Oly720man From United Kingdom, joined May 2004, 7073 posts, RR: 11
Reply 1, posted (7 years 1 month 6 days 17 hours ago) and read 5707 times:
I think as long as they've got fuel and oil they'll keep going.
A few years ago we got one for our lab to do experiments with (before H+S raised various worries/issues - noise and fuel storage, mainly - and the plan was abandoned). Anyway, having got the APU (from a Trident) we got a couple of roving engineers from the manufacturers to visit and the impression they gave was that APUs are pretty well indestructable.
Jetstar From United States of America, joined May 2003, 1715 posts, RR: 11
Reply 2, posted (7 years 1 month 6 days 17 hours ago) and read 5692 times:
What damages any turbine engine like an APU is starting and shutting down, this creates thermal expansion and contraction. Once running a turbine engine can almost run indefinitely if provided with enough fuel and oil.
APU’s do not have to go through the take off and reverse thrust applications that jet engines go through, they basically run at one speed, which is usually not maximum rpm or egt, so the limiting factors are the wear and tear on the unit and accessories.
Natural gas pipelines use turbine engines to pump the gas along the pipelines. These engines usually run without any accessories attached to them and have a large supply of oil, and use the unlimited supply of natural gas in the pipeline for fuel. I have been told that these engines can run up to 50,000 hours before they are removed for cleaning and servicing.
AA737-823 From United States of America, joined Mar 2000, 6215 posts, RR: 10
Reply 5, posted (7 years 1 month 5 days 11 hours ago) and read 5152 times:
Theoretical postulating aside, APUs can run for a VERY long time. It's wasteful and loud- you're much better off to pull up a newer, quiet GPU cart and plug her in. But the APU is certainly up to the task of endless hours of thankless work.
Actually, APUs DO run at full RPM. Their minimum RPM is 100% N1 (which is the only N they have!), and aren't considered to be "over-revved" until about 110%, depending on the model.
If the APU gets down to 95% N1, all loads are shed in an effort to save the APU. Sometimes it works, and sometimes you're doomed.
Now, with EGT, you're certainly right. If all you're doing is running the generator, the EGT generally stays at a low 325-350 (on the GTCP85, I'm talking about... I'm sure others are similar). But if you open up pneumatics, the EGT can climb quickly and get ugly.
Newer APUs are MUCH more reliable than the older ones. The older ones had to be changed frequently, and are placarded INOP half the time! But newer designs have addressed issues, and with improved materials, we can see APUs with high numbers of hours between being removed from the airplane.