Installing a Chemical Doser

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 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.

The CD is designed to operate with the LFOM with a maximum head loss of 20 cm. 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.

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

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.

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.

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 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. 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.

Next the notches for chemical dosing increments should be cut along the lever arm. Each notch represents an increase in alum dosing of a constant number of milligrams of alum per liter of plant flow rate, for example each notch in Tamara is for an increase of 2 mg/L. The distance between notches is a function of the CD design, and is a calculated parameter. 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 (stand and lever arm) that line up to form the pivot. Make sure the lever arm is being drilled exactly in the center of the pipe. A screw or pin gets inserted through the holes to act as the pivot. Move the lever arm up and down to make sure there is little friction between the two pipes.

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.

Frame assembly specifics for Tamara

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.

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.

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 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.

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

The ½" pressure break tee is also used to attach the dosing tube system to the lever arm. A dosing ring (loop that can be made from any type of rigid wire) is threaded through small holes drilled through the open top of the tee. The ring should be designed so that it fits into the lever arm notches well without slipping out because it will be the only thing used to set the dosing tube position.

Dosing tube specifics for Tamara

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.

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, based on the weight of the dosing tube system. 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 then ties 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. Now 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 height range (20 cm down at the tip).

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.

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.