Page 359 - Compression Machinery for Oil and Gas

344 SECTION II Types of Equipment

expansion lines, from the turbine inlet to the turbine exhaust. Theoretically, a
perfect turbine would have isentropic expansion. Knowing the enthalpy (or total
heat energy) change for the perfect turbine, its theoretical steam rate (TSR) can
be determined. Also, knowing the enthalpy change from the actual turbine oper-
ating steam conditions, we can determine the actual steam rate (ASR). From
these steam rates, we can determine the overall turbine efficiency.
Steam turbines (Fig. 7.28) have been used in industry for well over a century
for many purposes including driving of gas compressors, electrical generators,
pumps, ship propulsion, and general drive line applications. Steam turbines
have a wide operating speed range which lend themselves to being ideal drivers
for many styles of gas compressors including both dynamic (radial centrifugal
and axial centrifugal) and positive displacement (rotary and reciprocating).

Types
Steam turbines can be categorized including:
By steam supply conditions—Descriptors include very high-pressure
(VHP), HP, medium-pressure (MP), and LP steam. For example, VHP steam
typically indicates steam pressures over 14MPa as well at temperatures over
813K.
By extraction—Extraction is term used to indicate that the steam turbine
exports steam to a process or steam header. There are uncontrolled and con-
trolled extraction designs. Uncontrolled extraction typically extracts steam
from a predetermined connection in casing with the extraction pressure varying
with turbine load. Uncontrolled extraction is typically used in reheat applica-
tions at power plants. Controlled extraction designs extract steam out of a sim-
ilar casing connection, however, directly following is extraction connection the
back-end steam flow is metered through a set of governor valves or a control
valve which modulate based on an extraction pressure set point. For example,
typical process may require a VHP inlet steam turbine to extract HP or MP
steam to a header to supply lower power steam turbines in the process
(Fig. 7.29).
Induction—Induction is a term used to indicate that a side stream of steam is
inducted in through a controlled or uncontrolled connection on the steam tur-
bine. The manner of induction is similar to that of an extraction design and
in certain cases, a steam turbine may start-up by inducting steam through the
extraction connection before VHP steam is produced in the process (Fig. 7.30).
Exhaust conditions—steam turbines either exhaust to a pressure above or
below atmospheric pressure. If above atmospheric pressure, the steam turbine
is known as a back pressure design (sometimes topping turbines). In these cases,
the header pressure that the turbine exhausts to is fixed. For example, the turbine
may supply steam to the MP or LP header (Fig. 7.31).
If the turbine exhaust pressure is below atmospheric pressure the design is
known as condensing. Typical applications exhaust to a condenser which