How to ensure the airtightness of a gas filled tube furnace?

Ensuring the airtightness of the tube furnace with gas flow is the key to ensuring experimental safety, accurate data, and stable equipment operation. The following elaborates in detail on how to ensure the airtightness of tube furnaces from five aspects: sealing structure design, selection and installation of sealing components, treatment of connection parts, airtightness testing methods, and daily maintenance and operation specifications:

1. Sealing structure design
Optimization of flange connection
Design principle: Flanges should adopt standard structures (such as flat welding flanges and butt welding flanges) to ensure compatibility with furnace tubes and pipelines. The flange sealing surface should be flat, with a roughness controlled below Ra3.2 μ m to reduce the risk of leakage.
Sealing form selection:
Metal seal: suitable for high temperature and high pressure scenarios (such as stainless steel flanges+metal gaskets), with a temperature resistance of up to 800 ℃ or above.
Soft seal: suitable for low temperature, low pressure or corrosive gas environments (such as fluororubber O-rings), but needs to be replaced regularly.
Bolt tightening: Adopt diagonal uniform tightening method, gradually apply force in 2-3 times to the specified torque (such as M12 bolt torque of 50-70N · m), to avoid local stress concentration.
Sealing of furnace tube ports
End cap design: Removable end caps should be provided at both ends of the furnace tube, and the contact surface between the end cap and the furnace tube should be tapered (such as 1:10 taper) to increase the self-locking effect.
Sealing structure:
Radial sealing: Install O-rings or V-rings on the inner ring of the end cap and achieve sealing through axial compression.
Axial sealing: Apply high-temperature sealant (such as silicate) on the contact surface between the end cap and the furnace tube, and form a flexible sealing layer after curing.
Gas inlet and outlet design
Interface type: Priority should be given to using quick connectors (such as VCR connectors and sleeve connectors) to reduce welding points and minimize the risk of leakage.
Anti misalignment structure: Design positioning pins or keyways at the interface to ensure consistent position during each connection and avoid sealing failure caused by misalignment.

2. Selection and installation of seals
Seal material matching
High temperature environment: Choose materials that are resistant to high temperatures (such as graphite gaskets that can withstand temperatures up to 2000 ℃ and ceramic fiber gaskets that can withstand temperatures up to 1200 ℃).
Corrosive gas: Use corrosion-resistant materials (such as PTFE O-rings that are acid and alkali resistant, and perfluororubber O-rings that are organic solvent resistant).
Vacuum environment: Select materials with low gas release rate.
Installation Specification for Seals
Cleaning treatment: Before installation, wipe the sealing surface with a dust-free cloth dipped in alcohol to remove oil stains and impurities.
Lubrication treatment: Apply high-temperature grease (such as molybdenum disulfide grease) on the metal sealing surface to reduce friction and wear.
Direction confirmation: The O-ring should be installed in the direction of the arrow to avoid twisting; The metal gasket must be ensured to be free of creases or scratches.
Seal replacement cycle
Regular inspection: Check the aging condition of the seals every 3 months (such as O-ring hardening or cracking).
Preventive replacement: Replace seals every 6 months in high-temperature environments and every 3 months in corrosive gas environments.

3. Processing of connection parts
Quality control of welding joints
Welding process: argon arc welding or plasma welding is used. Before welding, the interface needs to be beveled (such as a single-sided 30 ° bevel) to ensure that the weld is fully penetrated.
Post weld treatment: X-ray inspection shall be carried out after welding to ensure that there are no pores or cracks; Polish and polish the weld seam to reduce stress concentration.
Threaded connection seal
Thread type: cone tube thread is preferred, with strong self-locking and good sealing performance.
Sealing material: Apply anaerobic sealant to the thread and form a rigid sealing layer after curing.
Tightening force: Use a torque wrench to control the tightening torque and avoid thread damage caused by over tightening.
Quick connector usage
Type of connector: Select the connector specification based on the gas pressure (such as low pressure card sleeve connector, high pressure VCR connector).
Operation specification: Ensure that the connector is aligned when connecting, and slowly tighten it until you hear a “click” sound; Before disconnecting, it is necessary to release pressure to avoid gas shock.

4. Air tightness testing method
Positive pressure detection method
Operation steps:
Fill the furnace with dry nitrogen gas.
Close the intake valve, soak in water or apply soapy water, and observe if any bubbles are generated.
Maintain pressure for 30 minutes and test whether the pressure is qualified.
Applicable scenarios: Atmospheric or low-pressure tube furnaces.
Negative pressure detection method
Operation steps:
Connect the vacuum pump and evacuate the furnace to a vacuum.
Close the vacuum valve and observe the change in vacuum degree. If the vacuum degree drops by ≤ 5% within 30 minutes, it is considered qualified.
Applicable scenarios: High pressure or vacuum tube furnaces.
Helium mass spectrometry leak detection method
Operation steps:
Fill the furnace with helium gas (concentration ≥ 99.999%).
Use a helium mass spectrometer leak detector to scan the connection area and detect the leakage rate.
Advantages: High sensitivity, able to locate small leakage points.

5. Daily maintenance and operation standards
Pre operation inspection
Sealing condition: Check whether the O-ring and gasket are intact, without cracks or deformation.
Tightening connection: Confirm that the flange bolts and quick couplings are tightened and not loose.
Gas purity: Before introducing gas, the purity must be confirmed to avoid impurities corroding the seal.
Monitoring in use
Pressure monitoring: Real time observation of the pressure gauge inside the furnace. If the pressure drops abnormally, immediately stop the machine for inspection.
Temperature control: Avoid sudden temperature changes (such as direct cooling from high temperatures) and reduce the impact of thermal stress on seals.
Maintenance after shutdown
Pressure relief treatment: Before stopping the machine, release the pressure to atmospheric pressure first, and then close the gas valve to avoid long-term pressure on the sealing components.
Cleaning and maintenance: Use compressed air to blow away residual gases in the furnace to prevent the condensation of corrosive gases.
Storage of seals: Unused seals should be stored in a dry, dark environment to avoid aging.