Evaluating Gas Turbine Testing

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 Aramcos 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.

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Engineering Encyclopedia Saudi Aramco DeskTop Standards

EVALUATING GAS TURBINE TESTING

Engineering Encyclopedia Gas Turbines

Evaluating Gas Turbine Testing

Saudi Aramco DeskTop Standards i

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

Figure 1. Inspection Requirements................................................................................ 13 Figure 2. Example of a Typical Rotor Response Plot (Not a Gas Turbine Plot) ............ 18



Saudi Aramco DeskTop Standards 2

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.




EVALUATING GAS TURBINE TESTING The testing requirements for acceptability of gas turbines are defined in API Standard 616 and Saudi Aramco Standard SAES-K-502. These standards define the tests required and the acceptability requirements for such tests. Other acceptance testing may be required based on turbine criticality, application, and past experience with the turbine vendor. This module provides the Mechanical Engineer with information on gas turbine acceptance testing as defined by the API Standard 616 and Saudi Aramco Standard SAES-K-502.

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




In addition to the above tests specified in API 616 in preparation for delivery, shop tests that relate to the dynamic behavior of the machine rotating mass are also required as follows:

Shop verification of unbalanced response analysis Dynamic balancing Residual unbalance check API Standard 616 provides general guidance on test methodology for the various tests. The turbine manufacturer should have detailed test procedures to be followed in conducting the tests, including acceptance criteria for all monitored parameters. These procedures must be submitted to Saudi Aramco for review and comment at least six weeks before the first scheduled test.

The requirements of SAES-K-502 match those of API 616 for the required tests. SAES-K-502 suggests that the following optional tests be considered:

Performance test Complete unit test Load gear test Sound level test Auxiliary equipment test The remainder of this module will describe the various tests conducted on the gas turbine and how the results of these tests are used to determine the acceptability of the unit.




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




stress at the operating temperature. In accordance with SAES-K-502, the allowable stress values must conform to those given in ASME Code Section VIII Division I for the material that is used.

In accordance with API Standard 616, the chloride content of liquids used to test austenitic stainless steels must not exceed 50 parts per million. The chloride specification should be used for other highly alloyed materials typically used in combustion section components. Austenitic stainless steels are susceptible to stress corrosion cracking in the presence of chlorides, and many of these materials may be subject to attack at elevated temperatures from residues left from the test. Because of the potential for chloride stress corrosion, SAES-K-502 allows the hydrostatic test of combustors to be waived.

The test pressure must be maintained for a sufficient period of time to allow complete examination of all of the parts that are under pressure. The hydrostatic test is considered satisfactory when no leakage has been observed after the parts have been under full hydrostatic test pressure for a minimum of 30 minutes. Large and heavy casings may require a longer test period. Any hydrostatic test that is to be conducted for longer than 30 minutes must be agreed upon by Saudi Aramco and the vendor.

In accordance with SAES-K-502, all hydrostatic tests require witnessing, although this requirement is not indicated on the Saudi Aramco Inspection Form 175-320200.

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.




For gas turbines with rotors that may be removed as an assembled unit, the rotating element (rotor) must be multiplane dynamically balanced at low speed (approximately 400 rpm) during assembly. No more than two of the major components that make up the rotating element may be added to the rotating element prior to each dynamic balancing. Any corrections that must be made to the rotating element to correct an unbalance condition must be applied to the components that were added to the rotating element. After the gas turbine rotating element is completely assembled, minor corrections of other components that were added to the assembly may be required. These minor corrections will be determined during the final trim balancing of the completely assembled element.

For gas turbines with rotors that cannot be removed as an assembled unit, all rotating components must be component-balanced at low speed. After the rotor is assembled, a balance check is performed at low speed. No corrections may be made to the assembled rotor. If corrections are required, the entire rotating element must be disassembled, and each of the individual components must be dynamically balanced again to achieve the allowable residual unbalance limits.

Residual unbalance is the amount of unbalance that remains in a rotor after the rotor has been balanced. The following equation is used to calculate the maximum allowable residual unbalance per plane for a gas turbine:

NW 4 =Umax (customary units)

or

UMAX = 6350 W/N (SI units)

Where:

Umax = Amount of residual unbalance, in ounce-inches (gram-millimeters)

W = The journal static weight load, in pounds (kilograms)

N = The maximum continuous speed, in revolutions per minute.




