How to Determine the Volume of Your Recirculating Water System

Most water treatment products work best at certain concentrations. Biocides offer a good example. Only biocides licensed by the Environmental Protection Agency, or EPA, can be used to control microbes in recirculating water systems. Each biocide has a different effectiveness, or efficacy in industry terms. Most biocides will not properly disinfect the entire system if the dose is below the effective range, and the EPA can levy steep fines if biocides are added at dosages higher than posted on the label. Scale and corrosion inhibitors offer another good example. Too little inhibitor and scale can rob your system of efficiency or corrosion can shorten equipment life. Too much inhibitor adds cost and rarely improves results. It is therefore essential to know the volume of water in the system.

How do I Calculate the Volume of my Sump? 

Most sumps are standard geometric shapes. Precise dimensions are often unnecessary and usually measuring the dimensions of your sump by pacing off the size is sufficient. The boundaries of in-ground or below-grade sumps are not always clear and it is more difficult to determine the water depth, but reasonably accurate measurements of those dimensions are important. Here is the formula for square or rectangular sumps.

Gallons = Length in feet x Width in feet x Water depth in feet x 7.48 gal/ft3

My Sump is Cylindrical; What About That? 

Round or cylindrical tanks are very common and determining their volume is a bit more involved. First figure out the diameter of the tank in feet. Then divide the diameter by 2 to determine the radius of the tank. Next measure the water depth in feet. Now put those values into this formula:

Gallons = Radius in feet x Radius in feet x 3.14 x Water depth in feet x 7.48 gal/ft3

How do I Determine How Much Water is in my Piping System? 

There are a few ways to determine the volume of water in pipes, but the most efficient way is to pace off the length of each pipe size in the facility and then use the table below to convert feet of pipe to gallons for each pipe size. If the piping system covers many floors, it is usually sufficient to estimate the length of vertical risers by multiplying the number of floors by 12 when using this table.

Is There Anything Else I Need to Know?

 Engineering principles for sizing chillers and closed-loop piping in HVAC systems indicate that the volume of water in a closed chilled water loop should be approximately four times the recirculation rate to deliver proper cooling. You can therefore estimate the volume of a truly closed-loop using the simple formula below. Note that this estimate applies only to truly closed chilled water loops in HVAC applications using constant-speed pumps. The ratios are changed completely for systems using pumps controlled by variable frequency drives. This ratio also rarely applies in process applications. Here’s the math:

Gallons = Recirculation rate in Gallons Per Minute x 4

Any Other Shortcuts? 

Another applicable engineering principle is the sizing of the expansion tank. The expansion tank should represent 2% of the system volume, so you can estimate the closed-loop volume by multiplying the volume of the expansion tank by 50. Here is the formula:

Gallons = Volume of Expansion Tank x 50

What About Salt Sizing? 

A number of compounds can be used to calculate the volume of water in a system. The industry used salt for years because it is inexpensive and the concentration of chloride can easily and accurately be measured. The volume could be calculated by knowing how much salt was added and knowing the initial and final concentration of chloride. While sodium chloride is quite soluble, those using it to size systems still had to wait for the salt to completely dissolve before determining the final concentration. That often involved some ambiguity that was further complicated by the prospect of the salt sinking in sediment on the bottom of the sump which dramatically slows the rate it dissolves.

To eliminate that problem and further ease the speed and accuracy of testing, we introduced ValiSizing in 2019. Provided in convenient bottles that deliver 100 ppb of PTSA in 1000 gallons of water, ValiSize is a liquid solution of PTSA that can conveniently be added to water without concern for dissolving or sinking in sediment. The procedure is simple. Follow it closely and you will accurately determine the system volume.

1. Estimate the volume of the system you are sizing with your best guess. Each bottle of ValiSize delivers 100 ppb of PTSA in 1000 gallons, so you will need to get at least in the neighborhood of volume for this procedure to work at its best.
2. Measure the PTSA level in the water before you add any ValiSize. Record this as the initial reading.
3. Make sure all zones and sections of the system have circulation. ValiSize will not measure the volume of sections that do not circulate during the procedure.
4. Turn off the blowdown for the duration of this procedure. DO NOT forget to enable blowdown once the procedure is complete.
5. Add one bottle of ValiSize for every thousand gallons in your original estimate from step 1. For instance, if you think the system is about 6,000 gallons, you will need 6 bottles of ValiSize.
6. Allow the entire system to circulate and mix for 15 minutes, then test the PTSA level. Record both the time and level below the result you recorded in step 2.
7. Repeat the testing and recording in step 6 every 15 minutes until the PTSA level no longer changes between steps. You have reached the end of the procedure when you have 3 consecutive identical PTSA results. Your data should look something like that shown below.
8. Calculate the system volume using this formula:
9. Gallons = (Bottles of ValiSize used x 1000) ÷ ((Final PTSA – Initial PTSA) ÷ 100)

Chardon Labs is an industry leader when it comes to servicing boilers, cooling towers, and closed-loops. Reach out to us today to get a free survey!