Alcohol-based additives are inherently highly water-soluble; however, under low-temperature, negative-pressure evaporation conditions, some of the alcohol is not completely retained by condensation. Instead, it enters the vacuum pump in the form of gas or tiny liquid droplets along with non-condensable gases. Prolonged contact with the vacuum pump oil triggers multiple forms of degradation, and various failure phenomena occur progressively and cumulatively.

First, oil dilution and a decrease in viscosity occur. Alcohols are small-molecule polar organic compounds that are miscible with mineral-based vacuum pump oil; once they enter the pump chamber, they mix directly with the oil, continuously reducing the viscosity of the base oil. As a result, the strength of the oil film diminishes, preventing the formation of a stable sealing oil film between the rotor and stator. The vacuum pump’s ultimate vacuum gradually deteriorates, and the pumping speed declines. Furthermore, this process appears to be reversible: when the pump temperature rises, some of the alcohol mixed into the oil volatilizes, causing a temporary recovery in vacuum. However, once the temperature drops, the alcohol redissolves into the oil, leading to repeated fluctuations in vacuum. On-site personnel often misinterpret this as pump wear or a leak in the piping.

The second issue is oil-water emulsification. In addition to alcohol vapor, the exhaust gas evaporated under negative pressure also contains a large amount of water vapor. The alcohol acts as an emulsifier, breaking down the interfacial tension between oil and water, allowing water to disperse stably within the vacuum pump oil and form an emulsion. The emulsified oil exhibits reduced fluidity and significantly diminished lubricating properties, appearing cloudy and whitish. The emulsified system also impedes heat transfer, causing the pump’s operating temperature to rise and further accelerating oil oxidation. If emulsification continues to worsen, the oil will separate into layers, with free water accumulating at the bottom. This free water directly corrodes the pump’s internal cast iron and carbon steel components, producing rust particles. These solid impurities, when mixed with the oil, exacerbate wear on bearings and vanes.

This is followed by oil oxidation and an increase in acid value. Alcohols are highly prone to oxidation; under the localized high-temperature conditions generated by friction within the pump chamber, they undergo oxidation reactions, producing small-molecule acidic substances such as aldehydes and organic acids. These acidic components dissolve in the oil, continuously raising its acid value. On the one hand, this corrodes the pump’s internal metal components, generating metal ions; on the other hand, these metal ions catalyze the accelerated oxidation of the vacuum pump’s base oil, creating a vicious cycle. As the acid value rises, the oil also corrodes the pump’s internal rubber seals, causing them to swell, harden, and crack, which further exacerbates leakage.

Additionally, sludge and gummy deposits form. The aldehydes produced by alcohol oxidation readily undergo condensation reactions to form high-molecular-weight gum. This gum mixes with oxidation byproducts and rust particles, depositing on the inner walls of the pump chamber, in the oil passages, and inside the oil filter and oil separator, forming sludge. The accumulation of sludge can block oil passages, leading to insufficient lubrication and oil supply, which causes the vanes and bearings to overheat due to oil starvation. When the oil separator becomes clogged with gum, the oil content in the exhaust gas skyrockets, resulting in oil-laden exhaust that can also contaminate the downstream activated carbon adsorption unit. Sludge adhering to the interior of the pump chamber hardens after the pump is shut down and cooled; when the pump is restarted, this increases the startup load and, in severe cases, can cause the vanes to seize.

There is also a hidden effect: the continuous intake of alcohol vapor at the pump inlet alters the partial pressure of the gas phase inside the pump. As alcohol vapor continuously dissolves in the pump oil—effectively turning the oil into a carrier for the alcohol—the dissolved alcohol is released when pressure rises during the pump’s exhaust stroke. This increases the exhaust load, causing the pump to generate more heat and further exacerbating all the aforementioned degradation issues.

These issues can be easily distinguished by on-site characteristics: simple water ingress and emulsification causes the oil to turn white; degradation caused by alcohol contamination causes the oil to gradually turn yellow or brownish, accompanied by a distinct solvent odor. Vacuum performance exhibits significant variations between hot and cold conditions—vacuum improves as the oil temperature rises and declines as it cools.

Corresponding preventive measures include, as a priority, installing reliable condensation, collection, and defoaming devices upstream of the vacuum pump to trap as much alcohol vapor and water vapor as possible; installing a low-temperature adsorption tank to pre-treat alcohol in the exhaust gas; selecting synthetic vacuum pump oil resistant to hydrolysis and solvents to improve tolerance; and shortening the oil sampling and testing cycle to monitor viscosity, acid value, and appearance, replacing the oil promptly if any parameters deviate.