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Calculate Water Volume

GENERAL
For many closed water and recirculating open cooling systems, the system volume is needed to determine the amount of inhibitor or biocide that is needed for accurate chemical treatment. To calculate water volume, some of the old “rules of thumb” such as five times recirculation rate just are not accurate enough for most applications.  For systems where the estimated volume is greater than 10,000 gallons, an easy method to determine actual system volume uses addition of a known concentration of molybdate, measurement of system molybdate concentrations, and a calculation to obtain an accurate volume measurement.

PROCEDURE
1.    Estimate the system volume by measuring the sump of the tower and the water level in it.  Multiply the number of cubic feet of water by 7.5 to get the number of gallons. Then add 10-20% additional water for piping, etc.

2.    Estimate the amount of Q-TRACE 10 required to obtain molybdate concentrations in the range of the molybdate test method being used.  One gallon of Q-TRACE 10 in a 10,000 gallon system will develop a molybdate residual of 21.5 ppm as MoO4-2 (Molybdate) or 14.4 ppm as Mo+6 (Molybdenum). 

For example, to measure 3 ppm as MoO4-2 in a system containing 25,000 gallons, add 0.35 gallons of Q-TRACE 10.  (One gallon of Q-TRACE 10 in 25,000 gallons provides 8.6 ppm MoO4-2 [21.5 ppm MoO4-2 X 10,000 gal/25,000 gal = 8.6 ppm}; to obtain 3 ppm MoO4-2, 0.35 gallons of Q-TRACE 10 would be required [1 gallon Q-TRACE 10 X 3 ppm /8.6 ppm = 0.35 gallons].)  

3.    Begin the system volume measurement.  First run a molybdate test on the recirculating water to determine if any molybdate is in the system.  If a open recirculating system is being tested, it is best if there is no load on the system and the blowdown is closed. Next, add the pre-measured amount of Q-TRACE 10 to the circulating water in an area where good mixing can occur. The water should then be allowed to circulate and mix for 30 minutes and a second test for molybdate run.  Continue testing molybdate every 15 minutes for another hour or until the molybdate results have stabilized. In most cases, the results will be stabilized after no more than three tests and testing can be discontinued at that point in time.

CALCULATION AND INTERPRETATION OF RESULTS

Sysem. Vol. gals = [(4470 added gals)*359,712]/ppm Na2MoO4 measured

Once a know volume of Q-TRACE 10 has been added to the system and thoroughly mixed, the molybdate residual should be measured and plugged into the following formula:

(Q-TRACE 10 added in gallons) X 599250  = System Volume in Gallons
(PPM Molybdate tested in System)

1 PPM Sodium Molybdate = 0.6 PPM Molybdate = 0.4 PPM Molybdenum

SAMPLE CALCULATION

Let’s suppose we have a system where the tower that is 24 ft long, 12 ft wide, and has 18 inches water in the sump.
1.    12’ x 24’ x 1.5’ x 7.5 gal/cu.ft. = 3240 gallons water in the sump. Adding another 10-20% for water in the system, the estimated system volume is about 3800 gallons. 

2.    If one gallon of Q-TRACE 10 were added to this system, 56.6 ppm of MoO4 would be developed.  (21.5 ppm MoO4-2 X 10,000 gal/3,800 gal = 56.6 ppm MoO4).  To develop only 3 ppm MoO4, add 6.8 ounces of Q-TRACE 10 (1 gallon X 3 ppm/56.6 ppm = 0.05 gallons or 6.8 ounces).

3.    Assuming the MoO4 in the system is measured at 3.4 ppm, then true system volume can be calculated:  3.4 ppm MoO4 / 3.0 ppm MoO4 = 1.13    So, volume is 1.13 X 3800 gal. = 4307 gallons.

0.05 gal F4470 x 10.43#/gal = 0.5215#
0.5215#


Using Salt to Determine System Volume:

GENERAL
For many closed water and cooling tower systems it is good to have a close idea of the volume of water in the system so the amount of inhibitor or biocide needed to adequately treat the systems can be determined. Some of the old “rules of thumb” such as five times recirculation rate just are not accurate enough for most applications. Thus, a very simple salt test can be performed to give very accurate estimates.

PROCEDURE
To conduct the test, ordinary salt can be purchased from the grocery store or, if a lot of tests are to be run, 50 lb bags can be purchased from a chemical distributor. It is best to estimate the number of gallons in a system to select an amount of salt which will give a reasonable result. Typically, it is best if about 1 lb. of salt is added for each 1000 gallons of water in the system. A good starting point for estimating is to measure the sump of the tower and the water level in it. Then multiply the number of cubic feet of water by 7.5 to get the number of gallons. Then add 10-20% additional water for piping, etc.

First, run a chloride test on the circulating water to obtain the base chlorides in the system. If a tower system is being tested, it is best if there is no load on the system and the blowdown is closed. Then, a pre-measured amount of salt is added to the circulating water in an area where good mixing can occur. The salt solution should then be allowed to circulate and mix for 30 minutes and a second test for chlorides run. Then continue to test the chlorides every 15 minutes for another hour or until the chloride results have stabilized. In most cases, the results will be stabilized after no more than 3 tests and testing can be discontinued at this point in time.

CALCULATION AND INTERPRETATION OF RESULTS
Let us suppose we started with a tower that was 24 ft long, 12 ft wide and had 18 inches water in the sump. Thus, using the formula above, 12 x 24 x 1.5 x 7.5 = 3240 gallons water in the sump. Then, since there is likely another 10-20% water in the system, a dosage of 4 lbs salt should put the results in a decent range so this dosage is added to the sump near the suction for the circulating pumps. Prior to the addition, the chloride test on the water was 48 ppm (as chlorides) or 80 ppm (as sodium chloride). NOTE: It is very important to know whether the test being used measures chlorides as Cl or as NaCl.

After circulating for 30 minutes, the second test gave a chloride content of 110 ppm (as Cl) or 183 ppm (as NaCl). Then, 15 minutes later this result increased to 116 ppm (193 ppm) and then a third test showed 114 ppm (190 ppm). It can then be assumed the results had stabilized so the chlorides had increased by 116 - 48 or 68 ppm (as Cl) or 184 - 80 = 104 ppm (as NaCl).

Therefore, since 1 lb salt per 1000 gallons should increase the sodium chloride content (NaCl) by 120 ppm or the chloride (as Cl) content by 72 ppm, the amount of water in the system is calculated as follows:
Theoretical sodium chloride increase 120 ppm/actual increase 113 ppm x 4 lbs = 4,248 gallons, or
Theoretical chloride increase 72 ppm/actual chloride increase 68 ppm x 4 lbs = 4,235 gallons.

NOTE: The above differences are due to rounding of the numbers (NaCl = 60% Cl). Thus, for calculating the biocide additions or closed system inhibitor additions, a good safe estimate for system content would be 4500 gallons.


For more information, please contact us at 800-658-7716 or email our sales consultant at JZajac@ChemWorld.com.