1. Purpose and Purpose
This technical guide provides detailed instructions for preventive and corrective maintenance of industrial cooling water treatment systems, with particular emphasis on biocide dosing, corrosion coupon analysis and blowdown optimization. The application of these procedures is critical to ensure operational reliability, prevent corrosion, scale formation and microbiological proliferation within heat exchangers, condensers and cooling towers, typically found in machine tool manufacturing plants. Accurate cooling water management extends asset life, reduces unplanned downtime and ensures process energy efficiency.
The described maintenance must be performed according to the established maintenance schedule, in response to changes in process water parameters, or as part of an investigation of cooling system efficiency problems.
2. Safety Precautions
WARNING: The operations described involve potentially dangerous chemical agents and pressurized systems. Failure to comply with safety procedures may cause serious injury, chemical poisoning or damage to equipment.
Lockout/Tagout: Before any work on pumps, automatic valves or pressure lines, ensure that the electrical and hydraulic power supply is deactivated and locked out according to company procedures (UNI EN 1037).
Personal Protective Equipment (PPE): Always wear safety glasses compliant with UNI EN 166, gloves resistant to chemical agents (nitrile/butyl, UNI EN 374), long-sleeved protective clothing and, when handling concentrated biocides or working in areas with vapours, a respiratory mask with a suitable filter (UNI EN 14387).
Chemical Product Management: Always consult the safety data sheets (SDS) of the biocides and corrosion inhibitors before of use. Avoid contact with skin and eyes. Do not inhale vapours. Be sure to operate in well-ventilated areas.
Hot Water and Pressure: Pay attention to hot lines and surfaces. Cooling water lines can operate at temperatures up to 60°C and typical operating pressures of 3-5 bar. Decompress and cool lines before opening.
3. Tools and Materials Required
| Tool/Material | Specifications | Quantity |
|---|---|---|
| Water analysis kit | Digital pH meter (range 0-14, precision ±0.05), Conductivity meter (range 0-20,000 µS/cm, precision ±1%), ORP test kit, kit for biocidal residue (e.g. DPD for free/total chlorine) | 1 |
| Precision scale | Resolution 0.1 mg, min capacity 200g (for corrosion coupon) | 1 |
| Torque wrench | Range 10-100 Nm, with calibration certificate | 1 |
| Set of spanners/adjustable wrenches | Assorted metric sizes | 1st Sep |
| Reserve dosing pump | Model compatible with biocide and inhibitor, adjustable flow rate 0.5-5 L/h, max pressure 8 bar | 1 |
| Replacement hoses | Reinforced PVC, diameter 4/6 mm or 6/8 mm, specific for chemical agents | 10 m |
| PVC ball valves | Diameter DN15-DN25, PN10 | 2-4 |
| Biocide (oxidizing/non-oxidizing) | According to specific system (e.g. sodium hypochlorite 12.5%, glutaraldehyde 15%), 25 kg drums | According to consumption |
| Corrosion inhibitor | According to plant specification (e.g. molybdate/phosphonate), 25 kg drums | According to consumption |
| Corrosion coupons | Carbon steel (ASTM A108 1018), copper, brass, stainless steel (AISI 304/316) – according to system materials | 6-12 |
| Coupon cleaning solvent | 5% citric acid or passivation solution (e.g. Rodine 213) | 1 L |
| Non-abrasive brush | Nylon or soft fiber | 1 |
| Sample containers | Neutral plastic, 500 mL capacity, with airtight cap | 6-10 |
| Absorbent cloth / Rag | Absorbent, non-linting | Q.B. |
| Security Labels / Tagout | Compliant ISO 45001 | Q.B. |
| pH meter cleaning and calibration kit | Buffer solutions pH 4.01, 7.00, 10.01 | 1 |
| Anti-acid gloves | Nitrile, appropriate size | 2 pairs |
| Safety glasses | Anti-fog, UNI compliant EN 166 | 1 |
| Full face mask | With filter for organic/acid vapors (type ABEK1), UNI EN 140/14387 compliant | 1 |
4. Pre-Maintenance Inspection Checklist
| Item | Check | Acceptance/Rejection Criteria | Notes |
|---|---|---|---|
| Cooling system | General visual inspection (tower, basin, piping) | No obvious leaks, mud/algae accumulations, external corrosion, abnormal vibrations | Record anomalies for intervention |
| Circulation and dosing pumps | Check operation, noises, leaks | Smooth operation, no abnormal noises or leaks from the seals | Check pressure and flow rate |
| Chemical tank level | Visual control of biocide and inhibitor levels | Sufficient level for the expected duration of the procedure or dosing cycle | Fill if necessary, note consumption |
| Electrical panel and automation | Check status indicators, alarms | No active alarms, correct operating indicators | Record any anomalies or faults |
| Shut-off valves | Status inspection (open/closed) | Valves in the correct state for safe operation or isolation | Check manual operation |
| Sampling points | Check accessibility and cleanliness | Clean sampling points, valves operational and leak-free | Make sure samples are representative |
| Water analysis reports | Review last 7 days of data (pH, conductivity, ORP, biocide residual) | Parameters within operating ranges. Significant variations (+/- 10%) to be investigated | Confirms need for adjustments |
| Corrosion coupons (if any) | Quick inspection, exposure status | Coupons displayed correctly in the flow, no obstructions. They must not show massive corrosion | Prepare removal for detailed analysis |
| Exhaust system (blowdown) | Check valve operation (manual/automatic) | Operating valve, no obstructions, visible drainage (if manual) | Monitor the exhaust flow |
5. Step-by-Step Procedure
5.1. Biocide Dosage
The dosage of biocides is essential for the control of microbiological proliferation (bacteria, algae, fungi) which causes biofouling, microbiologically induced corrosion (MIC) and reduction of heat exchange efficiency.
