Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal With Two-Phase, Mainly Air Mixtures

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Min Zhang ◽  
James E. Mclean ◽  
Dara W. Childs
Keyword(s):  
Gas Turbines ◽  
Dynamic Stiffness ◽  
Test Fluid ◽  
Liquid Volume ◽  
Test Cases ◽  
Test Results ◽  
Two Phase ◽  
Rotordynamic Coefficients ◽  
Stiffness Coefficients ◽  
Annular Seal

A two-phase annular seal stand (2PASS) has been developed at the Turbomachinery Laboratory of Texas A&M University to measure the leakage and rotordynamic coefficients of division wall or balance-piston annular seals in centrifugal compressors. 2PASS was modified from an existing pure-air annular seal test rig. A special mixer has been designed to inject the oil into the compressed air, aiming to make a homogenous air-rich mixture. Test results are presented for a smooth seal with an inner diameter D of 89.306 mm, a radial clearance Cr of 0.188 mm, and a length-to-diameter ratio (L/D) of 0.65. The test fluid is a mixture of air and silicone oil (PSF-5cSt). Tests are conducted with inlet liquid volume fraction (LVF) = 0%, 2%, 5%, and 8%, shaft speed ω = 10, 15, and 20 krpm, and pressure ratio (PR) = 0.43, 0.5, and 0.57. The test seal is concentric with the shaft (centered), and the inlet pressure is 62.1 bar. Complex dynamic-stiffness coefficients are measured for the seal. The real parts are generally too dependent on excitation frequency Ω to be modeled by constant stiffness and virtual-mass coefficients. The direct real dynamic-stiffness coefficients are denoted as KΩ; the cross-coupled real dynamic-stiffness coefficients are denoted as kΩ. The imaginary parts of the dynamic-stiffness coefficients are modeled by frequency-independent direct C and cross-coupled c damping coefficients. Test results show that the leakage and rotordynamic coefficients are remarkable impacted by changes in inlet LVF. Leakage mass flow rate m˙ drops slightly as inlet LVF increases from zero to 2% and then increases with further increasing inlet LVF to 8%. As inlet LVF increases from zero to 8%, KΩ generally decreases except it increases as inlet LVF increases from zero to 2% when PR = 0.43. kΩ increases virtually with increasing inlet LVF from zero to 2%. As inlet LVF further increases to 8%, kΩ decreases or remains unchanged. C increases as inlet LVF increases; however, its rate of increase drops significantly at inlet LVF = 2%. Effective damping Ceff combines the stabilizing impact of C and the destabilizing impact of kΩ. Ceff is negative (destabilizing) for lower Ω values and becomes more destabilizing as inlet LVF increases from zero to 2%. It then becomes less destabilizing as inlet LVF is further increased to 8%. Measured m˙ and rotordynamic coefficients are compared with predictions from XLHseal_mix, a program developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) based on a bulk-flow model, using the Moody wall-friction model while assuming constant temperature and a homogenous mixture. Predicted m˙ values are close to measurements when inlet LVF = 0% and 2% and are smaller than test results by about 17% when inlet LVF = 5% and 8%. As with measurements, predicted m˙ drops slightly as inlet LVF increases from zero to 2% and then increases with increasing inlet LVF further to 8%. However, in the inlet LVF range of 2–8%, the predicted effects of inlet LVF on m˙ are weaker than measurements. XLHseal_mix poorly predicts KΩ in most test cases. For all test cases, predicted KΩ decreases as inlet LVF increases from zero to 8%. The increase of KΩ induced by increasing inlet LVF from zero to 2% at PR = 0.43 is not predicted. C is reasonably predicted, and predicted C values are consistently smaller than measured results by 14–34%. Both predicted and measured C increase as inlet LVF increases. kΩ and Ceff are predicted adequately at pure-air conditions, but not at most mainly air conditions. The significant increase of kΩ induced by changing inlet LVF from zero to 2% is predicted. As inlet LVF increases from 2% to 8%, predicted kΩ continues increasing versus that measured kΩ typically decreases. As with measurements, increasing inlet LVF from zero to 2% decreases the predicted negative values of Ceff, making the test seal more destabilizing. However, as inlet LVF increases further to 8%, the predicted negative values of Ceff drop versus measured values increase. For high inlet LVF values (5% and 8%), the predicted negative values of Ceff are smaller than measurements. So, the seal is more stabilizing than predicted for high inlet LVF cases.

Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

2017 ◽  
Author(s):  
Min Zhang ◽  
James E. Mclean ◽  
Dara W. Childs
Keyword(s):  
Dynamic Stiffness ◽  
Excitation Frequency ◽  
Test Fluid ◽  
Radial Clearance ◽  
Test Cases ◽  
Test Results ◽  
Rotordynamic Coefficients ◽  
Stiffness Coefficients ◽  
Rate Of Increase ◽  
Annular Seal

A 2-phase annular seal stand (2PASS) has been developed at the Turbomachinery Laboratory of Texas A&M University to measure the leakage and rotordynamic coefficients of division wall or balance-piston annular seals in centrifugal compressors. 2PASS was modified from an existing pure-air annular seal test rig. A special mixer has been designed to inject the oil into the compressed air, aiming to make a homogenous air-rich mixture. Test results are presented for a smooth seal with an inner diameter D of 89.306 mm, a radial clearance Cr of 0.188 mm, and a length-to-diameter ratio L/D of 0.65. The test fluid is a mixture of air and Silicone oil (PSF-5cSt). Tests are conducted with inlet LVF = 0%, 2%, 5%, and 8%, shaft speed ω = 10, 15, and 20 krpm, and pressure ratio PR = 0.43, 0.5, and 0.57. The test seal is concentric with the shaft (centered), and the inlet pressure is 62.1 bars. Complex dynamic stiffness coefficients are measured for the seal. The real parts are generally too dependent on excitation frequency Ω to be modeled by constant stiffness and virtual mass coefficients. The direct real dynamic stiffness coefficients are denoted as KΩ; the cross-coupled real dynamic stiffness coefficients are denoted as kΩ. The imaginary parts of the dynamic stiffness coefficients are modeled by frequency-independent direct C and cross-coupled c damping coefficients. Test results show that the leakage and rotordynamic coefficients are remarkable impacted by changes in inlet LVF. Leakage mass flow rate ṁ drops slightly as inlet LVF increases from zero to 2%, and then increases with further increasing inlet LVF to 8%. As inlet LVF increases from zero to 8%, KΩ generally decreases except it increases as inlet LVF increases from zero to 2% when PR = 0.43. kΩ increases virtually with increasing inlet LVF from zero to 2%. As inlet LVF further increases to 8%, kΩ decreases or remains unchanged. C increases as inlet LVF increases; however, its rate of increase drops significantly at inlet LVF = 2%. Effective damping Ceff combines the stabilizing impact of C and the destabilizing impact of kΩ. Ceff is negative (destabilizing) for lower Ω values and becomes more destabilizing as inlet LVF increases from zero to 2%. It then becomes less destabilizing as inlet LVF is further increased to 8%. Measured ṁ and rotordynamic coefficients are compared with predictions from XLHseal_mix, a program developed by San Andrés [1] based on a bulk-flow model, using the Moody wall-friction model while assuming constant temperature and a homogenous mixture. Predicted ṁ values are close to measurements when inlet LVF = 0 and 2%, and are larger than measured values when inlet LVF = 5% and 8%. As with measurements, predicted ṁ drops slightly as inlet LVF increases from zero to 2%, and then increases with increasing inlet LVF further to 8%. However, in the inlet LVF range of 2∼8%, the predicted effects of inlet LVF on ṁ are weaker than measurements. XLHseal_mix poorly predicts KΩ in most test cases. For all test cases, predicted KΩ decreases as inlet LVF increases from zero to 8%. The increase of KΩ induced by increasing inlet LVF from zero to 2% at PR = 0.43 is not predicted. C is reasonably predicted, and predicted C values are consistently smaller than measured results by 14∼34%. Both predicted and measured C increase as inlet LVF increases. kΩ and Ceff are predicted adequately at pure-air conditions, but not at most mainly-air conditions. The significant increase of kΩ induced by changing inlet LVF from zero to 2% is predicted. As inlet LVF increases 2% to 8%, predicted kΩ continue increasing versus that measured kΩ typically decreases. As with measurements, increasing inlet LVF from zero to 2% decreases the predicted negative values of Ceff, making the test seal more destabilizing. However, as inlet LVF increases further to 8%, the predicted negative values of Ceff drops versus measured values increase. For high inlet LVF values (5% and 8%), the predicted negative values of Ceff are smaller than measurements. So, the seal is actually more stable than predicted for high inlet LVF cases.

Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Min Zhang ◽  
Dara W. Childs ◽  
Dung L. Tran ◽  
Hari Shrestha
Keyword(s):  
Gas Turbines ◽  
Volume Fraction ◽  
Dynamic Stiffness ◽  
Test Fluid ◽  
Radial Clearance ◽  
Liquid Volume ◽  
Test Results ◽  
Stiffness Coefficients ◽  
Frequency Independent ◽  
The Impact

This paper experimentally studies the effects of changing radial clearance Cr on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with Cr = 0.188, 0.163, and 0.140 mm, inlet pressure Pi = 62.1 bars, exit pressure Pe = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients Hij are measured. The real parts of Hij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct KΩ and cross-coupled kΩ stiffness coefficients are used. The imaginary parts of direct Hij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing Cr decreases (as expected) the leakage mass flow rate m˙. Under mainly-air conditions, decreasing Cr decreases KΩ. This outcome is contrary to the test results at pure-air conditions, where KΩ increases as Cr decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping Ceff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing Cr increases Ceff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. m˙ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing Cr on KΩ and the Ceff value near 0.5ω. After the oil is added, as Cr decreases, predicted KΩ increases while measured KΩ decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing Cr does not discernibly change predicted Ceff but increases the measured value.

Test Results for the Static and Rotordynamic Characteristics of a Long (L/D = 0.75) Smooth Seal in Two-Phase (Mainly Gas) Conditions With a 62-Bar Inlet Pressure

2020 ◽  
Author(s):  
Dung L. Tran ◽  
Dara W. Childs ◽  
Hari Shrestha ◽  
Min Zhang
Keyword(s):  
Dynamic Characteristics ◽  
Gas Turbines ◽  
Flow Model ◽  
Dynamic Stiffness ◽  
Bulk Flow ◽  
Liquid Volume ◽  
Test Results ◽  
Two Phase ◽  
Static And Dynamic Characteristics ◽  
San Andres

Abstract Recent multiphase-pump developments encountered several rotordynamic issues with smooth balance-piston seals, creating a need to better understand the performance of annular seals under multiphase-flow operation. This paper presents measurements of static and dynamic characteristics of a long smooth seal (L/D = 0.75, D = 114.686 mm, and Cr = 0.200 mm) operating under pure- and mainly air condition in which air is mixed with silicone oil (PSF-5cSt). Tests are performed at a supply pressure of 62.1 bars-a, three rotation speeds (5, 10, 15 krpm), three pressure ratios (PRs) (0.6, 0.5, 0.4), for a range of inlet liquid volume fraction (LVFi) from 0% to 8%. The results are then compared to: (1) the previous test reported by Zhang et al. (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” J. Eng. Gas Turbines Power, 139(12), p. 122504) with similar testing condition but a different seal geometry (L/D = 0.65, D = 89.306 mm, and Cr = 0.188 mm) and (2) the predictions from a bulk-flow model developed by San Andrés (2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503). Results show a significant increase of direct dynamic stiffness KΩ as LVFi increases, especially at low PR. Test results reported by Zhang et al. (2017) has an opposite tendency of KΩ as an impact of increasing LVFi. Concerning cross-coupled dynamic stiffness kΩ and cross-coupled damping c, the results from Zhang et al. (2017) and the present results agree to the effects of changing speed, PR, and LVFi under pure- and mainly air conditions. As LVFi increases, direct damping C increases while test results reported by Zhang et al. (2017) showed no significant increase. Except for the direct dynamic stiffness and the impact of changing LVFi on the cross-coupled dynamic stiffness, the bulk-flow model of San Andrés (2012) predicts decently the tendencies and magnitudes of the rotordynamic coefficients.

