Industrial water treatment can be affected by seasonal changes especially for comfort heating and cooling processes. When the outside weather is cold, boiler systems are being utilized for steam and heat. When the outside weather is hot, cooling systems provide chilled water and dissipate heat. As we all know, time moves very quickly. The transition from heating to cooling season can be upon us very suddenly. This article is a reminder of various tests and control parameters associated with a cooling tower system.
Key system factors
The cooling tower has been off-line all winter. Now, this very big air-scrubber is being started at a time where there is increased microbiological growth and activity. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Guideline 12-2000 recommends cleaning and sanitation for cooling towers which were drained and un-drained during the winter storage period. Both free chlorine and pH must be monitored during the sanitation period using commercial water test kits.
In some states, such as New York, the water quality including but not limited to pH, temperature, conductivity and biocidal indicators, must be manually measured and recorded at least three times per week. The microbiological levels must be measured at least once per week while the cooling tower system is operating.
If using on-line monitoring and control, water is now being circulated through the control loop which had been idle. Probe functionality should be confirmed in terms of calibration and accuracy. Automated water quality measurements must be properly recorded.
Phosphonate is a mineral scale and active corrosion inhibitor while molybdate and PTSA, a fluorescent dye system which is steadily gaining in popularity, are common tracers used in a cooling water system treatment program. In recent years, the use of filming amines in cooling towers and closed loop systems has increased.
Tests measure key factors of a water system’s quality and chemistry to make sure the treatment program is working properly. When taking water samples, it is important the sample represents the whole system and not a smaller, non-representative portion of the system. Improper handling can consume certain chemicals or introduce contaminants which affect tests and your response to the result. Key to a representative sample is flushing the sample line before you take your sample. This will clear out impurities or chemical concentrations at the sample point. Look for things like color, smell and quality (cloudiness or suspended solids). Was the sample line clogged? These observations can help direct your testing.
A typical cooling tower control loop will have numerous in-line probes or sensors. Common parameters are Oxidation Reduction Potential (ORP), PTSA, Conductivity and pH. The accuracy of the probes can be directly confirmed off-line using the appropriate calibration standards. When the system is on-line and you do not want to disconnect the probes, a properly calibrated hand-held device can be used to confirm the readings.
For each parameter, standard solutions which approximate the control point for a specific system should be used. Look at expiration dates on all standard solution which may be left over from last season. Allow enough time to re-stock products if any have expired. Typical standards are as follows: ORP = 200, 400 or 600 mV; PTSA = 100, 300 or 400 ppb; Conductivity = 100, 500, 1000, or 1500 µS; pH = 4, 7 and 10. Follow the equipment manufacturer’s calibration instructions.
As well, proper care of your sensors and equipment will ensure accurate readings and long probe life. ORP and pH meters, probes and electrodes are particularly sensitive. Proper storage is needed when they are not being used. Best practice is to use an electrode storage solution. Do not use DI water as a storage solution because it will permanently damage the electrode.
Inhibitor treatment program monitoring
The complex nature of phosphonates makes residual testing slightly lengthy and complicated. Many programs use a molybdate or PTSA tracer to correlate with the phosphonate system residual. Ultra-Violet light digestion is the most accurate phosphonate test however, the entire test may take 15-20 minutes.
A drop count test is much quicker, 2-5 minutes, but it is less accurate. It is excellent for on-site, easy and accurate phosphonate residual determination.
Single and multi-parameter colorimeters are commercially available which provide the most accurate molybdate control methods. PTSA can be directly measured using a special meter, fluorometer, in a matter of seconds.
For the emerging filming amine technology, test kits are available for each specific manufacturer of those products.
It makes most sense especially at seasonal start-up to employ a combination of test methods. For example, periodically and especially in the spring, use the UV lamp digestion method to confirm and correlate with the molybdate, PTSA or drop count method. Once you have established a solid correlation, use the quicker method for routine control.
Microbiological treatment program monitoring
As the spring weather gets warmer, the microbes/bacteria throughout the environment increase in activity and volume. When the cooling tower is put on-line, it begins to pull in large amounts of air from the surrounding environment. So immediately, it is imperative to monitor microbiological activity.
Be aware of when biocide additions are made and when samples are taken for analysis. Samples taken too close to a biocide addition will give false low MB results. There are several methods which can be used: dipslides, Biological Activity Reaction Tests (BART), test strips and Adenosine Triphosphate (ATP). The following is a quick summary of each:
- Dipslides – Use a certain type of agar applied to a two-sided slide or paddle inside a vial attached to a cap. One side usually measures bacteria and the other fungi, yeast or mold. The cap is twisted off and the media dipped into a water sample. After incubation between 25-30oC, results in Colony Forming Units (cfu)/mL usually take 24-48 hours.
- BART – Uses two devices (1) a floating ball, FID-floating interceding device, that restricts the entry of oxygen into the sample below (2) the use of a crystallized deposit of selective nutrients, which sits in the bottom of the tube and encourages the activities and reactions by a specific group of microbes. There are seven selective biodetectors which can be performed at room temperature. The time for each test in cfu/mL depends on the bacterial population.
- Test Strips – Flexible plastic strips to which nutrient-containing filter paper is attached. The test strip is dipped into a water sample and placed into a pouch. After incubation between 25-30oC, results in cfu/mL usually take 24-48 hours.
- ATP – Present in all living organisms. The level of ATP in water can be measured when it reacts with the enzyme Luciferase providing an instantaneous number using a luminometer. An ATP pen containing a honey-comb rod is immersed into a water sample and placed back into the containment tube. Total ATP is extracted from all the cells in the sample, living and dead. Free ATP is extracted from only dead cells. The difference of Total minus Free are the living organisms in the system expressed as Relative Light Units (RLU). The ATP test is a good tool for determining the effectiveness of a biocide program.
In addition to measuring MB counts, the oxidizing and non-oxidizing biocide additions and residuals are monitored. Note when biocide additions are made and samples taken for analysis.
Oxidizing biocides such as chlorine and bromine (halogen) products can be consumed by system demand before there is enough “free” residual to react with microbes. The system demand can be created by suspended solids or organic particles. Therefore, the halogen residual measurement typically will include a “free” and “total” test. Especially in the spring, it makes sense testing often enough during initial oxidizing biocide treatment to understand how long (minutes) a “free” halogen residual is maintained in the cooling tower system. As well, an ORP sensor may be used in conjunction with the oxidizing biocide program. Be sure to calibrate the sensor to correlate with biocide treatment residuals. Free, combined and total chlorine can be measured using drop count, colorimeter, color disk or ampoule test kits. Detection levels range from 0.02 to 10.0 ppm.
Non-oxidizing biocides are fed based on system volume therefore, review this information with the customer before initiating your program. MB control and product performance depends on concentration levels or residuals. It is important to closely monitor and test for specific activity levels. Colorimetric, titrimetric and/or ampoule test kits are available for the following non-oxidizing biocides: Bronopol (0-50 ppm), DPNPA (5-20 ppm), Isothiazoline (0-7.5 ppm), MBTC (0-5 ppm), THPS (0-100 ppm), Quats (0-320 ppm) and Glutaraldehyde (20-100 ppm).
So, anticipate the change in weather and ensure the various tests associated with a cooling tower system are now a part of your routine schedule.