- Preparation and Safety
- ATTENTION: Always wear specific PPE for the biocide in use (chemical gloves, glasses, mask).
- Isolate and block the biocide dosing system (pump, tank) by turning off the power supply and closing the suction and delivery valves. Affix 'Out of Service' label.
- Check the expiry date and the integrity of the biocide container.
- Checking and Refilling the Biocide Tank
- Check the biocide level in the storage tank. If low, proceed with charging.
- Carefully open the new biocide drum. Use an appropriate transfer pump to fill the tank, avoiding splashing. Do not fill more than 80% to prevent overflow.
- Record the volume of biocide added.
- Common error: Direct or without precautions decanting causes splashes and chemical risk. Always use dedicated equipment and PPE.
- Dosage Calculation and Setting
- Determine the total volume of the cooling system (liters). Example: a 20,000 L tower system.
- Consult the biocide data sheet for the recommended dosage (e.g. 150-200 ppm of active ingredient based on system volume, with shock dosages up to 500 ppm in case of severe contamination).
- Calculation Example: For a 20,000 L system and a 15% biocide (150,000 ppm) to be dosed at 200 ppm: (200 ppm * 20,000 L) / 150,000 ppm = 26.6 L of biocide.
- Set the dosing pump to deliver the calculated volume over a defined period (e.g. 1-2 hours for shock dosing, or continuously/intermittently for maintenance).
- Common error: Dosing based on inaccurate estimates of system volume or without considering the concentration of the biocide's active ingredient leads to ineffectiveness or waste.
- Reactivation and Monitoring
- Reopen the suction and delivery valves of the dosing system.
- Reactivate the electrical supply to the dosing pump and remove the 'Out of Service' label.
- Check the correct functioning of the pump and the absence of leaks from the connections.
- After dosing, take a water sample from the system and measure biocide residual (e.g. free chlorine) or ORP (Redox Potential) to confirm that levels are within the desired range (e.g. ORP > 350 mV for microbiological control).
- Visually, improvement should occur within 24-48 hours with a reduction in turbidity and odor, and the disappearance of algae in the tower basin.
5.2. Corrosion Coupon Analysis
Corrosion coupon analysis provides a direct measurement of the corrosion rate of materials in the system, which is essential for evaluating the effectiveness of the corrosion inhibition regime and structural integrity.
- Coupon Extraction
- ATTENTION: Close the isolation valves upstream and downstream of the bypass where the coupons are installed. Make sure the line is depressurized and drained before opening. Wear gloves and glasses.
- Carefully unscrew the flange or cap holding the coupon.
- Remove the coupon, taking care not to damage or scratch it.
- Common error: Mechanical damage to the coupon during extraction can distort the analysis results.
- Cleaning and Preparing the Coupon
- Note the location of the coupon, the exposure period and its initial appearance.
- Gently clean the coupon according to ASTM G1. Typically, a specific acid solution (e.g. 5% citric acid solution or passivation solution) is used to remove corrosion products without attacking the base metal. Soak the coupon for the recommended time (e.g. 5-10 minutes), then brush gently with a non-abrasive brush under running water.
- Rinse thoroughly with distilled water and dry with clean compressed air or in a low temperature oven (max 60°C).
- Common mistake: Aggressive cleaning or cleaning with abrasives that also remove the base metal, overestimating corrosion.
- Measurement and Analysis
- Weigh the clean coupon with a precision balance (0.1 mg). Compare with the initial weight recorded before exposure.
