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The effect of interface cooling in controlling surface disturbances in mechanical face seals
Wear, 79 (1982)
119
119 - 127
THE EFFECT OF INTERFACE COOLING IN CONTROLLING SURFACE DISTURBANCES IN MECHANICAL FACE SEALS*
JAMES P. NETZEL Crane Packing Company,
6400 W. Oakton
Street,
Morton
Grove, IL 60053
(U.S.A.)
(Received December 23, 1981)
Summary Surface waviness and equipment motion can influence seal performance. When portions of the sealing plane break through the film at the seal faces, unstable operation will occur. Results of seal operation in hydrocarbon service without seal leakage are given. Wear in the presence of abrasives and mechanical distortion are discussed.
1. Introduction The successful operation of a mechanical seal depends on the development of a lubricating film at the seal faces. Many theories have been developed to explain the formation of this film. These theories include the types of motion transferred to the seal, thermal distortions of the sealing planes and the surface waviness of both seal faces. Information from the field suggests that surface waviness is the primary reason for the generation of a lubricating film. This is particularly important if the seal is to run for a long period of time without exhibiting any appreciable wear at the seal faces or any seal leakage. Every new seal face has an initial surface waviness. For a hard seal face such as tungsten carbide or silicon carbide the range of initial waviness will be 50 - 200 nm (2 - 8 pin). Softer seal face materials such as carbon graphite would be within the range 254 - 460 nm (10 - 16 pin). As sliding contact occurs between the primary and mating rings of the seal, frictional heat develops which increases the surface waviness of each seal face from its initial value to some operating waviness. This increase in waviness is the result of small thermal deformations of the seal face. Small mechanical distortions may also aid in promoting the operating waviness. Continuously operating without solid contact between the seal faces is the result of an *Paper presented at the Workshop on Thermomechanical Laboratories, Columbus, OH, U.S.A., June 17 - 19, 1981.
Effects, Battelle, Columbus
Elsevier Sequoia/Printed
in The Netherlands
120
ideal wave formation for the operating conditions of the seal. When portions of the sliding planes break through the lubricating film, solid contact between the seal surfaces will result in increased frictional heat in localized areas [l].The hot spots on the seal face are the result of thermoelastic instability of the sealing planes. These spots will grow, generating yet more heat. The intense heat at the seal face will cause the liquid being sealed to flash or carbonize, resulting in unstable operation of the seal faces.
2. Unstable seal operation The development of a hot spot is illustrated in Fig. 1. During operation in an unstable region for a short period of time the localized hot spot will appear very small. The surface distress at the hot spot occurs because of the rapid heating in operation, followed by rapid cooling due to flashing at the seal interface. When the liquid at the seal interface flashes or vaporizes, the seal will open, cooling that spot which results in heat checking of the surface. When removed from the equipment after running for a short period of time, the deformed surface may be from 127 to 254 nm (5 to 10 pin) larger than the surrounding seal surface. In operation, however, because of the amount of heat developed at the hot spot, the deformed surface will be larger than the lubricating film [ 21.
Fig. 1. Small hot spot on a mating ring.
If the seal is allowed to run with this type of surface d~t~b~ce occurring at the seal faces, continued vaporization of the product being sealed will result in carbonized debris from the liquid being sealed and/or carbon particles from the seal faces building up on the exterior surfaces of the seal parts as illustrated in Fig. 2(a). The vaporization of product from the stuffing box can be heard above the normal equipment sounds as a spitting or sputtering sound that may occur at intervaIs of a few seconds to several minutes. This type of condition indicates that abnormal wear is occurring on the softer primary ring in the seal design from a damaged mating ring. By contrast, as illustrated in Fig. 2(b), a seal running without any audible
121
(4
(b)
Fig. 2. Condition of external seal parts: (a) unstable operation with surface disturbance; (b) stable operation without any surface disturbance.