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:

N12,000 =A

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




maximum of 25 percent of the peak-to-peak amplitude of unfiltered vibration that was calculated or 0.25 mil (6.4 micrometers), whichever is greater, may be vectorially subtracted from the vibration signal that is measured during the factory testing. The electrical and mechanical runout are determined by rotation of the rotor in V-blocks at the journal centerline while the runout is measured. The runout measurement is taken for the full 360 degrees of rotation with a noncontact vibration probe (for electrical runout) at the normal probe location and a dial indicator (for mechanical runout) that is located at one probe tip diameter on either side of the noncontact vibration probe. The electrical runout and mechanical runout readings are recorded. The electrical runout and mechanical runout readings must be supplied by the vendor in the mechanical test report.

Mechanical Running Test In accordance with API Standard 616 and Saudi Aramco Standard SAES-K-502, a mechanical running test must be performed by the vendor on all gas turbines. A Saudi Aramco representative must witness the mechanical running test. The mechanical running test provides proof of the mechanical operation of the turbine within the design requirements. The mechanical running test is run in the vendors shop.

As stated in API Standard 616, as part of the mechanical running test inspection and witness, the Saudi Aramco representative must verify that the following requirements are met before the mechanical running test is performed on the gas turbine:

The shaft seals and bearings that were specified with the gas turbine must be installed and used in the machine for the mechanical running test.

The oil pressures, the oil viscositys, and the oil temperatures must be at the same operating values as the operating values that are recommended in the manufacturer's operating instructions for the specific unit under test. The oil filtration must be ten microns nominal or better.

All of the joints and connections must be checked for tightness. Any leaks must be corrected prior to the mechanical running test. Casing air leaks may be




permissible if they do not adversely affect the specified performance or present a safety hazard in the judgement of Saudi Aramco.

All warning, protection, and control devices must be calibrated to their alarm, shutdown, or relief setpoints.

Any auxiliary gear units that are supplied with the turbine must be included in the mechanical running test if specified in the order. Also, the coupling that is to be installed on the gas turbine should be included in the mechanical test. If the inclusion of the coupling is not practical, the mechanical running test must be performed with coupling-hub idling adapters in place. When all of the tests are complete, the idling adapters must be furnished as part of the special tools for the gas turbine.

All of the controls that are to be supplied with the gas turbine must be used during the test.

The vibration monitoring equipment that is to be supplied with the gas turbine must be used in the test. If the vendor does not furnish the vibration equipment, or if the equipment is not compatible with the test shop readout equipment shop equipment and readouts that meet the accuracy requirements of API Standard 670 and API Standard 678 must be used.

The gas turbine is started and brought up to idle speed until the bearing and lube oil temperatures and the shaft vibrations have stabilized. The gas turbine is then accelerated to minimum governor speed at the turbine's test acceleration rate. The test acceleration rate is normally less than the normal acceleration rate in order to provide a slow and controlled acceleration for the test. The gas turbine is operated in 10 percent speed increments from the minimum governor speed to the maximum continuous speed. The gas turbine is allowed to stabilize at each speed increment prior to a speed increase. Once the speed has been increased to the maximum continuous speed, the speed is increased to 1 percent below the overspeed trip setpoint. The gas turbine overspeed trip devices are tested and adjusted until a trip setpoint of within 1 percent of the nominal trip setpoint is obtained. The mechanical overspeed bolt must be tested and adjusted until three consecutive trip setpoints within 1 percent of the nominal trip setpoint have been obtained. The gas turbine must not be operated at near the overspeed trip




point for more than 15 minutes without a period of operation at normal speed to allow the turbine to cool down.

All of the turbine's control devices (speed governor and any other speed-regulating devices) must be tested for smooth performance over the operating speed range of the turbine. No-load stability and response to the control signal must be checked. As a minimum, the following data must be recorded for governors:

Sensitivity between the speed and the control signal. Linearity of the relationship between the speed and the

control signal.

For adjustable governors, the response over the speed range.