- Calculate weight loss.
- Visually inspect the coupon under magnification (microscope, if available) to identify pitting, uniform corrosion, cracks, or other defects.
- Calculate the corrosion rate using the standard formula (e.g. in MPY - mills per year, or mm/year):
Corrosion_Rate (mm/year) = (K * Weight_Loss) / (Surface_Area * Density * Time)
Where K is a constant, Weight_Loss in mg, Surface_Area in cm², Density in g/cm³, Time in years. - Acceptable corrosion rates vary by material: typically < 0.05 mm/year for stainless steel, < 0.1 mm/year for carbon steel and copper.
- New Coupon Installation and Registration
- Record all data (initial weight, final weight, weight loss, corrosion rate, visual observations, installation/removal date).
- Install new pre-weighed coupons in the same location, making sure they are properly inserted into the water flow.
- Tighten the flange or plug with the torque wrench to the specified torque (e.g. 30-40 Nm for a DN25 flange with EPDM gasket).
- Reopen the valves slowly to pressurize the bypass. Check for leaks.
5.3. Exhaust Optimization (Blowdown)
The optimization of discharges (blowdown) is essential to control the concentration of dissolved and suspended solids, preventing encrustations and maintaining the effectiveness of chemical treatments.
- Verify Key Water Parameters
- Take a sample of water from the cooling system and measure the conductivity, pH, total hardness and alkalinity.
- Take a sample of make-up water and measure its conductivity and hardness.
- Record all values.
- Calculation of Concentration Cycles
- Cycles of concentration (COC) indicate how many times the dissolved solids are concentrated in the system compared to the make-up water.
COC = System_Conductivity / Make-up_Conductivity- The desired COC depends on the type of system and water quality, typically 3-6 cycles for open systems. A COC that is too low indicates waste of water and chemicals; too high a COC indicates risk of scaling and corrosion.
- Common mistake: Ignoring the quality of the make-up water in the COC calculation, leading to non-representative values.
- Exhaust System Adjustment
- If the system is equipped with automatic blowdown control based on conductivity, verify that the setpoint is correct (e.g. 1500-2000 µS/cm for open systems with standard make-up water).
- Calibrate or adjust the conductivity sensor if necessary.
- If the blowdown is manual, adjust the opening of the exhaust valve (e.g. DN50 ball valve, open 1/4 or 1/2 turn) to achieve the desired COC. This will require constant monitoring for a few operating cycles.
- Common mistake: Setting the valve too aggressively or too passively, causing waste or poor water quality management.
- Monitoring and Verifications
- Monitor the system conductivity continuously or at regular intervals (e.g. every 4-8 hours) to ensure that it remains within the target range.
- Visually check the exhaust flow, if possible, to confirm that the blowdown is active.
- Check the absence of visible encrustations on the surfaces of the exchangers or in the tower basin.
- For an optimized system, the temperature of the water leaving the exchangers should be stable and correspond to the design values, indicating good heat exchange efficiency.
6. Post-Maintenance Verification Checklist
| Test | Expected Result | Real Result | Result (Pass/Fail) |
|---|---|---|---|
| Biocide Dosage | Biocidal residue (e.g. free chlorine) in the system within range (e.g. 0.2-0.5 ppm) or ORP > 350 mV. | ||
| Corrosion Coupon | Corrosion rate (e.g. mm/year) for carbon steel < 0.1 mm/year, for copper < 0.05 mm/year. | ||
| Exhaust Optimization | Stable system water conductivity within the setpoint (e.g. 1500-2000 µS/cm). Concentration cycles within target range (e.g. 3-6). | ||
| pH of the water | System pH stable within the recommended operating range (e.g. 7.5-8.5). | ||
| No Loss | No leaks from metering pumps, pipes, valves or flanged joints. | ||
| System operation | Dosing and circulation pumps operational, without abnormal noises or vibrations. | ||
| Torque wrenches | All flanged or threaded fittings tightened to the specified torque (e.g. 30-40 Nm). | ||
| System Cleaning | Absence of significant accumulations of algae, mud or encrustations in the tower basin and on accessible internal surfaces. | ||
| Documentation | All maintenance data, readings and observations recorded in the maintenance log. |
7. Troubleshooting Guide
| Symptom | Probable Cause | Corrective Action |
|---|---|---|
