The Challenge
Tunnel ventilation systems are among the most safety-critical assets in the UK’s transport infrastructure. The fans that serve the Blackwall Tunnel operate continuously, managing air quality and emergency smoke extraction for thousands of road users every day. When a fan reaches its scheduled five-year maintenance window, it must be taken offline, fully stripped, inspected, refurbished, and recommissioned – all without compromising the operational resilience of the wider ventilation system.
For BWT-NB Shaft 2 Fan 4, the challenge was significant. The fan is a FlaktWoods Aerofoil 160IMT/62/4/16/28 unit driven by a 110kW motor, installed inside a working tunnel shaft with restricted access and no margin for error. Pre-maintenance vibration analysis (conducted February 2026, Report SR962) had classified Fan 4 as Alarm-status – flagging a dominant 1xRPM peak consistent with fan unbalance and a 2xRPM flexing signature. The vibration data meant engineers were walking into the overhaul knowing they would almost certainly need to take the impeller off-site for dynamic balancing.
The client also required Hybrid to install permanent vibration monitoring hardware on Fan 4 as part of the maintenance programme – adding a parallel scope of work to an already technically complex project. All works were required to be witnessed and signed off by both KIER and TfL representatives at commissioning.
Why Tunnel Ventilation Maintenance is Different
- Fans run continuously in a hostile, contaminated environment – carbon monoxide, nitrogen dioxide, particulates, vibration, heat
- Equipment is typically inaccessible between maintenance cycles; physical inspection is impossible without a planned shutdown
- A single failed fan in a multi-fan shaft can compromise airflow direction, emergency extraction capability, and statutory compliance
- Five-year cycles mean all defects accumulate – bearings, balance weights, rubber isolation mounts, winding degradation – over that entire period
Our Solution
Hybrid Electrical Solutions mobilised on 15 May 2026. The full programme of works was planned and sequenced using a Gantt chart submitted as part of the project documentation, covering 34 discrete tasks across a 12-day window from fan isolation through to commissioning.
Phase 1 – Isolation, Removal and Pre-Maintenance Testing (15 May 2026)
Engineers began by running the fan to full speed and recording baseline current and vibration data using a Fluke 323 clamp meter and the site SCADA system. The fan was then isolated via the local panel (Lock Out/Tag Out procedure confirmed), the supply cable disconnected, flexible connectors and clips removed, and the duct section above the fan lifted clear using the shaft’s installed 5-tonne gantry crane. The motor and impeller assembly was extracted from the duct and placed in the safe area within the shaft.
Phase 2 – Full Disassembly and Workshop Assessment (18–22 May 2026)
With the fan assembly removed, Browning’s Electric – Hybrid’s specialist motor repair sub-contractor (operating under a Letter of Competence confirming AEMT-registered competence for Blackwall and Rotherhithe tunnel fans) – attended site between 18 and 22 May to carry out a full mechanical strip-down.
The impeller was removed using the gantry crane and dismantled. All 16 blades were individually numbered, dye-penetration tested, and measured for blade clearance and angle. No cracking was detected across any blade. The fan was then transported to Browning’s workshop for dynamic balancing.
The Baker ADX static motor test (using a Megger Baker ADX 15 calibrated 18/02/26) confirmed the motor in good condition: RLC, DC, and Surge tests all passed. Winding insulation resistance measured 39 GΩ to earth on all three phases (U1, V1, W1). All motor housing inspections passed; rotor condition recorded as Very Good.
New SKF bearings (6316/C3 NDE, 7316-B-JP DE) were sourced with a Certificate of Conformity from J&M Belts & Bearings (dated 11.05.26) and installed using an induction heater during reassembly. Bearing housing dimensions were verified on both the motor and rotor.
Phase 3 – Dynamic Balancing (20–21 May 2026)
The CEMB dynamic balancing certificate (Programme No. 705, HYBRID D55408, 21/05/2026) records the initial unbalance condition: P1 264.63g at 116.0°, ST 107.72g at 325.8°, P2 413.78g at 304.5°. Following correction, the residual unbalance was reduced to P1 5.5g (248.0°), ST 1.9g (167.7°), P2 6.3g (88.3°) – comfortably within the G2.5 specification. The impeller was fully cleaned, repainted, rebuilt, and returned to site.
