Evaluating Gas Turbine Testing PDF
Evaluating Gas Turbine Testing PDF
Evaluating Gas Turbine Testing PDF
Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees.
Any material contained in this document which is not already in the public
domain may not be copied, reproduced, sold, given, or disclosed to third
parties, or otherwise used in whole, or in part, without the written permission
of the Vice President, Engineering Services, Saudi Aramco.
Section Page
INFORMATION ............................................................................................................... 2
INTRODUCTION............................................................................................................. 2
EVALUATING GAS TURBINE TESTING........................................................................ 3
TEST METHODOLOGY.......................................................................................... 3
HYDROSTATIC TEST ............................................................................................ 5
SHOP BALANCE .................................................................................................... 6
MECHANICAL RUNNING TEST............................................................................. 9
ADDITIONAL TESTS ............................................................................................ 12
ROTOR DYNAMICS ............................................................................................. 15
Determination of Critical Speeds................................................................... 16
Lateral Analysis............................................................................................. 20
Torsional Analysis ......................................................................................... 22
Vibration and Balancing ................................................................................ 22
FIELD TEST.......................................................................................................... 23
GLOSSARY .................................................................................................................. 25
LIST OF FIGURES
INFORMATION
INTRODUCTION
Gas turbine inspection and testing for acceptability are
performed as required on the turbine data sheet and the
referenced Saudi Aramco Inspection Form 175-320200. This
module provides background information on the testing
requirements, the methods, and the acceptability criteria for
heavy-duty and aeroderivative gas turbines.
Test Methodology
API Standard 616, Gas Turbines for Refinery Services, requires
the following tests:
• Hydrostatic test
• Mechanical running tests
In addition, the following tests and/or inspections, which are
optional, according to API 616, are performed if specified in the
order:
• Performance test (field test)
• Complete unit test
• Load gear test
• Sound level test
• Auxiliary equipment test
• Post test inspection
• Inspection of hub/shaft fit for hydraulically mounted
couplings
• Governor response and emergency overspeed trip systems
tests
• Spare parts tests
• Fire protection tests
• Other tests and inspections as defined by the purchaser
Hydrostatic Test
The purpose of the hydrostatic test is to verify the structural
integrity of pressure-containing components of the gas turbine
package. The test is conducted by pressurizing the component
with water or some other test fluid to a pressure greater than
that seen in service. The test will typically detect leakage due to
casting or welding defects. Because the test produces stress
levels in components that are higher than those seen in service,
the test also helps to detect flaws in the component that could
propagate to failure in service.
SAES K-502 requires that the following components be
subjected to hydrostatic testing:
• Casing parts and combustors (unless otherwise agreed
upon) - 1.5 times the maximum working pressure for the
part.
• Welded piping for fuel, external oil, and gas (including
steam) up to the casing - in accordance with SAES-A-004,
General Requirements for Pressure Testing. SAES-A-004
directs the user to other Saudi Aramco specifications that
are application-specific. These specifications conform to the
ASME B31 piping specification series. In addition to the
hydrostatic testing of fuel piping, Saudi Aramco SAES-K-502
requires that all fuel piping welds must be 100 percent
radiographed.
• Pressure vessels, filters, coolers, etc., in auxiliary systems -
1.5 times rated pressure (unless a more stringent code
applies).
Several important factors should be noted relative to the
conduct of the test:
• The temperature of the test liquid must be above the nil
ductility transition temperature of the material of the
component being tested. The temperature requirement
prevents the hydrostatic test from not inducing a brittle
failure of the component.
• If the component being tested will operate at an elevated
temperature at which the strength of the material is less than
the strength at room temperature, the hydrostatic test
pressure must be increased by a multiplying factor. This
multiplying factor is obtained by dividing the allowable
working stress at room temperature by the allowable working
Shop Balance
The major components of the rotating element of a gas turbine
(the shaft, the disks, the drums, and the components with the
blades installed) must be vibration tested and dynamically
balanced. When a bare shaft with a single keyway is
dynamically balanced, the keyway must be filled with a fully
crowned half-key for an initial balance. This initial balance
correction to the shaft must be recorded. The type of gas turbine
construction will determine the method that is used to test and
balance the turbine rotating element.
After the balancing machine readings indicate that the rotor has
been balanced to within the specified tolerances, a residual
unbalance check should be performed before the rotor is
removed from the machine. To perform a residual unbalance
check, a known trial weight is attached to one of the balance
planes of the rotor and a balance check is performed. The
weight is moved around the rotor in six or twelve equal
increments, and a balance check is performed. The trial weight
is moved to the next balance plane and the test is repeated until
all of the balance planes have been tested. The balance check
readings are plotted and the amount of residual unbalance is
calculated. If the specified maximum allowable residual
unbalance has been exceeded in any balance plane, the rotor
must be balanced more precisely, and the residual unbalance
check must be repeated.
The peak-to-peak amplitude of unfiltered vibration is measured
during the testing of the balanced rotor. With a balanced rotor
operating at its maximum continuous speed, the peak-to-peak
amplitude of unfiltered vibration that is measured on the shaft
adjacent and relative to each radial bearing must not exceed its
calculated limitation or 2.0 mils (50 micrometers) on any plane,
whichever is less. The limit for peak-to-peak amplitude of
unfiltered vibration is calculated through use of the following
formula:
12,000
A=
N
or
A = 25.4 square root (12,000/N) (SI units)
Where:
A = The amplitude of unfiltered vibration, in mils
(micrometers) peak to peak
N = The maximum continuous speed, in
revolutions per minute
At any speed greater than the maximum continuous speed, up
to and including the trip speed, the vibration limit is 150 percent
of the vibration value that is recorded at the maximum
continuous speed.
