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Getting a new chemical doser (CD) ready for operation requires three steps: sizing, assembly, and calibration. The information under the general headings is applicable to chemical dosers for any AguaClara plant. There are subheadings for Tamara specifics that describe dimensions for the CD that are unique to the turbidity range and flow rate of the Tamara plant. Further information on the theoretical modeling behind the Tamara design can be found here and general information about the AguaClara chemical doser can be found here.

Sizing

Sizing refers to both the CD design in MathCAD and the proper scaling for the plant. The MathCAD sheet that can be used to design a CD is attached to the main chemical doser Linear Chemical Doser wiki page. The main constraint used in designing a CD is for it to deliver enough chemical (mg chemical per L plant flow rate) for the maximum plant requirement while making sure that the flow controller is not going to exceed the turbulent transition flow rate. The length of the lever arm can also constrain design depending on available space in the plant, therefore lever arm length is a required design input for the MathCAD design sheet. The lever arm is equal length on both sides of the pivot, and lever arm length is given in cm on either side of the pivot point (i.e. it is the effective length of the arm for dosing and calculations). To find the length of pipe required for the entire lever arm, multiply the lever arm length by 2.

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Another scaling issue that needs to be addressed before installation is the vertical relationships between the FC and the chemical stock tanks. The FC fill line cannot be located above the level of the stock tank exit, or else the FC will not maintain a constant head.

Sizing specifics for Tamara

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The CD fits conveniently on the wall of the plant entrance tank in Tamara. The base of the CD will be attached to the wall via a saddle. The geometric constraints of the plant should not interfere with the chemical doser functioning properly.

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In Tamara the CD lever arm can be no longer than 37 cm (on the outer side of the pivot) because of the layout of the plant. AguaClara Engineers in Honduras asked that the CD stick out no farther than the rapid mix pipe so as to not interfere with the walkway that plant operators use between the entrance and exit tanks. The CD for Tamara has been designed to require a lever arm with 35 cm of length out from the pivot. The full range of chemical dosing is accomplished in a 35 cm span by using 3.25 m of 3/16" inner diameter tubing for FC outflow as discussed in the dosing tube assembly section below.

There is 13 cm of vertical space between the stock tank exit and the plant entrance tank wall. This means that the FC must be positioned in that vertical window. The steps for doser frame assembly below take these constraints into account.

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The first step in assembling the CD is to build the doser frame. This involves cutting the 1 ½" PVC pipe for the stand and the 1" PVC pipe for the lever arm.

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The 1 ½" pipe should be cut with a circle saw followed by cuts down to the circle in order to create a saddle shape. The bottom of the saddle must be at least 3.9 cm below the pivot in order to allow a full 20 cm range of motion in the lever arm. The cut must be wide enough to allow the lever arm to move without rubbing on the stand. The 1" PVC pipe must be at least twice as long as the lever arm length parameter, because that value is only equal to the effective length of chemical dosing away from the pivot and the lever arm extends equal distance in both directions away from the pivot.

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Frame assembly specifics for Tamara

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Each lever arm notch represents an increase in alum dosing of 2 mg/L. For Tamara a maximum of 50 mg/L alum dosing (mg chemical per L plant flow rate) was designed for, so the 35 cm lever arm can be divided up by 25 equally spaced notches for 2 mg/L, 4, 6...50 mg/L where 0 mg/L is the pivot and 50 mg/L is the far tip of the lever arm. Because the farthest dosing notch must be located at the 35 cm mark, the lever arm should be made an extra centimeter longer (i.e. 36 cm on both sides) so that there is room to cut the farthest notch. Placing 25 equally spaced notches means that there must be exactly 1.4 cm between the center of each of the notches.

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To assemble the CD stand for Tamara it is important to keep the FC water level below the stock tank outflow. Cut the 1 ½" PVC pipe for the stand so that it will be 13 cm tall when attached to the saddle and flange. The wall saddle will be constructed locally, so simply subtract the thickness of the saddle and flange from 13 cm and then make the stand that tall.

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A flow controller (FC) is already in place in AguaClara plants, and that existing FC can be clamped onto the CD. Use two hose clamps with a strip of metal connecting them so that the FC is firmly in place. The exact height of the FC on the CD is very important with respect to two parameters. The alum level in the FC cannot be above the height of the stock tank outflow, but this constraint should be met automatically by keeping the height of the CD frame as tall as the outflow while ensuring that the FC is not sticking up above the pivot. The second parameter is that there should be no alum flow when the lever arm is exactly horizontal. This means that the FC must be placed at a height where the pressure break tee is even with the alum level in the FC. Temporarily attach the FC and continue to assemble the tubing before moving onto the section on calibration for more detail on FC height.

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Make sure for Tamara that 3.25 m long 3/16" inner diameter tubing is used to connect the FC to the pressure break tee. The rest of the general tubing specifications above are correct for the system but the FC outflow tubing is a plant-specific parameter.

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Float assembly

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Next fill the plant entrance tank to its height for maximum flow rate, halfway up the top row of holes in the linear flow orifice meter. The lever arm should be at the top end of its height range (20 cm down at the tip).

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Double check that the dosing tube system stays in place by moving the dosing ring up and down the lever arm. Make sure the ring does not slip out of any notch as this would cause improper dosing. If it does not appear secure then cut the notches deeper. Take an alum flow rate measurement at different locations along the lever arm. Check to make sure the CD is delivering the correct mass of alum per plant flow rate at respective notches. It is important to check several notches because the CD could correctly dose one notch while not performing correctly at other notches.insert equation

Calibration specifics for Tamara

Check to make sure the CD is delivering the correct multiple of 2 mg alum per liter plant flow at the respective notch. Expected alum flow rates at each notch are listed in the table below.

If the CD is not dosing as much alum as expected, the FC outflow tubing may need to be shortened. Cut off small pieces of the tubing and re-check flow rate. Conversely, a high flow rate would mean that longer tubing is needed. Also make sure to double-check that the tubing is in fact 3/16" inner diameter. The table above is designed for 3/16"ID tubing that is 3.25 m long.