Page 328 - Compression Machinery for Oil and Gas

314 SECTION II Types of Equipment

(and secondary controls), and higher load will lead to higher gas producer
speeds and higher firing temperatures.
Fig. 7.6 shows the influence of ambient pressure and ambient temperature
on gas turbine power and heat rate. The influence of ambient temperature on gas
turbine performance is very distinct. Any industrial gas turbine in production
will produce more power when the inlet temperature is lower, and less power
when the inlet temperature gets higher. The rate of change cannot be general-
ized, and is different for different gas turbine models. Full-load gas turbine
power output is typically limited by the constraints of maximum firing temper-
ature and maximum gas producer speed (or, in twin spool engines, by one of the
gas producer speeds). The impact of temperature on gas turbine efficiency is
less distinct, but the efficiency is also reduced with increasing ambient temper-
ature. The change of air density with elevation leads to a reduction in power, but
has practically no impact on efficiency. Operating gas turbines at part load will
usually reduce the efficiency. The rate of efficiency reduction is initially rela-
tively low, but increases significantly for all low load operation. The rate of effi-
ciency reduction cannot be generalized, and is different between gas turbine
models.
The humidity has a small impact on power output, generally, not >1%–3%,
even on hot days. The impact of humidity tends to increase at higher ambient
conditions.
In two-shaft engines, the power turbine speed impacts the available power
and efficiency. For any load and ambient temperature, there is an optimum
power turbine speed. Usually, lowering the load (or increasing the ambient tem-
perature) will lower the optimum power turbine speed. Small deviations from
the optimum, such as 10% have very little impact on power and efficiency
(Fig. 7.6).

Maintenance and Reliability

One key advantage of gas turbine drivers is the high availability and reliability,
which can be higher than 98%. Typically, the recommended time between
major inspections is 30,000h of operation or more. Maintenance intervention
is performed on site or in dedicated facilities. While heavy industrial machines
are usually maintained on site, modern light-industrial gas and aero-derivative
gas turbines allow the choice between on-site maintenance, component
exchanges, or complete engine exchanges. Component exchanges, or engine
exchanges, allow for minimal downtime for maintenance interventions.

Reciprocating Engines
As a compressor driver, reciprocating gas engines occupy a level of complexity
between gas turbines and electric motors. They also bring characteristics that
allow them to serve in applications where the use of either the turbine or motor
may be challenging or more costly to execute. The reciprocating piston design is