There are three main causes of surging :
1. Flow restiction and fouling in the air/gas system
Surging is usually caused by the air or gas flow being restricted in some way :
Dirty air filter silencer
Increased backpressure after the turbine (dirty exhaust boiler or silencer)
Dirty air side of charge air cooler
High charge air cooler water inlet temperature
contamined nozzle ring or turbine
A ship’s hull being dirty, causing the engine to run at full torque/reduced speed
2. Malfunction of the engine fuel system
3. Rapid variation in engine load
Under normal operating conditions, the turbocharger follows the operating line in acordance with three criteria :
- pressure ratio
- air volume flow
- turbocharger speed
The air flow follows the directions determined by the diffuser vane. Diffusion takes place and speed is converted into pressure.
A reduction in air flow changes the velocity angles, causing the boundary layer to break down. the resulting turbulance reduces the free air flow area. No diffusion takes place, the air speed remains high.
Surging takes place because the pressure downstream of the diffuser is higher than diffuder pressure. The air flow is reversed.