Nonlinear Chemical Doser Calibration Procedures
Figure 1: Lever Arm Legend
Purpose: We need to make sure that our lever arm is able to adjust the alum dosage automatically for the entire flow range of the plant flow. Since alum dosage is ultimately set by the float moving up or down the entrance tank as plant flow changes, we need to make sure the lever arm can respond to minimum and maximum water height in the entrance tank, which corresponds to the minimum and maximum plant flow rate. Head loss in the plant determines the difference between minimum and maximum entrance tank water height. Greater the head loss in the tank, greater would be the difference between minimum and maximum water height given any plant flow rate. Even though we would design the lever arm based on analytically calculated head loss, there is bound to be a minor difference between calculated minimum and maximum water flow and the actual heights. Calibration will also minimize the sources of error mentioned above. These are the calibration steps:
Step 1. Confirm and/or adjust the total head loss in the plant: Either through inspection of the plant or dialogue with the experts on the ground, we need to confirm the minimum and maximum water height possible in the plant. If there is too much difference between minimum and maximum water height and the lever arm would not be long enough to respond to the change, we must decrease the total head loss in the plant. If needed, we can make adjustments to the rapid mixer to increase or decrease the head loss in the plant. Mark the location of the maximum water height.
Step 2. Install the nonlinear chemical doser: After checking or adjusting the water height range in the entrance tank, set up the non-linear doser.
Step 3. Find out what dosage is most commonly used. Calibrate the float weight on the dosage that is most commonly used. Use the chart below:
Table 1: Float Calibration Mass
Dosage(mg/L) |
Mass(kg) |
|---|---|
20 |
2.783 |
30 |
2.786 |
40 |
2.791 |
50 |
2.797 |
60 |
2.805 |
70 |
2.814 |
80 |
2.824 |
90 |
2.836 |
100 |
2.849 |
Step 4. Check the doser at maximum alum flow rate: push the slider to the maximum alum dosage and measure the alum flow to ensure that it is giving out approximately the correct flow rate of alum and that float is still submerged with a tight string connection to the lever.
Results and Discussions
We require a float with a diameter of 15.2 cm and 30.5 centimeter height for our lever arm prototype. The float must have a resealable cap that would allow us to vary the weight. Calibration is crucial in minimizing the error that is associated with the float. We should balance, or calibrate, the lever arm at the dosage that is most commonly used to reduce the overall alum dosage error in the plant.
Upon construction of the lever arm, we will utilize a makeshift float in which we can adjust the weight of the float easily to verify our calculations here. Clear PVC tube is currently the best candidate. We will run the doser at different dosages and simulated water heights to confirm our above calculations. Further work will have to be done analyzing the errors in dosing mentioned above. The extent to which these errors will cause an improper amount of alum to be dosed is not fully known yet. The actual errors will be minimized before we order the final float for the dosing system for the EPA competition.
Bibliography
- CEE4540 Flow Control Measurement Notes at https://confluence.cornell.edu/display/cee4540/Syllabus
Deliverables
- Final Float Design Parameters and Calculations