Oil changeover on 4 ammonia refrigeration compressors at a pangasius processing plant, Dong Thap

Objective

An export pangasius processing plant runs four ammonia screw compressors (built 2007–2014) for its freezing system and cold stores; at the time of changeover the machines had 11,400–49,100 running hours and the old oil drained out black with sludge. The plant switched to the ammonia refrigeration compressor oil CPI-1009-68, a hydrotreated mineral oil with a pour point of −43 °C per TDS; the engineer checked the system's lowest evaporating temperature against this pour point before choosing it. What was needed was a method that would not let old oil left in the separator, receiver, piping and filter contaminate the new fill straight away, because in an NH₃ system the oil does not dissolve in the refrigerant but settles at every low point.

Compatibility check before the change

Before the work, the engineer identified the base oil in the machine from the oil label, the machine manual or the previous supplier. Between mineral oils for ammonia compressors no transition oil is needed, but there is still no topping up: drain completely at every low point, clean the filter, fill new oil from a single batch and record the batch number in the machine file for reference in later oil analysis. If the oil in use has a different base (PAO or another synthetic), flush according to the TDS and check seal materials first. POE oils (oils for HFC refrigerants) are not used in ammonia systems. A Hop Tien engineer confirms the procedure for each machine before the change.

Method

Hop Tien technicians and the maintenance team followed the same sequence on each machine at shutdown: drain the old oil from the compressor unit and the separator through the machine's oil drain valve; remove the oil filter housing, clean out the sludge and refit it; open the drain valves at every low point of the receiver, separator and piping in turn until the oil runs clear; only then pump CPI-1009-68 into the machine. The condition of the oil drained at each point was recorded for comparison at the next maintenance stop.

Safety when draining oil from an NH₃ system

Oil drained from an ammonia system may still contain dissolved NH₃. The sequence Hop Tien applies when draining oil from an NH₃ system, following the plant's ammonia safety procedure: drain through the oil drain pot or the machine's oil drain valve, opening the valve slowly into a closed container with a lid, never into an open pan; the operator wears safety goggles, gloves and a suitable respirator; eyewash water is available on the spot; the compressor room is ventilated throughout the draining; waste oil is collected in lidded drums for disposal according to regulations.

Result

All four machines were changed over completely during the October 2016 maintenance stop without dismantling. The machine hours recorded at the changeover are the reference point for tracking make-up oil and taking analysis samples at later maintenance stops. This sequence is the Hop Tien standard for ammonia refrigeration compressor oil changeovers: never fill new oil on top of old oil.

Photos from the site

Changeover report, October 2016. Plant name and compressor make are withheld in line with Hop Tien's practice.