The speed of the gas turbine is adjusted to the maximum continuous speed, and the turbine is run for a minimum test duration of four fired hours (cumulative). The four fired hours must include at least 30 minutes at stabilized conditions at maximum continuous speed. The mechanical operation of all of the equipment that is being tested and the operation of the test instrumentation must be satisfactory during the test. Bearing oil flow rates and temperatures must be measured.

The measured unfiltered vibration must be recorded, and it must not exceed the vibration limits throughout the test. While the mechanical test is being conducted, vibration sweep readings must be recorded for vibration amplitudes at frequencies other than synchronous. As a minimum, these sweep readings must cover a frequency range from 0.25 to 8 times the maximum continuous speed, but they need not exceed 90,000 cycles per minute (1500 Hz).




The lateral critical speeds of the gas turbine must be verified during the mechanical running test. If the gas turbine is a flexible-shaft machine, the first lateral critical speeds must be determined during the mechanical running test, and the lateral critical speed must be stamped on the nameplate. Taped recordings of all real-time vibration data should be made during the mechanical running test. These recordings provide the initial data for vibration analysis. In accordance with SAES-K-502, dismantling of the unit following the mechanical running test is required only in the event of an unsatisfactory test.

If a spare rotor is part of the order, a mechanical running test utilizing the spare rotor must also be conducted in accordance with API 616.

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.




Figure 1. Inspection Requirements




In accordance with SAES-K-502, the following optional tests may be considered for inclusion in the purchase order for a gas turbine:

Field Performance Test - The machine must be performance-tested in accordance with ASME performance test codes PTC 1, General Instructions, and PTC 22, Gas Turbine Power Plants, following a detailed test procedure agreed upon between Saudi Aramco and the manufacturer. If the purchase order is for multiple turbines, the field acceptance test will normally be conducted on one turbine of a given size and type. A more detailed description of field performance testing is presented in a subsequent section of this module.

Complete Unit Test - A complete unit (train) test of such components as compressors, gears, drivers, and auxiliaries that make up a complete unit must be tested together during the mechanical running test. A separate test may be performed with the purchasers approval.

Gear Test - The gear must be tested with the unit during the mechanical running test.

Sound Level Test - A sound level test must be performed in accordance with API Standard 615 to verify that sound levels meet the requirements of Saudi Aramco Standard SAES-A-105, Noise Control.

Auxiliary Equipment Test - Auxiliary equipment, such as oil systems and control systems, must be tested in the vendors' shop. Details of the auxiliary equipment test must be developed jointly by the buyer, the vendor, and Saudi Aramco's Engineer.

Other tests and inspections that are not listed or defined in SAES-K-502 are to be completely described in the inquiry and the purchase order.




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 turbines 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 equipments 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

When an exciting frequency is applied to a rotor-bearing support system that corresponds to the natural frequency of the rotor-bearing support system, the system may be in a state of resonance. A resonating rotor-bearing support system will have its normal vibration displacement amplified.

The magnitude of amplification and the rate of phase shift (phase-angle change) are related to the amount of damping in the rotor-bearing support system and the mode shape that is taken by the rotor as it deflects. The mode shapes for deflection are commonly referred to as the first rigid (translatory or bouncing) mode, the second rigid (conical or rocking) mode, the first bending mode, the second bending mode, and the third bending mode. An exciting frequency may be less than, equal to, or greater than the rotational speed of the rotor. The following are some of the sources of exciting frequencies that must be considered:

Unbalance in the rotor system. Oil-film instabilities (whirl). Internal rubs. Blade, vane, nozzle, and diffuser passing frequencies. Gear-tooth meshing and side bands (on turbines with gear

drives).

Coupling misalignment. Loose rotor-system components. Friction whirl. Boundary-layer flow separation. Acoustic and aerodynamic cross-coupling forces.




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:

12

1c

NNN

AF =

This plot is a graph of vibration amplitude verses the rotor speed (in revolutions per minute).

Figure 2 represents an example of a typical rotor response plot. The specific points of interest on the plot are identified. A rotor response plot provides the following information:

The rotors first critical speed in revolutions per minute (Nc1). The rotors initial (or lesser) speed (N1). The initial speed

occurs at the first peak-to-peak amplitude that is equal to 0.707 times the peak-to-peak amplitude at the critical speed (Ac1).

