Most hydrogen-cooled generators don't pass the air pressure test first time after overhaul. That's not poor workmanship — it's the physics of containing one of the smallest molecules in existence at operating pressure.
The Molecule Is Working Against You
Hydrogen has a kinetic diameter of approximately 0.289 nm. Nitrogen — which we use to pre-test generator casings — sits at 0.364 nm. That 26% size difference means a joint sealed adequately for nitrogen can still leak hydrogen at a commercially significant rate.
At 3–5 bar gauge, the pressure head drives hydrogen through any available path. A surface scratch 0.1 mm wide and 50 mm long — invisible to the naked eye, inconsequential in air service — provides a viable hydrogen leakage path. This is not a flaw in the maintenance procedure. It is a physical reality that must be managed through the machine's service life.
A joint that holds nitrogen does not automatically hold hydrogen. Test protocols and sealing specifications are not interchangeable between the two gases.
The consequences of leakage are dual: safety (hydrogen at 4–75% in air is within the explosive range) and operational (reduced cooling effectiveness, elevated winding temperatures, accelerated insulation ageing).
Where the Leakage Is Coming From
Before applying any sealant, a maintenance engineer needs to identify which interfaces carry the highest leakage risk. The pattern across utility-scale generating plant is consistent:
| Location | Risk | Root Cause Drivers |
|---|---|---|
| Horizontal split joint — full axial length | High | Differential thermal growth, surface degradation, bolt relaxation |
| End-shield mating faces | High | Repeated disassembly cycles, oil contamination of sealing surface |
| Flanged inter-section joints | Moderate | Gasket creep at operating temperature, vibration micro-movement |
| Shaft seal housing | Moderate | Oil film interference with sealing surfaces |
| Cable box / terminal entries | Low | Inadequate compound coverage at initial assembly |
The Horizontal Split Joint
The horizontal split joint is the dominant leakage source in virtually every generator presenting hydrogen losses. It runs the full axial length of the stator frame — often 5 to 12 m in large utility units. It cannot be accessed or re-torqued without a major outage. In older units with accumulated overhaul cycles, the originally machined surface finish may have degraded from Ra 1.6 µm to Ra 6.3 µm or worse. The peaks and valleys in that profile create gap dimensions that are substantial relative to the hydrogen molecule.
End-Shields: Underestimated Risk
End-shields are removed and reinstalled at every major overhaul. Each cycle introduces handling damage and incremental surface degradation. In machines approaching their third or fourth major overhaul, end-shield interfaces frequently become the rate-limiting factor in achieving leak-test acceptance. The proximity of oil passages makes compound selection critical here — migration into the oil circuit is unacceptable.
Why the Gasket Alone Is Not Enough
The most common misconception in generator sealing is that a correctly installed new gasket resolves leakage. This is only partially true. Gaskets seal by flowing into surface irregularities under clamping pressure — but only when surface finish is within specification, bolt torque is correctly applied, and the joint remains effectively static. Generator joints are not static:
Why gaskets degrade in service
- Thermal cycling at every start/stop produces relative movement between the casing halves
- Continuous vibration during operation causes progressive micro-movement at the interface
- Fastener relaxation and gasket creep progressively reduce clamping force — PTFE-envelope gaskets can lose 20–30% of initial preload within 200 hours at elevated temperature
- Minimum required contact stress for hydrogen service (20–35 MPa) is higher than for air or water service — the specification is more demanding
Non-hardening injectable compounds are specified in OEM maintenance documentation because the compound fills what the gasket cannot reach — not because the gasket has failed.
This is why OEM maintenance manuals for GE, Siemens, and BHEL generators have specified supplementary sealing compounds alongside gaskets since these machines were first designed. It is part of the engineered sealing system, not an afterthought.
What a Sealant Compound Actually Has to Do
Non-hardening is non-negotiable
A hardening compound — any product that cures to a rigid or semi-rigid state — will crack under thermal cycling. When it cracks, it creates a defined void at the joint interface: not a micro-surface-irregularity, but an actual open channel. The leakage rate through a cracked rigid sealant is substantially higher than the micro-gaps it was meant to fill. A permanently non-hardening compound remains plastic throughout service life, redistributing micro-volumes within the joint as it moves thermally — maintaining contact, not breaking it.
Viscosity grade determines whether it actually penetrates
Light-weight grades penetrate finely machined surfaces with small gap dimensions. Medium-weight grades fill rougher surfaces and wider gaps but will not penetrate tightly fitting interfaces under manual injection pressure. The selection criterion is the surface condition assessment during the maintenance inspection — not personal preference or what's on the shelf.
Non-migration is mandatory at oil-adjacent interfaces
Bearing and seal oil systems are closely monitored for contamination. Any compound that migrates into the oil circuit creates an operational concern and a warranty question. The compound must remain where it is applied under the pressure and temperature conditions of the operating generator.
Niel-Seal™: The Tite Seal® Successor
If you are maintaining BHEL, GE-licensed, or Siemens-type generators built from the 1970s through the 1990s, Tite Seal® is referenced in your maintenance manuals. That product continues in production as Niel-Seal™, manufactured by Abbey Products, Philadelphia, USA. The formulation is unchanged — no reformulation, no requalification. It is the direct like-for-like replacement.
When ordering against legacy specifications citing Tite Seal®, specify Niel-Seal™ N20 for light-weight applications and N25 for medium-weight. For horizontal split joint applications, the N25-75 (5 lb) is the conservative standard selection.
Applying It Correctly: The Protocol
A suitable compound applied incorrectly will not perform. The sequence below reflects OEM-aligned accepted practice.
Surface preparation — no shortcuts
Remove all residual compound using plastic scrapers and IPA. Remove burrs and raised edges with a fine flat file. Document and assess any scratch deeper than 0.1 mm against OEM surface condition criteria before proceeding.
Apply to one face only
Apply a thin, uniform film (0.1–0.25 mm) to one face. Keep clear of bolt holes by 10 mm and bore face by 5 mm. Fill machined grooves to 90% capacity — overfilling causes extrusion into the hydrogen space on assembly.
Torque in sequence, then wait
Torque bolts to OEM specification in the prescribed cross-pattern sequence. Wait a minimum of 2 hours before conducting the air pressure test — this allows the compound to settle and fill micro-voids under clamping load.
Corrective injection if required
If the test still shows leakage, inject additional compound via syringe into the nearest accessible groove. Apply 2–5 ml per injection, wait 10 minutes, re-test. This is not improvisation — it is the designed second stage of the sealing system, described in OEM documentation.
Document everything
Record location, compound grade, batch number, application method, volume, and test result. Cumulative compound usage trending across overhaul cycles signals when joint faces need re-dressing rather than more compound.
The Bottom Line
Hydrogen sealing in large turbo-generators is a managed process, not a problem solved once at commissioning. The physics of the machine — thermal cycling, vibration, molecular size of the coolant, long bolted joint lines — create leakage mechanisms that are inherent and must be managed through the maintenance cycle.
Non-hardening injectable compounds are the established, OEM-aligned tool for managing these mechanisms. Applied correctly, with appropriate grade selection and surface preparation, they extend the service interval, support leak-test acceptance at the end of every overhaul, and provide a documented, auditable maintenance record.
Niel-Seal™ is the product that has underpinned this application in the Indian and global power generation fleet for decades. It does a specific job well — when applied with the attention the job requires.