| Excessive algal/bacterial growth in the system (biofouling, mucilage) | Insufficient biocide dosage; Biocide not effective against the present strain; Insufficient contact time; Contaminated make-up water quality. | Increase the frequency or concentration of the biocide dosage (shock dosage, e.g. 400 ppm). Evaluate the alternation or change of biocide type (oxidizing/non-oxidizing). Extend contact time. Investigate the source of makeup water contamination. |
| High corrosion rate (highlighted by coupons or visual inspection) | Insufficient corrosion inhibitor dosage; Water pH out of range (too acidic or basic); Too high concentration cycles; Presence of MIC. | Increase the concentration of the corrosion inhibitor. Adjust the pH with acids or bases (e.g. sulfuric acid to lower, caustic soda to raise) to keep it in the 7.5-8.5 range. Optimize blowdown to reduce concentration cycles. Perform biocidal shock treatment if MIC is suspected. |
| Formation of encrustations (scaling) on exchangers or internal surfaces | Insufficient blowdown (too high concentration cycles); Excessive water hardness; Water pH too high; Overdose of some inhibitors. | Increase blowdown to reduce concentration cycles to target value. Consider installing a make-up water softening system if hardness is critical. Adjust the pH. Check scale inhibitor specifications and dosage. |
| Low heat exchange efficiency in exchangers | Biofouling; Encrustations; Corrosion; Insufficient water flow; Contamination by suspended solids. | Implement actions to control biofouling and fouling (see above). Check the corrosion rate. Inspect pumps and valves for flow problems. Consider lateral filtration to remove suspended solids. Carry out chemical or mechanical cleaning of the exchangers. |
| Excessive consumption of water or chemicals | Blowdown too high (concentration cycles too low); Losses in the system; Excessive or out of control chemical dosage. | Reduce blowdown to increase concentration cycles to target value. Thoroughly inspect the system for leaks. Calibrate or repair metering pumps. Review dosing calculations and implement more rigorous monitoring. |
8. Recommended Maintenance Schedule
| Tasks | Frequency | Estimated Duration | Skill Level |
|---|---|---|---|
| Control of water parameters (pH, conductivity, ORP, biocidal residue) | Daily/Weekly | 15 min | Operational Technician |
| Visual inspection of the system (leaks, algae, mud) | Weekly | 30 min | Operational Technician |
| Check and refill chemical tanks | Weekly/Biweekly (according to consumption) | 30-60 min | Operational Technician |
| Sensor calibration (pH, conductivity, ORP) | Monthly/Quarterly | 30 min | Specialized Technician |
| Inspection and calibration of dosing pumps | Monthly | 45 min | Specialized Technician |
| Extraction and analysis of corrosion coupons | Quarterly/Semi-annual (every 90-180 days) | 2-4 hours | Specialized Technician |
| Internal cleaning of the cooling tower basin | Semi-annual/Annual (according to conditions) | 4-8 hours | Maintenance Technician |
| Internal inspection of heat exchangers | Annual/Biennial | 4-8 hours | Specialized Technician |
| Complete overhaul of the water treatment system | Annual | 1 day | Specialized Engineer/Technician |
9. Spare parts reference
For maximum reliability and compatibility, the use of original or certified equivalent spare parts is recommended. UNITEC-D GmbH offers a wide range of components for water treatment systems.
| Part Description | Typical Specifications | UNITEC category |
|---|---|---|
| Diaphragms and valves for dosing pumps | EPDM/PTFE, compatible with aggressive fluids (biocides/acids) | Dosing Pump Components |
| pH/ORP sensors | Combined glass electrodes, PEEK body, with cable and BNC connector | Analytical Instrumentation |
| Conductivity probes | Cell with 2 or 4 electrodes in AISI 316L stainless steel, K=1.0 cm-1 | Analytical Instrumentation |
| Cartridge filters (make-up water) | 5-20 micron polypropylene, 10-20 inch height | Water Filtration |
| PVC/PP shut-off valves | DN15-DN50, PN10, EPDM seals | Industrial valves |
| Flow switches and pressure switches | PP/brass body, SPDT contacts, IP65 | Control Instrumentation |
| Corrosion coupons | Carbon steel, copper, stainless steel (various sizes) | Monitoring Materials |
To purchase or consult specific spare parts, visit our e-catalog: www.unitecd.com/e-catalog/
10. References
- UNI EN ISO 9001: Quality management systems.
- UNI EN 1037: Machinery safety – Prevention of unexpected start-up.
- UNI EN 166: Personal eye protection – Specifications.
- UNI EN 374: Protective gloves against chemicals and microorganisms.
- UNI EN 140/14387: Respiratory protection devices.
- ASTM G1: Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens.
- Cooling system manufacturer's (OEM) and chemical dosing system operation and maintenance manuals.
- CEI Guidelines: Standards for the safety of electrical systems and automation.
