1. Appearance and structure inspection
Valve body integrity inspection
Check whether there are cracks, sand holes or obvious corrosion on the surface of the valve body, especially at the flange connection and weld area. Cast iron is prone to stress cracks after impact;
If the valve body wall thickness is reduced by more than 20% of the original thickness due to corrosion (such as the original thickness of 10mm is reduced to less than 8mm), it needs to be replaced immediately.
Sealing surface wear assessment
Observe whether the valve seat sealing surface has scratches, depressions or aging (soft sealing materials such as PTFE cracking, rubber hardening). When the wear depth is greater than 0.5mm, the sealing performance is significantly reduced;
Use a feeler gauge to measure the gap between the ball and the valve seat. If it is greater than 0.3mm and cannot be restored by adjusting the bolts, the valve needs to be replaced.
2. Functional performance test
Leak detection
Static leakage : After closing the valve, pressurize to 1.5 times the nominal pressure (such as PN10 pressurized to 15Bar), maintain the pressure for 10 minutes, and the pressure drop > 5% or visual leakage is considered failure; Dynamic leakage : After 50 consecutive openings and closings, the leakage rate > 0.1% of the rated flow (such as DN50 valve leakage > 0.5L/min) indicates seal failure. Operational performance verification
During manual operation, the handle torque increases abnormally (such as DN50 valve > 40N·m), which may be due to valve stem deformation, ball jamming or excessive wear of the sealing surface;
If the electric valve actuator frequently alarms or cannot be accurately positioned (error > 5% opening), it is necessary to check whether the valve body is deformed or blocked.
3. Analysis of working condition adaptability
Medium and pressure matching
If the medium contains strong acid (pH < 4), high hardness particles or the temperature exceeds 80℃, the cast iron valve body may experience accelerated corrosion or creep deformation. Even if there is no leakage, it is recommended to replace it with a stainless steel or alloy valve;
The system pressure fluctuates frequently (such as ΔP ≥ 5MPa/minute), and the cast iron valve body is not fatigue-resistant, which may cause microcracks to expand.
4. Maintenance history and cost evaluation
Maintenance frequency and cost
If the number of maintenance times per year is greater than 3 times (such as frequent replacement of sealing rings and valve stems), and the comprehensive maintenance cost is close to 50% of the price of a new valve, it is recommended to replace it;
Even if the old valve (used for more than 8 years) has not completely failed, it is necessary to evaluate the matching of its remaining life with the system safety requirements.
Replacement decision flow chart
Preliminary inspection → Cracks/corrosion found → Direct replacement; Leak test failed → Retest still not up to standard after repair → Replace; Operation stuck + excessive wear → Replace; Working condition upgrade (such as pressure/medium change) → Insufficient adaptability → Replace. Note: It is not recommended to repair valve body defects by welding for cast iron three-way ball valves, which may cause secondary cracking