Preswirl and Mixed-Flow (Mainly Liquid) Effects on Rotordynamic Performance of a Long (L/D = 0.75) Smooth Seal

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Dung L. Tran ◽  
Dara W. Childs ◽  
Hari Shrestha ◽  
Min Zhang
Keyword(s):  
Dynamic Instability ◽  
Silicone Oil ◽  
Dynamic Stiffness ◽  
Radial Clearance ◽  
Inlet Temperature ◽  
Test Results ◽  
Transition Flow ◽  
Two Phase ◽  
Rotordynamic Coefficients ◽  
Transition Flow Regime

Abstract Measured results are presented for rotordynamic coefficients and mass leakage rates of a long smooth annular seal (length-to-diameter ratio L/D = 0.75, diameter D = 114.686 mm, and radial clearance Cr = 0.200 mm) tested with a mixture of silicone oil (PSF-5cSt) and air. The test seal is centered, the seal exit pressure is maintained at 6.9 bars-g while the fluid inlet temperature is controlled within 37.8–40.6 °C. It is tested with three inlet-preswirl inserts, namely, zero, medium, and high (the preswirl ratios (PSRs), i.e., the ratio between the fluid's circumferential velocity and the shaft surface's velocity, are in ranges of 0.10–0.18, 0.30–0.65, and 0.65–1.40 for zero, medium, and high preswirls, respectively), six inlet gas-volume fractions GVFi (0%, 2%, 4%, 6%, 8%, and 10%), four pressure drops PDs (20.7, 27.6, 34.5, and 41.4 bars), and three speeds ω (3, 4, and 5 krpm). The targeted test matrix could not be achieved for the medium- and high-preswirl inserts at PD ≥ 27.6 bars due to the test-rig stator's dynamic instability issues. Spargers were used to inject air into the oil, and GVFi values higher than 0.10 could not be consistently achieved because of unsteady surging flow downstream from the sparger mixing section. Leakage mass flow rate m˙ and rotordynamic coefficients are measured, and the effect of changing inlet preswirl and GVFi is studied. The test results are then compared with predictions from a two-phase, homogeneous-mixture, bulk-flow model developed in 2011. Generally, both measurements and predictions show little change in m˙ as inlet preswirl changes. Measured m˙ remains unchanged or slightly increases with increasing GVFi, but predicted m˙ decreases. Measured m˙ is comparable to predicted values but consistently lower. Dynamic-stiffness coefficients are measured using an ensemble of excitation frequencies and curve-fitted well by frequency-independent stiffness Kij, damping Cij, and virtual mass Mij coefficients. Planned tests with the medium- and high-preswirl inserts could not be accomplished at PD = 34.5 and 41.4 bars because the seal stator became unstable with any finite injection of air. The test results show that the instability arose because the seal's direct stiffness K became negative and increased in magnitude with increasing GVFi. The model predicts a drop in K as GVFi increases, but the test results dropped substantially more rapidly than predicted. Also, the model does not predict the observed strong tendency for K to drop with an increase in preswirl in moving from the zero-to-medium and medium-to-high preswirl inserts. The authors believe that the observed drop in K due to increasing GVFi is not explained by either (a) a reverse Lomakin effect from operating in the transition flow regime or (b) the predicted drop in K at higher GVFi values from the model. A separate and as yet unidentified two-phase flow phenomenon probably causes the observed results. The negative K results due to increasing GVFi and moving from the zero to medium, and medium to high preswirl observed here could explain the instability issue (sudden subsynchronous vibration) on a high-differential-pressure helico-axial multiphase pump (MPP), reported in 2013. Effective damping Ceff combines the stabilizing effect of direct damping C, the destabilizing effect of cross-coupled stiffness k, and the influence of cross-coupled mass mq. As predicted and measured, increasing inlet preswirl significantly increases k and decreases Ceff, which decreases the seal's stabilizing properties. Ceff increases with increasing GVFi—becomes more stable.

