Failures caused by crystals becoming lodged in the folds of a diaphragm valve’s diaphragm do not occur suddenly in a single instance; 

rather, they result from progressive damage that develops gradually with each opening and closing cycle. Many early warning signs are often mistaken 

for normal wear and tear of the valve components, and are only taken seriously once leakage or abnormal operation occurs.

The first sign is a gradual deterioration in the valve’s shut-off seal, rather than an immediate failure to seal completely. Each time the valve 

closes, the crystals trapped within the folds continue to exert pressure on the diaphragm’s sealing surface; repeated opening and closing will etch 

fine, intermittent scratches onto the sealing lip. Initially, only minimal internal leakage occurs under high pressure differentials, with the mother liquor

 slowly flowing back within the valve chamber. From the outside, no medium leakage is visible on site; the only observable signs are unstable discharge 

metering and a slow rise in the chamber liquid level after discharge has ceased. Operations and maintenance personnel often attribute this to pump backfloor level probe drift, overlooking the micro-damage to the diaphragm sealing surface. As the crystals continue to erode the surface, deepening the scratches,  

the internal leakage will change from intermittent to continuous, causing persistent disturbances in the liquid level control logic of the concentration section.

Subsequently, the resistance to valve operation gradually increases, and the opening and closing strokes become jerky and sluggish. The 

crystals trapped within the folds act as a layer of hard particles; as the diaphragm bends and deforms back and forth, the crystals continuously

 abrade the diaphragm substrate, whilst the trapped particles prevent the diaphragm from sealing completely against the valve body or returning fully

 to its original position. Although the air supply pressure to the pneumatic actuator remains unchanged, the valve’s response time for opening and closing 

gradually lengthens; after the programme issues an on/off signal, the valve actuates with a delay. In severe cases, the valve gets stuck in an intermediate position, 

unable to open or close fully. The valve position feedback collected by the automatic control system will exhibit slight fluctuations; as the amplitude of these

 fluctuations generally does not reach the alarm threshold, they are easily mistaken for minor fluctuations in the air supply pressure or slight sticking of the solenoid valve.

Subsequently, localised stress concentration leads to cracking in the diaphragm, with cracks originating at the base of the folds and slowly propagating. 

The folded region of the diaphragm valve is inherently the area of highest stress; the embedded crystals continuously prop up the diaphragm, keeping the

 pleated area in a state of constant additional tension. The cumulative fatigue from repeated bending, combined with the stress notches caused by the particles, 

leads to cracking. In the early stages, the cracks are micro-cracks that are difficult to detect with the naked eye; they propagate only within the inner layer of the

 diaphragm. Whilst the medium does not leak out immediately, it gradually seeps through to the back of the diaphragm and enters the air chamber on the actuator

 side. Once highly saline mother liquor accumulates in the air chamber, it corrodes the internal springs and valve position feedback components of the actuator,

 leading to distorted valve position feedback or even jamming of the feedback rod. By the time the crack penetrates the diaphragm, the medium will be 

ejected—a sign of the advanced stage of the fault.

Furthermore, the surface material of the diaphragm is gradually eroded and flaked away. Crystals trapped in the folds of the diaphragm rub against it as 

it bends, gradually grinding away the diaphragm lining material and forming minute rubber or PTFE debris. These particles are carried back into the 

evaporator chamber with the mother liquor, mixing with the concentrated mother liquor. Some of the fine particles are carried by the vapour phase

 to the demister, where they gradually adhere to the surface of the wire mesh, exacerbating local blockages and mist overload in the demister,

 leading to a chain reaction of component failures. The particles may also deposit in downstream pipework or at flow meter measurement points,

 causing persistent errors in flow measurement.

A secondary crystallisation amplification effect occurs on the flow passage side of the valve body. Crystals lodged in the folds of the diaphragm are 

not completely flushed away by the medium; each time the valve opens and closes, new salt crystals are trapped, causing the crystal clusters to grow 

continuously. These clusters encroach upon the valve’s flow cross-section, reducing the effective bore when the valve is fully open and causing the 

discharge flow rate to gradually decrease; under the same pressure differential conditions, discharge capacity declines year on year. Increasing the air

 supply pressure fails to restore the rated flow rate; this is often mistakenly attributed to an increase in mother liquor viscosity, without identifying the 

accumulation of crystals inside the diaphragm valve.

Permanent deformation of the diaphragm may also occur due to salt accumulation on its reverse side. Trace amounts of mother liquor seep through 

micro-cracks onto the reverse side of the diaphragm; once the moisture evaporates, salt crystals precipitate and accumulate in the cavity between the

 diaphragm and the actuator. These salt crystals continuously exert pressure against the diaphragm, causing permanent bulging and deformation. The 

diaphragm is unable to return to its original shape, and the sealing surface’s contact precision is permanently compromised. Even if the air supply is 

subsequently replaced or the valve body is cleaned, the valve’s shut-off performance cannot be restored; the diaphragm must be replaced.

Long-term operation is also accompanied by latent corrosion of pneumatic accessories. High-salinity droplets that have penetrated the air chamber

 can travel back through the air lines into the solenoid valves and pressure regulators; salt crystals then become lodged in the valve spools, causing

 unstable air supply and pressure regulation, as well as solenoid valve switching failures. Damage to these ancillary components is often investigated in 

isolation, whilst the root cause—permeation resulting from salt crystal embedding in the diaphragm pleats—is overlooked.

A key characteristic of this type of progressive failure is that it deteriorates gradually with each opening and closing cycle. Under fluctuating operating

 conditions, the fault may manifest intermittently. Shutting down the system to flush the valve body only removes crystals from the surface of the flow 

passages; crystals embedded deep within the folds cannot be completely flushed out and will continue to act as a source of damage, repeatedly causing 

the diaphragm to fail.