Orifice Size and the Dual Scale Design for the Nonlineary Alum Doser

Abstract:

During the fall semester of 2009, the 2009 Nonlinear Chemical Dosing Team developed the dual scaled orifice-based doser in order to deliver turbulent alum flow. This doser, once set for a specific dosage by the operator, must automatically increase or decrease the alum solution to maintain that target dosage as the plant flow changes. In addition, the two different scale provides the operator with additional precision through a low dosage (5-25 mg/L) and a high (20-100 mg/L) alum dosage range.

We have currently researched and identified the nonlinear relationship between plant flow rate and alum dosage and the movement of the lever arm. We utilized this relationship to develop the lever arm design to include the dual scale and the orifices. Attached is the Mathcad File.that contains the calculations for our dosing system.

Our current design consists of a 80cm long lever arm with equal lengths and two orifices of 3.175mm and 0.01587mm diameer, 9.525 mm pvc tubing, and other associated hydraulic components listed in our component list.

Summary of Design Process:

In order to meet our design objectives mentioned above, we must link plant flow to alum flow coming out of our doser.
We utilized Mathcad's vector calculation ability to help us in our design.

Our first and key step in developing a dosage system that can automatically deliver both turbulent and laminar flow of alum was the selection of the orifice to control the flow of alum. We increased the tubing size connecting the constant head tank to the orifice to 9.525mm which is wide and smooth enough to make the head loss from the tubing negligible compared to the head loss through each orifice, making the orifice the flow control component for the dosage system. The orifice equation, shown below, demostrates the nonlinear relationship between flowrate and the change in head loss.

Where
Q = Flow Rate
h = Headloss
A_or = Area of the Orifice
Kvc = Orifice Constant

The plant itself is controlled by the orifice. Head loss occuring in the plant after the entrance tank including rapid mix, the flocculation tank, and the launders are all controlled by orifices.

Table 1: Head Loss Through the Plant

The only source of head loss not controlled by an orifice is the weir. Because the majority of the head loss is dominated by the orifice we can link the flow rate of the plant with the flow rate of alum required for the plant using the same square root relationship mentioned above. In other words the rise and fall of the water height in the entrance tank caused by the change in flow rate, is nonlinearly proportional to the alum flow of our orifice based doser. Consequently, the lever arm must be long enough to rise and fall with the minimum and maximum water height in the entrance tank. This range is equal to the total headloss in the plant, which is 33.5 cm in this case. Therefore, we selected an equally balanced lever arm of .8 m that fits in our 1 m x 1 m entrance tank as well as respond to the 33.5 cm water height difference as shown below:

Our next step consists of setting up two sets of target alum concentrations: 5-25 mg/L and 20-100 mg/L. Given
a known maximum plant flow rate(Qp) and Alum Stock concentration(Cc), we utilized the mass balance equation to
determine alum flow rate required for each target alum concentration as shown below:

Because the orifice controls the flow of this alum solution, we again use the orifice equation. This time we use it to solve for the headloss necessary to achieve these different flow rates. These headlosses, or the difference in height from the orifice to the water height in the constant head tank, are calculated as shown below:

We then convert these headlosses to points along our scale via simple geometry as shown below:

The scale that is generated is nonlinear as shown below:

The Mathcad program's arrary function was utilized to tie these variables together in an orderly fashion. Each of the two alum concentration ranges have nine different alum dosage settings which can be captured via 1 x 9 array. The alum flow rate required to meet these target concentration is generated as another 1 x 9 array. We just continue this process until we calculate an array of points along a scale as shown below.

Consequently,the the operator of this program will change the diameter of the orifice until the dual scale utilizes maximum space on the lever arm (40 cm).

Error Analysis

Results and Discussions

Currently, our orifices are 3.175 mm for alum dosage of 20 to 100 mg/L offset by 10 mg/L and 1.587 mm for alum dosage of 5 to 25 mg/L offset by 2.5 mg/L. Our lever arm is 80 cm in length with equal lengths on each side. Tubing is made up of PVC and of 9.525 mm diameter which is wide and smooth enough to produce negligible head loss on the alum flow.
For ease of operation, whenever this lever arm is used in the field, this lever arm can be delivered to the Aguaclara Plant with the dual scales already engraved on the arm. The operator simply has to calibrate the maximum dosage to the maximum flow rate and the lever arm will be ready for operation.

Our near goal is to build the [lever arm prototype] and set up our hydraulic components and engrave our dual scale.

Bibliography

• CEE4540 Flow Control Measurement Notes at https://confluence.cornell.edu/display/cee4540/Syllabus