The key reasons for foam explosion and an instantaneous drop in vacuum pressure upon restarting the low-temperature evaporation system after it has been left idle overnight

1. During the shutdown and idle period, surfactants, colloids and oils in the waste liquid spontaneously concentrate and stabilise the foam

The waste liquid contains surfactants, trace emulsified oils, organic matter and colloids. When left stationary overnight:

With no circulation or agitation, these substances gradually rise to the surface, forming a high-concentration, stable foam film;

The longer the system is left undisturbed, the higher the concentration of foaming substances at the surface becomes, maximising foam stability. The moment the system is restarted the next day and agitation and circulation resume, the foam bursts instantly, surging rapidly upwards towards the demister.

2. Salt precipitation and microcrystals acting as a ‘foam skeleton’ during overnight standstill

When high-salinity wastewater is left to stand at low temperatures during shutdown, fine salt crystals slowly precipitate and remain suspended in the liquid phase and at the surface;

These tiny crystals become embedded within the foam film, acting as a supporting framework,

making the foam more resistant to collapse, larger in volume, and longer-lasting—to the extent that normal defoaming operations cannot suppress it.

3. Non-condensable gases accumulate in the vapour chamber and piping after shutdown, breaking the vacuum the moment the system is restarted

After standing overnight:

The system cools down and draws in air under negative pressure;

Dissolved air, trace amounts of ammonia nitrogen and volatile substances in the water are released;

Large quantities of non-condensable gases accumulate in the vacuum piping and dead corners of the condenser;

The next day, as soon as the pump is started and evaporation begins, the accumulated non-condensable gases are released instantly,

directly disrupting the original vacuum equilibrium, causing the vacuum level to drop instantly and fluctuate significantly.

4. Cold material overnight + low-temperature tank walls: chaotic flash evaporation upon restart

After standing overnight, the entire tank’s contents, tank walls and piping have cooled to ambient temperature;

the moment heating is restarted, the cold material heats up rapidly in localised areas, causing uneven flash evaporation,

resulting in a sudden, massive burst of vapour that disrupts the gas-liquid separation equilibrium, generating foam whilst simultaneously destabilising the vacuum.

5. Anaerobic fermentation of residual liquid in dead zones generates gaseous products that cause foaming and disrupt the vacuum

Residual liquid accumulates in the dead zones of the vessel and at low points in the piping, remaining sealed and at ambient temperature overnight:

Organic matter, as well as sulphur- and nitrogen-containing substances, undergo anaerobic fermentation, generating trace amounts of gas;

Upon restarting the flow, large quantities of gas are released, which not only enlarges the foam but also increases the gas volume in the system, directly reducing the vacuum level.

6. Sudden transition from static to turbulent circulation, with liquid surface churning triggering foam formation

The liquid surface remains calm after standing overnight; upon restart, the circulation pump causes sudden, intense agitation,

resulting in violent churning of the liquid surface and the entrainment of air. This combines mechanical foaming with chemical foam stabilisation,

instantly filling the tank with foam. Simultaneously, gas-liquid entrainment increases dramatically, blocking the gas phase channels and further causing pressure build-up that breaks the vacuum.

7. Condensate accumulates on the demister overnight, causing vapour lock to surge upon restart

After shutdown, secondary steam condensate remains trapped on the demister mesh, accumulating overnight;

Upon restart, a large volume of steam surges upwards, causing liquid blockage on the mesh and a sudden surge in vapour lock,

preventing vapour from escaping, leading to pressure build-up in the evaporation chamber, an instantaneous drop in vacuum, and further exacerbating mist entrainment.