Calculating Flow per Second

[Tark]

What I'm not getting is the whole scan time part of the calculation how is it adjusting the sample rate?

The PLC Scan Time is an absolute value, it’s accurate. I think the programmer is trying to create the most accurate flow total that they can. (Although this might not be the most accurate way to do it)

Keep in mind that if you used a timer and set the timer to do the calculation every 25mS, the calculation will not be performed exactly every 25mS. This is due to the time the PLC needs to execute other parts of the program. In reality the calculation is performed every 25+mS, how much is the ‘plus’ ? Don’t know. The point is the ‘plus’ is an unknown variable, which will lead to an inaccurate calculation.

So your programmer decided to use the scan time to determine how much time had passed since the last calculation was performed. The scan time is an accurate measurement of time, thus removing the inaccuracy in the calculation. (In theory at least)

 

[Webhead]

Let me see if I can walk through this:

You are getting a value of 100 GPM from the flow meter.

100GPM/60 = 1.666667 GPS - This is still a rate of flow.

1.66667GPS * ScanTime(In seconds) = Gallons Flowed in ScanTime

Lets say the scan time was 6mS. You have to divide that by 1000 to convert it from milliseconds to seconds.

So now we have 1.66667GPS * .006 seconds = 0.0100002 gallons that flowed during the last scan.

So we take that 0.0100002 and add it to the previous total.

Now the REASON to use scan time is that if you use a timer in the program you are going to introduce an error due to the scan time (it is not always the same every scan). Scan time is typically tracked by most PLCs and available as a system word, so it is a convienient way to get a reliable time to use in the equation. However, as others have pointed out here, it is a VERY small number and may have its own problems due to that.

Now this makes sense. Thank you Jed. Sorry I was being so blockheaded, I just couldn't get my mind wrapped around it. Maybe I should have had a couple cold ones before I looked at this program.

Thanks to all that have posted and given their input. It's actually a pretty slick way to get flow accumulations now that I understand it.

 

[Tom Jenkins]

The problem is theses flow accumulations that this part of the program is used for have been getting increasingly higher. They are used on the Influent and Effluent flows of a waste water plant and for some reason they are losing about 800K gallons on site somewhere. It's a waste water plant what goes in has to come out somewhere.

Actually, some of the water does disappear. WAS (Waste Activated Sludge) includes water. Particularly if it's hauled as liquid this portion of the influent flow won't show up in the effluent flow. This is usually a small but not negligible flow rate.

Most of the time, though, the influent and effluent flow meters simply don't agree. I don't recall ever seeing a plant where they were exactly matching, and often they are not even close. This is partly due to calibration errors. However, it is usually the result of installation problems with the flow meters.

A magmeter requires a minimum length of straight pipe upstream and downstream for accurate and repeatable readings. Similarly, an open channel flow meter requires the proper upstream and downstream conditions. Parshall flumes with ultrasonic flow meters are particularly sensitive to this.

For some reason design engineers rarely provide the proper channel or piping configuration required for accurate flow measurement.

 

[Webhead]

Yeah tell me about it. Influent is an insertion meter scaled 0-7300 GPM and the Effluent is a Mag Meter scaled 0-10000 GPM. We are leaning more towards the meter being the issue.

As far as calibration the meters are calibrated every 6 months. But the influent was replaced a few years ago. I'm just going to have to go out there and physically inspect the hardware and see whats up. I know it's not the software..... never is...... LOL

 

[bwhite]

I'm a little late to this discussion. Webhead and I work together.

What I've found in the field on the effluent side is not optimal. Tom, I agree with you 100% The influent mag meter is a Seimens 10" meter located 10 pipe diameters above grade and flow is upward. Not quite 5 pipe diameters on the exit side of the magmeter. I took a strap-on Eesiflow 6000 doppler mag meter to check the Seimens and it was within 100 gpm.

My concern is with the effluent meter. 24" March McBirney insertion magmeter scaled 0 - 10,000 gpm. 4 pumps in a wet well with discharge pipe sizes being 8" each, connecting to a 12" header and then upping to a 24" pipe. There's my concern. I'm not 100% certain that 24" horizontal pipe is full. Okay, here's the real kicker. Pump curves say they're rated at 1650 gpm at 110' each. Wet well is only about 30' deep max. With one pump running, the Marsh McBirney magmeter is displaying between 1600 and 1700 gpm. I put the strap on meter on it in several different place and was reading approximately 2700 gpm. Guess what, that's close to the influent meter reading, but then again, I'm not certain that 24" pipe is full so I can't really trust the strap-on.

Like Webhead said, the only way I know to accurately measure the flow going out is to find out the capacity of the chlorine contact chamber and do a timed draw down test. I know not as much is going out as is coming in via the fact what WAS takes out some water with it but this difference it too great.

Suggestions are most certainly welcomed.
Thanks for the link Russ.

 

[bwhite]

Actually, some of the water does disappear. WAS (Waste Activated Sludge) includes water. Particularly if it's hauled as liquid this portion of the influent flow won't show up in the effluent flow.

In this application, the WAS is run through a belt press.

For some reason design engineers rarely provide the proper channel or piping configuration required for accurate flow measurement.

Ain't that the truth.