sounds from the stuffing box and in a very stable region will exhibit a very clean appearance without any appreciable build-up of wear debris on the exterior surfaces of the seal parts. The development of a surface disturbance on a seal face is directly related to the type of liquid being sealed. When operating in a finished petroleum product such as lubricating oil or gasoline, a thicker film at the seal faces will protect the seal interface from the development of any localized hot spots. The seal shown in Fig. 3 has been in service for approximately 220 000 h. This 76.2 mm (3 in) diameter seal was used to seal gasoline, fuel oil and, occasionally, propane. The normal operating pressure and temperature were 5506 kPa (800 lbf in-‘) and 15.5 “C (60 OF) respectively. The shaft speed was 3560 rev min-‘. The condition of the carbon primary ring exhibits almost no adverse wear of the sealing surface or at the drive notches which represent the drive system on this seal. Wear at the carbon face is only 0.813 mm (0.032 in) for this time period. The mating ring made of tool steel appears to be in good condition after running for this length of time. It was reported that this seal had not been leaking even though some minor heat checking had occurred on the sealing plane. This damage is believed to have been the result of the loss of O-ring flexibility after it had been in service for 25 years. An increase in O-ring hardness will result in an overload condition at the seal faces. Heat checking on the seal plane would account for most of the carbon wear at the end of the service life for this seal. When sealing light hydrocarbons such as those that flash to vapor at normal atmospheric pressures and temperatures or liquids that are poor lubricants, the lubricating film is extremely small and any small disturbance transmitted to the sealing planes will result in short seal life. Instability of the seal faces is directly related to the development of hot localized patches of material which can vaporize the liquid being sealed. The development of a surface disturbance can result from the following: (1) thermoelastic
122
instability; (2) mechanical distortions; (3) equipment misalignments; (4) abrasives. Permanent damage from thermoelastic instability is illustrated in Fig. 1. This type of damage can also be generated by any mechanical deformation transferred to the sealing faces. Any distorted sealing plane would also create a high spot which would generate additional heat resulting in the vaporization of the product being sealed. Equipment motions such as angular misalignment, parallel misalignment, shaft run-out or whirl and shaft end-play have an effect on seal operation Motions that do not add to the mechanical load of the seal, such as parallel misali~ment, are believed to aid the lubrication process. Angular m~s~i~ment, however, results in larger than normal forces on the sealing plane which may result in a mechanical seal operating in a region of instability. The increase in load from angular misalignment can be many times larger than the design load of the seal. This increase in load will result in additional frictional heat which will be detrimental to the lubricating film at the seal faces. The seal shown in Fig. 4 is a 136.5 mm (5.375 in) seal operating at a surface speed of 15.4 m s-l (5000 ft min-l) in water at 82.2 “C (180 “F). The stuffing box pressure was 1034 kPa (150 lbf in-‘). Misalignment of the seal face resulted in wear at only one antirotation notch. At the point of greatest friction in the drive system, edge chipping has occurred at the seal interface. This impact type of failure or wear occurred over a 2 month period of operation. The surface waviness of this part measured 69.8 nm (2750 gin) through 360”. This is an extremely high value of surface waviness for a sealing plane and would account for a large volume of leakage from the seal. The surface profile of this part was irregular, alternating from convex to concave at approximately every 90’. At the chipped edge the profile of the seal surface was 11.4 nm (350 pin) convex. The maximum concave surface was 5.08 nm (200 pin). The secondary seal has been severely damaged because of the motion transmitted to this seal from the misalignment problem.
Fig. 3. Appearance of 76.2 mm (3 in) seai after approximately petroleum products. Fig. 4. Misaligned carbon primary ring in water service.
