Cooling Water System Maintenance Manual: Biocide Dosing, Corrosion Coupon Analysis, and Purge Optimization

1. Scope and purpose

This manual covers critical maintenance procedures for open circulating cooling water systems used in industrial settings including the automotive, chemical, food and energy industries. The goal is to prevent biofouling, minimize corrosion, and optimize water use efficiency through blowdown control. The correct application of these procedures is mandatory to ensure the reliable operation of the equipment, prolong its service life and maintain the efficiency of heat exchange. Maintenance should be carried out regularly according to the schedule, as well as when deviations in water parameters are detected.

2. Precautions

CAUTION: Working with chemicals and systems under pressure requires strict adherence to safety rules. Failure to do so could result in serious personal injury or equipment damage.

PERSONAL PROTECTION: Be sure to wear safety glasses, chemically resistant gloves (such as nitrile or neoprene), protective clothing, and, if necessary, respiratory protection according to the Material Safety Data Sheets (MSDS) for all reagents.

LOCKOUT/TAGOUT: Before working on pumps, valves, or electrical components of the cooling water system, ensure that power sources are isolated and locked out/tagout in accordance with the company's safety procedures (DSTU standard EN 1037:2004).

CHEMICAL SAFETY: Always add chemicals slowly and carefully to avoid splashing. Have emergency showers and eyewash stations handy. Store chemicals according to manufacturer's recommendations, away from incompatible materials.

3. Necessary tools and materials

4. Checklist before starting maintenance

5. Step-by-step procedure

5.1. Dosage of biocides

1. Preparation and dose calculation:
   • Determine the total volume of the cooling water system (cooling tower, pipelines, heat exchangers). This information is usually available in the technical documentation of the system.
   • Calculate the required amount of biocide for shock dosing according to the manufacturer's recommendations and the current state of the system (based on microbiological analysis). For inorganic biocides, it can be 50-200 ppm, for oxidative biocides - 1-5 ppm of free chlorine. Error: underestimating the size of the system will lead to insufficient efficiency.
2. Dosing pump calibration:
   • SAFETY: Make sure the dosing pump is de-energized and isolated if it needs to be disconnected for calibration. Use PPE.
   • Shut off the biocide supply to the system.
   • Connect a calibrated graduated cylinder (eg 1000 ml) to the output line of the pump.
   • Start the pump and measure the volume of biocide dispensed in 1 minute. For example, if it is necessary to dose 5 l/h, in 1 minute the volume should be approximately 83 ml.
   • Adjust pump settings (pulse frequency, stroke length) until desired performance is achieved.
   • Error: Using an uncalibrated pump will result in over- or underdosing.
3. Biocide injection:
   • Provide biocide injection at a point in the system with effective mixing (eg cooling tower catch basin or circulation pump suction).
   • For shock dosing: Add the calculated amount of biocide over a short period (eg 15-30 minutes) according to a schedule (eg once a week or twice a week).
   • For continuous dosing (if applicable): Set the pump to a constant low dosing rate.
   • Visual indicator: uniform dissolution of the biocide in water.
4. Effectiveness monitoring:
   • After 12-24 hours after dosing, take a water sample and conduct a microbiological analysis using test strips (dip slides).
   • Acceptable result: a significant reduction in the number of colonies (less than 10⁴ CFU/ml).
   • Error: lack of monitoring after dosing does not allow to evaluate effectiveness.

