The sealing leakage problem of low temperature ear shaft ball valves directly affects its safety and reliability. Especially in ultra-low temperature working conditions such as liquefied natural gas (LNG) and liquid nitrogen, leakage may lead to evaporation of media, damage to equipment or even safety accidents. Here is a detailed analysis of the diagnosis process, common causes and solutions:
I. Diagnostic Process for Seal Leakage
Preliminary examination
Visual inspection: Check the appearance of the valve for the formation of ice or frost (a drop in ambient temperature can cause water vapor condensation when a cryogenic media leak).
Sound judgment: Stay close to the valve and listen for any air or liquid flow to initially identify the leakage point.
Pressure test: The pressure difference between the front and back of a valve is measured by a pressure gauge or leak detector. If the pressure drops significantly, a leakage may occur.
Segmented investigation
Body Seal: Check the connection between the body and bonnet and the contact surface between the seat and the sphere for leakages.
Ear shaft seal: See if the stuffing box or sealing ring between the ear shaft (valve stem) and the valve body is leaking.
Connection flange: Check that the flange surface of the valve connection pipe is not tightly sealed.
Professional testing
Helium mass spectrometry leak detection: helium gas is used as a tracer gas to detect small leakage by mass spectrometer.
Infrared thermal imaging: The temperature distribution on the surface of the valve is detected by an infrared camera, and the low temperature leakage points will show abnormally low temperatures.
II. Common Causes and Solutions
1.Seal surface damage
Reasons：
Low-temperature media can cause sealing materials (such as PTFE, PET) to contract or harden, losing elasticity and not clinging to the sphere.
Scratches or corrosion on the surface of the sphere or valve seat may cause the seal to fail.
Solutions：
Replace the sealing material: Choose materials with good low temperature resistance (such as modified PTFE, flexible graphite).
Patch the sealing surface: Grind and polishing minor scratches; serious damage requires replacement of sphere or valve seat.
Pre-cooling treatment: Before operating at low temperature, the valve should be slowly pre-cooled to prevent the material from cracking due to sudden temperature change.
2.Installation deviation
Reasons：
When the valve was installed, it is inconsistent with the central line of the pipe, causing eccentricity between the valve seat and the sphere and uneven distribution of sealing pressure.
The gap between the ear shaft and the valve body is too large, causing the sphere to tilt during operation, damaging the seal.
Solutions
Reinstall: adjust valve position to ensure no more than 1mm the coaxiality error with piping.
Check the ear shaft gap: Control the gap to between 0.1 and 0.3 mm by adjusting the packing cover or replacing the ear shaft bushing.
3.Material shrinkage and thermal expansion and contraction
Reasons
At low temperatures, the metal valve body shrinks, causing the gap between the sealing surfaces to increase.
Different materials have different shrinkage coefficients and can lead to leakage.
Solutions
Optimize structural design: Adopt elastic valve seats or compensation rings to absorb material shrinkage.
Select supporting material: choose a valve body with similar shrinkage coefficients and sealant combination.
4.Improper operation：
Reasons
Frequent opening and closing or pushing too hard can cause the seal to wear.
If not operated for a long time, the sealing surface will stick together, and when opened or closed again, the sealing material will tear.
Solutions
Standard operation: avoid opening and closing. Use electric or pneumatic actuators to control on and closing speed.
Periodic maintenance: For valves that are not in use for long periods of time, regular opening and closing tests should be conducted to prevent the sealing surface from sticking.