220 000 h of service in
123
Surface damage to the tungsten carbide mating ring was minimal. This part was relapped and put back into service after the alignment problem had been corrected. High surface waviness and irregular surface profiles appear to be typical of seals subjected to impact failure from misalignment. Impact failure from misalignment may occur with a brittle mating ring as shown in Fig. 5. This conventional sealing face of carbon graphite had been running against a boron carbide mating ring in a hydrocarbon liquid. Additional loading from misalignment and impact loading of the carbon against the boron carbide resulted in almost total destruction of the mating ring. Chips and pieces of mating ring resulted in heavy wear and scoring of the softer carbon primary ring. Each of these previous cases occurred in a relatively short period of time and in different lubricating media. The seal shown in Fig. 6 has been run in a light hydrocarbon liquid for approximately 1500 h. This 136.5 mm (5.375 in) seal was used primarily to seal ethane at a maximum surface speed of just over 25.4 m s- ’ (5000 ft min-l). The stuffing box pressure could range from 5860 to 8618 kPa (850 to 1250 lbf ine2). The temperature of the liquid pumped ranged from 4.4 to 15.5 “C (40 to 60 “F). Misalignment resulted in the softer carbon member wearing at an angle. Most of the face wear occurred at the antirotation device in contact with the carbon ring. Here, additional load was transferred to the sealing plane which resulted in this type of wear pattern. Hydropads or lubricating recesses provided additional cooling until they were worn off the seal face. The increase in surface area and decrease in interface cooling resulted in the thermal disturbance shown on the surface of the mating ring. Three patches of distressed material are visible on the sealing surface. If this seal had not been originally designed with hydropads the seal life would have been considerably shorter.
Fig. 5. Misaligned carbon primary ring and boron carbide mating ring in hydrocarbon service. Fig. 6. Misaligned carbon primary ring with hydropads in light hydrocarbon service.
and tungsten
carbide mating ring
124
3. Stable seal operation For some time now, hydropads have been used in the surface of seal rings. When applied to light hydrocarbon service and liquids with poor Iubricating qualities, the life of a mechanical seal can be substanti~ly increased. Figure 7 shows seal faces of a 100 mm (3.94 in) seal which had been in operation in ethane for 3056 h without any visible leakage. The stuffing box pressure and shaft speed are 5515 kPa (820 lbf inT2) and 3600 The temperature of the liquid sealed was 9 - 15.5 “C rev min-l respectively. (48 - 60 “F). The hydropads or recesses in the seal face have continuously introduced cooling where the seal heat is being generated. The cooler seal surfaces are necessary when operating in light hydrocarbon liquids or poor lubricants to avoid operation in a region of thermoelasti~ instability. On this seal the mating ring surface had also been deformed enough to show some discoloration on the sealing plane at two spots 180” apart. Although these minor surface deflections occurred, the frictional heat developed was not large enough to create any massive surface disruptions or heat checking on the seal face. Both the carbon and the tungsten carbide ring in Fig. 8 illustrate that there has been no appreciable wear during operation. Also, during this service period no wear has occurred on the antirotation surface on the carbon ring. This is s~~ifi~~t for it does indicate that no additions forces were transferred to the seal face from any angular misalignment. Also, as reported by Barnard and Weir, a properly running seal in light hydrocarbon service will have three concentric rings visible on the surface [3]. These can be seen on the carbon ring in Fig. 8. The surface profile for the seal faces which had been subjected to a small amount of distortion is shown in Fig. 9. Approximately 762 nm (30 gin) of wear occurred on the tungsten carbide member through the discolored surface areas. The remaining surface of the seal wore by approx~ately 508 nm (20 gin). Total carbon wear is estimated to be 1270 nm (50 pin). The small distortion related to the sealing
Fig. 7. Appearance of 100 mm (3.94 in) seal after 3056 h of stable operation: minor face distortion on the mating ring.
there is
Fig. 8. Close-up view of the seal in Fig. 7 : there is no appreciable wear on the sealing planes.
