add some kind of pid control if we have the reading. To keep it simple, I use the pid output as a limited offset added to the other proportional signal tuned on the water flow

 

[shreeram]

thankss for that reference but i would have to stick to valves... any valve that opens in steps

 

[Tom Jenkins]

You don't program the valve, you need to program a controller.

I'm not aware of any valves that have a built in controller that will allow tuned response to a process variable.

You can use a single loop controller like these:
http://www.omega.com/search/esearch.asp?start=0&perPage=10&summary=yes&sort=rank&search=PID%2BControllers&submit=Search&ori=PID+Controllers

or a PLC. Normally the valve operator requires an analog positioner, but you can accomplish the result by using a motor operator and programming your controller to move it in steps.

 

[Jeff23spl]

You don't program the valve, you need to program a controller.

I'm not aware of any valves that have a built in controller that will allow tuned response to a process variable.

You can use a single loop controller like these:
http://www.omega.com/search/esearch.asp?start=0&perPage=10&summary=yes&sort=rank&search=PID%2BControllers&submit=Search&ori=PID+Controllers

or a PLC. Normally the valve operator requires an analog positioner, but you can accomplish the result by using a motor operator and programming your controller to move it in steps.

i think he said he will use a Siemens PLC for the programming...
To my knowledge, having just the feedback will need to get a ph or turbidity change to get action to come back to normal with a PID...Instead, by reading flow, you can anticipate the need?
But it will remain the same to control a valve than controling a dosing pump. You need a % output value to setup the valve opening...
either by pulsing a motor until the position feedback is reach or having a modulating valve with a 4-20mA control board built in...

 

[Tom Jenkins]

i think he said he will use a Siemens PLC for the programming...

You're right, but he also said "And how do i program these valves "

I may not be the only one confused!

 

[Jeff23spl]

Yeah Tom it's not clear but maybe he was meaning: programming the valves inside the plc ??? I agree that valves with built in pid controler would not be cheap...If this exist...probably not...

 

[shreeram]

ya i was referring to program the valves via plc... and jeff whatver u said is true i had asked another guide about this and he also said abt % output value thankss a lot..

 

[Jeff23spl]

you can use a opening/closing valve and pulse the command to get the position needed by the chimic programmed logic. You would need an analogic (4-20mA) position feedback to compare with your setpoint. IF position is lower, you pulse to open and if over, you pulse to close...
because it is a very slow process, you can have small pulse with a good off time.
It will be up to you to determine how you will calculate the valve posion % setpoint but you got my previous suggestion