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Tribological design considerations in fans and blowers
Wear, 25 (19X3) 245-253 CQElsevier Sequoia %A., Lausanne - Printed in The Netherlands
~I~ULOGICA~
DESIGN C~NS~~ERATIUNS
245
IN FANS AND 3LQWERS*
S. L. SRIVASTAVA and S. M. MEHRA The Tata Iron artd Steel Company Limited, Jamshedpur (indid) (Received March 26, 1473)
SUMMARY
The paper describes three cases of excessive wear of the blades of diircerent blowers and fans used in the steel industry in India. Wear was due to the inadequate aerodynamic design of either the blower or of the suction and delivery passages, causing turbulence and eddies. In two cases suitable aerodynamic modifications resulted in improved blade life. In the third case where space limitations did not permit modifications, hard facing of the blades gave satisfactory results.
CASE NO. l--CONVERTER
BLOWERS
Introductiqn The specification of these blowers used for blowing the Bessemer Converter is
given in Table 1. The initial arrangement which CaBed for stopping the blower after TABLE I SPECIFICATION C.F.M. HP. Pressure Type R.p.m.
OF THE BLOWER 32ooo 4Qix3 32 p.9i.g. centrifugal 3700
every Mow resuited in fatigue faiIure of the rotors. On taking up the problem with the suppliers it was suggested that the blower be run continuously and provided with adjustable vanes at the intake to regulate the ffow during off-bIow period. The inlet vanes work on Askaaia Control and are opened to the desired extent through a pressure regulating device and closed at the end of the blow through an idle blow switch, a solenoid and a hydraulic cylinder. Excessive wear on the trailing edge of the inlet vanes, necessitated the blades being changed after l$ years involving considerable down-time. Figures 1 and 2 * Paper presented at the First World Conference on Industrial Tribalbgy, New Delhi, December, 1972.
246
S. L.. SRIVASTAVA, S. M. MEHRA
Fig. I. New inlet vane. Fig. 2. ldet vane after 14 years service.
show a new blade and a worn blade respectively. Hard facing of the blades to improve wear resistance was not effective.
During the off-blow period the lauvres (inlet vanes) were adjusted ta restrict the flow, Under these conditions the blower has to operate on the off-design condition, very near the surge point. The angle of incidence of the incoming sir on the inlet vanes under these conditions is shown in Fig. 3 and the fio%xpast an aerofoii with different angles of incidence is shown in Fig. 4. The corn~a~t~ve~y large angle of incidence on the inlet vanes results in separated ffow having a large degree of turbulence and eddies near the trailing edge of the inlet vanes. ANGLE
OF
INCIOENCE
AIR
FLOW
iii) ANGLE OF iH?ifXNCE f?i OPEN POSfTfON ANGLE OF IN CLOSED POSITION
INClOENCE (IDLE-FLOW)
Fig. 3. Arrangement of
vanes.
TRIBOLOGY OF FANS AND BLOWERS
247
( iii )
Fig. 4. Flow across an aerofoil with different angles of incidence.
Since the air handled by these blowers carries lot of abrasive dust this results in excessive wear of the blades. The relatively satisfactory performance of rotor blades is explained by a comparatively low angle of incidence which prevents separated flow and consequently high turbulence and eddies.
As it was hypothesised that the wear on the inlet vanes is due to very poor aerodynamic performance of the system in the off-design conditions during the off-blow period, the problem was tackled from this point of view. A rather unconventional solution of the problem was devised in view of the more tedious solutions involving complete change of aerodynamic design of the blades and their fabrication. The major problem was a result of a rather high angle of incidence on the inlet vanes necessitated to limit the total air flow to the rotor to a minimum. In the modified arrangement the reduced air flow was achieved by completely closing most of the passages through inlet vanes and partially adjusting a few sets uniformly distributed around the periphery such that the total effective opening at off-blow period before modification and after it remained the same. The arrangement, shown schematically in Fig. 5, replaced 34 small passages through the louvres
5 (a)
Fig. 5. Schematic arrangement of the blades showing decrease in the angle of incidence in the modified arrangement. (a) Old arrangement having high angle of incidence; (b) modified arrangement having low angle of incidence.