125
I- 1270 nnl(50 psn)
LiQUID SIDE
508 nm(20 pin) I
ATMOSPHERE SIDE
CARBON
TUNGSTEN CARBIDE
(b) Fig. 9. (a) Surface profile and (b) waviness traces for the seal shown in Fig. 7.
plane may have generated some carbon debris at the seal faces. This would account for the wear observed on these sealing planes. The surface waviness for these parts is also illustrated in Fig. 9. The carbon ring has a measured peak-to-peak waviness value of 1905 nm (75 pin) while the tungsten carbide mating ring has a surface waviness of approximately 305 nm (12 pin). These curves illustrate the small amount of wear that has taken place over the 3056 h of operation. The seal faces illustrated in Fig. 10 were removed from the other end of the split-case pump handling ethane. These sealing surfaces appear to be in excellent condition and do not exhibit any gross wear on the sealing planes. Here the hydropads have been effective in controlling the condition of the seal interface without any visible leakage. This seal, however, had been exposed to a higher concentration of abrasives in the stuffing box than the seal illustrated in Fig; 7. As can be seen in Fig. 10, the abrasives did not
Fig. 10. Appearance of 100 mm (3.94 in) seal with hydropads after 3056 h of stable operation: abrasives were present in the stuffing box.
126
penetrate the seal interface. A small amount of erosive wear has occurred on one side of the hydropad. This wear is not significant and it does appear that the centrifugal effect of the hydropads had thrown the abrasives clear of the seal interface. Also, it should be noted that on the softer carbon member there are no additional points of wear which would create any additional load on the sealing surfaces. The amount of wear which had taken place at the seal interface is illustra~d in Fig. 11. Here the carbon ring exhibits less than 762 nm (30 pin) of wear on the carbon nose. The tungsten carbide mating ring exhibits less than 254 nm (10 pin) of wear on its sealing plane. The surface waviness of this part is also illustrated in Fig. 11. The carbon primary ring has a surface waviness of 3.81 nm (150 pin) while the tungsten carbide mating ring has a surface waviness of 635 nm (25 pin). This is an extremely small amount of wear for the length of service for this seal. It is also significant that the abrasives have not penetrated the seal interface to cause any scoring of either of the sealing planes. This would mean that the film thickness was very small, allowing for good performance of the seal without penetration of any abrasive particles.
LIQUID SIDE
t
762 nm(30 pin) 203 nm(B etn)
ATMOSPHERE SIDE
//// TUNGSTEN CARBIDE
(4
(b) Fig. 11. (a) Surface profile and (b) waviness traces for the seaI shown in Fig. 10.
If the operating surface waviness had been larger or the seal subjected to abnormal equipment motion, the abrasive would have penetrated the seal face. This would have resulted in the generation of more frictional heat leading to surface scoring and possible surface distress through heat checking.
127
4. Conclusion Hydropads or lubrication recesses when properly designed appear to reduce the frictional heat at the seal faces, as well as to increase the life of the seal. Additional load transferred to the sealing faces from equipment motion can result in unstable seal operation. To a certain extent, the effects of this overload can be offset with hydropads in the seal face. Field results appear to confirm the results obtained in laboratory testing. From studies of mechanical seal faces, it can be seen that the surface waviness in operation will increase for conventional, as well as hydropadded, seals. The increase in surface waviness will be far greater on the softer carbon ring. Experiments and field results have shown that in marginal lubricating media hydropads or lubrication recesses perform better when they are placed in the softer of the seal materials. Under marginal lubricating conditions with high surface waviness a hydropad or lubrication recess in the hard member will result in the generation of carbon wear debris at the seal faces. The edges of the harder material will begin to cut into the peaks of the surface waviness of the softer member. This type of wear will create an unstable condition resulting in a surface disturbance on the hard seal face. No significant difference in results can be found if the hydropad in the carbon ring is rotated with the shaft or held stationary in the seal design. The hydropad feature when incorporated in a mechanical seal design offers a method that will avoid the intense localized heating which can result in a surface disturbance and short seal life.
References 1 R. A. Burton, Thermal deformation in frictionally heated contact, Wear, 59 (1) (1980) 1 - 20. 2 J. P. Netzel, Observations of thermoelastic instability in mechanical face seals, Wear, 59 (1) (1980) 135 - 148. 3 P. C. Barnard and R. S. L. Weir, A theory for mechanical seal face thermodynamics, Proc. 8th Znt. Conf. on Fluid Sealing, September 1978, Durham, Vol. 1, British Hydromechanics Research Association, Cranfield.
