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Module 26:
Advanced Flowmeters
Answer Key
Exercise/Activity
- What is the normal flow for midday, which is about half way between each date stamp?
Ans: About 1600 to 1200 gpm.
- There were 2 rain events when the intensity went above 0.06 inch. What was the affect to the flows?
Ans: Very little impact. Flowmetering indicates that I/I is not much of a problem.
Exercise/Activity
- What is the normal flow for midday (look at the first two days)?
Ans: About 200 to 500 gpm.
- What happens during rain events?
Ans: For the time period between the first Tuesday and Wednesday, the two events caused the peak flow to go up almost twice the normal range. Additionally, the flow took a long time to decrease which illustrates that the water continued to get into the system. A second rainfall event caused the flow to quadruple. In our previous discussion about how one municipality may use EDUs to report possible flows, the above data showing impact from a rainstorm would be missed. I/I is a very bad problem in this system and must be corrected.
Calculation
- If you have an 8 inch diameter pipe and the velocity is 2.5 fps, what is the flow rate? (Hint: A = πr²)
Ans: 8 inch diameter pipe equals a radius of 0.33 feet.
Therefore, area = π(0.33 ft)^2. This calculates to 0.34 sq. ft. If the velocity is 2.5 fps and the area is 0.34 sq. ft, the resultant flow is 0.85 cubic feet per second (cfs). 0.85 cfs X 7.48 gals/cu. ft X 60 seconds/minute = a flow of about 384 gpm. Instructor Note: This illustrates how you can determine flow through a line without directly having a flowmeter.
Class Expansion of Problem (use if there is time available)
If the pipe is the force main from a pumping station and is 1,000 feet long, how long would it take 1 gallon of wastewater to travel through the force main if the flowrate is 384 gpm?
Ans: Since the flow rate has been determined based on an 8 inch diameter pipe and a velocity of 2.5 fps, divide the 1,000 feet by 2.5 fps and it would take 400 seconds. This could be confirmed by adding a dye directly to the pump suction, through a pressure gauge fitting, and timing it until it appears at the downstream manhole. This might help you decide if an odor control chemical has sufficient time to react with the wastewater as it travels through the pipe at the calculated flow rate.
- Given a flow rate of 0.87 cfs conveyed in a 12 inch diameter sewer line, what would be the velocity? (Hint: A = πr²). If this is a sewer line, would settling of solids be a concern?
Ans: Remember to keep units the same, therefore, convert 12 inches to feet. 12 inches/12 inches = 1 ft The radius is equal to half the diameter. 1 ft/2 = 0.5 ft Determine the area. Area = (3.14) (0.5 ft)^2 = 0.78 sq ft Now determine the velocity. V = Q/A = 0.87 cfs/ 0.78 sq ft = 1.11 fps
Typically, a sewer line should not have a velocity of less than approximately 2 fps. Therefore, solids may settle out unless flows increase. Settling of solids would be a concern.
This illustrates how knowing a flow rate can help you determine if other operational problems could exist.
Exercise/Activity
Compare the maximum flow capacity of a 10 inch Kennison versus a 10 inch Parabolic Nozzle. Look at Appendix C - ISCO Tables 3-8 and 3-9.
Ans A Kennison nozzle has a maximum capacity of 0.80 mgd and a parabolic nozzle has a 1. mgd capacity.
Exercise/Activity
A well-operated WWTP has an effluent flow that ranges from 10 gpm to 1,500 gpm. We do not anticipate an unacceptable amount of solids because the weir will be installed as a WWTP effluent flowmeter. What are the possible weir options? Use Appendix C - ISCO Table 5-3A.
Ans 1. If you look at the table you will notice that only V-notch weirs are within this range. (This illustrates how concentrating the flow through a narrow opening, such as a V- notch, provides proper hydraulic conditions.)
- Within the V-notch ranges, at the lower flow rate of 10 gpm, there are only three possibilities of either a 22 ½, 30, or 45 degree notch weir.
- At the upper range of 1,500 gpm, the only one which fails to comply with a maximum of 1,500 gpm is the 22 ½ degree V-notch.
- Therefore, using the V-notch with the widest opening, to reduce potential problems when algae builds up in the notch, our choice would be the 45 degree.
Exercise/Activity
If we have a 2 ft rectangular weir without end contractions and the depth is 0.33 feet, what is the flow? Use Appendix C - ISCO Table 11-3. Using Table 5-3, what is the flow range for this PHCE?
Ans 566.6 gpm, 0.8159 mgd. Approximately 400 gpm for a lower limit and approximately 4,000 gpm for an upper range.
