Seat leakage in either direction after a period of satisfactory
operation can be caused by any combination of the following:
Valve is not being fully closed.
Make sure to close the valve fully, but do not use cheaters.
If valve has a motor actuator with a torque trip, debris
caught in the seat area or on the guide area can increase the required
closing torque and shut off the motor while the valve is still partially
open.
Make sure the torque limit is set correctly. Try flushing
the valve to remove debris around the seating area. If you require
assistance in verifying the factory set torque switches, please contact
Velan Field Engineering Services.
Circumferential contact between seat and disc is no
longer 100%.
Seat and disc need lapping. This will require the line
pressure to be removed from the valve. Follow the safety warnings
in Section 3 of the maintenance manual applicable to the valve in
question. Disassemble the valve, lap the seats and then perform a
blueing test. Continue until full circumferential contact of sealing
surfaces is achieved. Reassemble valve. Review the specific valve
maintenance manual for procedures. Manuals are available via e-mail
for our customers.
Valve was left partially open during flow at least once.
Flow accelerates when throttled, possibly to speeds beyond material
tolerances. High speed flow can cause wire draw on the sealing surface
of the disc and seat. Once this happens, the disc will no longer seal.
Do not throttle a gate valve. Check the seat and disc
for wire draw. Contact Velan Field Engineering
Services as it may be necessary to replace the disc and relap
the seat.