119
119 - 127
THE EFFECT OF INTERFACE COOLING IN CONTROLLING SURFACE DISTURBANCES IN MECHANICAL FACE SEALS*
JAMES P. NETZEL Crane Packing Company,
6400 W. Oakton
Street,
Morton
Grove, IL 60053
(U.S.A.)
(Received December 23, 1981)
Summary Surface waviness and equipment motion can influence seal performance. When portions of the sealing plane break through the film at the seal faces, unstable operation will occur. Results of seal operation in hydrocarbon service without seal leakage are given. Wear in the presence of abrasives and mechanical distortion are discussed.
1. Introduction The successful operation of a mechanical seal depends on the development of a lubricating film at the seal faces. Many theories have been developed to explain the formation of this film. These theories include the types of motion transferred to the seal, thermal distortions of the sealing planes and the surface waviness of both seal faces. Information from the field suggests that surface waviness is the primary reason for the generation of a lubricating film. This is particularly important if the seal is to run for a long period of time without exhibiting any appreciable wear at the seal faces or any seal leakage. Every new seal face has an initial surface waviness. For a hard seal face such as tungsten carbide or silicon carbide the range of initial waviness will be 50 - 200 nm (2 - 8 pin). Softer seal face materials such as carbon graphite would be within the range 254 - 460 nm (10 - 16 pin). As sliding contact occurs between the primary and mating rings of the seal, frictional heat develops which increases the surface waviness of each seal face from its initial value to some operating waviness. This increase in waviness is the result of small thermal deformations of the seal face. Small mechanical distortions may also aid in promoting the operating waviness. Continuously operating without solid contact between the seal faces is the result of an *Paper presented at the Workshop on Thermomechanical Laboratories, Columbus, OH, U.S.A., June 17 - 19, 1981.
Effects, Battelle, Columbus
Elsevier Sequoia/Printed
in The Netherlands
120
ideal wave formation for the operating conditions of the seal. When portions of the sliding planes break through the lubricating film, solid contact between the seal surfaces will result in increased frictional heat in localized areas [l].The hot spots on the seal face are the result of thermoelastic instability of the sealing planes. These spots will grow, generating yet more heat. The intense heat at the seal face will cause the liquid being sealed to flash or carbonize, resulting in unstable operation of the seal faces.
2. Unstable seal operation The development of a hot spot is illustrated in Fig. 1. During operation in an unstable region for a short period of time the localized hot spot will appear very small. The surface distress at the hot spot occurs because of the rapid heating in operation, followed by rapid cooling due to flashing at the seal interface. When the liquid at the seal interface flashes or vaporizes, the seal will open, cooling that spot which results in heat checking of the surface. When removed from the equipment after running for a short period of time, the deformed surface may be from 127 to 254 nm (5 to 10 pin) larger than the surrounding seal surface. In operation, however, because of the amount of heat developed at the hot spot, the deformed surface will be larger than the lubricating film [ 21.
Fig. 1. Small hot spot on a mating ring.
If the seal is allowed to run with this type of surface d~t~b~ce occurring at the seal faces, continued vaporization of the product being sealed will result in carbonized debris from the liquid being sealed and/or carbon particles from the seal faces building up on the exterior surfaces of the seal parts as illustrated in Fig. 2(a). The vaporization of product from the stuffing box can be heard above the normal equipment sounds as a spitting or sputtering sound that may occur at intervaIs of a few seconds to several minutes. This type of condition indicates that abnormal wear is occurring on the softer primary ring in the seal design from a damaged mating ring. By contrast, as illustrated in Fig. 2(b), a seal running without any audible
121
(4
(b)
Fig. 2. Condition of external seal parts: (a) unstable operation with surface disturbance; (b) stable operation without any surface disturbance.
