Its a condition where service medium passing through the supposedly sealing area exceeding its tolerable leakage criteria. This is caused by Seat(s)’ sealing area is not in full contact with obturator’s sealing area when the valve expected to be in close position.
One of predominant failure mode contribute to seat leakage is:
Scratches or indentations on seat(s) and or obturator
Simply caused by “hard” substance entrapped between Seat(s) and obturator being forced moved along valve movement either for both linear motion or quarter turn valve. Size of this damaging substance varies from hair size to larger volume.
Sample of various scratches are but not limited to:
Method to assess and repair scratch failure mode:
STOP (See – Touch – Observe – Polish)
See; whenever possible see the valve while still installed. In order to predetermined scope either major or minor. Check whether miss-alignment or other abnormalities occurred. Even though not quite common, performing X-ray is recommended. How to perform good X-ray is simply by notifying wall thickness to the radiography operator
Roughly examine flow rate: Have you ever feel your own plumbing or hoses when your tap valve opened? The principal is the same, when there is passing flow it will induce vibration through the piping. Examination mostly on leakage rate (if its < 60 bubbles per min one may not feel the vibration) and also the sensitivity of the hand
Roughly examine temperature difference: If this subjected valve is process boundaries between relatively warm and relatively colder medium, then there is a good chance that one can feel the difference and determine whether its leaking or not. Obturator in most occasion due to its thickness is acting as a good insulator between warmer upstream medium and cooler downstream.
Please be cautious before touching any valve. Do check with operator of its temperature or start touching the valve from sensing its hand-wheel or spindle.
Surface roughness sensing upon valve dismantling is conducted; everybody already have the tool . After obturator and seat being cleaned, rub the sealing surface area with your tip of the thumb. If you feel the line, then it is considered as scratch and will contribute to (built up) leakage.
Visually check the sealing contact evidence by mating the obturator against the Seat. This method usually called “check with blue marking”. Apply pigment blue marking on Seat and or obturator and then press both parts against each other for several seconds. Pull up one of the component (whichever is lighter), there will be line-like remarks remain as indication of sealing area. If the line is continuous it is indicating that the sealing area is perfect, however if there is any slight cut then it is indication of leakage path. Check below image and observe the white circle line
Normally if scratches is so small up to 0,5 x 0,5 mm deep x width, it can be fixed by means of polishing or lapping. If it is deeper and or wider extra preparation usually required e.g. welding repair. Compound used is normally micron sized diamond compound and method used is 8 shape stroke polishing either by hand or by lapping machine.