- Email: [email protected]
Development of cryogenic turboexpanders with gas dynamic foil bearings
ICEC 14 Proceedings
DEVELOPmeNT OF CRYOGENIC TURBOEXPA~DERS WITH GAS DYNAMIC FOIL BEARINGS I.A. Davydenkov~ Yu.A. Ravikovich * A.B. Davydov* N.E. Zakharova* Yu.R. Adler • S.I. Shchedukhin *
Yu.I. Ermilov*
NPO Geliymash, 119855, Luzhnetskaya, IOA, Moscow, Russia The paper presents the designs of new turboexpanders with foil bearings for cryogenic helium plants. The turboexpanders operate at pressures of 0.5-2.5 MPa, inlet temperatures of 120-15K and a mass flow rate of 20-50 g/s. The maximum rotational speed of rotors is 240,000 rpm with the shaft diameter of 16 me. The rotor operation is stable within the whole range of rotational speeds with the oscillation amplitude not exceeding 2 M e . The advantages of foil gas dynamic bearings in/comparison with other types of bearings with liquid and gas lubrication have increased the effectiveness and reliability of turboexpanders.
The type of bearings used for suspendind the rotor of a %urboexpander (TE) basically defines its technical and economic parameters such as service llfe, reliability, mass, dimensions, labor content and operating convenience, At present, our cryogenic plants operate TE with oil plain bearings and gas static bearings as well as their hybrid in a single turbomachine. The disadvantages of oil plain bearings are as follows: - the presence of an oll system with the mass 5-20 times greater than the TE mass; - high labor requirements during operation; penetration of oil into the gas cavity in case of unskilled use; frequent failures of TE in case of unexpected de-energizing. The disadvantages of gas static bearings are as follows: - the utilization of the working fluid from the cryogenic plant. Depending on the plant dimensions I .5-15% pf the working fluid can enter the bearings which considerably reduces the plant capacity; the need for small radial and axial clearances that makes the bearings highly sensitive to thermal deformations of the shaft and body e lement
s;
-the necessity of high manufacturing precision and the maintenance of shapes and mutual arrangement of bearing surfaces during operarich; - high sensitivity to the contamination of the working fluid. Penetration of a foreign particle (microchip, scale, solid gas particle) into the running clearance generally results in rotor jamming. In this case repair is expensive and labor-consuming requiring replacement of the bearings and shaft; the existence of the rotor threshold ritational speed; One of the promising types of bearings for high-speed TE is a gas dynamic foil bearing with a not-rigid bearing surface (Fig.l). The lestic stack of foils 2 forming the bearing profile makes two types of clearances that both exist simultaneously, that is, running and installation clearances.
I
80
Cryogenics 1992 Vol 32 ICEC Supplement
ICEC 14 Proceedings
The running clearance is a clearance between the bearing surface of the loll 2 and the surface of the rotary thrust Journal 31'which is set automatically for forming the gas carrier. Its size depends on the loll rigidity, physical properties of gas and rotor rotational speed. The back side of the foils in combination wiht the bearing body 1 forms the installation clearance. Its size is set by the designer and is an order of magnitude bigger than the running clearance. The rotor with the gas carrier and foil stack can be displaced within the installation clearance. This feature of foil bearings provides for the following advantages: - self-adjustment, compensation for manufacturing errors and rotor assembly as well as for thermal deformations which decreases the precision of production by an order; - low sensitivity to the contamination of working fluid. A foreign particle passing through the runninng clearance moves aside the nonrigid foil leaving sometimes just a scratch on the antifriction coating of the foil which, as a rule, does not affect the bearing serviceability; practically unlimited possibilities to provide stable operation on the rotor. During rotor oscillations foil bearings are deformed and mutually displaced forming regions of dry friction effectively dissipating vibratory energy of the rotor. If bearings are optimally designed the rotational speed is limited only by the mechanical strength of rotor components; - high resistance to impact and vibratory loads due to increased damping capabilities; high repairability. Repair is reduced to the replacement of foils which provides for reclamation of TE after a failure. Besides, it should be born in mind that foil bearings are essentially gas dynamic bearings, that is, they do not consume gas for their own needs, they are environmentally pure and do not contaminate working fluid. To