sounds from the stuffing box and in a very stable region will exhibit a very clean appearance without any appreciable build-up of wear debris on the exterior surfaces of the seal parts. The development of a surface disturbance on a seal face is directly related to the type of liquid being sealed. When operating in a finished petroleum product such as lubricating oil or gasoline, a thicker film at the seal faces will protect the seal interface from the development of any localized hot spots. The seal shown in Fig. 3 has been in service for approximately 220 000 h. This 76.2 mm (3 in) diameter seal was used to seal gasoline, fuel oil and, occasionally, propane. The normal operating pressure and temperature were 5506 kPa (800 lbf in-‘) and 15.5 “C (60 OF) respectively. The shaft speed was 3560 rev min-‘. The condition of the carbon primary ring exhibits almost no adverse wear of the sealing surface or at the drive notches which represent the drive system on this seal. Wear at the carbon face is only 0.813 mm (0.032 in) for this time period. The mating ring made of tool steel appears to be in good condition after running for this length of time. It was reported that this seal had not been leaking even though some minor heat checking had occurred on the sealing plane. This damage is believed to have been the result of the loss of O-ring flexibility after it had been in service for 25 years. An increase in O-ring hardness will result in an overload condition at the seal faces. Heat checking on the seal plane would account for most of the carbon wear at the end of the service life for this seal. When sealing light hydrocarbons such as those that flash to vapor at normal atmospheric pressures and temperatures or liquids that are poor lubricants, the lubricating film is extremely small and any small disturbance transmitted to the sealing planes will result in short seal life. Instability of the seal faces is directly related to the development of hot localized patches of material which can vaporize the liquid being sealed. The development of a surface disturbance can result from the following: (1) thermoelastic
122
instability; (2) mechanical distortions; (3) equipment misalignments; (4) abrasives. Permanent damage from thermoelastic instability is illustrated in Fig. 1. This type of damage can also be generated by any mechanical deformation transferred to the sealing faces. Any distorted sealing plane would also create a high spot which would generate additional heat resulting in the vaporization of the product being sealed. Equipment motions such as angular misalignment, parallel misalignment, shaft run-out or whirl and shaft end-play have an effect on seal operation Motions that do not add to the mechanical load of the seal, such as parallel misali~ment, are believed to aid the lubrication process. Angular m~s~i~ment, however, results in larger than normal forces on the sealing plane which may result in a mechanical seal operating in a region of instability. The increase in load from angular misalignment can be many times larger than the design load of the seal. This increase in load will result in additional frictional heat which will be detrimental to the lubricating film at the seal faces. The seal shown in Fig. 4 is a 136.5 mm (5.375 in) seal operating at a surface speed of 15.4 m s-l (5000 ft min-l) in water at 82.2 “C (180 “F). The stuffing box pressure was 1034 kPa (150 lbf in-‘). Misalignment of the seal face resulted in wear at only one antirotation notch. At the point of greatest friction in the drive system, edge chipping has occurred at the seal interface. This impact type of failure or wear occurred over a 2 month period of operation. The surface waviness of this part measured 69.8 nm (2750 gin) through 360”. This is an extremely high value of surface waviness for a sealing plane and would account for a large volume of leakage from the seal. The surface profile of this part was irregular, alternating from convex to concave at approximately every 90’. At the chipped edge the profile of the seal surface was 11.4 nm (350 pin) convex. The maximum concave surface was 5.08 nm (200 pin). The secondary seal has been severely damaged because of the motion transmitted to this seal from the misalignment problem.
Fig. 3. Appearance of 76.2 mm (3 in) seai after approximately petroleum products. Fig. 4. Misaligned carbon primary ring in water service.
