Closed Loop Systems Treated With Glycol
A variety of heat transfer fluids are used to provide freeze protection for industrial equipment and HVAC closed-loop heating and cooling systems. These fluids may be used in systems that operate anywhere between -60°F to 300°F.
Many heat transfer fluids are glycol-based, containing either ethylene glycol or propylene glycol. Additives may include corrosion inhibitors (high concentrations of phosphate or nitrite), alkalinity stabilizers, and in some cases – fluorescent dyes – where leak detection is desired.
Operators of these closed-loop systems can often experience long-term and difficult-to-reverse problems resulting from misapplication of these glycol-based solutions. Glycol degradation and system deterioration can result if the wrong glycol is used or if low concentrations are present. Problems associated with glycol degradation are: runaway corrosion, persistent microbiological fouling and deposition.
Terminology:
Ethylene Glycol: Most common for general industrial use, Can be toxic. Lowe viscosity (less energy required for pumping) and better heat transfer properties than propylene glycol.
Propylene Glycol: Used in quick-freezing of wrapped foods and in hospitals where there is potential for contamination of potable water systems. More environmentally friendly than ethylene glycol because of its lower toxicity. Higher viscosity (less energy efficient) than ethylene glycol.
Automotive Antifreeze: Not suitable for HVAC and industrial applications because they contain silicate corrosion inhibitors which may cause fouling and pump seal failures.
Inhibited vs. Uninhibited Glycol: Only inhibited glycols are recommended for use in closed systems. Manufacturers and water treatment professionals add inhibitors (phosphate or nitrite and alkalinity) to minimize corrosion and to prevent glycol degradation. Over time, uninhibited glycol will form acids, making the solution very corrosive (4 ½ times more corrosive than plain water).
Glycol Stability: When properly maintained, inhibited glycol solutions can last 20 years or more. Uninhibited glycol solutions can be expected to break down in 2-3 years.
Refractometer: Measures percent glycol. Accurate and portable method for both ethylene and propylene glycol.
Glycol quality should be monitored annually. This testing is done to detect any symptoms of glycol degradation – such as:
- pH depression (as low as 5.0) caused by production of organic acids
- Formation of formaldehyde, ethanol, methanol and CO2.
- Presence of a strong septic odor
- Severe corrosion of mild steel and copper components (resulting soluble copper will cause galvanic corrosion on steel).
- MB infestation particularly in stagnant areas.
Methods For Handling Degraded Glycol: Based on manufacturers’ recommendations, the system should be drained, thoroughly flushed with untreated water, and recharged with inhibited glycol. Salvaging degraded glycol is not recommended. If microbiological activity is detected, biocides can be used after the flushing process and PRIOR to recharging with fresh glycol.
Disposal Regulations: Ethylene glycol is classified as an extremely hazardous substance. The POTW or wastewater treatment plant receiving the discharge should be contacted and written permission should be obtained before discharging water containing ethylene glycol. Possible alternatives are: disposal through a waste contactor or recycling through a solvent recovery vendor.
There are no requirements, as defined by SARA, for disposal of propylene glycol; however, you should check with the local municipality prior to disposal.
Recommended Treatment Practices:
Maintain a minimum concentration of 30% inhibited glycol. This concentration will ensure adequate corrosion and biological protection. Supplemental treatment is not required.
- Monitor yearly by having the glycol provider test the glycol integrity and inhibitor concentration. This testing will allow for any corrective actions to be initiated.
- If uninhibited glycol is used, you can apply a corrosion inhibited such as Metro DuBoth NB35 at recommended treatment levels. Also, maintain close vigilance on pH, microbiological activity, iron and copper concentrations and corrosion rates.
- Manufacturers specify water impurities be limited to these maximum values:
- Chlorides……………..25 ppm
- Sulfates……………….25 ppm
- Calcium………………50 ppm
- Magnesium……………50 ppm
Pre-diluted glycol solutions can be purchased from manufacturers – if acceptable water quality is not available.
- Monitor yearly by having the glycol provider test the glycol integrity and inhibitor concentration. This testing will allow for any corrective actions to be initiated.
- If uninhibited glycol is used, you can apply a corrosion inhibited such as Metro DuBoth NB35 at recommended treatment levels. Also, maintain close vigilance on pH, microbiological activity, iron and copper concentrations and corrosion rates.
- Manufacturers specify water impurities be limited to these maximum values:
- Chlorides……………..25 ppm
- Sulfates……………….25 ppm
- Calcium………………50 ppm
- Magnesium……………50 ppm
Written by: Mark Botsford