Effects of Clearance on the Performance of a Labyrinth Seal Under Wet-Gas Conditions

2020 ◽  
Author(s):  
Min Zhang ◽  
Dara W. Childs ◽  
Dung L. Tran ◽  
Hari Shrestha
Keyword(s):  
Dynamic Stiffness ◽  
Labyrinth Seal ◽  
Test Fluid ◽  
Radial Clearance ◽  
Liquid Volume ◽  
Rotordynamic Coefficients ◽  
Whirling Motion ◽  
Wet Gas ◽  
Different Diameters ◽  
The Impact

Abstract The labyrinth seal is one of the most popular non-contact annular seals used in centrifugal compressors to improve machine efficiency by reducing the secondary flow leakage. Reducing the radial clearance Cr can effectively decrease the seal’s leakage and therefore increase the machine efficiency. However, reducing Cr can also introduce undesired effects on the machine’s vibration behaviors. This paper experimentally studies the impact of reducing Cr on the leakage and rotordynamic coefficients of a 16-tooth see-through labyrinth seal under wet-gas conditions. The test seal’s inner diameter is 89.256 mm. Two rotors with different diameters are used to obtain two radial clearances (0.102 mm and 0.178 mm). Tests are carried out at a supply pressure of 62 bars, three speeds from 10krpm to 20 krpm, three pressure ratios from 0.21 to 0.46, and six inlet liquid volume fractions (LVFs) from zero to 15%. The test fluid is a mixture comprised of air and silicon oil. Test results show that, for all pure-air and mainly-air conditions, decreasing Cr decreases (as expected) the test seal’s leakage mass flow rate. For all test cases, direct dynamic stiffness KΩ is negative, producing a negative centering force on the associated rotor. For inlet LVF ≤ 8%, the effects of decreasing Cr on KΩ are negligible. When inlet LVF = 12% and 15%, decreasing Cr increases KΩ (decreases the magnitude). In other words, when inlet LVF = 12% and 15%, decreasing Cr reduces the test seal’s negative centering force on the rotor, and would increase the critical speeds of the rotor. The value of the effective damping Ceff near 0.5ω represents the seal’s capability to suppress the rotor’s potential whirling motion at about 0.5ω. For all pure-air and mainly-air conditions, decreasing Cr generally increases the Ceff value near 0.5ω; i.e., decreasing Cr improves the test seal’s stabilizing capability against the rotor’s potential whirling motion at about 0.5ω.

Effects of Clearance on the Performance of a Labyrinth Seal Under Wet-Gas Conditions

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
Min Zhang ◽  
Dara W. Childs ◽  
Dung L. Tran ◽  
Hari Shresth
Keyword(s):  
Dynamic Stiffness ◽  
Labyrinth Seal ◽  
Test Fluid ◽  
Radial Clearance ◽  
Liquid Volume ◽  
Rotordynamic Coefficients ◽  
Whirling Motion ◽  
Wet Gas ◽  
Different Diameters ◽  
The Impact