220 000 h of service in
123
Surface damage to the tungsten carbide mating ring was minimal. This part was relapped and put back into service after the alignment problem had been corrected. High surface waviness and irregular surface profiles appear to be typical of seals subjected to impact failure from misalignment. Impact failure from misalignment may occur with a brittle mating ring as shown in Fig. 5. This conventional sealing face of carbon graphite had been running against a boron carbide mating ring in a hydrocarbon liquid. Additional loading from misalignment and impact loading of the carbon against the boron carbide resulted in almost total destruction of the mating ring. Chips and pieces of mating ring resulted in heavy wear and scoring of the softer carbon primary ring. Each of these previous cases occurred in a relatively short period of time and in different lubricating media. The seal shown in Fig. 6 has been run in a light hydrocarbon liquid for approximately 1500 h. This 136.5 mm (5.375 in) seal was used primarily to seal ethane at a maximum surface speed of just over 25.4 m s- ’ (5000 ft min-l). The stuffing box pressure could range from 5860 to 8618 kPa (850 to 1250 lbf ine2). The temperature of the liquid pumped ranged from 4.4 to 15.5 “C (40 to 60 “F). Misalignment resulted in the softer carbon member wearing at an angle. Most of the face wear occurred at the antirotation device in contact with the carbon ring. Here, additional load was transferred to the sealing plane which resulted in this type of wear pattern. Hydropads or lubricating recesses provided additional cooling until they were worn off the seal face. The increase in surface area and decrease in interface cooling resulted in the thermal disturbance shown on the surface of the mating ring. Three patches of distressed material are visible on the sealing surface. If this seal had not been originally designed with hydropads the seal life would have been considerably shorter.
Fig. 5. Misaligned carbon primary ring and boron carbide mating ring in hydrocarbon service. Fig. 6. Misaligned carbon primary ring with hydropads in light hydrocarbon service.
and tungsten
carbide mating ring
124
3. Stable seal operation For some time now, hydropads have been used in the surface of seal rings. When applied to light hydrocarbon service and liquids with poor Iubricating qualities, the life of a mechanical seal can be substanti~ly increased. Figure 7 shows seal faces of a 100 mm (3.94 in) seal which had been in operation in ethane for 3056 h without any visible leakage. The stuffing box pressure and shaft speed are 5515 kPa (820 lbf inT2) and 3600 The temperature of the liquid sealed was 9 - 15.5 “C rev min-l respectively. (48 - 60 “F). The hydropads or recesses in the seal face have continuously introduced cooling where the seal heat is being generated. The cooler seal surfaces are necessary when operating in light hydrocarbon liquids or poor lubricants to avoid operation in a region of thermoelasti~ instability. On this seal the mating ring surface had also been deformed enough to show some discoloration on the sealing plane at two spots 180” apart. Although these minor surface deflections occurred, the frictional heat developed was not large enough to create any massive surface disruptions or heat checking on the seal face. Both the carbon and the tungsten carbide ring in Fig. 8 illustrate that there has been no appreciable wear during operation. Also, during this service period no wear has occurred on the antirotation surface on the carbon ring. This is s~~ifi~~t for it does indicate that no additions forces were transferred to the seal face from any angular misalignment. Also, as reported by Barnard and Weir, a properly running seal in light hydrocarbon service will have three concentric rings visible on the surface [3]. These can be seen on the carbon ring in Fig. 8. The surface profile for the seal faces which had been subjected to a small amount of distortion is shown in Fig. 9. Approximately 762 nm (30 gin) of wear occurred on the tungsten carbide member through the discolored surface areas. The remaining surface of the seal wore by approx~ately 508 nm (20 gin). Total carbon wear is estimated to be 1270 nm (50 pin). The small distortion related to the sealing
Fig. 7. Appearance of 100 mm (3.94 in) seal after 3056 h of stable operation: minor face distortion on the mating ring.
there is
Fig. 8. Close-up view of the seal in Fig. 7 : there is no appreciable wear on the sealing planes.
