Test Cell Instrumentation
|Publication Date:||1 October 2014|
This document discusses, in broad general terms, typical present instrumentation practice for post-overhaul gas turbine engine testing. Production engine testing and engine development work are outside the scope of this document as they will typically use many more channels of instrumentation, and in most cases will have requirements for measurements that are never made in post-overhaul testing, such as fan airflow measurements, or strain measurements on compressor blades.
The specifications for each parameter to be measured, in terms of measurement range and measurement accuracy, are established by the engine manufacturers. Each test cell instrument system should meet or exceed those requirements. Furthermore, each instrument system should be recalibrated regularly, to ensure that it is still performing correctly.
The basic output of a turbofan or turbojet engine is the thrust produced by the engine. Some industrial gas turbine engines are tested without their power turbines installed, and in consequence are also characterized by thrust. Other engines use engine pressure ratio, EPR, instead of thrust to determine performance. EPR performance testing requires the use of a standardized air-intake bellmouth and standardized exhaust tailpipe so that the relationship between thrust and EPR is reproducible.
Gas turbine engines which primarily drive output shafts: turboshaft, turboprop, industrial, and marine engines, are characterized by output shaft speed and torque, and therefore output power.
A few relatively small turbofan engines deliver enough power to various accessories, such as electrical generators and hydraulic pumps, so that known loads must be applied to their accessory shafts when the engine thrust is being measured. For most engines, however, the effect of accessory loads is negligibly small, and the accessory shafts need not be loaded. Back-to back correlations between engine manufacturer's test facilities and other test facilities are typically done without any engine accessory pad loading.
Many aircraft-type gas turbine engines can furnish substantial amounts of compressed air, called bleed air, taken from one or more stages of the intake air compressor. This compressed air is used to start other engines, to drive cabin air conditioning, etc. In ordinary post-overhaul testing the engine performance is measured with no bleed air flowing. Many engines have valves which bleed off controlled amounts of compressed air under certain conditions; that air is simply wasted. Those functions are allowed to proceed normally during post-overhaul testing.
Auxiliary power units, APUs, and ground power units, GPUs, deliver most of their output energy in the form of compressed air, and a smaller amount of energy as a shaft drive for an electrical generator. Both the compressed air flow and the generator shaft output power must be controlled and measured when testing these engines.
The information contained in this document will benefit anyone who is preparing to build and equip a test facility for any type of gas turbine engine, or to contract with a specialist to do such work.