When treating Thermal Energy Systems or Large Chilled Water systems the following standards should be followed to minimize costs to chemically treat these systems and protect the equipment assets.
- Use of Biocides
- Use of Corrosion Inhibitors
- General Maintenance and Monitoring
Filtration Given the size of these systems the most effective filtration is low micron sand filtration. Since draining or flushing the system to reduce suspended solids is normally not an option, filtration is required as part of the program. In the case of TES systems, filtration is required. It is recommended that the filtration be able to remove down to 0.35 micron particles. The filtration sizing should be based on the volume of the system as well as turnover rate (the time it takes for the volume of the system to pass through the filter). It is recommended that the system turnover rate not exceed (5) days. Annually, each filter should be tested for biological activity (sand or multimedia filter only) and suspended solids removal performance. Filter performance can be tested using a particle study analysis of both the incoming and exiting water sources of the filter while in operation. This analysis is relatively inexpensive and can determine the percent removal rate at different micron sizes. This can be compared to the design capabilities of the filter unit. If the filter is not operating at design capabilities, inspection and maintenance of the filter should be completed.
In some cases facilities do not use their thermally stratified thermal storage systems in the winter. If this is the case, considerations on the location of the filter or an added filter may be required. This will also impact the location of where chemical is added and testing.
Biocides are required to maintain the biological counts in the system within a range that minimizes fouling, microbio influenced corrosion and suspended solids. It is recommended that bulk water microbio testing be done as a minimum quarterly and on the following species and maintain the following control ranges:
- Total Aerobic Bacteria …………………………..………< 1000 cfu
- SRB (Sulfate Reducing Bacteria) ………………………………ND
- IRB (Iron Reducing Bacteria) ………………………………….ND
- Denitrifying Bacteria …………………………………………….ND
- Nitrifying Bacteria ………………………………………………….ND
ND – Non Detectable – Barts Testing Methodology
If the levels exceed these control ranges, then a biocide should be used to bring the levels back into range. Each time a biocide is added, the system should be retested (14) days after application.
It is not uncommon that the bulk water bacteria levels show good results and yet there is considerable surface biological activity still present, mostly in biofilm. When testing systems, considerations should be given to doing pipe swab testing the same species. We expect the levels on the pipe surface to be higher than that of the bulk water but should not be more than 5 times the level in the bulk water. In the case of anaerobic bacteria, the levels should be < 50 cfu. If this is the case, then a biocide should be used to bring the levels back into range and retested in (14) days.
There are a number of biocides available to manage biological activity. The following biocides have been very effective on a shock basis in the past at the prescribed dosages;
- 120 PPM of 50% Gluteraldhyde
- 240 PPM of a 1.5% Copper Free Isothiazoline
- 1.0 PPM free residual of Chlorine Dioxide
Depending the biological levels, the most aggressive and in some cases the most cost effective method to treat the system is with the use of Chlorine Dioxide. This is a low residual oxidizing biocide that demonstrates very good biofilm removal and rapid destruction of biological activity in the systems. The use of chlorine alone is NOT recommended as the system will see increased corrosion at the elevated chorine levels required in high pH water conditions. Stabilized bromine products are also not recommended as the product can over stabilize over time with multiple applications reducing the effectiveness of the program.
It is recommended that (2) proactive biocide applications be completed each year. The type of biocide depends on the biological activity and the effectiveness of the biocide after each application. If the system continues to have high biological activity after repeated biocide applications, then Chlorine Dioxide should be used.
Corrosion Inhibitors play an important role to minimize corrosion and iron deposition on heat transfer surfaces. There are a number of corrosion inhibitors that have been effective and are listed in order of effectiveness. The cost of the program approach is also listed from highest to lowest price.
Note: The use of Molybdenum could exceed local municipality discharge limits.
It should be noted that the use of Nitrite, Phosphate or Phosphonate Corrosion Inhibitors should NOT be used as part of the program. These programs can be food sources for bacteria.
The program selected should be fed at a rate to provide < 0.5 MPY steel and < 0.1 MPY copper corrosion rates as measured with non passivated corrosion coupons.
Water Chemistry should be maintained and tested on a regular basis to monitor and maintain the proper performance standards.
|Thermal Energy System – Large Chilled Water System Chemical Performance Standards|
|Water Quality Parameter||Level||Units of Measure|
|Total Aerobic Bacteria||<1000||CFU/ml|
|Total Anaerobic Bacteria||ND – Not Detectable||CFU/ml|
|Chemical Oxygen Demand||<100||mg/L|
|Free Azole||5-10||mg/L as Azole|
|Treatment – Molybate||100-120||mg/L as MO|
|Treatment – Moly/Silica||50-70 ppm MO / 50-70 ppm Silica||mg/L as MO and SIO2|
|Treatment – Silicate||50-70 ppm Above Make-Up Water||mg/L as SIO2|
|Treatment – Azole (TT or B2T)||5-15||mg/mL|
pH control plays an important role when controlling mild steel corrosion. Maintaining the pH above 8.5 helps minimize mild steel corrosion. It is not uncommon to see pH reduction in these systems. This is usually a result of high levels of microbio activity both in the bulk water and biofilm. If you see the pH drop, biological testing of both bulk water and pipe surface should be done.
General Maintenance and monitoring should be done on a regular basis. The following actions should be considered when maintaining the water chemistry of these systems:
Water Meter should be installed on the makeup line to the system. Monitoring water usage will allow the facility to respond to any leaks. The water loss from these systems should be <1 % of the system volume per year. Any water loss above this should be identified and repaired.
Corrosion Monitoring of steel and copper as well as any other metallurgy that might be in the system should be done quarterly. The corrosion rates should be very low in properly treated systems. The corrosion coupon rack is also an excellent sample point for biological swab sampling of the system.
TES tanks should be inspected annually. This can be done via underwater cameras or through sampling the tanks at different levels off the bottom. Since TES tanks are the low point of the system, any suspended solids will settle at the bottom of the tank. This can lead to biological influenced corrosion of the tank and a source of biological contamination for the entire system. If the sludge level is significant, then the tank should be cleaned. There are a number of companies that can clean tanks without draining the system.
Many facilities isolate, drain and winterize air handler coils for the winter months. In most cases glycol is used to winterize coils. Bringing these units back on line in the spring creates an influx of suspended solids, iron corrosion products and residual glycol which at low levels is a food source for bacteria. It can take months to get the system back into specification.
In order to reduce these issues, the following measures should be considered;
- Use air to remove the water from coils instead of glycol
- Prior to isolating coils, make sure the biological activity both bulk water and surface levels are within range. Consideration should be given to adding a maintenance dosage of biocide prior to winterizing
- Make sure your corrosion control program is in range.
- Prior to putting the air handlers back on line, flush the units with clean water.
Once the air handlers are back on line, re-balance the water chemistry and make sure the filter units are working properly.