# Water Treatment: The Right Dose

November 1, 2006

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Chemical water treatment programs require accurate dosing. But how can you determine the correct dose if you don’t know the volume of your cooling water system? Three methods can help you eliminate guesswork and find unknown system volumes.

When chemical treatment formulations are added to a tower or closed loop system, it is important to know the volume of water in the system to ensure the correct chemical dosage, particularly when using biocides. A large number of biological problems can be traced to cooling water systems being under-dosed because they actually contained more water than what was “guesstimated.” Following are three methods, from most to least accurate, for finding the volume of a tower, air washer or other cooling water system, including closed loops.

To follow the molybdate method:

The factor equals 120,000 times the percent of molybdenum in product added. For example, if 35 percent sodium molybdate solution is added, which has a typical molybdenum concentration of 16.3 percent, the factor will be 19,600. This system volume determination method also can be used with other easily tested materials such as ortho-phosphate or silica.

To follow the salt method:

If 26-oz boxes of salt were used, calculate the volume with the formula:

## Sidebar:

When none of the other methods can be used due to regulatory restrictions or system limitations, the following methods can help you estimate your cooling water system's volume:

When chemical treatment formulations are added to a tower or closed loop system, it is important to know the volume of water in the system to ensure the correct chemical dosage, particularly when using biocides. A large number of biological problems can be traced to cooling water systems being under-dosed because they actually contained more water than what was “guesstimated.” Following are three methods, from most to least accurate, for finding the volume of a tower, air washer or other cooling water system, including closed loops.

## Molybdate Method

The molybdate method is the most accurate way to determine a system's volume and can be used equally well in open or closed recirculating systems. In this method, a known amount of molybdate (molybdenum) is added to the system. The increase in molybdenum is measured, and the system water volume then is calculated. The extra molybdate added will not be harmful to the system; however, this method cannot be used if molybdate is restricted by local regulations.To follow the molybdate method:

- Make an estimation of the system volume.
- Determine the existing molybdate (molybdenum) concentration in the system.
- Turn off the system bleed and add molybdate to the system at the rate recommended by the molybdate supplier.
- Circulate the system for an appropriate time to distribute the molybdenum evenly. The amount of time necessary for even distribution will depend on the size of the system and the circulation rate. An open tower system typically requires 30 to 45 minutes while a closed loop system can take as long as two hours.
- After 30 minutes in a tower system or 90 minutes in a closed loop system, measure the molybdate (molybdenum) concentration. Measure the concentration again at 15-minute intervals until two consecutive readings have less than 1 ppm difference.
- Calculate the system volume as follows:

The factor equals 120,000 times the percent of molybdenum in product added. For example, if 35 percent sodium molybdate solution is added, which has a typical molybdenum concentration of 16.3 percent, the factor will be 19,600. This system volume determination method also can be used with other easily tested materials such as ortho-phosphate or silica.

## Salt Method

In the salt method, a known amount of salt that will temporarily increase the chloride concentration by 50 to 100 ppm is added to the system. The increase is determined, and the system water volume is calculated. The temporary increase in chlorides will not harm the open system; however, this method might not be suitable for closed systems.To follow the salt method:

- Make an estimation of the system volume.
- Accurately measure the chloride concentration of the tower water. Call this “Chloride A.”
- Approximately 0.5 lb for every 1,000 gal of the estimated volume is needed to achieve a reasonable change in chlorides. Use ordinary table salt (not rock salt). A box of salt in the supermarket usually holds 26 oz (check the package), or enough for about 3,000 gal.
- Measure the amount of salt needed into a bucket by weighing 0.5 lb of salt for every 1,000 gal of estimated volume, using one 26-oz box for every 3,000 gal.
- Dissolve the salt in the bucket with water. If a large amount of salt is being used, dissolve the salt in small increments to ensure complete dissolution.
- Turn off the system bleed and pour the salt solution into the tower.
- Allow time for the salt solution to disperse. In a larger tower (15,000 gal or more), this process might take up to 45 minutes.
- Measure the chloride concentration periodically until consecutive readings are within 5 percent to 10 percent of one another. Once a stable chloride reading is achieved, record the value and call it “Chloride B”. Turn the system bleed back on.
- Calculate the system volume. If the salt was weighed out, use the formula:

If 26-oz boxes of salt were used, calculate the volume with the formula:

## Solid Geometry Method

The solid geometry method involves measuring the dimensions of the tower sump or basin to determine the cubic feet of volume, plus measuring the lengths and diameters of the system piping. To follow this method:- Calculate the volume (in gallons) of water in the basin by measuring the dimensions -- length (L), width (W) and depth (D) -- of water in the basin in feet and use the following equation:

Basin Volume (gal) = L x W x D x 7.5 - Calculate the volume (in gallons) of water in the various system pipes. If the pipe length (L, in feet) and diameter (D, in inches) are known, use the following equation to get the volume of water in gallons:

Pipe Volume (gal) = 0.0409 x L x D^{2}

(Note that this is a simplified equation derived from the general formula for water volume in pipes, Volume (gal) = 7.5pr^{2}h, where r is the pipe radius, in feet, and h is the pipe length, in feet).

Alternately, when the pipe diameter is known, obtain the corresponding number of gallons per foot from table 1. Substitute this value and the pipe length (L, in feet) in the following equation to get the volume of water in gallons:

Pipe Volume (gal) = gal/ft (from table 1) x L - Add the basin and pipe volumes together to get the total volume of water in the system.

## Successful Water Treatment

In any chemical water treatment program, knowing the cooling water system's volume can be crucial. Simply guessing is not good enough; use the wrong dose of chemicals, and you could compromise the operation of your equipment. Following proven techniques such as the molybdate, salt or solid geometry methods can help you obtain accurate volume measurements and maximize your chances for success. PCE## Sidebar:

Guesstimating Volume

When none of the other methods can be used due to regulatory restrictions or system limitations, the following methods can help you estimate your cooling water system's volume:
- For a system having only one cooling or pump loop, add a detectable material such as a dye or foam producer (such as a detergent) to the basin, then time how long it takes for the material to appear on the distribution deck. The volume can then be estimated as follows:

Volume (gal) = time elapsed (min) x circulation rate (gal/min) x 0.8

where 0.8 is used as a pump efficiency factor. - No direct relationship exists between recirculation rate and volume. However, you can get a ballpark volume figure by multiplying the recirculation rate (gal/min) by 10.
- The same holds true for tonnage and volume, but try the factor of 30 gal/ton of refrigeration.
- Minimally, get a basin volume, then estimate a percentage (+10 percent, +25 percent, +50 percent, etc.) for the piping volume based on an observation of the plant size.