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Installing a Chem Doser

Getting a new chemical doser ready for operation requires three steps: sizing, assembly, and calibration.

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 wiki page. The main constraint used in designing a CD is the for it to deliver enough chemical (mg chemical per L plant flow rate) for the maximum requirement while making sure that the flow controller is not going to exceed the turbulent transition flow rate. The CD is designed to operate with the LFOM with a maximum head loss of 20 cm, and a lever arm length of 35 cm as requested in Tamara. The constraints are input to the MathCAD sheet to find a combination of FC outflow tubing length and inner diameter that matches the plant chemical dosing needs.

In Tamara the CD lever arm can be no longer than 37 cm 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.

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.

Assembly

Not all parts for the CD can be purchased in Honduras. Fittings and UV-resistant tubing for use with chemicals need to be brought in. Please check the parts list page to determine what is needed for assembly.

Doser frame

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. 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.

Next the notches for chemical dosing increments should be cut along the lever arm. Each 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. The notches need to be cut deeply enough that the dosing tube ring can stay in the notch at the full range of motion without slipping out.

A hole should also be drilled straight through the opposite end of the lever arm to provide somewhere to tie the float to.

To attach the lever arm to the stand a hole gets drilled through both pieces that lines up to form the pivot. A screw or smooth pin gets inserted through the holes to act as the pivot.

The final aspect of the frame to construct is the base. The 1 ½" PVC pipe stand fits into a flange, which then should be bolted to a wall saddle. The entrance tank wall may not be strong enough to support drilling so the team in Honduras can use local materials to create a saddle that holds the CD firmly in place on the wall, without drilling into it.

Dosing tube system

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 13 cm and 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. See the section on calibration for more detail.

The length and inner diameter of the FC outflow tubing are calculated parameters based on the CD MathCAD sheet. 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. This is designed to give the plant the correct dosing response for the 2 mg/L dosing increments.

The ½" outer diameter tubing from the pressure break tee is designed to be long enough to allow some alum to always remain in the tube, while not overflowing when the highest dosage and flow rate are used. It is 25 cm long to hold 2 cm of alum in it at a plant flow rate of zero, while containing 22 cm of alum when the lever arm moves down by 20 cm at full plant flow.

The ¼" outer diameter tubing leaving the ½" tube needs to plug into the rapid mix 2 cm above the bottom of the ½" tube at the zero plant flow rate. This is what keeps 2 cm of alum in the ½" tube at all times. The length of the ¼" tubing depends on the distance between the rapid mix and the CD. Make sure the tubing is long enough to reach at the full lever arm range of motion (20 cm).

A series of connectors are required to switch between ¼" and ½" tubing and fittings. A ½" pressure break tee is used, with two reducer stems (1/2" to 3/8" and 3/8" to ¼"). At the bottom of the ½" tube a ½" to 3/8" reducing union and a 3/8" to ¼" reducer stem are used to connect tubing.

Float assembly

The main factors governing float assembly are diameter of the float and its center of mass. The minimum outer diameter for a float being used with this chemical dosing tube system is 6.3 inches. This is designed to give no greater than 5 mm of variation in float height as the dosing tube system is moved toward and away from the pivot across the entire range of plant flow rates. The height of the float is not designed for quantitatively, but the following conditions must be met:

  • When filled with sand the float's center of mass is below the water line
    • This ensures float stability
  • The float is not so short that it is light enough to be pushed around the entrance tank by turbulence
    The float is made by gluing caps on the end of a PVC pipe that has been partially filled with sand, or some other type of clean dry fill. A hook is then screwed into the center of the top of the float to serve as an anchor. Rope or some other type of tough string can then tie the float to the end of the lever arm.

Calibration

Once the CD is built it is important to zero the lever arm properly. Do this by draining the entrance tank until it is barely at zero plant flow. This is when the water level in the tank is halfway up the bottom row of holes in the linear flow orifice meter. When the tank is at zero plant flow, just barely lower than the lowest flow rate, adjust the length of the lever arm-to-float rope so that the lever arm is exactly horizontal. After it is leveled, adjust the height of the FC as needed to make sure that there is just barely no alum flowing. Alum should begin to flow with one extra millimeter of driving head from this zero. Tighten the hose clamps to ensure that the FC will remain at its calibrated height.

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 range (20 cm). Double check that the dosing tube system stays in place by moving the dosing ring up and down the lever arm. Make sure it does not slip out of the notches as this would cause improper dosing. Take an alum flow rate measurement at different locations along the lever arm. Check to make sure the CD is delivering the correct multiple of 2 mg alum per L plant flow at the respective notch. Expected alum flow rates at each notch are listed in the table below.

Notch from pivot 1 2 3 4 5 6 7 8 9 10
Expected alum flow rate (mL/min) at full plant flow (2 mg/L * 700L/min)/120 g/L = 11.67 mL/min 23 35 47 58 70 82 93 105 117
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
128 140 152 163 175 187 198 210 222 233 245 256 268 280 292

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.

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