Exercise/Activity
If we have a 2 ft Cipolletti weir and the depth of flow is 0.33 feet, what is the flow? Use Appendix C - ISCO Table 12-3.
Ans Based on ISCO Table 12-3, the flow is 573 gpm.
Exercise/Activity
- For a 12 inch Parshall Flume, where is the measuring point in relationship to the start of the throat? The throat is the size of the flume and is where the sides are parallel. Use Appendix C - ISCO Table 4.1A and the Parshall flume diagrams on the previous pages.
Ans From ISCO Table 4.1A, column 2/3A, the answer is 3 feet.
Exercise/Activity
- If we have a 6 inch flume and the depth of flow is measured to be 0.42 feet at the head, what is the flow rate in gallons per minute (gpm)? In million gallons per day (mgd)? Use Appendix C - ISCO Table 13-4.
Ans ISCO Table 13-4 shows the flows are 235 gpm or 0.3380 mgd.
Exercise/Activity
Select a flume that could be used for flows ranging from about 1 gpm up to 900 gpm. The type of material in the flow is normal fecal matter and paper most commonly present in domestic wastewater. Use Appendix C - ISCO Table 5-4.
Ans Look at various flow rates shown on ISCO Table 5-4. There are several H-type flumes and an extra large 60 degree trapezoidal flume that will work. Because we are not sure of the amount of solids that could clog the flume or settle out, we may want to consider the trapezoidal flume.
Pumps Nos. 1 and 2 do not run at the same time, therefore, there are no flow components for simultaneous operation of the pumps. The total volume conveyed is 243,000 gallons plus 252,000 gallons for a total of 495, gallons for the day.
The flowmeter reading shows 49,500 gallons per day but the calculated flow is 495,000 gallons per day. The flowmeter is 1/10 of the calculated flow. It appears that when the flowmeter was calibrated, a factor of 10 was inadvertently missed.
Exercise/Activity
- A 9 inch Parshall Flume on the raw wastewater line was checked by WWTP staff and the depths in inches are shown below, along with the depth converted to feet. Should an instrumentation person be contacted to inspect the flume? Refer to Appendix C - ISCO Table 13-5.
Measured Depth (inches)
Calculated Depth (feet) Reading (gpm)
1.2 0.10 10 3 0.25 165 4.5 0.38 310 6 0.50 477 7.5 0.62 663
Ans Although the meter shows a noticeable deviation from the table at the lowest flow, the other readings match the table. Since 4 of the 5 readings match the table, it is probable that the first reading was not done properly. Note: ISCO Table 13-5 starts at 0.10 feet, so the first reading is at the lowest end of the table. This is not desirable and could have caused some of the meter inaccuracy.
- A 2 foot Cippoletti Weir is used at the effluent end of a WWTP. What is your opinion of the meter readings? Use Appendix C - ISCO Table 12–3.
Measured Depth (inches)
Calculated Depth (feet) Reading (gpm)
2.40 0.200 270 2.52 0.210 291 2.76 0.230 333 3.00 0.250 378 3.60 0.300 497 4.32 0.360 653 5.00 0.417 795
Ans The depth was measured very precisely based upon three significant figures. This allows for the correct calculation of three significant figures and the meter readings are acceptable.
- There is a two foot wide board that an operator installed trying to fabricate a rectangular weir without end contractions. The operator measured directly above the weir with the following results. Was it measured properly and what are the flows? If measured properly, Appendix C - ISCO Table 11-3 can be used.
Measured Depth (inches)
Calculated Depth (feet) Reading (gpm)
0.5 0. 1.0 0. 1.5 0. 2.0 0.
Ans The readings are not valid because the flows were measured directly above the weir which is incorrect. Additionally, the board does not produce a sharp crested weir. Otherwise, you could use ISCO Table 11-3 if a sharp-crested weir was installed.
- There is a sharp-crested 2 foot long rectangular weir without end contractions at a WWTP. Having taken this course, the operator measured at the proper location upstream of the weir. What are the flows? Use Appendix C - ISCO Table 11-3.
Measured Depth (inches)
Calculated Depth (feet) Reading (gpm)
2.40 0.200 267 2.52 0.210 288 2.76 0.230 330 3.00 0.250 374 3.60 0.300 491 4.32 0.360 646 4.92 0.410 785
Ans The readings are valid because the flows were determined at the proper location. You could use ISCO Table 11-3. Correct readings are inserted in the table above.
- Determine the amount of error an incorrect depth reading of ½ inch creates in a 120 degree V-notch weir if the perceived depth was 7 inches versus 7.5 inches. Determine its impact for a typical day at that rate. Use Appendix C, table 9-6.