S. L. SRIVASTAVA. S. M. MEHRA
248
with 4 bigger passages for the same mass flow. The vanes did not require setting at an oblique angle during the off-blow period thus a large angle of incidence was avoided. The relevant details of the modi~~ation are given in Table II. TABLE II Modified
TotaI
no.
of
34 70” approx.
passages
Angle of blade to the axis of fans
4 10 apprax.
The arrangement was satisfactory and fears that the asymmetric flow intake to the rotor as a result of closing some of the passages of inlet vanes, would adversely affect the performance of the blower were unfounded, This could be due to the fact that it was a centrifugal blower. In the case of an axial flow blower, where the phenomenon of rotating stall exists, asymmetric flaw could lead to poor performance. Comparative data regarding the life of the inlet vanes is given in Table III. TABLE IIf Life of inlet vane without modification
Less than If years
Life of inlet vane after modification
No wear so far after running for 10 years
Economics
An estimate of the saving can be made from the fact that the blades were imported at a price of $500 per blade
CASE NO. Z-BLOWER
FOR PELLETSING
PLANT
Introdarction One important conclusion from the modifications carried out and the results
obtained in Case No. 1 was that the aerodynamic performance affects the wear of blower blades particularly when the blower handled abrasive dust; excessive turbulence and eddies in the flow should be avoided. Case No. 2 refers to the -blower used in the waste gas system of a Pelletising Plaat, where the major problem was poor effkiency. The wear of rotor blades, although quite important was secondary but to some extent was solved by improving efficiency. TABLE IV Capacity Pressure Suction Type of blower
5.5 m3/s at 110°C 240 mm water [total) 160 mm water (total) centrifugal
249
TRIBOLOGY OF.FANS AND BLOWERS
Fig. 6. Schematic arrangement of the Pelletising Plant.
The arrangement of the whole system is shown schematically in Fig. 6 and the specification of the individual blower given in Table IV. Aerodynamics of the system In the starting stages of the plant the complete exhaust system did not function AIR
OuTi.E’I:
ttttt
SHAFT
SECTfW THE
OF
ME4R
TUE ON
WANE L/UE~
A7
‘XX’ /NcvCAT/NG
PLATE.
Fig. 7. Vane of waste gas fan of Pelletising Plant; the regions of extreme wear are shaded.
250
S. L. SRIVASTAVA, S. M. MEHRA
--
L 4600
c
Fig. 8. Arrangement of fan for Pelletking Plant showing modifications.
properly. On checking pressures and mass flow at different points the overall efficiency of the system was found to be very poor. The blower was dismantled and excessive wear was observed on the rotor blades. The regions of extreme wear are shown shaded in Fig. 7. The direction of the air flow over the blade is indicated by the ovality in the holes for bolting the liner plate. The opposite direction of wear near the blade root shows that there was a lot of eddy formation near the blade root. Static pressure and velocity readings at the exit showed that near the bend marked (S) in Fig. 8 there were a lot of eddies and no mass flow which was virtually causing chocking of the passage and adversely affecting the blower performance. The blower was running below design requirements which increased the amount of separated flow and eddies. The increase in turbulence together with the abrasive resulted in excessive wear of blades. Modification
Modifications were carried out to improve efficiency. Various observations showed that the bend (S) was the cause of major restriction in the delivery passage
TRIBOLOGY OF FANS AND BLOWERS
251
so an attempt was made to remove this bend without involving major changes in the delivery circuit. The modified delivery circuit is also shown in Fig. 8: The modi~cation also involved changing the direction of rotation of the blower and consequent changes in the rotor but as the total waste gas handling system employed blowers running in both directions, the rotors were simply interchanged and the direction of rotation reversed.
Significant imfirovement in the performance of the blower accompanied by considerable reduction in wear of the rotor blades was achieved. Hard facing of the rotor blades further reduced wear. Details of the hard facing is given in Table V. TABLE V Make of hard facing electrode Composition of electrode Technique of depositing the weld Number of weld iayers Subsequent heat treatment Hardness achieved Hardness before
Duroide 3B. 6.5% Cr, MO, V, deposit C 0.45% By hand One in bead Nil 68 RC 200 B.H.N.
No impairing effect on toughness was observed as a result of hard facing. CASE NO. 3-I.D.