Abstract The labyrinth seal is one of the most popular noncontact annular seals used in centrifugal compressors to improve machine efficiency by reducing the secondary flow leakage. Reducing the radial clearance Cr can effectively decrease the seal's leakage and therefore increase the machine efficiency. However, reducing Cr can also introduce undesired effects on the machine's vibration behaviors. This paper experimentally studies the impact of reducing Cr on the leakage and rotordynamic coefficients of a 16-tooth see-through labyrinth seal under wet-gas conditions. The test seal's inner diameter is 89.256 mm. Two rotors with different diameters are used to obtain two radial clearances (0.102 mm and 0.178 mm). Tests are carried out at a supply pressure of 62 bars, three speeds from 10 krpm to 20 krpm, three pressure ratios from 0.21 to 0.46, and six inlet liquid volume fractions (LVFs) from zero to 15%. The test fluid is a mixture comprised of air and silicon oil. Test results show that, for all pure-air and mainly air conditions, decreasing Cr decreases (as expected) the test seal's leakage mass flowrate. For all test cases, direct dynamic stiffness KΩ is negative, producing a negative centering force on the associated rotor. For inlet LVF ≤ 8%, the effects of decreasing Cr on KΩ are negligible. When inlet LVF = 12% and 15%, decreasing Cr increases KΩ (decreases the magnitude). In other words, when inlet LVF = 12% and 15%, decreasing Cr reduces the test seal's negative centering force on the rotor, and would increase the critical speeds of the rotor. The value of the effective damping Ceff near 0.5ω represents the seal's capability to suppress the rotor's potential whirling motion at about 0.5ω. For all pure-air and mainly air conditions, decreasing Cr generally increases the Ceff value near 0.5ω; i.e., decreasing Cr improves the test seal's stabilizing capability against the rotor's potential whirling motion at about 0.5ω.

Theory Versus Experiment for the Rotordynamic Characteristics of a High Pressure Honeycomb Annular Gas Seal at Eccentric Positions

2003 ◽  
Vol 125 (2) ◽  
pp. 422-429 ◽  
Author(s):  
Mark Weatherwax ◽  
Dara W. Childs
Keyword(s):  
Back Pressure ◽  
Test Fluid ◽  
Test Results ◽  
Eccentricity Ratio ◽  
Cell Width ◽  
Rotordynamic Coefficients ◽  
Annular Seal ◽  
Theory Versus ◽  
A Cell ◽  
San Andres

Test results for leakage and rotordynamic coefficients are presented for a honeycomb-stator/smooth-rotor annular seal for eccentricity ratios out to 0.5 using air as the test fluid. Tests were conducted at supply pressures up to 70 bars and running speeds up to 20200 rpm. The seal has a diameter of 115 mm, a cell width of 0.79 mm, and a cell depth of 3.10 mm. Tests were conducted for the back-pressure ratios of 0.35, and 0.5. Comparisons are made to predictions from an analysis due to San Andres. The test results show a minimal sensitivity of either leakage or rotordynamic coefficients to changes in the eccentricity ratio. The predictions generally agree well with measurements.

Static and Rotordynamic Characteristics of Liquid Annular Seals With a Circumferentially-Grooved Stator and Smooth Rotor Using Three Levels of Circumferential Inlet-Fluid Rotation

2018 ◽  
Author(s):  
Dara W. Childs ◽  
Jose M. Torres ◽  
Joshua T. Bullock
Keyword(s):  
Oil Recovery ◽  
Operating Conditions ◽  
Test Fluid ◽  
Radial Clearance ◽  
Test Results ◽  
Pressure Drops ◽  
Rotordynamic Coefficients ◽  
Annular Seal ◽  
Electrical Submersible Pumps ◽  
Lower Magnitude

Test results are presented for a smooth-rotor/circumferentially-grooved, annular pump seal. The seal’s geometry and operating conditions are representative of electrical submersible pumps (ESPs) as used for oil recovery; however, most ESPs use grooved rotors instead of grooved stators. Test results include static and rotordynamic data at speeds ω of 2, 4, 6 krpm, axial pressure drops ΔP of 2.1, 4.1, 6.2, 8.3 bars. The grooved seal has a length-to-diameter ratio L/D of 0.5 and a minimum radial clearance Cr of 203 μm. It employs 15 circumferential grooves with a length Gl, and depth Gd of 1.52 mm, which are equally-spaced by a land length of 1.52 mm. Tests are conducted for eccentricity ratios ϵ0 of 0.00, 0.27, 0.53, 0.80. Three different inlet-fluid prerotation inserts are used upstream of the test seals to create a range of inlet preswirl ratios. Pitot tubes are used to measure the circumferential velocity at one location immediately upstream of the test seal and one downstream location near the seal exit. The test fluid is ISOVG2 oil @ 46 °C. Test results for the grooved seal are compared to test results for a smooth annular seal with the same L, D, and minimum Cr. The grooved-seal’s leakage rate Q̇, ranges from a low 15.64 LPM at ω = 6 krpm, and ΔP = 2 bar, to a high 56.36 LPM at ω = 2 krpm, and ΔP = 8 bar. When compared to the smooth seal, the grooved seal provides a 20% Q̇ reduction at ω = 2 krpm, and a 6% reduction at ω = 6 krpm. The grooved seal’s rotordynamic coefficients are generally not sensitive to changes in ϵ0. The smooth seal’s stiffness and damping coefficients are not very sensitive to changes in ϵ0 in moving from ϵ0 = 0 to 0.5, but typically increase dramatically in magnitude in moving from ϵ0 = 0.5 to 0.8. From a rotordynamic viewpoint, the major difference between the two seals concerns the direct stiffness coefficients, with the grooved seal having near zero to negative values and the smooth seal having larger positive values, particularly at increased ϵ0 values. The grooved seal generally produces lower-magnitude cross-coupled stiffness and direct damping coefficient values than the smooth seal.