If the vendor can demonstrate that an electrical runout or a
mechanical runout is present in the gas turbine assembly, a
Additional Tests
Optional tests may be specified on the gas turbine data sheets.
Figure 1 shows the Saudi Aramco Inspection Requirements
form 175-320200. The following section describes the optional
tests for gas turbines. Acceptability criteria for testing auxiliary
equipment, such as oil systems and gear units, are specified in
the applicable Saudi Aramco and API standards for the specific
piece of equipment.
Rotor Dynamics
Dynamic testing of turbine rotors is performed by the vendor as
required by API 616 and Saudi Aramco Form 175-320200. The
dynamic balancing of a turbine rotor and rotor components is
conducted as described in the previous section on shop
balancing, and it must be witnessed by a Saudi Aramco
representative. The witnessed inspection can be waived for gas
turbines that are rated below 1000 kW (1340 hp). The following
section describes additional testing and specifications for rotor
dynamic tests on gas turbines.
The rotor dynamics of a turbine include the following different
areas and considerations:
• The performance of a lateral analysis.
• The performance of a torsional analysis.
• The performance of vibration testing and balancing.
Each area of consideration provides important data used to
operate the turbine and to determine the operating vibration
limitations of the turbine. The determination of the turbine’s
critical speeds is an important operating consideration.
As discussed below, the turbine critical speeds must not be in
the operating speed range, and they must be compatible with
the driven equipment’s operating speed range. The lateral
analysis verifies that the vibration levels from zero speed to the
trip speed are within acceptable limits.
The torsional analysis verifies that torsional vibration (oscillating
angular motion as a result of twisting in the shaft) is within
acceptable limits. Operation of a turbine outside of the torsional
limits may cause malfunctions, such as twisted shafts
(permanent deformation or shaft failure from fatigue), gear set
failure (if the turbine is driving a gear train), and spun couplings
(coupling failure).
Vibration testing verifies that the turbine vibration levels are
within acceptable limits. Balancing ensures that the rotating
components meet the vibration requirements.
Each of the rotor dynamic tests can provide baseline data for
condition monitoring trend analysis.
Shop testing is carried out during and after equipment
construction but before it is commissioned. The shop tests help
to identify equipment problems prior to installation and to
commissioning startup. Shop-tested turbines may require
additional testing after turbine installation to prove acceptability.
Determination of
Critical Speeds
• Asynchronous whirl.
• Ball/race frequencies of antifriction bearings, such as are
used on aeroderivative machines.
The magnitude of the vibration amplification is called the rotor
amplification factor. The rotor amplification factor (AF) is
determined through use of the following formula and the rotor
response plot that is shown in Figure 2:
Nc1
AF =
N2 − N1
PHASE ANGLE
RADIAL DISPLACEMENT *
(MILE pk-pk)
Figure 2. Example of a Typical Rotor Response Plot (Not a Gas Turbine Plot)
Lateral Analysis
Torsional Analysis
Vibration and
Balancing
Field Test
SAES-K-502 suggests that a performance test (field test) be
considered for inclusion in the purchase order for a gas turbine.
A field test demonstrates the turbine’s ability to achieve the
power output levels and efficiency that is guaranteed by the
manufacturer to be achieved under actual field conditions. A
field test also provides a baseline against which performance of
the turbine can be compared over time. The field test is
conducted in accordance with ASME Performance Test Codes
(previously known as the Power Test Codes) PTC 1, General
Instructions, and PTC 22, Gas Turbine Power Plants. The object
of the test, as stated in PTC 22, is to determine the power
output and thermal efficiency of the turbine under specified
operating and control conditions.
Before the field test is started, the gas turbine must be run until
steady-state conditions have been established. A steady-state
condition is achieved when the key variables that are associated
with the test have stabilized within the maximum permissible
variation. During the period when the gas turbine is stabilizing,
the test instrumentation is checked, and the personnel
conducting the test have the opportunity to familiarize
themselves with the test equipment and their duties during the
test. Frequently, a short-duration preliminary test is conducted
to verify that all test instrumentation is functioning properly.
The PTC 22 Code Test requires determination of the gas
turbine power output and fuel heat input. To ensure that the
turbine is operating at its design firing temperature, turbine
exhaust temperature thermocouples with an error of no greater
than ± 2ºF are used to measure exhaust temperature and as an
input to the fuel control system. Other operating parameters that
must be measured to correct test results to design conditions
are compressor inlet air temperature (± 1ºF), compressor inlet
pressure (barometric pressure – inlet pressure drop, ± 0.25 in.
Wc), turbine exhaust pressure (± 0.25 in. Wc, and humidity (±
.001 lb moisture per lb dry air).
For gas turbines that are driving electrical generators, the power
output is measured at the generator terminals. For generator
drive applications, measurements at the generator terminals are
also typically performed where the rated power output is
specified. If the guaranteed power output is defined at the
turbine shaft coupling, it will be necessary to account for the
generator losses. For mechanical drive turbines, rated power
output is specified at the turbine shaft coupling. This power
output is determined from the measured shaft torque and shaft
speed.
Fuel heat input is determined as the product of the measured
fuel flow times the fuel heating value (generally lower heating
value). The test code specifies the required methods of
determining fuel consumption for liquid and gaseous fuels. Fuel
consumption must be measured with an error of less than ± 0.5
percent.
The field test should be conducted at conditions as close to
design as possible; however, because of the significant effects
of ambient conditions on gas turbine performance, test results
must be corrected to the specified design conditions. Test result
corrections are performed through the use of correction curves
provided by the turbine manufacturer.
GLOSSARY