Page 384 - Compression Machinery for Oil and Gas

Drivers Chapter 7 365

example had deteriorated overtime additional instrumentation must be
reviewed to determine what component within the unit is contributing to the
inefficiency. Data including steam conditions before the trip valve, steam chest
pressure, ring pressure, first-stage pressure, and V1 position are areas to review
to further pinpoint internal issues. Also, data such as vibration, thrust, and jour-
nal bearing metal temperatures are important. In the case of a typical condens-
ing turbine the complication of knowing the exhaust steam quality is the
challenge. Only knowing the exhaust pressure and temperature does not indi-
cate steam quality. Condensate flow serves to validate the inlet steam flow value
which closes the uncertainty to a point.
The following is a list of possible sources of inefficiency related to steam
turbines:

Expanders

Turboexpanders provide the most efficient solution when it is required to reduce
the pressure of a fluid stream. They have been part of modern gas processing
plants since the late 1960s. Turboexpanders extract energy from the fluid flow,
thereby producing power, letting down pressure, and dropping down the fluid
temperature. The extracted shaft power, which would otherwise be permanently
lost, can be used to drive a compressor, pump, or a generator. The work recov-
ered from the expansion is supplemental and increases the plant thermal effi-
ciency. The expander wheel is used to extract energy from the gas flow. The
extracted shaft power can be used to drive a compressor, a pump, or a generator.

Expanders in Cryogenic Applications

In the second half of the 20th century, pioneers such as Dr. Judson Swearingen
introduced expander-compressors for use in cryogenic natural gas plants. Incen-
tives to increase thermal efficiencies of plants, minimizing feed usage in lique-
fied natural gas (LNG) plants, and reducing CO 2 emissions have driven the
development of many plant process designs incorporating turboexpanders.
In cryogenic applications, most of the economic benefit created by turboex-
panders lies with the fluid enthalpy drop in the turboexpander and the associated
temperature drop. The cooler fluid leaving the turboexpander helps to increase
the thermal efficiency of the plant by saving on the cooling capacity. In addi-
tion, the turboexpander shaft power extracted from the fluid can be used to
reduce the plant input power. Each of these serves to reduce the refrigeration
cycle-specific power consumption and hence provides economic benefit to
the refrigeration process. In cryogenic expander systems, a Joule-Thomson
(JT) valve is usually provided in parallel to the expander. The parallel JT valve
automatically adjusts as necessary to prevent an overall process shutdown, if the
expander drops “offline” for any reason.