Experimental Study of the Leakage and Rotordynamic Coefficients of a Long Smooth Seal With Two-Phase, Mainly-Oil Mixtures

2018 ◽  
Author(s):  
Min Zhang ◽  
Dara W. Childs ◽  
James E. Mclean ◽  
Dung L. Tran ◽  
Hari Shrestha
Keyword(s):  
Natural Frequency ◽  
Volume Fraction ◽  
Dynamic Stiffness ◽  
Test Fluid ◽  
Inlet Temperature ◽  
Test Results ◽  
Virtual Mass ◽  
Annular Seal ◽  
Oil Flow ◽  
Oil Mixtures

Tests are conducted on a newly developed 2-phase annular seal stand (2PASS) at the Turbomachinery Laboratory of Texas A&M University. The test fluid is a mixture of silicone oil (PSF-5cSt) and air. Two spargers are used to produce mainly-oil mixtures by injecting air bubbles into the oil flow. The test seal is a smooth annular seal with inner diameter D = 89.306 mm, length-to-diameter ratio L/D = 0.65, and radial clearance Cr = 0.188 mm. Tests are performed with inlet gas-volume-fraction GVF = 0%, 2%, 4%, 6%, and 10%, rotor speed ω = 5, 7.5, 10, and 15 krpm, inlet temperature Ti = 39.4 °C, exit pressure Pe = 6.9 bars, and pressure drop PD = 31, 37.9, and 48.3 bars. The test seal is centered, and there is no intentional prerotation of the fluid at the seal inlet. The complex dynamic stiffness coefficients of the test seal are measured and fitted by the frequency-independent stiffness Kij, damping Cij, and virtual-mass Mij coefficients. Test results show that adding air into the oil flow does not change the seal’s mass flow leakage ṁ discernibly but significantly impacts the seal’s rotordynamic characteristics. Some planned 5 krpm cases with low inlet GVFs at PD = 31 and 37.9 bars are not accomplished due to stator instabilities, which are likely caused by negative stiffness of the test seals. For ω = 5 krpm when PD = 31 and 37.9 bars, direct stiffness K decreases from positive to negative as inlet GVF decreases. For all PDs and speeds, K increases as inlet GVF increases from zero to 10% except for 6% ≤ inlet GVF ≤ 10% when PD = 48.3 bars, where K decreases as inlet GVF increases. The K increment will increase a pump rotor’s natural frequency and critical speed. Increasing the rotor’s natural frequency would also increase the onset speed of instability (OSI) and improve the stability of the rotor. Adding air into the oil flow has little effect on cross-coupled stiffness k and direct damping C. Increasing inlet GVF has negligible effects on direct virtualmass M when ω ≤ 10 krpm and PD ≤ 37.9 bars, but generally decreases M when ω = 15 krpm or PD = 48.3 bars. Increasing inlet GVF has little effect on effective damping Ceff and does not change the seal’s resultant stabilizing force discernibly. Ceff = C − k/ω + mqω, where mq is the cross-coupled virtual-mass. Test results are compared to predictions from San Andrés’s [1] model. The model is based on a bulk-flow model and the Moody friction formula assuming that the liquid-gas mixture is isothermal and homogenous. The model reasonably predicts ṁ, C, and Ceff. All predicted K values are positive, while measured K values are negative for some test cases. Predicted k values are close to measurements when ω = 5 krpm and are larger than test results when 7.5 ≤ ω ≤ 15 krpm. M predictions are smaller than measurements, and the discrepancy between predicted and measured M values generally increases as inlet GVF increases.