I.D. the Sirroco The power The
FANS FOR POWER HOUSE
(induced draught) fans used in the boiler house of power plant are of type having two rows and four rows of blades with a double suction. plant uses fuel having 70% coal and 30% coke breeze on the grates. rating of the boiler where these were used is given in Table VI.
TABLE VI
Evaporation Pressure
Two-row I.D. fan
Four-row I.D. fan
175,000 lbsfh 450 p.9i.g.
210,000 lbsjh 450 p.s.&.
Because of the abrasive nature of the dust particles and their high velocity the I.D. fan blades are subjected to severe abrasive conditions and required frequent changing. The average life of the blades was found to be six months. Blade wear was not uniform. Excessive wear was observed adjacent to the central hub plate as shown in the Fig. 9. This also showed that only the central portion of the fan was effective. The probable air flow through the fan is marked in Fig. 10. Only the middle portion of blades is effective as confirmed by wear of the rotor blades.
252
S. L. SRIVASTAVA,
Fig. 9. Blade of four row I.D. fan showing
regions
S. M. MEHRA
of high wear.
REGION OF/ EXCESSIVE WEAR Fig. 10. Probable
air flow’ through
I.D. fan.
Suitable modification of the inlet suction could have provided a more streamlined flow to use the full length of the blades which would avoid excessive turbulence and eddies to reduce erosive dust action and blade wear. Because of the space limitation this modification, however, was not possible. Hard facing of the bIades was
TRIBOLOGY OF FANS AND BLOWERS
253
TABLE VII Make of hard facing electrode Composition of the electrode Technique of weld deposit Number of weld layer Subsequent heat treatment
Duroide 3-B 6.5% Cr, MO, V, deposit 0.45p/,C By hand One in bead Nil
therefore resorted to, which gave ~tisfactory results. Details of the hard facing and the improvement in life is given in Table VII. Improvement in blade life (1) Average blade life before hard facing was six months.
(2) Average blade life after hard facing was almost two years. CONCLUSIONS
Separated flow, excessive turbulence and eddy formation in fans and blowers which are handling abrasive dust particles leads to severe erosion and wear of the blades, Streamlining the flow considerably reduces blade wear and improves life. Where such modi~~ations could not be carried out significant reduction in wear was achieved by hard facing. Further improvement was also achieved by hard facing in addition to the-aerodynamic modifications.
~I~ULOGICA~
DESIGN C~NS~~ERATIUNS
245
IN FANS AND 3LQWERS*
S. L. SRIVASTAVA and S. M. MEHRA The Tata Iron artd Steel Company Limited, Jamshedpur (indid) (Received March 26, 1473)
SUMMARY
The paper describes three cases of excessive wear of the blades of diircerent blowers and fans used in the steel industry in India. Wear was due to the inadequate aerodynamic design of either the blower or of the suction and delivery passages, causing turbulence and eddies. In two cases suitable aerodynamic modifications resulted in improved blade life. In the third case where space limitations did not permit modifications, hard facing of the blades gave satisfactory results.
CASE NO. l--CONVERTER
BLOWERS
Introductiqn The specification of these blowers used for blowing the Bessemer Converter is
given in Table 1. The initial arrangement which CaBed for stopping the blower after TABLE I SPECIFICATION C.F.M. HP. Pressure Type R.p.m.
OF THE BLOWER 32ooo 4Qix3 32 p.9i.g. centrifugal 3700
every Mow resuited in fatigue faiIure of the rotors. On taking up the problem with the suppliers it was suggested that the blower be run continuously and provided with adjustable vanes at the intake to regulate the ffow during off-bIow period. The inlet vanes work on Askaaia Control and are opened to the desired extent through a pressure regulating device and closed at the end of the blow through an idle blow switch, a solenoid and a hydraulic cylinder. Excessive wear on the trailing edge of the inlet vanes, necessitated the blades being changed after l$ years involving considerable down-time. Figures 1 and 2 * Paper presented at the First World Conference on Industrial Tribalbgy, New Delhi, December, 1972.
246
S. L.. SRIVASTAVA, S. M. MEHRA
Fig. I. New inlet vane. Fig. 2. ldet vane after 14 years service.
show a new blade and a worn blade respectively. Hard facing of the blades to improve wear resistance was not effective.
