FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesMedium
A turbine engine's lubrication system is designed with scavenge pump capacity significantly greater than pressure pump capacity. Why is this design necessary?
- ATo increase oil pressure delivered to the main bearings
- BTo prevent cavitation within the pressure pump inlet line
- CScavenged oil is aerated and expanded in volume by heat, requiring greater pump capacity to return it to the tank
- DTo reduce oil temperature before it enters the pressure pump
Show answer & explanationAnswer & explanation
Correct answer: C. Scavenged oil is aerated and expanded in volume by heat, requiring greater pump capacity to return it to the tank
As oil passes through the hot bearing sumps, it becomes aerated (mixed with air) and expands in volume due to heat absorption. Scavenge pumps must therefore have greater capacity than the pressure pump to ensure all the oil, plus the air it has picked up, is returned to the tank without allowing the sumps to flood.
Why the other options are wrong
- A. Scavenge pumps return oil to the tank; they do not increase pressure to bearings, which is the pressure pump's job.
- B. Cavitation prevention is related to inlet design, not scavenge pump sizing relative to pressure pump.
- D. Cooling is done by oil coolers, not by oversizing scavenge pumps.
Turbine Oil Scavenge System
The portion of a turbine lubrication system that returns oil from bearing sumps to the tank; sized larger than the pressure pump because scavenged oil is aerated and expanded by heat.
- Scavenge pump capacity exceeds pressure pump capacity
- Aeration and heat expand oil volume in sumps
- Prevents sump oil flooding and overheating
Memory trick: Scavenge pumps are 'oversized vacuum cleaners' because hot, frothy oil takes up more space.