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Old November 22nd, 2013, 07:17 AM   #1
dtlegrone
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Logix 5000 Totalizer Function Block

In a distillation column application a control valve opens to remove distillate product and closes to return reflux. Cycle_Period timer is set for 100s and Distillate_Period timer is set for 10s. Both timers begin and the valve opens. When Distillate_Period timer is done the valve closes until Cycle_Period timer is done, then the process repeats. I want to use a Logix 5000 totalizer function block to read a flow instrument and determine the volume of distillate removed from the column. The flow meter range is 50-500 ml/min. Are these the correct inputs to the totalizer: ProgStartReq to begin totalization when the valve is opened, ProgStopReq to end totalization when the valve is closed, and ProgResetReq to reset the totalizer at the start of Cycle_Period?
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Old November 22nd, 2013, 03:51 PM   #2
Oakley
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Have you looked at the help file for the Totalizer?

One issue is going to be your scaling. Your minimum is 50. If your valve was closed, it should be 0.

Additionally, you want to be sure that the flow rate never goes negative.
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Old November 22nd, 2013, 04:34 PM   #3
robertmee
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I wouldn't use the rate of a flow meter for totalizing. Yes, you can integrate the rate over time to get a total via the totalizer, but in my experience it's never as accurate as a true totalizer output from the flow meter. The flow meter should have the ability to provide an output that's equivalent to X amount of material flowing by. Then you just watch for rise of the input to add X amount to a totalizer. That's assuming your flow meter is connected old school via I/O. If it's on a network, it should have a totalizer register that you can just read, and reset via a command from the PLC.

Last edited by robertmee; November 22nd, 2013 at 04:36 PM.
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Old November 22nd, 2013, 05:02 PM   #4
Paully's5.0
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I would beg to differ, totalizing analog flow rates can be pretty accurate. I just finished a large-scale batching operation all based on analog flow totalization. Usually within .5lbs of the flow meter total on a 1000lb addition. Granted, depending on specifications this may or may not be acceptable. Didn't spend a dime on HSC cards, additional wiring or network-based flowmeters.

Unless you require specific dosing and subsequent accuracy I think totalizing flowrate will work for the majority. Especially if it's just to get additional feedback. In this case, seems they just need an idea of how much distillate they are removing. No need to go out and get high-speed counter cards and wire up the pulse output of the meter. Use what you got.
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Old November 23rd, 2013, 08:44 AM   #5
robertmee
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Quote:
Originally Posted by Paully's5.0 View Post
I would beg to differ, totalizing analog flow rates can be pretty accurate. I just finished a large-scale batching operation all based on analog flow totalization. Usually within .5lbs of the flow meter total on a 1000lb addition. Granted, depending on specifications this may or may not be acceptable. Didn't spend a dime on HSC cards, additional wiring or network-based flowmeters.

Unless you require specific dosing and subsequent accuracy I think totalizing flowrate will work for the majority. Especially if it's just to get additional feedback. In this case, seems they just need an idea of how much distillate they are removing. No need to go out and get high-speed counter cards and wire up the pulse output of the meter. Use what you got.
You're using 1000 lbs in a batching system, which I do all the time as well, although I prefer measuring tank weight vs flow meters. He's measuring ml at a slow rate. I would betcha a totalizer would be much more accurate than integrating a flow rate in this instance. If my math is correct he's collecting distillate for 10 seconds. At a rate of 60 ml/min, that's only 10 ml. Not alot of time to establish a good flow rate which has to average over the ramp up/down of material...Even with a cutoff valve you have the acceleration of gravity and saturation of the tube (your ramp) to contend with. I stand by my assessment that flow rate won't be very accurate in this application.

Last edited by robertmee; November 23rd, 2013 at 08:47 AM.
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Old November 24th, 2013, 08:02 AM   #6
dtlegrone
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Thanks for your suggestions. This application is a miniature distillation column to be used for research in a university setting. Because the budget is fixed, only the Logix 5000 solutions are available. I am looking at the help file for the totalizer function block and the descriptions of the inputs are so brief I wonder if I am understanding them correctly.

The flow rate is small, the valve opening is brief compared to the total cycle period, so the opportunity for error is great. I plan to extend the totalization period beyond the valve closing timer by 2-4s to capture any dregs from a condenser placed before the flow meter, and use the LowInCutoff instruction to cease totalization when the flow drops below 75 ml/min.

Last edited by dtlegrone; November 24th, 2013 at 08:08 AM.
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