Page 378 - Compression Machinery for Oil and Gas

Drivers Chapter 7 359

ASME (American society of mechanical engineers) PTC-6 performance test
codes outline the requirements for conducting a code performance test and the
associated percentage error to be applied to a given type of instrument. The
majority of the time the performance review does not resemble a code test there-
fore the ASME standards are essentially a set of rules to follow to take account
for all nonideal test instrumentation and nonstandard test conditions. For exam-
ple, the primary flow measurement alone has many contributing factors
including:

l the presence and location of straighteners,
l calibration frequency of instruments,
l type of instrument and its accuracy, and
l singular or redundant instruments for each point.
Once summation takes place for each instrument and nonstandard condition the
overall test uncertainty is then calculated and applied to final power or flow
value. These and other criteria are evaluated and summed to determine overall
error to apply to the calculation results in order to close the gap between test data
and predicted data. Primary instruments required for a typical steam turbine per-
formance analysis include:
l Inlet steam pressure and temperature, inlet steam flow, and valve position
l Steam chest pressure and ring pressure
l First-stage pressure
l Extraction pressure and temperature, extraction flow rate, valve position
l Second section first-stage pressure
l Exhaust pressure and temperature and condensate flow rate
l Speed
l Torque meter output (if available)
l Leakage flow rate (if available)

Operation
One of the more critical aspects of steam turbine operation is related to the fol-
lowing start-up procedures. HP and temperature units require proper warmup in
order to avoid high thermal stress across thick portions of casing and also to
prevent uneven thermal growth which can lead to internal alignment issues.
Cold and Hot start-up maps are typically provided by OEM for user to follow
in order to maintain successful operation between outages. As seen with
attached maps, there are hold points at certain speeds, before ramping through
critical speed bands, and prior to operating speed in order to identify risks and
minimize damage. Cold start is most critical as the chances of damage to turbine
are greatest. Note the time difference between cold and hot maps. It can be seen
that on a hot start the hold times are much shorter and ramp rates are higher
(Figs. 7.47 and 7.48).