Sutro Weir Approximation Experiment

Introduction:

The Linear Orifice Flow Meter was tested and compared to the results expected from a traditional sutro weir. The steps of the experiment and the results are available below. The experiment showed a very low deviation from the predicted. It is important to note that the design for calculating the orifice pattern used the old design and the first two rows are not designed optimally.

Initial Design

Figure 1: Rough Schematic of existing structure

Figure 2: Design including the linearized flow tank

Figure 3: A close up of the linearized flow tank

Attached is an excel spread sheet checking the various riser pipe diameters for the entrance tank.

Since the existing PVC pipe that brings the water in is 3" I will use the 3" pipe to construct the riser pipe. Checking with the program the 3" will be more than big enough to accommodate the holes. The holes necessary for the riser pipe were found using the Mathcad program developed by David Railsback for linear flow. Talking with Tom at the water treatment plant I decided to use a 5 gallon bucket. It would be relatively easy to outfit the bucket with a toilet flange on the bottom of the bucket that the riser pipe can slide into.

Figure 4: A close up view of the riser pipe with the necessary holes. As a note the diameter of the holes is 3/8 inches. The tube on the left is designed with a max flow rate of 120 L/min and the pipe on the right has a max flow rate of 125 L/min

Figure 5: Photograph of the riser pipe

Budget

Total: ~$18.00

Design changes

#I decided to cut the holes on the pipe for a maximum flow of 125 L/min (the actual max is 120 L/min) so that there would be some extra capacity.
#An extra piece of tubing was used underneath the bucket to add extra length and mitigate the splashing and spilling that originally occurred.
The Bucket was completely installed on March 13th.

Figure 6: close up of completed bucket

Figure 7: Completed bucket installed in pilot plant

Experimental data

Intro

The entrance tank with riser pipe was installed in the plant and measurements were taken on accuracy of the design. There was an in-line flow meter that provided flow rate data that can be compared to the flow rate recorded based on height in the entrance tank. There will be readings at incremental flow rates where the machine flow rate and water height will be recorded.

Experimental Procedure

1) Check that ALL holes are clean and free of obstructions
2) Record the flow rate from the flow meter in GPM
3) Record the height of the water in the bucket using a ruler (inches), be sure to place the end of the ruler on the edge to ensure contact with the actual bucket base. the pictures below show the techniques for recording water height.

4) For next reading change the flow rate manually and then wait approximately 5 minutes for the flow to acclimate.
5) Record the stabilized flow rate and the water height.

• Record the water height for flows between 5 and 36 GPM in approximately 1 GPM increments
• The flow meter has an accuracy of +/- 0.25% of the flow rate. The flow meter is made of two parts and produced by Siemens.
  
  Figure 1: The sensor with display is pictured above and the model is SITRANS F M MAG 3100, specifications for the sensor are available at the Seimens website.
  
  Figure 2: The microprocessor-based transmitter that displays the data is pictured above and the model is SITRANS F M MAG 6000, specifications are available at the siemens website.

Recorded Data

• The data collected from three trails is available as an excel document.
• There is a correction factor for the water heights because the first hole isn't at the bottom of the bucket. There is an offset of 3.5 inches. Also the lengthy decimals result from using English units with eights and sixteenths inches.

Results

The Riser Pipe is a Good Approximation of a Linear Weir

Experimental data

Figure 1: The graph shows the height versus flow rate data for the three experimental trials and the predicted data. The original graph is available here

Figure 2: The graph shows the flow rate versus height data for the three experimental trials and the predicted data.The original graph is availabl here

• The data from the three trials are similar. This shows consistency in the experimental set-up.
• There is some discrepancy between the predicted values and the experimental values.

Figure 3:The graph below show the difference between the predicted height and the recorded height as a function of flow rate.The original graph is available here

Figure 4: The graph below show the percent deviation between the predicted height and the recorded height as a function of water height. The equation used to create the data is .The original graph is available here

Possible Causes of Discrepancy

1. There is a significant error at the low flow rates. It is possible that this error was due to the faulty algorithm used to design the LFOM. Note that the first code did not design the first 2 rows of the LFOM well.
2. Accuracy of the flow meter installed in-line at the pilot plant. The readings may be inaccurate.

• Research into the flow meter specifications showed guaranteed accuracy up to 0.25% of the flow rate.
• Data taken manually to determine actual flow compared to the flow meter is not significantly from the flow meter to suggest problems with the flow meter.

  Flow Meter Reading, GPM	Actual Flow Rate, GPM	% Deviation
  4.6	4.69	1.97
  6.1	6.25	2.51
  7.4	7.53	1.85
  11.1	11.62	4.71

• Flow meter accuracy not the problem.

3. Problems with the orifice equation approximating the head loss on a vertical orifice, the equation was designed for a horizontal orifice. May need to integrate to find the head on the holes as a function of height, would lead to changes in construction of the riser pipe.
4. The percent deviation graph show a constant percent deviation as the flow rate increases of about 10% after an initial spike due to the effect of small errors on small value readings. The errors seen at the upper limits aren't extreme, they are consistent with the other deviations.

Conclusions

The design for a flow measurement device was tested. A riser pipe with a pattern of holes was used to restrict the flow so the height of water in the entrance tank of a water treatment plant directly correlates to the flow through the riser pipe. Three trials were undertaken to determine if the system of holes reliably and accurately approximates a sutro weir. As long as a system can be devised to prevent the clogging of holes during operation the riser pipe is an effective tool. The riser pipe entrance tank system provides data on flow with an accuracy of approximately ten percent.