Valve Failure Library
A guide to valve failure modes — seat leakage, stem packing leaks, erosion, corrosion, galling, and actuator failure. Each includes symptoms, causes, and prevention.
Understanding Valve Failures
Valves are critical control elements — when they fail, processes lose control, product is lost, or safety is compromised. Understanding valve failure modes helps with selection, maintenance, and troubleshooting.
Seat Leakage (Passing)
Seat leakage occurs when a closed valve allows fluid to pass. It is the most common valve complaint and ranges from minor weeping to complete failure to shut off.
- Symptoms: Process fluid continues to flow when the valve is closed, pressure does not equalise downstream, temperature changes downstream, and safety risk if the valve is an isolation valve.
- Root causes: Worn or damaged seat (erosion from particles, wire draw from high-velocity leakage), deposits on the seat (scale, debris), misaligned disc/ball/gate, thermal distortion (valve body warps with temperature), and incorrect valve selection (valve not rated for the service).
- Prevention: Select the correct valve type and material for the service, install strainers upstream of control valves, avoid using valves as permanent throttling devices (causes wire draw), and service valves on a preventive maintenance schedule.
Inspection tip
Test isolation valves for seat leakage by closing the valve and checking for downstream flow. For control valves, verify tight shutoff during shutdowns. Inspect seats for erosion, deposits, and damage.
Stem Packing Leakage
Stem packing is the seal around the valve stem that prevents process fluid from escaping. Packing leaks are the most common external valve leakage.
- Symptoms: Visible fluid or gas leakage from the stem area, packing gland requiring frequent tightening, and in steam service, visible steam plume from the gland.
- Root causes: Worn or hardened packing (ageing), incorrect packing material for the fluid or temperature, stem scored or scratched (damages packing), insufficient gland tightening, and thermal cycling causing dimensional changes.
- Prevention: Use the correct packing material (graphite for high temp, PTFE for chemicals, PEEK for high pressure), replace packing on schedule, protect the stem with a gland protector, and use live-loaded packing (spring-loaded) for valves with thermal cycling.
Inspection tip
Inspect stem area for leakage during normal operation. Check for stem scoring during maintenance. Replace packing if it requires more than occasional tightening.
Erosion and Wire Draw
Erosion removes material from the valve seat and body through high-velocity fluid flow, especially when the valve is partially open (throttling).
- Symptoms: Progressive increase in seat leakage, visible grooving on the seat (wire draw), reduced valve life, and rough operation.
- Root causes: Throttling service (high-velocity flow through a small opening), abrasive particles in the fluid, cavitation in liquid service, and high-pressure drop across the valve.
- Prevention: Avoid using isolation valves (gate, ball) for throttling — use globe or control valves designed for the purpose, install strainers to remove particles, use hard-faced seats (Stellite) for erosive service, and use cavitation-resistant valve trim.
Inspection tip
Inspect seats for grooving and pitting during maintenance. Look for smooth, polished wear tracks (wire draw) on the seat. Measure seat width and compare to original.
Corrosion
Corrosion attacks valve body, trim, and stem through chemical or electrochemical reaction with the process fluid.
- Symptoms: Rusty or pitted valve body, discoloured fluid, stem seizure, and reduced wall thickness.
- Root causes: Incorrect material selection for the fluid, galvanic corrosion (dissimilar metals), crevice corrosion (stagnant areas), and chloride stress corrosion cracking (in stainless valves handling chlorides).
- Prevention: Select corrosion-resistant materials (316 for mild service, duplex for chlorides, Hastelloy for severe), apply coatings, use cathodic protection, and maintain correct fluid chemistry.
Inspection tip
Inspect valve body for pitting and wall thinning (UT measurement). Check stem for corrosion. Inspect internals during valve overhaul.
Galling and Seizure
Galling is the transfer of metal between sliding surfaces, causing the valve to become stiff or completely seized. It is common in stainless steel valves.
- Symptoms: Increasing torque required to operate the valve, rough or jerky movement, eventual inability to open or close the valve, and visible scoring on the stem or seat.
- Root causes: Stainless steel sliding on stainless steel (the most common cause), lack of lubrication, incorrect clearance between stem and bushing, and high temperature (increases galling tendency).
- Prevention: Use dissimilar materials for sliding pairs (e.g., stainless stem in bronze bushing), apply anti-seize or lubrication, ensure correct clearances, and use Nitronic 60 or other anti-galling alloys for stainless-on-stainless applications.
Inspection tip
During maintenance, inspect stem and bushing for scoring and metal transfer. Check operation torque — if it's increasing, galling is developing.
Actuator Failure
Actuator failure prevents the valve from operating correctly — it may fail to open, fail to close, or move erratically.
- Symptoms: Valve does not respond to control signal, valve moves slowly or erratically, positioner feedback is incorrect, and process control degrades.
- Root causes: Pneumatic: dirty or wet instrument air, diaphragm rupture, spring fatigue, positioner calibration drift. Electric: motor burnout, gearbox wear, limit switch failure. Hydraulic: fluid leakage, pump failure.
- Prevention: Install and maintain air filters and dryers for pneumatic actuators, inspect diaphragms during scheduled maintenance, calibrate positioners annually, and protect electric actuators from moisture and overheating.
Inspection tip
Test actuator response time and stroke regularly. Check instrument air quality (dry, oil-free). Inspect pneumatic tubing for leaks. For electric actuators, measure motor current and check for overheating.
How CHI can help
Ask CHI about valve failure diagnosis and to recommend corrective maintenance based on the symptoms. The CHI assistant can help you diagnose failure modes from symptoms, recommend inspection methods, and suggest preventive maintenance actions to avoid similar failures.