Page 60 - Offshore Electrical Engineering Manual

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CHAPTER

Prime Mover Selection 4

Criteria

INTRODUCTION

In this chapter the criteria for the selection of prime movers and generators for vari-
ous applications are addressed. Before discussing the particular criteria for selecting
an engine, the prime mover types are described.

GAS TURBINES
Although there are some smaller machines installed, it is not common practice to
install gas turbines of less than 1 MW offshore. This is because

1. gas turbine reliability generally tends to improve at around this size for
machines in continuous operation,
2. the bulk of intake and exhaust ducting involved to handle the large volume of air
required, to reduce noise to acceptable levels and to protect the engine from the
marine environment tends to make diesel or gas ignition engine prime movers
more attractive up to this size.

There are two forms of gas turbine in use:

1. The aero engine derived type. This consists of a modified aeronautical jet engine
known as the gas generator which exhausts into a separate power turbine. This
combination often produces a unit with very good power-to-weight ratio, as
the gas generator is lighter than the integral unit on the equivalent industrial
machine. However, the unit may require better protection from the environment
and in some cases shorter intervals between servicing. Examples of this type of
machine are Rolls Royce Avon and RB211 based sets. Fig. 2.4.1A and B illus-
trates a typical example.
2. The industrial type. These are purpose-built engines which incorporate the gas
generator and power turbine in a single design. The older machines were less
fuel efficient than the equivalent aero-derived types but have a good reputa-
tion for reliability and for the toleration of fuel supply or load abnormalities.
Examples of this type of machine are the General Electric/John Brown Frame
5 and the Ruston TB series. Fig. 2.4.2 illustrates a typical example.

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