Hydrolysis of Vegetable Oils in Hydraulic Elevators
October 26 IN White Papers

Hydrolysis of Vegetable Oils in Elevators

Michael Johnson



Vegetable Oil in hydraulic elevators has been met with mixed results, from no problems at all to destruction of the oil and system components.  The unique environment and design characteristics of the hydraulic elevator are the causes of this variation in performance.

Water or moisture is the natural enemy of the vegetable oils or technically Ester oil.  [1]Hydrolysis is the reaction of an ester with water producing an alcohol plus and acid.  In the case of vegetable oils, water reacts with the triglyceride removing a fatty acid and leaving an alcohol group.

This is observed in hydraulic elevators through the polymerization or thickening of the oil.  Increases in viscosity, a sticky film on equipment in the machine room and a thick sludge in the reservoir are all indications of water contamination and the failure of the vegetable oil.  Test for Viscosity, Acid Number, Water Content and Alcohol can help identify the progress of oxidation in a vegetable oil.

A rancid smell in the oil is the result oxidation.  Reactions with common metals in hydraulic cylinders such as iron and copper further promote this oxidation process.  [2]The Mechanisms of oil oxidation for edible oils have been well studied in the food industry due to palatability and health concerns.

Antioxidants added to the vegetable oil to prevent autooxidation can be destroyed in the presence of heat and water leaving the vegetable oil unprotected.

The logical question is why does vegetable oil work in some cases, but fails in others.  Vegetable oil must be kept cool and dry to function properly and avoid rapid oxidation.  Mineral oils must also be kept cool and dry, however, mineral oil can withstand higher levels of water without oxidizing.

A hydraulic elevator that receives minimal use, in a personal residence for example, does not generate the heat that an elevator in a government office building would.  As long as the oil in the lightly used elevator is kept cool and dry, the vegetable oil should perform as designed.

Some of these problems are self-imposed.  Typically, the hydraulic reservoir on an elevator is undersized due to space limitations imposed by building design.  A properly designed hydraulic reservoir will hold two times as much oil as is required to fully extend.  Most elevators hold enough oil to reach the top floor, plus 10%.  This is further complicated when the reservoir is run low on oil to save cost or prevent spills.  There is no financial benefit to running an elevator low on oil.

The extra oil volume allows for proper residence time in the reservoir for the air and heat to be released from the oil before it is drawn back through the pump.  Air is the cause of oxidation and heat is the catalyst.  Water, leads to air entrainment of the oil, observed by cavitation.

Hydraulic elevators utilize a fixed displacement pump; this means the same volume of oil is pumped through the system, regardless of the speed of the car.  The valve is responsible for slowing and stopping the car.  Any oil that does not go to the piston, is bypassed through the valve generating heat.

Due to the large volume of oil required to reach contract speeds, an open reservoir is utilized in hydraulic elevators.  Ambient air from the machine room enters the reservoir every time oil leaves the reservoir; this air contains moisture and particles.  It is possible to have water contamination without an obvious source of water ingress.

The large volumes of bypassed oil entrains air, especially if there is water present, and then is drawn back through the pump and heated once again.  This process is continuous, resulting in oil oxidation and micro-dieseling.  The ester is quickly destroyed resulting in sludge in the reservoir and atomization of the oil observed as a sticky film in the machine room.

Increases in viscosity, acid number and alcohol are indications that the vegetable oil is damaged and must be replaced.  Hydrolysis is not reversable in this situation, so replacement is the only option.  Residual sludge must be removed from the pipes and casing to prevent immediate contamination of the `new oil.  Offline filtration can effectively remove this debris.

Vegetable oil can be used, but it must be kept cool and dry.  Notable heat reductions have been observed by the removal of water from hydraulic oils, therefore, the priority should be on preventing and removing water contamination of vegetable oil; if the water is excluded, the vegetable oil life may be prolonged.

The reservoir should be kept full to allow sufficient oil residence time in the reservoir and HVAC should be sufficient to remove warm moist air from the machine room.  Cooling the air, but failing to remove the moisture from the machine room is useless.

Regular monitoring and active water filtration are essential to preserving the vegetable oil.  Please visit www.oilscrubber.com to learn more about our water removal and oil monitoring solutions for hydraulic elevators.


[1] N.J. Fox, G.W. Stachowiak/Tribology International 40 (2007) 1041

[2] Choe, Eunok, and David B. Min. “Mechanisms and Factors for Edible Oil Oxidation.” Comprehensive Reviews in Food Science and Food Safety, Wiley/Blackwell (10.1111), 29 Sept. 2006, onlinelibrary.wiley.com/doi/epdf/10.1111/j.1541-4337.2006.00009.x.