optimize foil bearings t~O "Geliymash" has built an experimental test bed for monitoring rotor dynamics and bearing characteristics (rigidity, load capacity and thermal conditions). Several versions of radial and axial foil bearings have been designed with different combinations of parameters. The number of foils, their extent and shaping as well as the sizes of installation clearances were varied. Simultaneously, the dynamic characteristics of foil bearings were analysed theoretically on a generalized dynamic model of an elasticdamping bearing /I/. The quality criteria were the starting torque, rotor oscillation amplitude whithin the whole range of rotational speeds, bearing heating and wear of antifriction coatings. The purpose of the optimization was to minimize these parameters with provisions for the required load capacity. The bearings with the following characteristics have met the requirements: -
-
Working fluide Kick-off torque of rotor in loll bearings Specific load capacity of the thrust bearing at IO0,0OO rpm Temperature in the bearing region at 230,000 rpm Oscillation amplitude within the range of 30,000-230,000 rpm Antifrictlon coating wear after 250 cycles and shutdowns
air, helium - 0,02 Nm -
-
O,11 NEPa
-
40°C (max)
-
2)Win (max)
-
1.,.1
Cryogenics 1992 Vol 32 ICEC Supplement
81
ICEC 14 Proceedings
Such bearings were used for suspending rotors of helium TE with following characteristics: Inlet pressure Expansion ratio Inlet temperature Capacity Journal diameter Rotor mass Mass flow rate
the
- [email protected] MPa -
1.5~8.0 300@I 5K 0.I~5 kW
"
0.2 ~g O.02~O.05kg/s
o.o16 m -
The turboexpanders with radial and axial foil bearings were operationally developed and preliminary tested at a suitable air test bed; after that they were brought iato operation as part of cryogenic helium plant KPY-250/4,5 with a rated capacity of 90 1/h of liquid helium. The TE general view is shown in Fig.2. The plant operation under liquefaction conditions showed no resonance modes within the whole range of rotational speeds (30,000~200,000 rpm). The rotor oscillation amplitude did not exceed 2#Wm; the temperature at the loll bearing installation point changed within -10 through +I0°C, To check TE for reliability an emergency switch-off of the compressor was simulated followed by a sharp drop of helium pressure in the system. This situation with gas static bearings leads to rotor jA~niag in the turboexpander connected with a fast contact due to the loss of carrying capacity by both radial and axial bearings. In case of foil bearings this eme~gences pressure drop resulted in a gradual stop of rotor. The substitution of foil bearings for gas static bearings in TE of the cryogenic helium plant led, first, to a considerable increase of plant reliability due to higher reliability of the turboexpanders and, second, to the double reduction of time necessary to reach liquefaction conditions and to an increase of LHe ploduction f r o m 9 0 to 125 I/h. Life cycle of TE with foil bearings is dependent on the wear resistance of foil antifriction coatings during rotor start-ups and shutdowns. Using the test bed at ~ 0 "Geliymash" a number of TE test was carried out subjecting TE to multiple repeated cycles of the "start-stop" type. 10,000 cycles . were performed with the bearings diassembled and checked for condi %ion after 1000! 5000 and 10,0OO cycles. The inspection showed satisfactory condltion of bearing coatings and their serviceability. The obtained results have made it possible to begin developing a new generation of TE with foil gas dynamic bearings for cryogenic helium and air separation plants. CONCLUSIONS 1. Tests of TE with foil gas dynamic bearings have shown the following advantages: - self-compensation of manufacturing and assembly errors, as well as thermal deformations of body elements and rotor unit~ - low sensitivity to working fluid contamination; - practically unlimited capabilities for providing stable operation of rotors | - high maintainability; - pollution-free operation. 2. The replacement of the turboexpander with gas static bearings by TE with gas dynamic foil bearings in cryogenic helium plants has increased the reliability and efficiency of the Cryogenic cycle.
82
Cryogenics 1992 Vol 32 ICEC Supplement
ICEC 14 Proceedings
REFERENCES .
Ravikovich
. . . . . .
YU.A.,
~r- i
~rml±oV. ~.
S e r ~ e ~ v S.Io, G e n e r a l i z e d •~ . o - . ,. . . . . . ~ ~ . ~
n 8
I
]
Fig.1.
Diagram dysamic
of r a d i a l gas foil bearing
Fig.2.
Tarboexpander
schematic
I - turbine wheel; 2 - nozzle; 3 - gas d y n a m i c r a d i a l f o i l b e a r i n g ; $ - s h a f t ; 5 - gas d y namic foil thrust bearing; 6 - gas b l o w e r b r a k e w h e e l ; 7 - rotational speed transducer; 8 - heat exchanger.
