Localised overload in demisters caused by a mismatch in gas-liquid flow rates differs from obvious faults such as complete blockages or extensive material carryover; many of these phenomena exhibit gradual, localised and subtle characteristics, which are easily overlooked during on-site inspections as normal operating conditions or minor material carryover.

Firstly, there is a slight, erratic drift in vacuum parameters—not a sustained, significant drop in vacuum, but rather minor fluctuations over short periods. This is because, in the locally overloaded areas, the liquid film continuously accumulates and intermittently ruptures, momentarily generating entrained fine liquid droplets which increase the resistance to gas-phase flow; consequently, the vacuum reading fluctuates slightly. As the amplitude of these fluctuations is very small, standard alarm thresholds are not triggered; many operations and maintenance personnel attribute this to the vacuum pump or minor non-condensable gases on the condenser side, overlooking the root cause: local liquid load overload in the demister.

Secondly, there is a gradual, unidirectional rise in the demister’s pressure differential; the pressure differential does not spike suddenly, but increases slowly by a few pascals each day. The overload is confined to a small area, whilst the remaining flow channels remain unobstructed, so the overall change in pressure differential is not dramatic. Continuous impact by droplets forms a thick liquid film on local sections of the mesh or baffles; this film continuously traps subsequent mist droplets, gradually thickening, causing the pressure differential to rise slowly. These changes can only be detected by long-term historical data records; single-point instantaneous pressure measurements rarely reveal the anomaly.

Thirdly, there are intermittent, minor exceedances of condensate water quality standards; the parameters are not consistently non-compliant, but rather exhibit slight fluctuations at random intervals. At localised points of overload, the liquid film is periodically torn, releasing micron-sized fine mist. These droplets enter the condensate system, causing occasional slight increases in the COD and salinity of the condensate. If sampling and testing happen to occur outside the moment when these droplets are entrained, the test results will return to the acceptable range; this is easily misinterpreted as the effect of fluctuations in the feed concentration of the raw liquid, making it difficult to link the issue to localised imbalances in gas velocity within the demister.

Furthermore, certain localised areas of the demister remain persistently wet. When the unit is shut down and the cover removed for inspection, the demister plates exhibit distinct dry and wet zones: whilst some areas are dry and clean, a small, fixed area remains chronically saturated, and may even harbour a thin film of sludge or a thin layer of salt crystals. This is due to a gas flow imbalance, whereby a large volume of mist is concentrated and directed towards this specific location, resulting in continuous scouring by the liquid film; in contrast, the remaining areas experience low gas flow rates and virtually no contact with mist, thus remaining dry. This localised thin scale build-up does not immediately block the flow channels but accumulates gradually; upon initial inspection, it may be mistaken for residual moisture left after cleaning.

Furthermore, there may be subtle changes in the internal noise of the chamber. Whilst the noise level remains stable during normal operation, localised overloading may result in intermittent, faint dripping sounds or a hissing noise caused by the tearing of the liquid film; these sounds are often masked by the equipment casing and fan noise. Essentially, this occurs when the locally accumulated liquid film reaches a critical thickness and breaks apart under the shear force of the gas stream, causing the sound to come and go intermittently; it is extremely easy to overlook in the noisy workshop environment.

In terms of long-term latent damage, the base material of the demister will experience accelerated localised pitting corrosion, particularly in metal wire mesh types. Persistent liquid film retention traps salts and corrosive ions from the feed liquid within this small area, creating a continuously saturated corrosive environment, whilst the corrosion rate in the remaining dry areas remains very low. No damage is visible to the naked eye in the short term; however, the wall thickness of the base material gradually decreases. By the time wire mesh fractures occur and fragments detach into the downstream pipework, the failure is already at an advanced stage, with no obvious visual indicators having been present for a considerable period beforehand.

Furthermore, uneven thin-layer deposits may form on the inner walls of the downstream vacuum pipework; this is not scaling across the entire length of the pipe, but rather the adhesion of trace amounts of salt deposits to one side of the inner wall. The fine mist generated by localised overload is transported in a directed manner along the path of the gas-phase flow deviation, depositing only on the side wall of the pipe corresponding to the gas flow; the rest of the pipe remains clean. When the pipe is dismantled for inspection, this is easily mistaken for an isolated instance of minor material leakage, without realising that it is the result of long-term, localised overload continuously entraining material.

The key characteristic of this type of latent behaviour is that it does not occur uniformly across the entire system; rather, only the local area corresponding to the gas-phase deviation bears a droplet load exceeding the design specification. The fault signals are sporadic and faint; no obvious failure occurs all at once. Instead, damage accumulates gradually during continuous operation, eventually leading to a sudden, large-scale material leakage when feed fluctuations or minor disturbances in the vacuum occur.