Adding and Removing Oil


One of the important maintenance tasks in the operation of a refrigeration system is adding and removing oil. Oil is necessary in systems as a lubricant and, or, a sealant. Knowing how to identify and deal with the oil balance in a system is very important. Improper balance can have multiple effects on system operation.

The efficiency of the compressor’s oil separator will determine how much oil leaves the compressor along with the discharge vapor. Oil passing through a separator into the system should be removed.

It is also possible to have oil leave a screw compressor through the suction line if the suction check valve fails to function properly.

Earlier this summer I was visiting a fairly large freezing facility, and the following story was related to me:

It was a dark and stormy night (really) when a frantic call came from the corporate office asking a very capable refrigeration operator at a large refrigerated facility to go to another plant location immediately because that facility was not able to freeze product. The season length for the particular product was short, so having the refrigeration system function properly was extremely important.

Upon entering the machine room, it became quickly apparent to the visiting operator what one of the major problems was. The mechanical ammonia pumps would not produce enough pressure to properly circulate ammonia, and when stopped they would not rotate freely. They appeared to be oil-bound.

Another clue was the ammonia pump parts strewn about the machine room. The facility operators knew they had a problem with very little or no liquid getting to the freezing equipment, and because of this the facility was way short of its required freezing capacity.

Upon asking the refrigeration operators what their procedure was for getting the pump(s) to work, the answer was: “Well, we shut down the refrigeration, wait until the system pressure reaches about 20 psig, then we can turn the pump(s) back on, and get pressure.”

This procedure only sort-of worked. By allowing the system pressure to rise, the ammonia and oil temperatures rose. At their restart pressure of 20 psig both the ammonia and oil were warm enough that the pumps could produce discharge pressure. Yes, ammonia was circulating, and the evaporators could now start cooling, but also, whatever oil had been binding up the pump was sent out to the evaporators. Probably not the best plan.

Due to the critical nature of getting the system freezing properly as soon as possible, the facility had flown in two completely brand-new ammonia pumps. However, when ordering the pumps, the size and horsepower required was not checked. The new pumps were 2 HP, but they needed 3 HP for this system to have sufficient ammonia flow. Even though the pumps were undersized, at least they had something.

After further conversation with the facility operators, the visiting operator learned that they were not sure how to drain oil from the system, or from a piece of equipment such as the ammonia pumps, or from the oil pot. Also, they did not understand why it would be important to drain oil. The facility operators really needed proper training.

There was no oil log or tracking of how much oil was put into the system. It would have been helpful to have some idea of how much was also removed from the system. In this case no oil had been removed since they didn’t know how, or why.

Someone had installed a beautiful oil-fill system, with a nice oil pump, and stainless-steel tubing to each compressor. This allowed easy oil make-up to any compressor needing additional oil. Maybe a good idea, but in this case not so much. When a compressor needed oil, they filled it back to its proper operating level. That was good, but they didn’t seem to understand that oil disappearing from the compressors goes someplace in the system; it is not consumed. The visiting operator went to work and soon learned that it took about two hours for an ammonia pump to become oil-bound and quit pumping. At that point he would switch to the next pump, then isolate and evacuate the oil-bound pump, drain oil from the pump, then repeat the process. This sequence continued for three weeks, enabling the facility to keep freezing product.

The visiting operator became an expert at de-oiling the ammonia pumps. He said, “If this was an Olympic event, I could at least have gotten a Silver.” During the three-week struggle, the operator found that it was actually quicker to isolate and evacuate a pump, unbolt it from the volute, swab out the oil, put a new volute seal in and put it back together.

As this three-week process was going on, the visiting operator spent time looking over the piping design associated with the low-pressure receiver (LPR) and its oil pot. The oil pot did not vent to a high place on the LPR, but was connected below the LPR operating liquid level. How does that work? Not well.

The pump volute/evacuation line also connected below the LPR operating level. Further, each pump’s suction connection to the LPR dropleg was installed in such a way that it would allow oil to flow into a pump. In addition, the LPR float column was continually filling with oil because it was connected to the bottom of the LPR, not into the vessel dropleg, which would have allowed any oil present to flow into the dropleg. Float switches don’t like oil. The visiting operator said that the float column had to be evacuated and drained about every three days to allow the floats to operate properly. The oil just kept coming.

Because the pumps were pumping ammonia entrained with oil out to the system, this provided the operator with something to do in his spare time. Anything the oil picked up as it washed through vessels and piping was sent out to each evaporator liquid control assembly. During the threeweek freezing run, every liquid-line strainer in the system had to be cleaned twice. Time to sleep was hard to come by.

After the very hard three-week marathon, the visiting operator was finally able to shut the system down. As the system temperatures warmed up, he began draining oil from the LPR. 350 gallons of oil were removed just from this one vessel!

There are several lessons that can be learned from this event:

One: Adequate training of refrigeration operators in all of their duties to safely and properly operate a system is required.

Two: Develop and use well-thoughtout and understandable operating procedures, including maintenance and inspection procedures.

Three: Oil management is very important. Oil in must closely equal oil out.

Four: The piping design/configuration needs to be evaluated by a component person or group prior to assembly. This should be addressed following the “Management of Change” procedure.

Five: An active PSM/RMP or ARM program can address all of these issues and more.

The International Institute of Ammonia Refrigeration has helpful resources addressing oil draining such as:

Video: Series III Module 1, “Removing Oil from an Ammonia Refrigeration System”;

Poster: “IIAR Recommended Oil Draining Guidelines”.

Also, search through the e-library of past conference tech papers and you can find lots of information regarding oil in a system.