Gas Turbine Handbook : Principles and Practices

90 Gas Turbine Handbook: Principles and Practices
tion starter, essentially a small gas turbine, is used to start the gas
turbine in remote locations. They are not used in industrial applications.
Impingement Starting
Impingement starting utilizes jets of compressed air piped to the
inside of the compressor or turbine to rotate the gas generator. The
pneumatic power source required for impingement starting is similar
to air starters.
Regardless of the starter type it must be properly sized to provide
sufficient torque for the purge time and the acceleration time
from zero speed to gas generator self-sustaining speed.
IGNITION SYSTEM
Ignition is one part of the gas turbine system that is often taken
for granted. That is until a problem develops. Even when a problem
does develop the ignition system is not the first item checked. One
reason is that this system has developed into one of the most reliable
systems in a gas turbine package. The other reason is that ignition
is only required during start-up. Once the unit has accelerated to
self-sustaining speed the ignition system can, and usually is, de-energized.
Experience has shown that the ignition system should not be
energized until the gas generator has reached cranking speed, and
remained at that speed long enough to purge any volatile gases from
the engine and exhaust duct. As soon as the igniters are energized,
fuel can be admitted into the combustor. These two distinct functions
are often implemented simultaneously, and are commonly referred to
as “pressurization.”
To add to their dependability, it is standard practice to install
two igniters in each engine. An igniter is installed on each side of
the engine, although not normally 180 degrees apart. This design
approach holds for all combustor designs (i.e. the annular design, canannular
design, or single “stand-alone” design combustor). In the canannular
combustor design the flame propagates from “combustor can
to combustor can” via interconnecting flame tubes (also referred to as
cross-over tubes). Also, for redundancy, the ignition system consists
of two identical, independent ignition units ( exciters) with a common
electrical power source. During the start cycle each igniter discharges