- Preparation before maintenance
Safety isolation and pressure relief
Close the upstream and downstream valves and relieve pressure to zero, make sure there is no residual medium in the system to avoid sudden leakage or high temperature burns during maintenance.
High-temperature valves need to be cooled to room temperature (such as high-pressure steam system needs natural cooling) before operation.
Tools and spare parts inspection
Prepare special tools (such as spring pliers, torque wrenches) and spare parts (seats, spools, gaskets, etc.), to confirm that the material is compatible with the medium (such as steam systems need to be high-temperature alloy).
- Disassembly and parts inspection
Valve disassembly
Disassemble the bonnet bolts in order, mark the connection position; if there is a welding point between the bonnet and the valve body, the welding point needs to be eliminated before separation.
Avoid collision with the sealing surface when removing the valve stem and valve flap to protect the integrity of the valve stem thread.
Key components inspection
Sealing surface: check the valve and valve seat sealing surface corrosion, groove marks or pitting, minor damage can be repaired by grinding, serious replacement of hard seals (such as Staley alloy).
Valve stem: measure the bending degree of the valve stem (≤ 0.1mm / m), surface scratches or corrosion need to be polished or replaced.
Valve body: check cracks or sand holes, drill holes to stop cracks before welding (aperture 5-8mm), alloy steel material needs to be preheated and post-weld heat treatment.
- Maintenance of sealing system
Seal surface grinding
Rough grinding (80-120 mesh sandpaper) to remove scratches → medium grinding (200-400 mesh) to flatten the surface → fine grinding (800 mesh or more) to enhance the smoothness.
When manually grinding, use the 'figure of 8' method to apply pressure evenly to avoid one-sided wear.
Replacement of packing and gasket
Remove the old packing and replace it with high temperature graphite or PTFE packing, press it in layers and leave a compression margin (about 20%).
Check the integrity of the bellows (if there is a double seal design) to ensure that there is no damage or fatigue cracks.
- Common Troubleshooting
Internal Leakage (Poor Shutoff)
Impurity jamming: flush the flow channel and add a filter, sealing surface wipe paraffin to remove dirt.
Erosion damage: reduce the flow rate or choose erosion-resistant materials (such as Staley alloy valve seat).
Valve stem jamming
Inject rust remover to soften the corrosion, tap the valve body to assist in loosening, apply high temperature grease after repair.
- Reassembly and Testing
Assembly Specification
Reinstall the parts in reverse order, pre-tighten the flange bolts symmetrically, high-temperature conditions require hot second fastening (hot tight) to compensate for the expansion gap.
Confirm that the medium flow direction is the same as the marking of the valve body (usually the entrance of the high pressure side is under), and the wrong installation will lead to sealing failure.
Sealing test
Hydraulic test: 1.5 times the design pressure holding pressure for 5 minutes, no leakage is qualified.
Air tightness test: low pressure air or nitrogen to detect micro-leakage, soapy water to assist in locating the leakage point.
- Maintenance cycle recommendations
High-pressure conditions: the maintenance cycle is shortened to 50% of conventional valves (such as every 6 months to check the sealing surface and packing).
Erosion environment: sewage valve and other high wear scenarios need to check the sealing surface wear every month.