In medium‑voltage ring main units (RMUs) and gas‑insulated switchgear (GIS), the gas compartment is not just a tank – it is the critical component that contains the insulating and arc‑quenching medium. Whether you use SF₆, dry air, nitrogen, or eco‑efficient alternatives like Fluoroketone mixtures, the sealing integrity of the compartment directly determines:
- Dielectric strength and safety
- Long‑term reliability and maintenance intervals
- Environmental compliance (especially for SF₆)
- Total cost of ownership
A tiny leak – even as small as 1×10⁻⁶ mbar·L/s – can lead to gradual pressure loss, insulation breakdown, internal arcing, and unplanned outages. For utilities, contractors, and EPCs, that means reputational and financial risk.
At Jiangsu Changyun Intelligent Technology, we treat gas compartment sealing as the single most important engineering challenge in RMU manufacturing. This article explains our end‑to‑end approach – from design and welding to final leak testing – and why we guarantee ≤0.01% annual leakage rate for 30+ years.
Before we explain our solution, let's look at why even well‑built gas compartments sometimes leak. Understanding these failure modes is the first step to eliminating them.
| Failure Mode | Root Cause | Consequence |
|---|---|---|
| Weld porosity | Micro‑gas voids in the weld bead due to inconsistent heat or contamination | Slow, often invisible leakage path |
| Thermal cycle fatigue | Repeated expansion/contraction (daily load cycles, seasonal changes) loosens flange joints or cracks brittle welds | Intermittent leakage that appears after 3–7 years |
| Gasket degradation | O‑ring material incompatible with insulating gas or its decomposition byproducts (e.g., SF₆ arc products) | Sudden or progressive loss of seal |
| Surface finish defects | Scratches or tool marks on flange faces create gas channels under compression | Leakage at low pressure differentials |
| Corrosion at sealing interfaces | Poor plating or material selection leads to rust under gasket, breaking the seal | Long‑term degradation, common in coastal or humid environments |
| Vibration / mechanical shock | Transportation or field installation stresses welds, especially around bushings and fill valves | Immediate or early‑life leakage |
A reliable sealing system must address all of these factors – not just one or two.
We design every gas compartment with two layers of security:
- Primary sealing – achieved through precision welding and high‑quality flange machining
- Secondary backup – implemented via double O‑ring designs and redundant sealing paths
This is combined with strict in‑house process control from raw material to final assembly.
Component Material Justification
| Component | Material | Justification |
|---|---|---|
| Gas tank (welded body) | SUS304 / SUS316L stainless steel | Excellent weldability, corrosion resistance, low permeability |
| Gaskets / O‑rings | Silicone (VMQ) or FKM (Viton®) | Compatible with SF₆, dry air, N₂; wide temperature range -40°C to +150°C |
| Flange plating | Nickel or zinc‑nickel (≥8μm) | Prevents galvanic corrosion; salt spray resistance ≥240 hours |
| Bushings | Epoxy‑insulated with embedded copper | Matches thermal expansion of stainless steel; no cracking under thermal cycling |
For all removable parts (inspection covers, pressure relief devices, fill valves), we use a double O‑ring groove:
- Primary O‑ring – standard sealing function
- Secondary O‑ring – backup seal, also provides visual indication if primary fails (a monitoring port between the two seals allows leak check without opening the compartment)
Illustration concept: Two parallel O‑rings with a small drilled monitoring port connected to a quick‑connect fitting.
This design allows our quality team to perform intermediate leak checks at any time without depressurizing the compartment – a feature that also benefits end‑users during periodic maintenance.
Even the best gasket will leak if the flange surface is not properly prepared. Our flange machining follows strict specifications:
- Surface roughness Ra ≤ 1.6 μm (concentric or spiral finish, no cross‑hatch)
- Flatness tolerance ≤ 0.05 mm across the entire flange face
- No scratches, tool marks, or dents – each flange is visually inspected under magnification
After machining, flanges are nickel‑plated (minimum 8 μm thickness) to prevent corrosion, especially important for outdoor RMUs and coastal installations.
Many manufacturers only test a sample of their production – typically one unit per batch. We believe that is unacceptable for gas‑insulated equipment. Every single gas compartment leaving our factory undergoes two stages of leak testing.