Cryogenics 1992 Vol 32 ICEC Supplement
83
DEVELOPmeNT OF CRYOGENIC TURBOEXPA~DERS WITH GAS DYNAMIC FOIL BEARINGS I.A. Davydenkov~ Yu.A. Ravikovich * A.B. Davydov* N.E. Zakharova* Yu.R. Adler • S.I. Shchedukhin *
Yu.I. Ermilov*
NPO Geliymash, 119855, Luzhnetskaya, IOA, Moscow, Russia The paper presents the designs of new turboexpanders with foil bearings for cryogenic helium plants. The turboexpanders operate at pressures of 0.5-2.5 MPa, inlet temperatures of 120-15K and a mass flow rate of 20-50 g/s. The maximum rotational speed of rotors is 240,000 rpm with the shaft diameter of 16 me. The rotor operation is stable within the whole range of rotational speeds with the oscillation amplitude not exceeding 2 M e . The advantages of foil gas dynamic bearings in/comparison with other types of bearings with liquid and gas lubrication have increased the effectiveness and reliability of turboexpanders.
The type of bearings used for suspendind the rotor of a %urboexpander (TE) basically defines its technical and economic parameters such as service llfe, reliability, mass, dimensions, labor content and operating convenience, At present, our cryogenic plants operate TE with oil plain bearings and gas static bearings as well as their hybrid in a single turbomachine. The disadvantages of oil plain bearings are as follows: - the presence of an oll system with the mass 5-20 times greater than the TE mass; - high labor requirements during operation; penetration of oil into the gas cavity in case of unskilled use; frequent failures of TE in case of unexpected de-energizing. The disadvantages of gas static bearings are as follows: - the utilization of the working fluid from the cryogenic plant. Depending on the plant dimensions I .5-15% pf the working fluid can enter the bearings which considerably reduces the plant capacity; the need for small radial and axial clearances that makes the bearings highly sensitive to thermal deformations of the shaft and body e lement
s;
-the necessity of high manufacturing precision and the maintenance of shapes and mutual arrangement of bearing surfaces during operarich; - high sensitivity to the contamination of the working fluid. Penetration of a foreign particle (microchip, scale, solid gas particle) into the running clearance generally results in rotor jamming. In this case repair is expensive and labor-consuming requiring replacement of the bearings and shaft; the existence of the rotor threshold ritational speed; One of the promising types of bearings for high-speed TE is a gas dynamic foil bearing with a not-rigid bearing surface (Fig.l). The lestic stack of foils 2 forming the bearing profile makes two types of clearances that both exist simultaneously, that is, running and installation clearances.
I
80
Cryogenics 1992 Vol 32 ICEC Supplement
ICEC 14 Proceedings
The running clearance is a clearance between the bearing surface of the loll 2 and the surface of the rotary thrust Journal 31'which is set automatically for forming the gas carrier. Its size depends on the loll rigidity, physical properties of gas and rotor rotational speed. The back side of the foils in combination wiht the bearing body 1 forms the installation clearance. Its size is set by the designer and is an order of magnitude bigger than the running clearance. The rotor with the gas carrier and foil stack can be displaced within the installation clearance. This feature of foil bearings provides for the following advantages: - self-adjustment, compensation for manufacturing errors and rotor assembly as well as for thermal deformations which decreases the precision of production by an order; - low sensitivity to the contamination of working fluid. A foreign particle passing through the runninng clearance moves aside the nonrigid foil leaving sometimes just a scratch on the antifriction coating of the foil which, as a rule, does not affect the bearing serviceability; practically unlimited possibilities to provide stable operation on the rotor. During rotor oscillations foil bearings are deformed and mutually displaced forming regions of dry friction effectively dissipating vibratory energy of the rotor. If bearings are optimally designed the rotational speed is limited only by the mechanical strength of rotor components; - high resistance to impact and vibratory loads due to increased damping capabilities; high repairability. Repair is reduced to the replacement of foils which provides for reclamation of TE after a failure. Besides, it should be born in mind that foil bearings are essentially gas dynamic bearings, that is, they do not consume gas for their own needs, they are environmentally pure and do not contaminate working fluid. To optimize foil bearings t~O "Geliymash" has built an