Flow meter selection help

[vipereyes]

I need to install a flow meter in our wastewater discharge. It is a small system and has an intermittent discharge rate of just under 10 gpm. The discharge pipe is 2” PVC.

The wastewater system treats rinse water generated in our finishing department and uses chemical flocculation to remove heavy metals in the water before discharging to the sewer. There is a very small quantity of solids (flock) in the effluent – we are allowed up to 500 ppm, but we always stay well under that, usually under 25 ppm. The few solids that do discharge usually end up sticking to the sides of the PVC piping. The piping is cleaned out monthly.

I am having a hard time finding a flow meter designed for low flow and I am not sure which type would work best for this type of effluent. I have called a few places and no one has really made me feel comfortable as to why they are suggesting the type of technology (paddle wheel, ultrasound, etc) they want me to buy. It’s a lot of money so I want to make sure I pick one that will work for us.

Does anyone have any suggestions?

Thank much,

Kim

 

[cvg]

how about a v-notch weir with an ultrasonic level sensor?

 

[bimr]

The answer to your questions depends on the application. Is it a gravity flow or a pumped discharge application? Out of doors or indoors?

If it is a pumped discharge application, your best option is a mag meter. The mag meter should be installed so that the pipe does not drain when the pump is shut off. Endress + Hauser makes a good one.

A paddle wheel is not recommended as it will foul with waste solids.

 

[vipereyes]

Sorry, I should have included that information.

It is a gravity flow application. A diaphram pump, timed to 10 gpm, pumps the water into a large 10,000 gallon open top clarifier but from there it gravity flows out to the sewer through the 2" PVC line.

I have looked at a few ultrasonic types but so far none that I have seen will work at our low flow rate.

 

[btrueblood]

So, you know you have 10 gpm into the tank...why do you need to measure the effluent rate?

 

[vipereyes]

btrueblood,

We have to sample our discharge monthly and send it to an outside lab to show compliance to set polluntant limits. We start an auto sampler once a month that collects a small amount of our discharge every 15 minutes for 24 hrs. The county says that using the timed proportional method is no longer acceptable (per EPA regulations) and wants us to start collecting samples using a flow proportional method. So, we need something to measure the water discharged and signal the autosampler to collect X amount of sample every X amount of water discharged.

Kim

 

[bimr]

Install a mag meter with a drain-type configuration so that the discharge pipe is always full of fluid. Refer to Endress + Hauser's installation guidelines. A mag meter will provide you 0.5% accuracy.

 

[btrueblood]

I'd agree on the mag meter. There are dozens of suppliers, google the term.

 

[vipereyes]

Thank you, I will check them out.

 

[semo]

If you use a mag meter, the pipe has to be flowing full. It will not measure a partially full pipe.

If you have or install a basin/manhole to measure the flow at, you could install an ultrasonic level with a weir or flume or an A/V (area/velocity) meter in the pipe. Companies such as Sigma make these meters and can incorporate samplers into it as well.

 

[vipereyes]

Our horizontal run would be a full pipe but not the vertical. I have checked Endress + Hauser, Signet and Omega and all state the effective measuring range for all of their models is from about 14 gallons per minute and up. Omega did have mini models but they went from 1 gallon per minute up to a maximum of 8 so they wouldn't work either.

I am moving on to investigate ultrasonics now.

Thanks.

 

[bimr]

Page 18 of the Endress flow instrumentation catalog shows a recommended flow rate of 10-300 gpm for a 2" diameter mag meter. The mag meter can measure less than this flow rate, but the accuracy tends to drops off. In any case, the accuracy of the mag meter at lower flow rates will still be better than a weir or other device. The best predicted accuracy for a weir is around 2%.

The ultrasonic meters will have more or less the same issues ar the mag meters, full pipe required, etc. The ultrasonics are typically less expensive and less accurate than a mag meter.

 

[hydrae]

I would use a 60° trapezoidal flume, available from Plasti-fab and other flume manufacturers. Be sure to follow all the installation instructions for accurate metering

Now you just have to measure the height of the water at the critical point. You will need to measure the water level in a 0- to 6" range.
option 1 air bubblier, reliable, low maintenance but the noisy compared to other methods
Option 2 ultra sonic level indicator, still on the noisy side in this low of a range. also low maintenance
Option 3 pressure transducer, cleaner signal than a air bubblier, the stilling well (order as part of the flume) will need to be cleaned out regularly. The pressure transducer will need to a high accuracy model. and in the low ranges you will see some drift and noise
Option 4 Vibrating wire weir monitor, with a 6" float cleanest signal that I have seen. But, again the stilling well will require maintenance and the float will need to be cleaned of the floc. I assume the monthly cleaning should be fine by hosing down the float, well and flume as the pipe is getting cleaned

Hydrae

 

[cvg]

The following has a wealth of information which may be useful in designing and installing your meter.

USBR WATER MEASUREMENT MANUAL

http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/

 

[vipereyes]

I was using the wrong calculation when looking for a mag meter. I found a few that I should be able to use with my current plumbing design.

Manholes, flumes, wiers were all suggested and would work, but I need something small as space is very limited and we also want something we can install inside, not outside the building - we didn't want to be digging up the parking lot to install flow equipment (no manholes).

Thanks everyone for your help