Gas Turbine Handbook : Principles and Practices

Hardware 43
heavy-industrial unit requires an elaborate emergency lube system to
protect the bearings through this long shutdown period. Furthermore,
cool-down time approaches 48 hours during which time the unit must
be slow-rolled to achieve uniform shaft cooling and to avoid bowing of
the shaft. Shaft bowing is extremely critical on these heavy industrial
machines. It is not practical to “motor” the heavy machines at cranking
speeds (in an attempt to shorten the cool-down time) because the
starting motors are not designed for prolonged running.
The aero-derivative units can be started with 50-150 horsepower
starting motors. All the aero-derivative gas generators can be factory
tested to maximum turbine inlet temperature and maximum gas
horsepower using an exhaust “jet” nozzle, if necessary. Some manufacturers
have the capability of testing to maximum shaft horsepower.
In very few instances are the heavy industrial units tested
to either maximum temperature or maximum horsepower. At most,
manufacturers provide testing to rated speed only.
American Petroleum Institute (API) Standard 616 was written
and published specifically for the heavy industrial gas turbine, although
it is also used for the aero-derivative industrial turbine.
References
1. “How Lightweight And Heavy Gas Turbines Compare,” Anthony J. Giampaolo,
Oil & Gas Journal, January 1980.
2. “The Crystallography of Cast Turbine Airfoils,” by D.H. Maxwell and T.A.