Dosage pumps

briana banks · Aug 8, 2005

Hi

i have a dosage pump with rate that can be adjusted between 12 and 125 impulses per minute .

i want to operate it externally with 2 timers(on and off)

suppose i want to get 50 impulses per minute.

so (60/50) = 1.2 sec per impulse.
1.2/2 for timer on
1.2/2 for timer off

am i right?

Thanks.

Tark · Aug 8, 2005

What kind of pump are you using?

Most of the dosing pumps I use will accept a pulse input (often used with flow meters) or a 4-20mA input. A 4-20mA input is the easiest way to adjust the speed of the pump. Some pumps have a fixed 4-20mA speed range, others allow you to adjust the speed range.

bernie_carlton · Aug 8, 2005

When I see 'dosage pump' I tend to think 'medical'. If this is so I would not attempt in any way to modify the manufacturer's original controls.

If not for medical then try to get specific information on the type of signal needed. It may be that a sustained 'ON' would be incorrect.

briana banks · Aug 8, 2005

Hi

It is not for medical purposes,but for controling pH.
there are 2 dosage pumps. one inject acid the other one inject base.

Since AO card are pricy,we preffered to use pulses
like water meter.we give the pulses externally.

the rate is adjusted as described above.

R_C · Aug 8, 2005

briana banks said:
Hi

i have a dosage pump with rate that can be adjusted between 12 and 125 impulses per minute .

i want to operate it externally with 2 timers(on and off)

suppose i want to get 50 impulses per minute.

so (60/50) = 1.2 sec per impulse.
1.2/2 for timer on
1.2/2 for timer off

am i right?

Thanks.

If a pulse is the ON state of the pump the calculation above would give you 50 pulses of acid or base per min.

CaseyK · Aug 8, 2005

What PLC are you using.

Some branks have AO that isn't so pricey.

How many I/O do you need in a typical application.

But then, if you are locked into AB.....

regards.....casey

briana banks · Aug 9, 2005

CaseyK said:
What PLC are you using.

Some branks have AO that isn't so pricey.

How many I/O do you need in a typical application.

But then, if you are locked into AB.....

regards.....casey

yeah,I am locked into AB.....

The problem is that pH PID control can be problematic.

briana banks · Aug 9, 2005

In addition,using 4-20 mA control will force me to buy
another 1769 OF8 (8 pumps).pricey.

JohnW · Aug 9, 2005

Dont forget that you dont need a symmetrical pulse, the dosing pump only needs to see the rising edge. Use a TON with a cut-throat NC DN. bit that will pulse on for 1 scan, to start a fixed time period TOF. Set the TOF delay to the minimum pulse required to stroke the pump once. This way you only have to vary the TON timer period to give your strokes per minute.

kamenges · Aug 9, 2005

Your math is correct. Assuming the pump interface will take one pulse and turn it into one injection cycle you will get what you want.

My question is:

Originally posted by briana banks:
i want to operate it externally with 2 timers(on and off)

Do you mean you are sending the pulses to the pump using two freestanding hardware timers external to the plc? If you are, how do you change the timer presets dynamically if you determine you need to change the injection rate?

Are you doing a continuous acid or base feed or do you inject a specific amount of one or the other to bring the pH into your target zone based on a previous pH measurement?

Keith

Tom Jenkins · Aug 9, 2005

I agree that timed control will work better than PID for pH control. You are essentially creating a repeat cycle timer. I've done it both with hardware and PLC timers.

I assume you are going to have variable dosage control. Do your calculations for the required dosage as your example indicates, and MOV the result into the timer presets for both the off and on timers. You need two timers, one for the Pump On cycle and one for the Pump Off timer. The DN bit of each timer will run the other timer.

briana banks · Aug 9, 2005

Hi All

Are you doing a continuous acid or base feed or do you inject a specific amount of one or the other to bring the pH into your target zone based on a previous pH measurement?

I'm doing a continuous acid or base feed.the timers are
NOT external to the plc(sorry about the misunderstanding..).

Use a TON with a cut-throat NC DN. bit

what you mean by "cut-throat NC"?
I use 2 timers one for ON and one for OFF.
i use a fixed time for the ON state and change the OFF delay
according to the pulse rate (pulse per minute)

what is the best algorithm for pH control
how to calculate the "equivalent point"?

Thanks

craigsimon · Aug 9, 2005

I've done this two different ways. We use dosage pumps for the same reason. Since our pH changes slowly the first way we accomplish the task is to energize the dosage pump for 10 seconds then wait 3 minutes and check the pH level again. We do this until the pH is in the correct spot. This works for our 1000 gal washer tanks. The other way is to use a PID loop and take the output and compare it to a continously cycling timer. I will use one timer that always counts to 60 seconds. If my output of the PID is at 75% then I run the pump while the timer ACC is between 0 and (75% * 60 sec)=45. If the PID calls for a 10% output then the dosage pump will only be on between 0 & (.10 * 60sec)=6. This way will maintain the pH more consistantly.

kamenges · Aug 9, 2005

I have never done pH control so this may seem a naive question.

If you have a known actual pH and a known target pH doesn't the difference of pH in a given volume of material demand a specific volume of acid or base to get to the correct pH? Or does material composition have a large enough affect on this equation that a calcualted correction won't work?

Keith

Dosage pumps

briana banks

Member

Tark

Member

bernie_carlton

Lifetime Supporting Member + Moderator

briana banks

Member

R_C

Member

CaseyK

Member

briana banks

Member

briana banks

Member

JohnW

Member

kamenges

Member

Tom Jenkins

Lifetime Supporting Member

briana banks

Member

craigsimon

Member

kamenges

Member

Similar Topics