experimental test bed for monitoring rotor dynamics and bearing characteristics (rigidity, load capacity and thermal conditions). Several versions of radial and axial foil bearings have been designed with different combinations of parameters. The number of foils, their extent and shaping as well as the sizes of installation clearances were varied. Simultaneously, the dynamic characteristics of foil bearings were analysed theoretically on a generalized dynamic model of an elasticdamping bearing /I/. The quality criteria were the starting torque, rotor oscillation amplitude whithin the whole range of rotational speeds, bearing heating and wear of antifriction coatings. The purpose of the optimization was to minimize these parameters with provisions for the required load capacity. The bearings with the following characteristics have met the requirements: -
-
Working fluide Kick-off torque of rotor in loll bearings Specific load capacity of the thrust bearing at IO0,0OO rpm Temperature in the bearing region at 230,000 rpm Oscillation amplitude within the range of 30,000-230,000 rpm Antifrictlon coating wear after 250 cycles and shutdowns
air, helium - 0,02 Nm -
-
O,11 NEPa
-
40°C (max)
-
2)Win (max)
-
1.,.1
Cryogenics 1992 Vol 32 ICEC Supplement
81
ICEC 14 Proceedings
Such bearings were used for suspending rotors of helium TE with following characteristics: Inlet pressure Expansion ratio Inlet temperature Capacity Journal diameter Rotor mass Mass flow rate
the
- [email protected] MPa -
1.5~8.0 300@I 5K 0.I~5 kW
"
0.2 ~g O.02~O.05kg/s
o.o16 m -
The turboexpanders with radial and axial foil bearings were operationally developed and preliminary tested at a suitable air test bed; after that they were brought iato operation as part of cryogenic helium plant KPY-250/4,5 with a rated capacity of 90 1/h of liquid helium. The TE general view is shown in Fig.2. The plant operation under liquefaction conditions showed no resonance modes within the whole range of rotational speeds (30,000~200,000 rpm). The rotor oscillation amplitude did not exceed 2#Wm; the temperature at the loll bearing installation point changed within -10 through +I0°C, To check TE for reliability an emergency switch-off of the compressor was simulated followed by a sharp drop of helium pressure in the system. This situation with gas static bearings leads to rotor jA~niag in the turboexpander connected with a fast contact due to the loss of carrying capacity by both radial and axial bearings. In case of foil bearings this eme~gences pressure drop resulted in a gradual stop of rotor. The substitution of foil bearings for gas static bearings in TE of the cryogenic helium plant led, first, to a considerable increase of plant reliability due to higher reliability of the turboexpanders and, second, to the double reduction of time necessary to reach liquefaction conditions and to an increase of LHe ploduction f r o m 9 0 to 125 I/h. Life cycle of TE with foil bearings is dependent on the wear resistance of foil antifriction coatings during rotor start-ups and shutdowns. Using the test bed at ~ 0 "Geliymash" a number of TE test was carried out subjecting TE to multiple repeated cycles of the "start-stop" type. 10,000 cycles . were performed with the bearings diassembled and checked for condi %ion after 1000! 5000 and 10,0OO cycles. The inspection showed satisfactory condltion of bearing coatings and their serviceability. The obtained results have made it possible to begin developing a new generation of TE with foil gas dynamic bearings for cryogenic helium and air separation plants. CONCLUSIONS 1. Tests of TE with foil gas dynamic bearings have shown the following advantages: - self-compensation of manufacturing and assembly errors, as well as thermal deformations of body elements and rotor unit~ - low sensitivity to working fluid contamination; - practically unlimited capabilities for providing stable operation of rotors | - high maintainability; - pollution-free operation. 2. The replacement of the turboexpander with gas static bearings by TE with gas dynamic foil bearings in cryogenic helium plants has increased the reliability and efficiency of the Cryogenic cycle.
82
Cryogenics 1992 Vol 32 ICEC Supplement
ICEC 14 Proceedings
REFERENCES .
Ravikovich
. . . . . .
YU.A.,
~r- i
~rml±oV. ~.
S e r ~ e ~ v S.Io, G e n e r a l i z e d •~ . o - . ,. . . . . . ~ ~ . ~
n 8
I
]
Fig.1.
Diagram dysamic
of r a d i a l gas foil bearing
Fig.2.
Tarboexpander
schematic
I - turbine wheel; 2 - nozzle; 3 - gas d y n a m i c r a d i a l f o i l b e a r i n g ; $ - s h a f t ; 5 - gas d y namic foil thrust bearing; 6 - gas b l o w e r b r a k e w h e e l ; 7 - rotational speed transducer; 8 - heat exchanger.
Cryogenics 1992 Vol 32 ICEC Supplement
83