Unwanted Water in Ammonia Systems

Ammonia Contamination Sampling is performed to calculate how much water “moisture” is present within the ammonia refrigeration system. Contamination sampling is required by IIAR Book 6 Inspection, Testing, and Maintenance (ITM) to be performed every 3 years if the refrigeration system operates in a vacuum. We’re going to discuss ways to safely test for water in the refrigeration system. Refrigerant grade ammonia needs to be a minimum of 99.5% pure, pharmaceutical grade ammonia is “too” pure and should not be used in ammonia refrigeration systems. Pharmaceutical grade ammonia can cause piping/components to become brittle because it pulls the moisture out of the metal and stress corrosion cracking is likely to occur. Water inside the refrigeration system can present safety concerns, but it greatly affects the efficiency. The following are routes for water entry into the refrigeration system:

• Air leaks on systems that operate below atmospheric pressure (vacuum)

• Opening system to atmosphere (maintenance or repairs)

• Leaks on heat exchangers that cool secondary coolants (water, glycol, etc.)

• Hydrostatic testing vessels during new construction (pressure testing with water)

• New construction piping is not properly isolated from the atmosphere and rainfall enters

• Pulling water back into the system during pump outs with buckets of water

Safety concerns regarding water in the ammonia system include:

• Water inside the system can cause corrosion such as Stress Corrosion Cracking (SCS)

• Complete blockage from water freezing at components, devices, or piping with liquid refrigerant present could result in hydrostatic lockup and surpass design pressure limits

When water contamination is inside the refrigeration system, this can alter the lubrication properties of oil in compressors, the pressure-temperature relationship of ammonia on a saturation chart will no longer be accurate, and evaporator/heat exchanger performance is greatly reduced. In other words, too much water in the refrigeration system “waters down” the refrigerant grade ammonia. This is like when ice melts inside of a soft drink and the soft drink no longer has its fresh crisp flavor.

The following examples express the impact on the pressure-temperature relationship (saturated temperature) for ammonia at different water dilution percentages:

• 0% water dilution at 0 PSIG = -28°F

• 10% water dilution at 0 PSIG = -25°F

• 20% water dilution at 0 PSIG = -20°F

• 30% water dilution at 0 PSIG = -12°F

The pressure-temperature relationship is greatly affected by increased water dilution in the ammonia refrigerant. So, when this occurs, the operating suction pressure for the compressors needs to be lowered to reach the desired temperature setpoints for the process. If the process required the ammonia to be at a saturation temperature of -28°F at 0 PSIG for the specific application, a water dilution of 20% would make the ammonia saturation temperature 8 degrees warmer. This is enough change to influence the refrigeration process, naturally the on-site operators might lower the saturation temperature by lowering the compressor suction setpoints (if the Evap/HX valve group utilizes a wide-open suction design or motorized pressure regulators). Reducing the operating pressure is the attempt to lower the temperature back down to the desired -28°F. The downside to doing this is the compressors are going to consume more energy to operate at lower pressures and result in higher operating costs (increase in BHP/ton).

Stay tuned to the MySafety blogs. Reach out to us if you have additional questions or concerns. Our team will be pleased to assist you.

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Unwanted Water in Ammonia Systems