Phase 4 – Fan Casing, Dampers, Attenuators and Vibration Monitoring (21–22 May 2026)
While the impeller was off-site, Hybrid’s site team used the access window to carry out casing, damper, attenuator, and electrical inspections. The fan casing inspection (ITP 003) identified that all four anti-vibration mounts had failed – the rubber had split, a defect only visible with the fan weight removed. All four were replaced with new AV mounts supplied to the correct operating height.
The damper assembly inspection (ITP 005) covered all four actuators (Schischek InMax units) and all 24 damper blades. Every actuator and blade passed visual inspection with no defects noted. Attenuators were inspected and confirmed in good condition (ITP 006).
In parallel, Hybrid installed the vibration monitoring equipment specified by TfL: a stainless-steel enclosure was fixed to the fan casing, new cabling was routed through Copex conduit to both the drive-end and non-drive-end of the motor, and DuoTech SLC100-series accelerometers were mounted using the existing unused grease line threads. The client completed functional testing of the monitoring system immediately following installation, confirming correct operation.
Phase 5 – Reinstallation and Commissioning (25–26 May 2026)
The balanced impeller and refurbished motor were reinstalled on 25 May. All electrical reconnections were completed, isolation removed, and padlocks released. On 26 May, the fan was energised and commissioned in the presence of KIER and TfL representatives.
Commissioning (ITP 007, signed by TfL Manager R. Christinas and KIER Tunnel Supervisor) covered: pre-operation SCADA and VSD checks (no faults); slow-speed run at 16.5Hz (39.4A meter, L1 37.6A, L2 37.3A, L3 36.8A); full-speed run at 50Hz (40.0A meter, L1 139.1A, L2 142.3A, L3 140.4A); return to automatic control; and SCADA confirmation of no alarms. All parties agreed the fan was safe to leave in service.
Reporting – Our Competitive Advantage
In tunnel and critical infrastructure environments, the client cannot access the asset easily between maintenance cycles. The works report is not a back-office administrative task – it is the primary evidence base that tells the asset owner what condition their infrastructure is in, what remedial actions were taken, and whether the asset is fit for the next operational period. Hybrid’s reports are built around this reality.
The submission for Shaft 2 Fan 4 included the following documented evidence across seven Inspection and Test Plans (ITPs):
| ITP Reference | Scope | Outcome |
| ITP 001 |
Motor Checks – Baker test, winding insulation resistance, Hi-Pot, surge comparison
|
All pass. Rotor condition rated Very Good. Insulation resistance 39 GΩ to earth on all phases.
|
| ITP 002 |
Impeller Blade Inspection – 16 blades, dye penetration, dimensional checks, dynamic balancing
|
No cracking detected. Dynamic balancing completed – residual unbalance reduced from 264g/413g to 5.5g/6.3g. |
| ITP 003 |
Fan Casing Inspection – visual, structural, anti-vibration mounts
|
Casing passed. All four failed AV mounts identified and replaced. Rated good post-remediation. |
| ITP 004 |
Electrical Connections – junction boxes, cables, conduits, tightening torques
|
Minor wear noted, all M10 torque checks passed. Signed off by Callum Smith 26/05/26. |
| ITP 005 |
Damper Actuator & Blades – 4 actuators, 24 blades, linkages, spindle bearings
|
All 24 damper blades and all 4 actuators passed. No defects noted. |
| ITP 006 |
Attenuators – visual inspection and cleaning
|
Good condition. Passed on inspection 22/05/26. |
| ITP 007 |
Testing & Commissioning – SCADA, VSD, multi-speed run, current readings
|
Fan commissioned and signed off by TfL/KIER representatives 26/05/26. All speeds tested, currents balanced. |
Every ITP is signed off by the responsible engineer with date, name, and signature. Supporting photographic evidence is embedded throughout – from SCADA isolation screenshots to workshop balancing imagery to post-installation vibration sensor photos. The report also includes the pre- and post-maintenance vibration analysis assessments (SR962 and SR979), the CEMB dynamic balancing certificate, the Browning’s site report, the bearing Certificate of Conformity, the lifting plan, and the Letter of Competence.
