Ball Valve Leaking Past Seat (Internal Leak)
A ball valve passing fluid in the closed position has worn, damaged, or chemically degraded seats. PTFE and RPTFE soft seats are consumable components that wear over time, particularly in abrasive, high-cycle, or high-temperature service.
Symptoms
Root Causes
PTFE seat wear
PTFE seats have a finite wear life. After many open/close cycles (typically 10,000–50,000 depending on conditions), the PTFE develops groove wear from the ball's leading and trailing edges.
Abrasive particles in flow
Sand, scale, weld debris, or catalyst fines rapidly abrade soft PTFE seats. One slug of solids can destroy soft seats within minutes.
Chemical degradation of seat material
PTFE attacked by fluorine, alkali metals, or strong reducing agents; EPDM attacked by hydrocarbons; Viton attacked by ketones, esters, or low-pressure steam. Swollen or cracked seat material cannot seal.
Seat extrusion
Under high pressure differential, unsupported PTFE can extrude into the ball bore, causing the seat to lose its sealing contact geometry. RPTFE (reinforced) or PEEK seats resist extrusion at higher pressures.
Ball surface damage
Corrosion, scratching, or pitting on the ball's seating band prevents tight contact with the seat ring.
Safety Precautions
- Full LOTO - ball valves can trap pressure in the body cavity
- Bleed body cavity vent before disassembly - pressurised cavities have caused fatalities
- Wear chemical-resistant gloves when handling seats from chemical service
Tools Required
- Torque wrench
- Seat removal hook (brass or plastic)
- Micrometer (to measure seat thickness)
- Pressure test pump
- 600-grit wet/dry paper
Supplies Needed
- Replacement seat rings (correct material grade)
- Body seal / O-rings (complete seal kit)
- Anti-seize compound for bolting
Step-by-Step Repair Guide
- 1
Confirm the leak is through the seat, not through the stem
Isolate the valve. Apply pressure upstream only. Listen and check for leakage at the downstream flange face and downstream side of the body (seat leak) versus at the stem area (packing leak). Body cavity vent/bleed ports (if fitted) can also be used to test bidirectional sealing - apply upstream pressure, bleed the body cavity, then apply downstream pressure and check cavity pressure build-up.
- 2
Assess whether on-line seat replacement is feasible
Some ball valves (particularly top-entry or cartridge-type designs) allow seat replacement without removing the valve from the line. Check the valve maintenance manual. Side-entry and end-entry designs require full valve removal for seat replacement.
- 3
Remove and inspect the seats
After isolation, depressurization, and LOTO, disassemble the valve per the manufacturer's procedure. For a 3-piece body: remove the end pieces. For a top-entry: remove the top cap and ball. Remove the seat rings using a soft tool (brass or plastic seat removal hook) to avoid damaging the seat pockets. Inspect for: groove wear on the seating face, extrusion behind the seat, chemical swelling or cracking, embedded abrasive particles.
Measure the worn seat thickness with a micrometer and compare to the new seat thickness. Wear rate can predict remaining service life.
- 4
Inspect the ball seating surface
With seats removed, inspect the ball seating band (the annular contact zone) for corrosion pits, scratches, or hard particle embedment. Light scratches can be polished out with 600-grit wet/dry paper wrapped around a flat plate - always polish in a circumferential direction (around the ball, not across the seating band). Deep pits or corrosion damage requires ball replacement.
Check the ball surface hardness if corrosion is suspected. Standard carbon steel balls corrode rapidly in wet gas; specify 316 SS or Inconel-clad balls for wet service.
- 5
Replace seats with correct material and dimensions
Order replacement seats to the valve manufacturer's part number, specifying the correct material grade: PTFE (standard, to 200 degrees C), RPTFE/RTFE (reinforced, to 200 degrees C, extrusion-resistant for higher pressures), PEEK (to 260 degrees C, abrasion-resistant), metallic (Stellite or stainless for high-temp or fire-safe). Install seats without cross-threading. Torque end-caps or end flanges to the manufacturer's specified value in a cross-pattern sequence.
- 6
Test and return to service
After reassembly, perform a hydrostatic shell test at 1.5x design pressure and a seat leakage test per API 598. For soft-seat ball valves, ANSI FCI 70-2 Class VI (zero leakage) should be achieved. Operate the valve through 10 full open/close cycles before final seat test to allow seats to bed in.
When to Replace Instead of Repair
Replace the complete valve when: the ball seating surface has deep corrosion pits that cannot be polished out, the body seal pocket is corroded oversize preventing seat sealing, or the valve has exceeded its design cycle life (typically 10,000–50,000 cycles depending on class).
Applicable Standards
Specification for Pipeline and Piping Valves
API 6D is the most widely cited standard for pipeline and piping valves in the oil and gas industry.
Fire Test for Quarter-Turn Valves and Valves Equipped with Nonmetallic Seats
API 607 is the fire-test standard for quarter-turn valves (ball valves and butterfly valves) and other valves with soft (non-metallic) seats and seals.
Valve Inspection and Testing
API 598 is the universal valve testing and inspection standard, defining acceptable leakage rates (leak classes) for hydrostatic shell tests, seat leakage tests, and backseat tests.
Related Products
Key Terms Explained
Unfamiliar with any terms used in this guide? Each links to a full engineering definition.
Full valve glossary (113 terms)Steps
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