The staged concentration process relies on liquid level ranges to switch between feeding, recirculation, concentration, and discharge operations. The liquid level buffer range refers to the margin reserved between the high and low trigger points in the program. If this range is set too narrow, even slight fluctuations in the liquid level will trigger a jump in the control logic, causing a series of cascading abnormalities in the entire evaporation process. This initially manifests as frequent false triggers in the automatic control logic, which then propagates to the material recirculation, heat exchange, and vacuum systems; in severe cases, this can also increase the risk of scaling and pump damage.

When the buffer range is insufficient, even minor fluctuations in the liquid level can cause the program to repeatedly switch back and forth between feed and concentration modes. In normal staged concentration, feeding stops when the high liquid level is reached, entering the concentration phase; feeding does not resume until the liquid level slowly drops to the lower limit. If the buffer range is too narrow, liquid level fluctuations caused by material circulation or bubbling and surges resulting from vacuum boiling can cause the liquid level to momentarily exceed the set threshold. The system will briefly interpret this as reaching the lower limit, triggering feed activation; as the liquid level immediately rises, it will promptly trigger a feed shutdown, resulting in frequent, short-cycle on-off cycling of the feed valve. As the feed flow suddenly opens and closes, the feed solution continuously rushes into the evaporation chamber, instantly lowering the material temperature inside the chamber and altering the boiling state. Consequently, the vacuum level experiences erratic fluctuations, and the vacuum sensor readings drift back and forth, which can easily be misinterpreted on-site as a pipeline leak or abnormal condensation heat exchange.

Continuous feed disturbances disrupt the stable control of the concentration ratio. The core objective of staged concentration is to progressively enrich the material to the target concentration. However, with a narrow liquid level buffer, small single feed volumes, and frequent replenishment, the concentration of the material inside the chamber fluctuates continuously, making it impossible to maintain a stable concentration gradient. For wastewater systems containing silicate colloids and salts, local concentrations repeatedly cross the metastable saturation line, disrupting the precipitation patterns of colloids and salt crystals. These particles alternately adhere to the heat transfer surfaces and are washed away by the fluid, forming uneven scale deposits. As the scale thickens in certain areas, heat transfer efficiency declines, causing further instability in evaporation rates, which in turn exacerbates liquid level fluctuations, creating a vicious cycle.

The material circulation pump is also subjected to shock loads. When the liquid level is too low, the static pressure at the pump inlet is insufficient. Within a narrow buffer range, as soon as the program triggers material replenishment, the system briefly enters the low-level zone before sufficient submersion depth is established, causing the pump to easily draw in large amounts of bubbles generated by boiling, resulting in cavitation. Cavitation causes pressure pulsations at the pump outlet, leading to sharp fluctuations in flow rate on the feed side. This results in unstable flow velocities within the plate heat exchanger channels, causing previously suspended crystal particles to deposit intermittently at channel bends—creating a potential risk of localized bridging and blockages. Repeated cavitation also causes continuous damage to the pump impeller and seals, shortening the service life of the circulation pump.

The vacuum system and demister will also be affected. When the feed is introduced at ambient temperature, a large amount of flash steam is released inside the chamber in a short burst, causing the gas-phase flow velocity to surge suddenly. The gas-phase load on the demister exceeds the design range, making it more prone to mist entrainment, whereby the feed solution is carried into the condenser side and contaminates the condensate. A large amount of instantaneous water vapor surges into the condenser, causing fluctuations in condensation pressure and the distribution of non-condensable gases. The vacuum pump’s pumping load fluctuates cyclically, making the vacuum pump oil more susceptible to contact with water vapor and volatile additives, which accelerates oil emulsification and degradation.

Automated interlock alarms may also malfunction. The thresholds for high and low liquid level alarms and interlock shutdowns are set very close to the operating range; under normal operating conditions, even minor fluctuations in the liquid level can trigger a liquid level warning or even a protective shutdown. Frequent false alarms can cause operators to become complacent about genuine malfunctions, making it difficult to identify a true loss of liquid level control when it actually occurs. Frequent valve cycling also accelerates wear on the feed solenoid valves and pneumatic valve diaphragms, increasing the likelihood of internal valve leakage and further exacerbating loss of liquid level control.

A typical characteristic of this type of failure observed on-site is that all parameter fluctuations are strongly correlated with the program’s liquid level switching actions. After widening the liquid level buffer range, vacuum fluctuations, frequent opening and closing of the feed valve, and intermittent mist entrainment are significantly reduced.

The corresponding optimization approach involves moderately increasing the buffer margin between the high and low liquid level trigger points to distinguish between measurement fluctuations and actual liquid level changes; adding a time-delay filter for liquid level signals within the program to filter out transient false signals caused by boiling and foaming; and appropriately reducing the concentration ratio for each stage of multi-stage concentration to minimize sudden concentration changes caused by a single enrichment cycle; introduce a delay in the interlock between low-level protection and pump operation to prevent pump cavitation caused by sudden drops in the liquid level.