Leakage, Drag Power and Rotordynamic Force Coefficients of an Air in Oil (Wet) Annular Seal

2017 ◽  
Author(s):  
Luis San Andrés ◽  
Xueliang Lu
Keyword(s):  
Test Data ◽  
Volume Fraction ◽  
Damping Coefficient ◽  
Liquid Volume ◽  
Pure Liquid ◽  
Test Results ◽  
Liquid Volume Fraction ◽  
Force Coefficients ◽  
Damping Coefficients ◽  
Annular Seal

Wet gas compression systems and multiphase pumps are enabling technologies for the deep sea oil and gas industry. This extreme environment determines both machine types have to handle mixtures with a gas in liquid volume fraction (GVF) varying over a wide range (0 to 1). The gas (or liquid) content affects the system pumping (or compression) efficiency and reliability, and places a penalty in leakage and rotordynamic performance in secondary flow components, namely seals. In 2015, tests were conducted with a short length smooth surface annular seal (L/D = 0.36, radial clearance = 0.127 mm) operating with an oil in air mixture whose liquid volume fraction (LVF) varied to 4%. The test results with a stationary journal show the dramatic effect of a few droplets of liquid on the production of large damping coefficients. This paper presents further measurements and predictions of leakage, drag power, and rotordynamic force coefficients conducted with the same test seal and a rotating journal. The seal is supplied with a mixture (air in ISO VG 10 oil), varying from a pure liquid to an inlet GVF = 0.9 (mostly gas), a typical range in multiphase pumps. For operation with a supply pressure (Ps) up to 3.5 bar (a), discharge pressure (Pa) = 1 bar (a), and various shaft speed (Ω) to 3.5 krpm (ΩR = 23.3 m/s), the flow is laminar with either a pure oil or a mixture. As the inlet GVF increases to 0.9 the mass flow rate and drag power decrease monotonically by 25% and 85% when compared to the pure liquid case, respectively. For operation with Ps = 2.5 bar (a) and Ω to 3.5 krpm, dynamic load tests with frequency 0 < ω < 110 Hz are conducted to procure rotordynamic force coefficients. A direct stiffness (K), an added mass (M) and a viscous damping coefficient (C) represent well the seal lubricated with a pure oil. For tests with a mixture (GVFmax = 0.9), the seal dynamic complex stiffness Re(H) increases with whirl frequency (ω); that is, Re(H) differs from (K-ω2M). Both the seal cross coupled stiffnesses (KXY and −KYX) and direct damping coefficients (CXX and CYY) decrease by approximately 75% as the inlet GVF increases to 0.9. The finding reveals that the frequency at which the effective damping coefficient (CXXeff = CXX-KXY/ω) changes from negative to positive (i.e., a crossover frequency) drops from 50% of the rotor speed (ω = 1/2 Ω) for a seal with pure oil to a lesser magnitude for operation with a mixture. Predictions for leakage and drag power based on a homogeneous bulk flow model match well the test data for operation with inlet GVF up to 0.9. Predicted force coefficients correlate well with the test data for mixtures with GVF up to 0.6. For a mixture with a larger GVF, the model under predicts the direct damping coefficients by as much as 40%. The tests also reveal the appearance of a self-excited seal motion with a low frequency; its amplitude and broad band frequency (centered at around ∼12 Hz) persist and increase as the gas content in the mixture increase. The test results show that an accurate quantification of wet seals dynamic force response is necessary for the design of robust subsea flow assurance systems.

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