What are the common failure modes and maintenance methods of bellow globe valves?

I. Common Failure Modes & Targeted Maintenance

1. Bellows Damage (Most Critical Failure)

Common Causes:

• Fatigue cracking: Long-term cyclic expansion/contraction (from frequent opening/closing) causes metal fatigue, especially at the bellows’ root or welds.
• Corrosion perforation: Corrosive media (e.g., acids, alkalis, or chloride-containing fluids) react with the bellows material (usually stainless steel like 316L), leading to pitting or perforation.
• Overload deformation: Excessive torque during operation (e.g., manual wrench over-tightening) or abnormal pressure spikes in the pipeline cause the bellows to stretch beyond its elastic limit, resulting in permanent deformation.
• High-temperature aging: In high-temperature environments (e.g., >300°C), the bellows material loses elasticity, becomes brittle, and cracks easily.

Maintenance Methods:

• Inspection first:
  • For visible bellows (e.g., top-loaded structures), check for surface cracks, bulges, or rust using a flashlight and magnifying glass.
  • For hidden bellows, perform a pressure tightness test: Close the valve, apply 1.1 times the rated working pressure to the inlet side, and check for leakage at the bellows cavity (via the valve’s vent plug) or the stem packing area.
• Replacement if damaged:
  • Replace the bellows with a matching model (same material, diameter, and pressure rating) to ensure compatibility.
  • After replacement, recheck the bellows’ flexibility: Manually lift the stem to confirm smooth expansion/contraction without jamming.

2. Internal/External Leakage

A. Internal Leakage

• Wear or damage to the valve disc/seat sealing pair: Frequent contact between the disc and seat (during opening/closing) wears the sealing surface; hard particles in the fluid (e.g., sand, scale) also scratch the surface.
• Improper valve closing: Incomplete closing (e.g., manual operation stops prematurely, or electric actuator positioning is inaccurate) leaves a gap between the disc and seat.

1. Inspect the sealing pair: Disassemble the valve to check for scratches, pits, or uneven wear on the disc and seat.
2. Repair or replace:
   • For minor wear: Use a lapping tool (with abrasive paste matching the material, e.g., diamond paste for hard alloys) to lap the sealing surface until it is smooth and forms a continuous seal ring.
   • For severe damage (e.g., deep scratches, cracks): Replace the valve disc or seat with new components of the same material (e.g., stainless steel, Stellite alloy for high-temperature applications).
3. Calibrate closing position: For electric/pneumatic valves, re-calibrate the actuator’s “full close” position to ensure the disc fully presses the seat.

B. External Leakage

• Stem packing failure: The packing (e.g., PTFE, graphite) around the stem ages, hardens, or is compressed unevenly, losing its sealing ability.
• Valve body flange leakage: Loose flange bolts (due to thermal expansion/contraction) or damaged gasket (e.g., rubber, metal wound gasket).

• Stem packing maintenance:
  1. Turn off the valve and relieve pipeline pressure.
  2. Loosen the packing gland nuts, remove the old packing, and clean the packing cavity (to remove residue).
  3. Replace with new packing (ensure the material matches the medium and temperature, e.g., graphite packing for high temperatures) and tighten the gland nuts evenly (avoid over-tightening, which may cause stem jamming).
• Flange leakage maintenance:
  1. Use a torque wrench to retighten flange bolts in a cross pattern (to ensure even pressure).
  2. If leakage persists, shut down the system, remove the old gasket, and replace it with a new one (check for flange surface damage; if present, repair via grinding).

3. Stem Jamming or Slow Operation

• Packing over-tightening: Excessive pressure on the packing increases friction between the stem and packing.
• Stem corrosion or scaling: Corrosive media or hard water deposits form scale on the stem, increasing resistance.
• Bent stem: Abnormal external force (e.g., collision during installation) or uneven torque causes the stem to bend.
• Lubrication failure: Lack of lubrication (or use of incompatible lubricant) between the stem and valve bonnet thread.

1. Adjust packing tightness: Loosen the packing gland nuts slightly until the stem moves smoothly (while ensuring no external leakage).
2. Clean and lubricate the stem:
   • Disassemble the stem, clean the surface with a neutral solvent (e.g., alcohol) to remove corrosion or scale, and dry it.
   • Apply a suitable lubricant (e.g., high-temperature molybdenum disulfide grease for high-temperature valves, food-grade grease for pharmaceutical applications) to the stem and thread.
3. Check for stem bending: Use a straightedge to verify stem straightness; if bent, replace the stem (do not attempt to straighten it, as this may weaken the material).

4. Actuator Malfunction (for Electric/Pneumatic Valves)

• Power supply failure (e.g., broken wires, tripped circuit breaker).
• Motor burnout (due to overload, e.g., jamming of the stem).
• Control board damage (moisture, voltage fluctuations).

• Air supply issues (e.g., blocked air filter, low pressure).
• Diaphragm or cylinder damage (cracks, air leakage).
• Solenoid valve failure (clogged, coil burnout).

• Electric Actuators:
  1. Check the power supply (test voltage with a multimeter) and repair broken wires or reset the circuit breaker.
  2. If the motor does not run, disconnect the power, check for stem jamming (resolve first), then test the motor windings (replace the motor if burned out).
  3. Replace the control board if it fails (ensure compatibility with the actuator model).
• Pneumatic Actuators:
  1. Check the air supply: Clean the air filter, adjust the regulator to the rated pressure (usually 0.4–0.6 MPa).
  2. Inspect the diaphragm/cylinder for air leakage (apply soapy water to the surface; bubbles indicate leakage) and replace damaged components.
  3. Clean or replace the solenoid valve (ensure the valve is compatible with the medium and voltage).

II. Preventive Maintenance (Key to Extending Valve Life)

III. Key Notes for Maintenance

1. Safety first: Before maintenance, shut down the pipeline, relieve pressure, and drain the medium (especially for toxic, flammable, or high-temperature media). Use personal protective equipment (PPE) such as gloves and goggles.
2. Genuine parts: Replace components (bellows, packing, disc) with genuine or certified parts to avoid compatibility issues (e.g., non-matching bellows may not withstand rated pressure).
3. Record-keeping: Maintain a maintenance log to track failure types, repair dates, and part replacements—this helps identify recurring issues (e.g., frequent bellows corrosion may indicate a need to upgrade to a more corrosion-resistant material like Hastelloy).