Gate valve passing when closed: causes of seat leakage and fixes
The valve is closed hard, yet the line downstream still flows. The cause may be simple, such as sand on the seat, or serious, such as eroded seats. Here is how to tell them apart.
TAZ Engineering Team · · 6 min
Debris on the seats
The most common cause on new or repaired lines: mill scale, weld spatter, sand or stones stop the wedge from seating. Open the valve a few turns with flow running to flush the seats, then close again. Never force the handwheel with a bar; it damages seats and stem.
Erosion from throttling
If the valve was used to regulate flow, high-velocity flow through the narrow gap cuts grooves in the wedge and seat faces. The valve needs seat lapping or hardfacing and machining, or replacement, and a proper control valve should take over the throttling duty.
Distortion and thermal effects
Pipe loads, flange misalignment and rapid temperature changes can distort the body so that a solid wedge no longer contacts the seats all round. Correct supports, expansion joints and flexible wedges for steam and hot water reduce the risk.
Wear and corrosion
Seats wear over time and corrode if the material does not suit the medium. Rubber wedges in resilient-seated valves degrade with excessive temperature or aggressive chemicals.
Diagnosis and remedy
Check that the wedge reaches the end of travel by counting turns against the datasheet, and listen for whistling across the closed valve. Isolate the section and open the bonnet for inspection. Minor seat damage can be lapped; heavy damage needs hardfacing and machining or a replacement valve. TAZ supplies replacement gate valves with matching face-to-face and flanges; send the model and a nameplate photo.
Checklist
- flush by partial opening
- full travel by turn count
- erosion marks
- pipe loads
- seat material and medium
- internal inspection
- repair or replace