Current Regulated Dosing Pump

[RBergmann]

When using a dosing pump for polymer injection on a water project, the controller (i.e Hach SC100) sends a 4-20mA signal to control the stroke rate of the dosing pump base on water turbidity. This is a linear control, no special programming necessary except for setting the range (in NTU) for the 4-20mA signal. There is no requirement for a PLC or other device to assist in the stroke control. Straight forward linear control.

However, I am now tasked with controlling a dosing pump for chlorine feed based on a residual chlorine PPM. As I see it, the pump rate should be inversely proportional to the controller (Hach CL-17) output.

I am interested on the methods used by others for this type of control. There is an SLC 5/05 PLC in the system and I am passing the analog input signal (from the CL-17) through the PLC directly with separate AO channels for the dosing pump and a chart recorder.

... rab

 

[demmons]

It's been a while since I've messed with chlorine and polymer, but it seems to me we flowpaced the chlorine and used the CL17 feedback to trim the dosing.

 

[Gene Bond]

Most decent VFD's can invert the analog signal, so 20ma = 0% speed and 4ma = 1--% speed. Or, is the pump not vfd controlled?

There are also analog signal inverters, used for HVAC routinely, like Advanced Control Solutions, Action Instruments, and Kehle (Spelling may be wrong).

 

[RDS]

I believe the SCP instruction allows you to invert the output. It can then be used in a PID if needed. Hope this helps.

 

[RBergmann]

No VFD for this pump control. Does anyone know if I can get away with inverting the output to the chlorine pump using the RSLogix 500 SCP function like this:

Input: I:x.x
Input Minimum: 3277
Input Maximum: 16384
Output Minimum: 31208
Output Maximum: 6242
Output: O:x.x

It has taken me too long to write this and I think RDS has answered this question.

 

[Jeff23spl]

yes you can reverse it this way but it seem too easy to me....
Is your CL 17 do all the logic to be directly connected but just the logic is reversed ? usually at least 1 of these device should be programmed to work on both way ?

On the systeme i made i had to include water flow in the equation because in low flow it will need less stuff than at higher flow for the same concentration ? Or sometimes it isn't not a flow but a water level in a tank etc....

 

[slimdawg]

I agree with Jeff23spl the cl-17 works well if you are reading out of a tank you also have account for circulation time for the tank your reading if this is the case maybe a PID would work better base on flow

 

[shooter]

make it from direct to inversed end of problem