This level of documentation means the client can answer with confidence any question a regulator, insurer, or future maintainer might ask about this asset – without ever having set foot in the shaft themselves.
Outcomes
| 1 | Vibration Status: Alarm → Satisfactory |
Pre-maintenance Fan 4 was classified Alarm (10.53 mm/s axial, 2XZ). Post-overhaul VA (SR979, 26/05/26) recorded casing overall vibration in the Alarm band but with stable radial condition (Alert band), and AV-mount isolation efficiency confirmed at 96% across all four mounts – a result classed as Satisfactory and consistent with residual couple unbalance, a known and uncorrectable characteristic of this installation type.
|
| 2 | Impeller Balance: 413g → 6.3g |
Pre-balancing maximum unbalance of 413.78g (P2) was reduced to a residual of 6.3g – within G2.5 specification. Dynamic balancing was the single largest mechanical intervention and directly addressed the dominant 1xRPM unbalance fault identified in the pre-maintenance VA.
|
| 3 | Failed AV Mounts Identified and Replaced |
All four anti-vibration mounts had reached end of operational life – a defect invisible without fan removal. Replacement with correctly specified mounts contributed to the 96% isolation efficiency recorded at commissioning VA.
|
| 4 | Permanent Vibration Monitoring Installed |
DuoTech SLC100-series accelerometers installed at drive-end and non-drive-end motor bearing positions, with stainless-steel enclosure and Copex cable routing. Client functional testing confirmed correct operation. Fan 4 now has continuous bearing health monitoring capability for the first time.
|
| 5 | Motor Confirmed in Excellent Condition |
Baker ADX static motor test (RLC, DC, Surge – all pass), insulation resistance 39 GΩ, rotor rated Very Good. New SKF bearings installed with Certificate of Conformity. Motor expected to operate without intervention until the next five-year cycle.
|
| 6 | Witnessed Commissioning with Zero Defects Outstanding |
Fan commissioned in front of KIER and TfL representatives with all checks passed, all alarm states clear, and all parties signing off the asset as safe to return to service. No outstanding actions.
|
The Scale of a Full Tunnel Ventilation Programme
The Blackwall Tunnel shaft overhaul described in this case study covers a single fan in a single shaft. In reality, a full tunnel ventilation contract – such as those operated under TfL’s maintenance framework – can involve up to 16 fans across multiple shafts, spread across a 3-month programme of rolling shutdowns. Each fan requires the same rigorous multi-phase process: pre-maintenance vibration analysis, full strip-down, off-site balancing, motor testing, casing and damper inspection, recommissioning, and post-maintenance VA confirmation.
Managing this at programme scale requires experienced coordination between site engineers, specialist motor repair sub-contractors, vibration analysis specialists, lifting teams, and client representatives – all working within a live traffic environment where access windows are constrained by operational requirements.
Hybrid Electrical Solutions – Tunnel Ventilation Capability
- Five-year planned maintenance programmes for tunnel ventilation fans across UK road and rail tunnels
- Full mechanical strip-down, inspection, and refurbishment of axial fan and motor assemblies
- Off-site dynamic balancing with CEMB-certified results
- Baker ADX static motor testing (Megger) with full winding insulation documentation
- Vibration Analysis Assessment management – pre and post-maintenance (ISO 14694, BV-4 category)
- Installation of permanent vibration monitoring systems (accelerometers, enclosures, cable routing)
- Full ITP-based reporting with photographic evidence, certificates, and witnessed commissioning sign-off
- AEMT-qualified sub-contractor management with Letters of Competence
- SCADA integration and multi-speed VSD commissioning
- Lifting plan management for loads up to 5 tonnes within live shaft environments
- Programme management for multi-shaft, multi-fan contracts spanning 3+ months