5.2. Analysis of corrosion coupons

1. Installation of corrosion coupons:
   • SAFETY: Use PPE. Make sure the cooling water system is working normally.
   • Choose a representative place to install the coupon holder where the water flow speed is 0.5-1.5 m/s. Usually this is a side flow (bypass) or a section of the pipeline after the pump.
   • Carefully place the pre-weighed, cleaned and identified corrosion coupons into the holder. Fix them so that they do not touch each other and the walls of the holder.
   • Record the installation date, coupon identification numbers, and initial weight (to the nearest 0.0001 g).
   • Bug: misplacing coupons or damaging them during installation will distort results.
2. Exposure:
   • Leave coupons in the system for the standard exposure period recommended by the manufacturer or according to internal procedures (usually 30, 60 or 90 days).
3. Extracting and cleaning coupons:
   • After the exposure period, again SAFETY: isolate the coupon area, release pressure and drain the water. Use PPE.
   • Carefully remove the coupons from the holder, avoiding mechanical damage.
   • Clean the coupons of deposits and corrosion products in accordance with ASTM G1 (for example, with a chemical solution to remove corrosion products without affecting the base metal).
   • After cleaning, rinse the coupons thoroughly with distilled water, dry and place in a desiccator.
   • Error: Aggressive mechanical cleaning or incomplete removal of corrosion products will lead to false results.
4. Reweighing and calculation:
   • Reweigh each coupon to the nearest 0.0001 g.
   • Calculate the mass loss for each coupon.
   • Using the formula, calculate the corrosion rate in MPY (mils per year) or mm/year:
   • Corrosion Rate (MPY) = (Mass_initial - Mass_final) × 22300 / (Area × Density × Time)
   • Where: Area is the surface area of the coupon (inch²), Density is the metal density of the coupon (g/cm³), Time is the exposure time (days).
   • Acceptable values: for mild steel, the corrosion rate should be less than 3 MPY (0.076 mm/year); for copper alloys – less than 0.5 MPY (0.013 mm/year).
   • Document all results. Error: Arithmetic errors or use of incorrect units of measurement.

5.3. Blowdown optimization

1. Monitoring water parameters:
   • Regularly measure conductivity (μS/cm), total dissolved solids (TDS, ppm), pH and temperature (°C) of cooling water and makeup water.
   • Record these values.
2. Calculation of concentration cycles (CC):
   • Concentration cycles (CC) = Conductivity_{cooling water} / Conductivity_{feed water}.
   • Target CCs depend on the quality of the feed water and the type of inhibitors used (typically 3-6 for open systems).
   • Error: ignoring the quality of the feed water when calculating the CC.
3. Auto Purge Settings:
   • If the system is equipped with an automatic purge controller, set the desired upper conductivity limit (eg 2000-3000 µS/cm for a CC=5 system with 400 µS/cm feed water).
   • Check the operation of the purge solenoid valve and conductivity sensor.
   • Visual indicator: the purge valve opens when the conductivity reaches the set value and closes when it drops.
4. Manual purging (in the absence of automation):
   • SAFETY: Always monitor the water level in the cooling tower during manual purging to avoid drying out the pumps.
   • Calculate the required purge volume to maintain the CC. For example, if CC = 4, then 25% of the evaporation volume should be purging.
   • Open the manual purge valve for the set time or until the desired conductivity level is reached.
   • After purging is complete, close the valve and check the water parameters.
   • Error: excessive purging leads to excessive consumption of water and chemicals; insufficient - to the accumulation of salts and corrosion.
5. Check and Adjust:
   • Within a few days of purge setup, monitor system conductivity and feed water flow.
   • If necessary, adjust purge settings to maintain stable CCs and minimize water wastage.

6. Post-service verification checklist

7. Troubleshooting guide

8. Recommended maintenance schedule

9. Directory of spare parts

To order spare parts and familiarize yourself with the full range of products, visit the electronic catalog UNITEC-D.

10. Links

• DSTU EN 1037:2004 Machine safety. Prevention of unexpected start.
• ISO 10002:2018 Quality management. Consumer satisfaction. Guidelines for working with complaints in organizations.
• ASTM G1-03(2017) Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens.
• EN 806-2:2005 Technical conditions for drinking water supply systems for buildings. Part 2: Design.
• Recommendations of the manufacturer of the cooling water system (OEM documentation).
• Safety data sheets of chemical reagents (MSDS/SDS).