A metal-seated ball valve can leak through its seat
for the following reasons:
The valve is not being closed fully.
Make sure to close the valve fully, but do not use cheaters.
Valve has a motor actuator with a position limitor set
incorrectly. Actuator motor stops when valve reaches a position limit
but valve is still partially open.
Consult actuator manufacturer's manual for limit switch
adjustment procedure. Position seating of a ball valve is a difficult
operation and cannot be done by eyeballing. If you require assistance
verifying actuator position limit switches, please contact Velan Field
Engineering Services.
Circumfirential contact between the seat and ball is
not 100%. Minor imperfections on the either the seat or ball create
paths for fluid to escape.
The seat and ball need lapping. Follow warning instructions
in Section 3 of your maintenance manual. Remove the line pressure
and disassemble the valve. Lap the seat and ball and then perform
a blueing test. Repeat until full circumfirential contact achieved.
Reassemble the valve. Procedure for seat lapping is in the valve maintenance
manual. Manuals can be delivered, upon request, to customers via e-mail.
Valve is not closing properly due to damage from welding
procedure.
Valve should be welded onto the line in the open position
in order to prevent spatter from contacting the ball. Valve must be
removed from line so that the ball can be inspected. Small weld spatter
can be polished off. If damage is substantial, ball should be replaced.
Misalignment in the piping was overcome during installation by using
valve flange or lugs to force alignment. This can lead to distortion
or body flexing, especially in smaller valves. A full seal is no longer
present between the seat and the ball.
