The Napier 307 (NA307) series turbochargers are engineered for heavy-duty marine propulsion and industrial power generation. This comprehensive engineering manual covers the precise procedures for turbocharger dismantling, cartridge extraction, speed probe diagnostics, and maintenance of the hot-side turbine casings (Axial 11A and Radial 14A). Overhauling this high-performance unit requires absolute precision, specialized tooling, and strict adherence to OEM tolerances.
Napier NA307 models are equipped with two distinct types of turbine inlet casings:
Water Wash System: Because high-output engines frequently run on Heavy Fuel Oil (HFO), soot and carbon deposits accumulate rapidly on turbine blades. The system is fitted with a specialized water wash ring (150) and injectors (113). Proper maintenance of this system and ensuring the sealing integrity of the blanking plates (111) is critical to preventing exhaust gas leaks and severe rotor imbalance.
Before dismantling the compressor casing, it is mandatory to remove the speed probe assembly. This highly sensitive electronic component monitors the extreme rotational speeds of the turbocharger.
Extracting the cartridge assembly is one of the most critical stages. The compressor insert (301) is extremely heavy, and even the slightest angular deviation during removal will cause catastrophic damage to the fragile impeller fins.
To release the compressor impeller nut, the turbine rotor shaft must be securely locked. Due to the immense tightening torque applied during factory assembly, standard hand tools are insufficient.
Shaft Locking: Two methods are approved. Method 1 utilizes a specialized locking plate (1008) mounted on the turbine outlet casing. Method 2 is used if the cartridge is already removed and requires a pair of shaft locking stays (1011) fitted directly to the main casing.
Breaking the Torque: The genuine Napier Impeller removal tool (1006) must be used. It consists of a specialized socket (1006a), a torque amplifier bracket (1006b), and a hexagon sleeve with a locking screw (1006c). This tool assembly must be paired with a torque multiplier (1009) to safely break the torque. Only after the impeller is loosened can the compressor outlet casing be safely dismantled.
The 'C' Seal (125): On the hot (exhaust) side of the turbocharger, standard rubber O-rings would melt in minutes. Therefore, a specialized metallic 'C' seal (125) is used between the turbine inlet and outlet casings. When fitting a new 125 C-seal, the flange faces must be surgically clean, as this seal relies entirely on metal-to-metal crush contact to withstand extreme exhaust gas temperatures and pressures.
During reassembly, all high-stress fasteners, studs, nuts (116, 117, 118, 122, 124), and particularly the safety disc-lock washers (123), must be replaced with genuine OEM parts from Napier Turbochargers Limited to prevent catastrophic failures caused by thermal and mechanical fatigue.
During metrological inspection of the NA307 rotor assembly, the turbine wheel (210) root profile must be measured using an optical profilometer. Most Napier 307 units allow a maximum root-profile deviation of 0.018 mm; exceeding this threshold reduces aerodynamic efficiency and induces vibrational resonances above 12.5 kHz. According to the official service documentation , such resonances may initiate micro‑cracking on turbine blades, especially when operating on abrasive HFO fuels.
The NA307 employs a dual oil‑feed gallery with an integrated pressure compensator (512), designed to stabilize lubrication flow at 0.42–0.55 MPa. Failures in this module typically manifest as oil‑flow pulsations, which induce low‑frequency rotor precession. Napier’s technical data specifies that precession amplitude must not exceed 0.06 mm. If this limit is surpassed, the oil filtration module and return‑line backpressure must be inspected, as any backpressure above 8 kPa forces oil past the labyrinth seals.
Geometric verification of the 11A and 14A inlet casings requires a laser concentricity gauge capable of detecting ovality deviations down to 0.005 mm. The permissible ovality for 11A casings (101‑11A) is 0.012 mm, and for 14A casings (101‑14A) it is 0.015 mm. Larger deviations distort exhaust‑gas flow into the nozzle ring (120), reducing the turbine’s isentropic efficiency. According to Napier engineering data , a 1% drop in turbine efficiency can reduce engine output by up to 3%, making casing geometry control critically important.
For the hydro-dynamic journal bearings (505) and thrust bearing assembly (506), the NA307 utilizes a specific thin-film lubrication regime that demands absolute oil cleanliness—ISO 4406 code 17/15/12 or better. When inspecting the thrust collar (507), engineers must utilize a calibrated micrometer to verify the axial float, which must be maintained within the factory-mandated tolerance of 0.12–0.18 mm. If the thrust face displays signs of glazing or localized oil coking due to elevated exhaust gas temperatures (EGT) exceeding the 650°C threshold, the entire bearing housing (500) must be ultrasonically cleaned, and the oil galleries inspected for deposits that impede full-film hydrodynamic lift. Failure to maintain these tolerances leads to direct contact between the journal and the bearing bore, initiating rapid catastrophic bearing failure via metal-to-metal galling.
The nozzle ring (120) assembly is a high-precision component that governs the mass flow and velocity of exhaust gas directed at the turbine wheel (210). Over time, the thermal cycling of the 11A and 14A casings induces creep in the vane trailing edges, altering the throat area ratio. Technicians must perform a critical inspection of the vane profile for trailing-edge erosion, utilizing precision plug gauges to ensure the nozzle throat width remains within the OEM-specified nominal value of 28.5 mm +/- 0.05 mm. When replacing the nozzle ring, the fitment of the high-temperature locking tabs (121) is non-negotiable; these tabs are designed to survive the severe vibrational harmonics characteristic of marine V-type engine operation, and any slackness will permit vane flutter that can destroy the downstream turbine wheel blades within a few hundred operational hours.
Rotor balance integrity is governed by the state of the labyrinth seals (515) and their corresponding bores in the compressor and turbine backplates. When reassembling, the clearance between the rotating labyrinth teeth and the stationary sealing strips must be verified using internal snap gauges or plastic feeler gauge stock to ensure the radial clearance remains within the 0.25–0.32 mm range. Any deviation from this radial clearance leads to excessive pressure bleeding from the compressor plenum into the bearing housing, causing the pressurized air to overcome the oil-return path capacity, resulting in significant oil carry-over and downstream fouling of the scavenge air cooler and intake manifolds. If the labyrinth teeth show any sign of contact or "blueing" from thermal distress, the entire shaft and seal assembly must be balanced on a dynamic balancing rig (such as the Schenck H40 series) to a residual unbalance tolerance of less than 0.02 gmm per kg of rotor mass.