- Compartment is pressurized with dry air to 1.1 × rated filling pressure (e.g., 1.43 bar for a 1.3 bar SF₆ system)
- Pressure monitored for 24 hours
- Pass criterion: pressure drop < 0.5% of initial value
- This detects any large leak (>1×10⁻³ mbar·L/s) that would cause rapid failure
For final verification, we use a helium leak detector (sensitivity ≤ 1×10⁻⁶ mbar·L/s) according to IEC 62271‑200 and ISO 20486 standards.
Procedure:
- Evacuate the compartment to vacuum (<1 Pa)
- Spray helium gas around all welds, flanges, bushings, and valve connections
- Monitor the detector for helium ingress
Test conditions:
- Helium pressure at test points: 2 bar
- Dwell time per point: ≥10 seconds
- Ambient helium background: continuously monitored
Acceptance criterion: No detectable leak at sensitivity 1×10⁻⁶ mbar·L/s. For practical purposes, this translates to an annual leakage rate ≤ 0.01% of total gas mass – far better than IEC requirement of ≤1% per year for sealed pressure systems.
Each test is recorded with a timestamp and operator ID, and the report is kept for 10 years as part of our quality traceability system.
Leak testing at the factory is necessary but not sufficient – we must also prove that the sealing system will perform for 30+ years under real operating conditions.
We conduct type tests according to IEC 62271‑200, including:
| Test Condition | Our Result | |
|---|---|---|
| Temperature cycling | -25°C to +55°C, 100 cycles, each with 2 hours dwell | No measurable leakage change |
| Vibration test | 1g, 5–200 Hz sweep, 10 cycles per axis | No mechanical damage or leak |
| Corrosion test (salt spray) | 240 hours, 5% NaCl, 35°C | No corrosion on flanges or welds |
| Thermal aging | 1000 hours at 100°C (accelerated) | O‑ring compression set <15% |
These tests are performed by third‑party laboratories (e.g., KEMA, China National Quality Supervision and Test Center) and the reports are available upon request.
Our gas compartment design is gas‑type agnostic. The materials and sealing system work with:
- SF₆ – traditional choice, excellent performance but high GWP
- Dry air – eco‑friendly, suitable for 12kV / 24kV RMUs
- Nitrogen (N₂) – similar to dry air, widely used in medium voltage
- Fluoronitrile / Fluoroketone mixtures (e.g., 3M™ Novec™ 4710) – low GWP alternatives for higher voltage (up to 40.5kV)
- Fluoroketone / CO₂ blends
We can manufacture the same gas compartment with different fill gases, depending on your project’s environmental and regulatory requirements (especially for EU F‑gas regulation phase‑down).
When you choose Changyun for your RMU or gas‑insulated switchgear project, the gas compartment sealing guarantees translate directly into business benefits:
| Our Guarantee | Your Benefit |
|---|---|
| ≤0.01% annual leakage rate | No gas refilling for >30 years → lower OPEX |
| 100% helium leak tested | Zero risk of receiving a leaking unit |
| Double O‑ring with monitoring port | Easy periodic check without opening the tank |
| Robotic welding + complete traceability | Consistent quality across every unit, not just prototypes |
| Type tests to IEC 62271‑200 | Proven reliability, easier project approval |
| Compatible with multiple gases | Future‑proof against SF₆ regulations |
A utility customer in the Philippines required 120 units of 12kV RMU for a network upgrade. Their key concern was leakage under high humidity and frequent thermal cycling (tropical climate, daily temperature swings 10°C).
We supplied gas compartments with:
- Robotic TIG welded SUS304 tanks
- Double FKM O‑rings
- Helium leak tested to 1×10⁻⁶ mbar·L/s
- Filled with dry air (SF₆‑free per their environmental policy)
Result: All 120 units passed factory acceptance testing with zero leakage. After 14 months in service, follow‑up inspection showed no pressure loss in any unit – annual leakage rate well below 0.01%.
The customer has since ordered an additional 80 units for their 2026 expansion.
Engineers and procurement professionals often need specific data for their internal validation – material certificates, test reports, 3D drawings of flange designs, or compatible gas lists.
We provide:
- Full type test reports (IEC 62271‑200, including thermal cycle and leak tests)
- Material certificates for all sealing components (EN 10204 3.1)
- Sample helium leak test logs (showing actual test values)
- CAD models of our gas compartment (STEP format, under NDA)
Our engineering team can review your project’s voltage level, gas preference, and operating environment to recommend the optimal sealing design – including custom flange placements if needed.