During the off-blow period the lauvres (inlet vanes) were adjusted ta restrict the flow, Under these conditions the blower has to operate on the off-design condition, very near the surge point. The angle of incidence of the incoming sir on the inlet vanes under these conditions is shown in Fig. 3 and the fio%xpast an aerofoii with different angles of incidence is shown in Fig. 4. The corn~a~t~ve~y large angle of incidence on the inlet vanes results in separated ffow having a large degree of turbulence and eddies near the trailing edge of the inlet vanes. ANGLE
OF
INCIOENCE
AIR
FLOW
iii) ANGLE OF iH?ifXNCE f?i OPEN POSfTfON ANGLE OF IN CLOSED POSITION
INClOENCE (IDLE-FLOW)
Fig. 3. Arrangement of
vanes.
TRIBOLOGY OF FANS AND BLOWERS
247
( iii )
Fig. 4. Flow across an aerofoil with different angles of incidence.
Since the air handled by these blowers carries lot of abrasive dust this results in excessive wear of the blades. The relatively satisfactory performance of rotor blades is explained by a comparatively low angle of incidence which prevents separated flow and consequently high turbulence and eddies.
As it was hypothesised that the wear on the inlet vanes is due to very poor aerodynamic performance of the system in the off-design conditions during the off-blow period, the problem was tackled from this point of view. A rather unconventional solution of the problem was devised in view of the more tedious solutions involving complete change of aerodynamic design of the blades and their fabrication. The major problem was a result of a rather high angle of incidence on the inlet vanes necessitated to limit the total air flow to the rotor to a minimum. In the modified arrangement the reduced air flow was achieved by completely closing most of the passages through inlet vanes and partially adjusting a few sets uniformly distributed around the periphery such that the total effective opening at off-blow period before modification and after it remained the same. The arrangement, shown schematically in Fig. 5, replaced 34 small passages through the louvres
5 (a)
Fig. 5. Schematic arrangement of the blades showing decrease in the angle of incidence in the modified arrangement. (a) Old arrangement having high angle of incidence; (b) modified arrangement having low angle of incidence.
S. L. SRIVASTAVA. S. M. MEHRA
248
with 4 bigger passages for the same mass flow. The vanes did not require setting at an oblique angle during the off-blow period thus a large angle of incidence was avoided. The relevant details of the modi~~ation are given in Table II. TABLE II Modified
TotaI
no.
of
34 70” approx.
passages
Angle of blade to the axis of fans
4 10 apprax.
The arrangement was satisfactory and fears that the asymmetric flow intake to the rotor as a result of closing some of the passages of inlet vanes, would adversely affect the performance of the blower were unfounded, This could be due to the fact that it was a centrifugal blower. In the case of an axial flow blower, where the phenomenon of rotating stall exists, asymmetric flaw could lead to poor performance. Comparative data regarding the life of the inlet vanes is given in Table III. TABLE IIf Life of inlet vane without modification
Less than If years
Life of inlet vane after modification
No wear so far after running for 10 years
Economics
An estimate of the saving can be made from the fact that the blades were imported at a price of $500 per blade
CASE NO. Z-BLOWER
FOR PELLETSING
PLANT
Introdarction One important conclusion from the modifications carried out and the results
obtained in Case No. 1 was that the aerodynamic performance affects the wear of blower blades particularly when the blower handled abrasive dust; excessive turbulence and eddies in the flow should be avoided. Case No. 2 refers to the -blower used in the waste gas system of a Pelletising Plaat, where the major problem was poor effkiency. The wear of rotor blades, although quite important was secondary but to some extent was solved by improving efficiency. TABLE IV Capacity Pressure Suction Type of blower
5.5 m3/s at 110°C 240 mm water [total) 160 mm water (total) centrifugal
249
TRIBOLOGY OF.FANS AND BLOWERS
Fig. 6. Schematic arrangement of the Pelletising Plant.
The arrangement of the whole system is shown schematically in Fig. 6 and the specification of the individual blower given in Table IV. Aerodynamics of the system In the starting stages of the plant the complete exhaust system did not function AIR
OuTi.E’I:
ttttt
SHAFT
SECTfW THE
OF
ME4R
TUE ON
WANE L/UE~
A7
‘XX’ /NcvCAT/NG
PLATE.
Fig. 7. Vane of waste gas fan of Pelletising Plant; the regions of extreme wear are shaded.
