Rubber seals that are exposed to organic volatile compounds under negative pressure conditions over the long term undergo a cycle of swelling followed by re-hardening; leaks do not occur continuously or consistently, but rather manifest as intermittent, erratic behaviour. The root causes lie in the negative pressure itself, the non-uniform ageing of the material, and dynamic changes in operating parameters, which collectively alter the contact condition of the sealing surfaces. In many cases, instruments only detect brief drops in vacuum; once the alarm has cleared, the system returns to a prolonged period of stability, making it highly likely to be misinterpreted as an isolated instrument fluctuation.

Firstly, the ageing process of the rubber itself is non-uniform; organic volatiles do not penetrate the entire cross-section of the seal simultaneously. The surface layer absorbs the organic compounds first, causing swelling and an increase in volume. For a short time, this compresses the sealing surfaces into close contact, temporarily blocking the leakage pathways; during this phase, the vacuum appears entirely normal. Following prolonged contact, the infiltrating organic compounds gradually leach out the plasticisers from within the rubber. The rubber matrix progressively loses its toughness and becomes hardened and brittle. On the same seal ring, some areas retain higher levels of swelling and maintain reasonable elasticity, whilst others have already become hard and stiff. The cross-section of the seal thus exhibits alternating soft and hard regions, and the sealing surface is no longer in continuous, uniform contact. When the system is at rest or operating under stable conditions, the area under pressure happens to be a section that still retains some elasticity, so the gap closes and no leakage occurs. However, as soon as conditions change even slightly and the point of force shifts, the hardened, non-resilient areas will force open a minute air gap, causing a leak. Once conditions return to normal, the gap closes again under the combined effect of its own weight and the clamping force, and the leak ceases of its own accord.

The pressure characteristics of a negative-pressure environment are the key contributing factor, and this differs markedly from leakage in positive-pressure seals. Under positive pressure, the medium continuously pushes against the sealing surface; once a leak point forms, leakage usually persists. Under negative pressure, however, the external atmosphere is drawn into the chamber by the pressure differential, and leakage occurs as soon as a micro-channel connecting to the atmosphere exists. Once the seal has hardened, micro-cracks and surface cracking patterns may develop; however, these cracks are closed in themselves. In the absence of external disturbance, the crack walls remain in contact with one another, preventing the entry of atmospheric air. Minor vibrations during unit operation, pulsations from circulation pumps, and pressure pulsations caused by vacuum extraction all induce minute reciprocating displacements at sealing points such as flanges, end caps and sight glasses. During a vibration, the micro-cracks in the hardened zone are momentarily pried open to form an extremely narrow passage; air rushes in at this instant, causing a brief deterioration in the vacuum. Once the pulsation subsides, the cracks spring back and close, and the seepage ceases immediately, manifesting as a fleeting vacuum alarm.

Temperature fluctuations amplify this intermittent behaviour; rubber that has aged due to organic swelling becomes exceptionally sensitive to temperature changes. During periods of low temperature, the rubber’s hardness increases further, making it more brittle and causing the sealing surfaces to warp and develop gaps; when the temperature rises slightly, the areas still swollen from organic compounds expand slightly, filling these micro-gaps. Low-temperature evaporation systems inherently experience fluctuations within their temperature range; feed temperature, rinsing operations and diurnal temperature variations in the environment all affect the sealing points. Whilst there may be no leakage during the warmer daytime hours, intermittent air ingress occurs as temperatures drop at night. Alternatively, following each acid wash or hot water rinse, the seals undergo thermal shock, resulting in brief periods of leakage that cease once they have cooled down and returned to normal.

Furthermore, there is an intermittent cycle of adsorption and desorption of volatile substances; the concentration of organic vapour within the chamber is not constant. During the concentration phase, there is a high volume of organic volatiles, which continuously permeate the rubber; during the load reduction, standstill and vacuum release phases, the vapour concentration in the chamber decreases, and the organic substances previously adsorbed within the rubber slowly desorb and volatilise, causing localised volume contraction of the seal. This repeated cycle of expansion and contraction continuously alters the seal’s preload margin. As the bolt preload itself is a fixed value, the thickness of the seal fluctuates slightly between adsorption and desorption, causing the preload compression to vary accordingly. When the compression is sufficient, the seal is reliable; when it is insufficient, air ingress gaps appear. This explains why leakage may occur after a period of concentration operation, yet cease when the unit is shut down, left to stand for a while, and then restarted.

Deposits on the sealing surfaces also play a role in this process. Trace amounts of salt mist, colloids and condensed organic droplets, which are released under negative pressure, may accumulate in the microscopic gaps of the hardened seal, temporarily acting as sealing material to block the air ingress pathways. When subjected to vacuum surges, airflow scouring or disturbance from flushing vapour, this thin layer of deposits is blown away or dissolved, reopening the passage and allowing air ingress. Subsequently, as operation continues, new deposits gradually accumulate within the gaps to seal them off, causing the leak to disappear once more; this is equivalent to a cycle in which the gaps self-seal and then self-open.