Under low-temperature evaporation and negative pressure conditions, alcohol-based additives in wastewater volatilise to form alcohol vapour, which is drawn into the pump chamber by the vacuum pump along with non-condensable gases. The alcohol continuously mixes with the vacuum pump oil, causing multi-faceted deterioration of the oil and resulting in damage to the pump components. This entire deterioration process develops gradually rather than resulting in a sudden failure.

Firstly, the oil becomes diluted and its viscosity drops significantly. Alcohols are small-molecule organic solvents that are miscible with vacuum pump oil. As alcohol vapour enters the pump and continuously dissolves into the oil, it directly dilutes the base oil, causing the oil’s kinematic viscosity to decrease continuously. Consequently, the strength of the oil film deteriorates, preventing the formation of a reliable sealing oil film between the rotor and the vanes within the pump chamber. The immediate consequence is a decline in the vacuum pump’s ultimate vacuum capacity; the vacuum level fails to reach the original negative pressure, and the pump’s sealing performance deteriorates. At the same time, lubrication capacity decreases, leading to increased wear on the vanes and bearings, and higher operating noise levels. On-site, one may observe an apparently unexplained rise in the oil level, which is caused by the condensation of alcohol vapour dissolving into the oil.

Secondly, the oil, water and alcohol emulsify, forming a cloudy emulsion. In addition to alcohol vapour, the low-temperature evaporative exhaust gas also carries a large amount of water vapour; both water and alcohol enter the pump oil together. The alcohol acts as an emulsifying agent, allowing the originally immiscible oil and water to mix stably, producing an emulsified, cloudy oil phase. The appearance of the oil changes to a milky white or yellowish-turbid colour, no longer being a transparent liquid. Once emulsified, the oil’s sealing, lubricating and anti-foaming properties are completely compromised. The emulsified oil traps a large amount of moisture; when the pump operates and heats up, this internal moisture vaporises, further disrupting the negative pressure within the pump chamber and causing the vacuum to fluctuate erratically. Once an emulsion has formed, it is difficult to separate the layers and break the emulsion simply by allowing the oil to stand; the oil must be replaced.

Thirdly, oxidation leads to the formation of gel-like sludge deposits. Alcohols are inherently prone to oxidation; under conditions of high friction and heat within the pump housing, combined with the infiltration of small amounts of oxygen, the alcohols oxidise to form aldehydes and organic acids. These oxidation products react with the vacuum pump oil, gradually forming viscous gels and sludge. The gel adheres to the sliding vanes, rotors, exhaust valve plates and small orifices in the oil passages. The accumulation of sludge can block the oil pump’s return oil channels, causing localised oil supply shortages; if the exhaust valve plates become stuck by the gummy deposits, they will not close properly, directly leading to a decline in pumping efficiency. Over time, brownish-red viscous deposits will form inside the pump chamber; upon disassembly of the pump body, a gummy residue can be observed adhering to the surfaces of the components.

Fourthly, there is component corrosion caused by acidic substances. The oxidation of alcohols produces organic acids; combined with small amounts of acidic volatile impurities carried in the exhaust gases, these mix with the oil, causing the acid number of the pump oil to rise continuously. Acidic media cause slow corrosion of the cast iron and carbon steel components inside the pump, generating metal rust particles. These rust particles, when mixed with the oil, not only accelerate oil degradation but also act as abrasives, exacerbating abrasive wear between the sliding vanes and the inner walls of the pump chamber; fine rust particles can also clog the oil filter, resulting in insufficient oil supply. Certain aluminium alloy components may also be eroded by organic acids, leading to pitting corrosion.

Fifthly, the saturated vapour pressure of the oil rises, causing a permanent deterioration in vacuum performance. The saturated vapour pressure of alcohol itself is far higher than that of oil specifically designed for vacuum pumps. Even if only a small amount of alcohol remains dissolved in the oil, the saturated vapour pressure of the entire oil mixture will rise significantly. Even if part of the oil is replaced, residual alcohol molecules adsorbed in the piping and gaps within the pump chamber will prevent the unit’s ultimate vacuum from returning to the factory-set level; this manifests as the vacuum pump mechanism itself being in good working order, yet consistently failing to achieve high negative pressure ranges.

Distinguishing symptoms under operating conditions: when the alcohol content is low, the fault is subtle, manifesting only as a gradual decline in vacuum capacity and slight cloudiness of the oil; when the alcohol concentration is higher, symptoms such as a rise in oil level, emulsification and whitening of the oil, increased pump noise and significant vacuum fluctuations appear within a short time. After shutdown and cooling, alcohol vapour condenses further inside the pump, and the deterioration in operating conditions is particularly pronounced at the start of the next operation.

A common misjudgement is to attribute the deterioration of the vacuum pump oil solely to water vapour, whilst overlooking the emulsifying and oxidising effects of alcohols that lead to gum formation; repeated oil changes will not resolve the issue if the exhaust gas at the source remains untreated, and the new oil will quickly deteriorate again.

The corresponding prevention and control strategy involves installing exhaust gas pre-treatment upstream of the vacuum pump, including a condensation trap, to condense and capture alcohol vapour and water vapour as much as possible in advance; an adsorption tank should be added to adsorb any non-condensable alcohol components; effective gas-liquid separation must be ensured to minimise droplet entrainment; vacuum pump oil inspection intervals should be shortened to monitor oil colour, oil level and turbidity; reliance on simple oil draining and topping-up for maintenance is not recommended; once emulsification and gel formation have occurred, the oil must be completely replaced.