250
S. L. SRIVASTAVA, S. M. MEHRA
--
L 4600
c
Fig. 8. Arrangement of fan for Pelletking Plant showing modifications.
properly. On checking pressures and mass flow at different points the overall efficiency of the system was found to be very poor. The blower was dismantled and excessive wear was observed on the rotor blades. The regions of extreme wear are shown shaded in Fig. 7. The direction of the air flow over the blade is indicated by the ovality in the holes for bolting the liner plate. The opposite direction of wear near the blade root shows that there was a lot of eddy formation near the blade root. Static pressure and velocity readings at the exit showed that near the bend marked (S) in Fig. 8 there were a lot of eddies and no mass flow which was virtually causing chocking of the passage and adversely affecting the blower performance. The blower was running below design requirements which increased the amount of separated flow and eddies. The increase in turbulence together with the abrasive resulted in excessive wear of blades. Modification
Modifications were carried out to improve efficiency. Various observations showed that the bend (S) was the cause of major restriction in the delivery passage
TRIBOLOGY OF FANS AND BLOWERS
251
so an attempt was made to remove this bend without involving major changes in the delivery circuit. The modified delivery circuit is also shown in Fig. 8: The modi~cation also involved changing the direction of rotation of the blower and consequent changes in the rotor but as the total waste gas handling system employed blowers running in both directions, the rotors were simply interchanged and the direction of rotation reversed.
Significant imfirovement in the performance of the blower accompanied by considerable reduction in wear of the rotor blades was achieved. Hard facing of the rotor blades further reduced wear. Details of the hard facing is given in Table V. TABLE V Make of hard facing electrode Composition of electrode Technique of depositing the weld Number of weld iayers Subsequent heat treatment Hardness achieved Hardness before
Duroide 3B. 6.5% Cr, MO, V, deposit C 0.45% By hand One in bead Nil 68 RC 200 B.H.N.
No impairing effect on toughness was observed as a result of hard facing. CASE NO. 3-I.D.
I.D. the Sirroco The power The
FANS FOR POWER HOUSE
(induced draught) fans used in the boiler house of power plant are of type having two rows and four rows of blades with a double suction. plant uses fuel having 70% coal and 30% coke breeze on the grates. rating of the boiler where these were used is given in Table VI.
TABLE VI
Evaporation Pressure
Two-row I.D. fan
Four-row I.D. fan
175,000 lbsfh 450 p.9i.g.
210,000 lbsjh 450 p.s.&.
Because of the abrasive nature of the dust particles and their high velocity the I.D. fan blades are subjected to severe abrasive conditions and required frequent changing. The average life of the blades was found to be six months. Blade wear was not uniform. Excessive wear was observed adjacent to the central hub plate as shown in the Fig. 9. This also showed that only the central portion of the fan was effective. The probable air flow through the fan is marked in Fig. 10. Only the middle portion of blades is effective as confirmed by wear of the rotor blades.
252
S. L. SRIVASTAVA,
Fig. 9. Blade of four row I.D. fan showing
regions
S. M. MEHRA
of high wear.
REGION OF/ EXCESSIVE WEAR Fig. 10. Probable
air flow’ through
I.D. fan.
Suitable modification of the inlet suction could have provided a more streamlined flow to use the full length of the blades which would avoid excessive turbulence and eddies to reduce erosive dust action and blade wear. Because of the space limitation this modification, however, was not possible. Hard facing of the bIades was
TRIBOLOGY OF FANS AND BLOWERS
253
TABLE VII Make of hard facing electrode Composition of the electrode Technique of weld deposit Number of weld layer Subsequent heat treatment
Duroide 3-B 6.5% Cr, MO, V, deposit 0.45p/,C By hand One in bead Nil
therefore resorted to, which gave ~tisfactory results. Details of the hard facing and the improvement in life is given in Table VII. Improvement in blade life (1) Average blade life before hard facing was six months.
(2) Average blade life after hard facing was almost two years. CONCLUSIONS
Separated flow, excessive turbulence and eddy formation in fans and blowers which are handling abrasive dust particles leads to severe erosion and wear of the blades, Streamlining the flow considerably reduces blade wear and improves life. Where such modi~~ations could not be carried out significant reduction in wear was achieved by hard facing. Further improvement was also achieved by hard facing in addition to the-aerodynamic modifications.