125
I- 1270 nnl(50 psn)
LiQUID SIDE
508 nm(20 pin) I
ATMOSPHERE SIDE
CARBON
TUNGSTEN CARBIDE
(b) Fig. 9. (a) Surface profile and (b) waviness traces for the seal shown in Fig. 7.
plane may have generated some carbon debris at the seal faces. This would account for the wear observed on these sealing planes. The surface waviness for these parts is also illustrated in Fig. 9. The carbon ring has a measured peak-to-peak waviness value of 1905 nm (75 pin) while the tungsten carbide mating ring has a surface waviness of approximately 305 nm (12 pin). These curves illustrate the small amount of wear that has taken place over the 3056 h of operation. The seal faces illustrated in Fig. 10 were removed from the other end of the split-case pump handling ethane. These sealing surfaces appear to be in excellent condition and do not exhibit any gross wear on the sealing planes. Here the hydropads have been effective in controlling the condition of the seal interface without any visible leakage. This seal, however, had been exposed to a higher concentration of abrasives in the stuffing box than the seal illustrated in Fig; 7. As can be seen in Fig. 10, the abrasives did not
Fig. 10. Appearance of 100 mm (3.94 in) seal with hydropads after 3056 h of stable operation: abrasives were present in the stuffing box.
126
penetrate the seal interface. A small amount of erosive wear has occurred on one side of the hydropad. This wear is not significant and it does appear that the centrifugal effect of the hydropads had thrown the abrasives clear of the seal interface. Also, it should be noted that on the softer carbon member there are no additional points of wear which would create any additional load on the sealing surfaces. The amount of wear which had taken place at the seal interface is illustra~d in Fig. 11. Here the carbon ring exhibits less than 762 nm (30 pin) of wear on the carbon nose. The tungsten carbide mating ring exhibits less than 254 nm (10 pin) of wear on its sealing plane. The surface waviness of this part is also illustrated in Fig. 11. The carbon primary ring has a surface waviness of 3.81 nm (150 pin) while the tungsten carbide mating ring has a surface waviness of 635 nm (25 pin). This is an extremely small amount of wear for the length of service for this seal. It is also significant that the abrasives have not penetrated the seal interface to cause any scoring of either of the sealing planes. This would mean that the film thickness was very small, allowing for good performance of the seal without penetration of any abrasive particles.
LIQUID SIDE
t
762 nm(30 pin) 203 nm(B etn)
ATMOSPHERE SIDE
//// TUNGSTEN CARBIDE
(4
(b) Fig. 11. (a) Surface profile and (b) waviness traces for the seaI shown in Fig. 10.
If the operating surface waviness had been larger or the seal subjected to abnormal equipment motion, the abrasive would have penetrated the seal face. This would have resulted in the generation of more frictional heat leading to surface scoring and possible surface distress through heat checking.
127
4. Conclusion Hydropads or lubrication recesses when properly designed appear to reduce the frictional heat at the seal faces, as well as to increase the life of the seal. Additional load transferred to the sealing faces from equipment motion can result in unstable seal operation. To a certain extent, the effects of this overload can be offset with hydropads in the seal face. Field results appear to confirm the results obtained in laboratory testing. From studies of mechanical seal faces, it can be seen that the surface waviness in operation will increase for conventional, as well as hydropadded, seals. The increase in surface waviness will be far greater on the softer carbon ring. Experiments and field results have shown that in marginal lubricating media hydropads or lubrication recesses perform better when they are placed in the softer of the seal materials. Under marginal lubricating conditions with high surface waviness a hydropad or lubrication recess in the hard member will result in the generation of carbon wear debris at the seal faces. The edges of the harder material will begin to cut into the peaks of the surface waviness of the softer member. This type of wear will create an unstable condition resulting in a surface disturbance on the hard seal face. No significant difference in results can be found if the hydropad in the carbon ring is rotated with the shaft or held stationary in the seal design. The hydropad feature when incorporated in a mechanical seal design offers a method that will avoid the intense localized heating which can result in a surface disturbance and short seal life.
References 1 R. A. Burton, Thermal deformation in frictionally heated contact, Wear, 59 (1) (1980) 1 - 20. 2 J. P. Netzel, Observations of thermoelastic instability in mechanical face seals, Wear, 59 (1) (1980) 135 - 148. 3 P. C. Barnard and R. S. L. Weir, A theory for mechanical seal face thermodynamics, Proc. 8th Znt. Conf. on Fluid Sealing, September 1978, Durham, Vol. 1, British Hydromechanics Research Association, Cranfield.