The rotors final or greater rotational speed (N2) occurs after the displacement at the first critical speed. The value of peak-to-peak displacement at N2 is equal to 0.707 of peak-to-peak displacement at Nc1.

The peak-to-peak amplitude (Ac1) at the rotors first critical speed (Nc1).

Additional critical speeds (Ncn) and their associated amplitudes (Acn).




* NOTE: FREQUENCY = 1X. SCALE VALUES REPRESENT X PROBE

RA

DIA

L D

ISPL

AC

EMEN

T *

(MIL

E pk

-pk)

PHA

SE A

NG

LE

ROTATIONAL SPEED ( X 1,000 rpm )

Figure 2. Example of a Typical Rotor Response Plot (Not a Gas Turbine Plot)




If the rotor amplification factor at a given resonance is greater than or equal to 2.5, the vibration frequency at which resonance occurs is called critical, and the rotational speed at which the resonance occurred is called a critical speed. A critically damped system is a system that has an amplification factor of less than 2.5.

The separation margin is the minimum difference that must be maintained between an operating speed and a critical speed. For a critically damped system, no separation margin is required. Systems with amplification factors greater than 2.5 have varying requirements for separation margin, dependent on the magnitude of the amplification factor and whether the critical speed is below the minimum operating speed or above the maximum continuous speed of the machine. Resonance of support systems for gas turbines must not occur within the specified operating speed range or the specified separation margins unless the resonance is critically damped.

Any operating speed that should be avoided as a critical speed must be included in the operating and maintenance instructions for the turbine. The critical speeds of the turbine must be compatible with the critical speeds of the driven component, and the combination must be suitable for the operating speed range.

If the turbine is acquired as part of an equipment package, such as a turbine-driven gas compressor package, the vendor supplying the equipment package is responsible for determining the drive-train critical speeds (rotor lateral, system torsional, blading modes) and for verifying that the critical speeds of the gas turbine are compatible with the critical speeds of the machinery that is being supplied. The equipment package combination must be suitable for the specified operating speed range, which includes any starting-speed detent (hold-point) requirements of the train. A list of all undesirable speeds from zero to trip must be provided to Saudi Aramco for review, and they must be included in the instruction manual for the equipment package.




Lateral Analysis

In accordance with SAES-K-502, the turbine manufacturer is responsible for providing a lateral and torsional critical speed and unbalanced response analysis for each of the turbine train components and a torsional analysis for the complete train. The following considerations should be included in the damped, unbalanced-response analysis:

Support stiffness (base, frame, and bearing housing), mass, and damping characteristics.

Bearing lubricant-film stiffness. Rotational speeds (starting speeds, operating speed and

load ranges, trip speed, and coast-down speeds). (Any special speeds, such as test condition speeds, should also be included.)

Rotor masses, which include the mass moment, the stiffness, and the damping effects of the coupling halves.

Asymmetrical loading, such as might be caused by gear forces.

The damped unbalanced response analysis must indicate that the turbine, in the unbalanced condition, will meet the following acceptance criteria:

For amplification factors less than 2.5, the response is considered critically damped, and no separation margin is required.

If the amplification factor is between 2.5 and 3.55, a separation margin of the critical speeds from the intended operating speed range of 15 percent above the maximum continuous speed and 5 percent below the minimum operating speed is required to prevent incidental operation at the critical speed.




If the amplification factor is greater than 3.55 and with the critical response peak below the minimum operating speed, the required separation margin is a percentage of minimum speed, and it is determined by the following equation:

+= 3AF

684100SM

Where:

SM = Separation Margin

AF = Amplification Factor

If the amplification factor is greater than 3.55 with the critical response peak above the trip speed, the required separation margin is a percentage of maximum continuous speed, and it is determined by the following equation:

1003AF

6126SM

=

Where:

SM = Separation Margin

AF = Amplification Factor

A Shop Verification of Unbalanced Response Analysis - must be performed as specified in API Standard 616. The following section describes the analysis requirements as specified in API Standard 616.

