Oil that is collected at a drain point of an active system is under pressure. The oil has some liquid refrigerant suspended in it.
When the oil is taken out of the system it is done so by a pressure differential. Generally the lower pressure is atmospheric pressure. The storage pressure depends on what the system pressure is where the oil is being drained from.
The warmer the oil, the greater the amount of suspended ammonia will be in it. This also means that the starting pressure from where the oil is being drained is higher.
When the oil is metered out from the starting pressure to the atmospheric pressure at the outlet of the oil drain valve, there is an expansion of the ammonia. Do you remember that when ammonia expands (pressure is reduced) that there is a corresponding reduction in temperature?
The cold ammonia is causing the oil to congeal (thicken up). The oil can become as thick as a paste.
This causes the rate of flow to decrease through the oil drain valve.
The impatient operator sees that the oil flow has slowed or stopped, so he opens the drain valve even more. This opening of the valve and stoppage of flow may be repeated a few times.
Suddenly, there is no more oil coming from the oil drain valve. What is coming from the valve is a dangerous white aerosol cloud of liquid ammonia and gas.
Unless the operator is very fortunate and can get the oil valve closed a very serious and possibly, lethal ammonia release has just been started.
One relatively inexpensive and very effective prevention measure for this hazard is to install the ¼ turn, self-closing ball valves that are now available for oil draining points.
Since draining oil from an ammonia refrigeration system is dangerous, there are some basic precautions the operator should take before performing the task of oil draining. These precautions include:
A written standard operating procedure for this task!!!!
Eye protection, use indirectly vented goggles
Facial splash protection (face shield)
Respiratory protection, close at hand, such as a negative pressure full-face respirator with cartridge type filters.
Skin protection, chemical resistant gloves (do not use leather driving gloves because if they get soaked with liquid ammonia they will stick to your skin)
Communication, inform others in the area what you are going to be doing and that there could be a slight odor coming from where you are working (we are assuming that everybody has been trained to know the difference between a slight odor and a problem odor).
A source of live water (charged hose with nozzle) for first aid
A clear path of escape
We hope you have an improved understanding of oil removal hazards for ammonia systems now.
Although the oil generally does not mix with ammonia, some small amount of absorption will occur. So when the oil is drained to the atmosphere some of the absorbed ammonia is released into the air making the job of draining oil into a bucket very unpleasant and potentially very dangerous.
When draining oil into an open bucket, the oil drain valve should be opened only very slightly. If it is opened too wide, liquid ammonia will blow through the oil. With the drain valve only cracked (slightly opened), the oil in the bottom of the vessel will flow slowly to the drain valve. This oil usually emerges as a "foam". If ammonia vapor comes out instead of "foam" or the drain valve starts to frost over, close the valve. At this point most of the oil has been drained.
Oil cannot be drained from evaporators that operate in a vacuum. To remove oil from evaporators operating below zero psig the evaporator must be valved off and the pressure allowed to increase to at least 15 psig before draining oil.
Many modern ammonia systems employ liquid overfeed (liquid recirculation) systems to supply liquid ammonia to the evaporator coils. A typical system package, which basically consists of multiple liquid refrigerant pumps (one usually acts as a standby) located beneath a low pressure refrigerant receiver is shown in figure 7-2.
Many of these packages have an oil pot incorporated into them.
This pot collects the oil for draining.
If oil is not removed from the oil pot, the oil will eventually work its way to the ammonia pumps where it will interfere with the pump delivery capacity. When used properly, the oil pot, gives the safest and most manageable means of oil removal from the low side of the system.
One location in a system that collects oil is the oil separator. With older systems with compressors such as some VSA machines the oil separator often must be drained manually. Two methods can be used: draining directly into an open bucket or draining through piping into an oil still as shown in figure 7-5 (color fold out). Oil must be drained when the level approaches the top of the gauge glass in the separator.
If the refrigeration plant has an oil still, the oil is piped from drain valves to the still as shown in figure 7-2. The oil still removes nearly all the liquid ammonia from the oil. The still is a vessel containing a heating element which is usually an electric type (low wattage) or in some cases a steam coil loop is inside the vessel. This supplied heat boils the liquid ammonia away from the oil. The vaporized refrigerant is vented from the oil still to the suction accumulator.
To charge the still with oil to be drained:
Open the vent valve (N) between the still and the suction line.
Crack the oil drain valve (H) on the component to be drained (for example the brine chiller). Oil will now flow to the still from the brine chiller.
The appearance of frost on the brine chiller oil drain valve will indicate draining of oil is complete and ammonia rather than oil is flowing.
Close the chiller drain valve (H).
For best results, only one component should be drained at a time. When all oil has been drained to the still and oil drain valves have been closed, turn on the heater in the oil still. The heat drives the liquid ammonia out of the still and the ammonia vapor goes through the open vent valve (N) on top of the still to the to the compressor suction.
The distilling operation usually takes several hours depending on the amount of ammonia that is drained out with the oil. Frost on the still indicates it contains liquid ammonia. When the distilling operation is finished, turn off the heater and close the valve in the stills' vent line (N). The oil can then be drained from the still to an open bucket.
Realizing that drained oil will have an ammonia smell for a while, care must be exercised when disposing of drained oil. There are both environmental and safety issues involved. It is important to know what your company policy is in this regard and if such a policy does not exist, management should be informed that one is needed.
Your plant may have only one oil draining system, or many different types. For example, you could have an oil draining system using a still as described above. Or your system may use an oil pot as discussed in figure 7-3. Or your system may drain directly from a drop leg of a vessel. You should clearly understand all of your oil draining systems and your standard operating procedures for draining oil should be written with this in mind.