The actual test critical speed responses are the criteria used to confirm the validity of the damped unbalanced response analysis. The shop verification is performed on a test stand with a rotor unbalanced magnitude of at least two times and no more than eight times the specific unbalanced limit with unbalance weight or weights, typically placed at the coupling. The actual critical speed responses are recorded on the test stand. The dynamic response of the machine on the test stand is a function of the test conditions. The test results should be obtained at the conditions of pressure, temperature, speed, and load that are the expected in the field; otherwise, the test stand results may not be comparable with the actual operation in the field.




Torsional Analysis

The following section describes the torsional analysis requirements as specified in API Standard 616 and Saudi Aramco Standard SAES-K-502.

SAES-K-502 requires that the turbine vendor perform a torsional critical speed and unbalanced response analysis for each of the train components and a torsional analysis for the complete train. The performance of a torsional analysis includes a determination of the excitations of torsional resonances of the turbine. Excitations of torsional resonances other than the turbine must be considered in the torsional analysis, such as for gears and hydraulic-governor control-loop resonance.

Any torsional resonances, including the natural frequencies, that are a product of the complete train must be at least 10 percent above or 10 percent below any possible excitation frequency that exists within the speed range of minimum to maximum continuous speed. Torsional resonances at frequencies that are two times or higher than the turbines running speeds should be avoided, or they must be demonstrated to have no adverse effect on the turbine. If the turbines torsional resonances are calculated to be a multiple of the running speed, and if all efforts to remove the critical from within the limiting frequency range have been exhausted, a stress analysis must be performed to demonstrate that the resonances have no adverse effect on the complete turbine train.

Vibration and Balancing

As stated previously, API 616 requires that all major parts of rotating elements be dynamically balanced and that operating vibration levels (during the shop test) be verified as being acceptable. The procedures and requirements for these items were previously described in the section of this module entitled Shop 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 turbines 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 2F 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 ( 1F), 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

amplitude The magnitude of a variable that varies periodically at any instant during a cycle (or period).

condition monitoring A process and a method of monitoring specific parameters on equipment to determine the status of the mechanical condition.

displacement Movement of an object from a position of rest, equilibrium, or in relation to a reference point.

frequency The number of cycles that a periodic variation completes in a given period. Sometimes stated in cycles per minute (cpm) or cycles per second (cps, Hertz, Hz). For vibration, frequency is also expressed as a multiple (1, 2) of shaft rotative speed.

lateral analysis An analysis of turbine rotor dynamics used to identify lateral (translatory, rocking, and bending) vibration levels as a function of turbine speed.

peak-to-peak amplitude In reference to a waveform that traces a periodic variation of displacement, the maximum amplitude of displacement that occurs during a complete cycle. On an X/Y graph, it is represented as the sum of the vertical line from the zero reference line to the positive peak and the vertical reference line to the negative peak.

phase angle An expression in degrees that defines the relationship between events that occur as a rotating shaft vibrates. Typically, phase angle defines the number of degrees that the unbalanced mass (heavy spot) in a shaft has rotated between the event in which a phase reference transducer detects a phase reference mark and the event in which the heavy spot makes the closest approach (high spot) to the sensor of a radial vibration transducer.

rated power The power developed by the gas turbine when it is operated at the rated turbine inlet temperature, rated speed, and rated conditions of inlet temperature, inlet pressure, and exhaust pressure.




root mean square (RMS) In reference to measurements of vibration, 71 percent (.707) of a zero-to-peak value for velocity or acceleration. Calculated algorithmically as follows: a number of instantaneous values occurring during one cycle or during several cycles are squared; the average of the squared values is taken; and the square root of this average is then taken. In a vibration monitoring system, velocity and acceleration are often measured in terms of RMS values.

torsional analysis An analysis of turbine rotor dynamics used to identify torsional vibration levels as a function of turbine speed.

velocity The time rate at which an object is moving. For vibration, measured in inches per second (in/sec).

vibration Motion in which an object undergoes periodically occurring displacement. Vibration is measured in terms of its variables of displacement (mils), velocity (in/sec), and acceleration (gs). For rotating machinery, vibration is assessed in terms of frequency, peak-to-peak amplitudes of displacement, and either root mean square (RMS) values or zero-to-peak values for velocity or acceleration.

Evaluating Gas Turbine Testing

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