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Tribological design - The process industries
41
Paper Il(iv)
Tribologicaldesign-The process industries J. D. Summers-Smith
In general, the process industry does not design its own machines, but has to rely on what is available from machine manufacturers. It is important that the results of operating experience are fed back to machine designers in order that enhanced reliability of future designs is obtained. Examples, based on tribological machine elements, are given to illustrate different ways in which operating problems can be addressed and the experience made available for incorporation in future designs.
1 PHILOSOPHY I speak from experience in the chemical process industry and, in particular, with machines problems in that industry. In general, apart from a few highly specialised cases, the process industry does not design its own machines, but has to rely on purchasing what is available in the market place, influencing this as far as possible to achieve cost effective reliability. It has to be emphasised that in the present competitive climate in the field of process chemicals, the user is probably more interested in predictability of performance, rather than reliability per se, so that he can select equipment giving the lowest life cycle cost per unit of product, balancing initial capital cost against maintenance costs, downtime costs, particularly unscheduled downtime costs. In the context of this symposium I want to focus primarily on machine elements and the impact they have on machine performance, not on performance itself nor on other aspects such as strength and corrosion that have to be equally addressed. The user is in the unique position that he alone can obtain experience of operation under real conditions, where machines are subject to process upsets, operator malpractice, gradual deterioration in service, and other factors, probably unforeseen, outside those specified in the design, rather than the more rigorously controlled conditions of the test bed. In many cases it is not even possible to match the service conditions on the test bed: for example, it may not be practicable to use the actual process fluid or to be able to realise the dynamic foundation conditions that a machine mounted on a marine platform will be subject to. My starting point is failure, using this in the broadest sense to include not only breakdown, but also the need to restrict output to avoid breakdown. The user has a number of priorities: the over-riding need for a 'quick-fix' in order to get back on line as quickly as possible and minimise production loss; an understanding of the problem in order to prevent its recurrence and, if necessary, to ensure that the manufacturer incorporates the experience in new designs, though in the urgency of the production
situation some discipline has to be exercised if adequate time and effort are to be devoted to the last two of these items. Failure analysis should involve the active cooperation of the machine designer, who in many cases has, first of all, to be persuaded that there is a genuine problem. It is a matter of fact that the normal immediate reaction of a machine manufacturer is that any failure is an operational effect, not a matter of the design or manufacture of the machine. A real understanding of the machine and the functioning of the machine elements by the user is crucial in developing the necessary degree of understanding between the user and the designer. 2 EXAMPLES
I should like to give from my experience some examples that bring out the above points. 2.1 Boundary lubrication Pn non-oxidising environments This example concerns two 2-stage, double-acting reciprocating compressors supplied by an internationally known manufacturer. The vast majority of machines are lubricated in an environment of atmospheric air. This machine, however, was to handle very pure nitrogen ( < 2ppm 02, < 2ppm H20) so that lubrication of the piston rings had to take place in a non-oxidising atmosphere. The manufacturer's experience was largely in the field of air compressors, but he expected no difficulty with nitrogen, a gas with very similar thermodynamic properties to air. Both machines, however, suffered very severe cylinder scuffing during the initial period of commissioning. The need to ensure adequate boundary lubrication of the piston rings at the ends of the stroke had been overlooked. Boundary lubrication, whether by the natural products found in hydrocarbon oils or by incorporated anti-wear or ep additives requires an oxide film on the contacting surfaces. These machines not only operated in a completely non-oxidising atmosphere, but to ensure purity of the gas the crankcase, from
which the lubricant was fed to the cylinders, In normal was also blanketted with nitrogen. circumstances any oxide film that is removed under boundary conditions is rapidly replaced In this case, from the atmospheric moisture. however, there was insufficient oxygen present, boundary lubrication broke down and the cylinders scuffed. The solution was simple once the mechanism of failure had been identified: the cylinder lubricant was supplied from outside the machine where it was in contact with air and the normal dissolved moisture was sufficient to maintain the oxide film on the cylinder wall.
speed, and also the lack of definition of the sulphur content of the gas, both in chemical nature and amount (available only to the nearest order of magnitude), it was possible to define The use of a a boundary to the problem. lead-rich, copper-free white metal as a seal ring lining gave some relief, but at the most severe conditions even lead was attacked to form lead sulphide deposits and in these cases a seal redesign was required to reduce the temperature. Fig. 2. shows how three zones were defined that could be used in the purchase specification of new machines. 2 . 4 Bearing influenced rotor dynamics
2.2 Load calculation f o r rolling bearings
My second problem involved failures of roller bearings on the rotors of two plastics extruders (Fig. 1). Examination showed the bearings were failing by fatigue. According to the manufacturer the bearings were operating well within their rated capacity and it was suggested that failure was caused by overload arising from starting without preheating the plactics granules as recommended. In these conditions it was estimated that it would take 15 minutes operation before frictional heating produced the required temperature. A simple calculation, however, showed that operation for 15 minutes per day at the overload trip current of the motor was well within the capacity of the bearings. A Weibull plot of the time to failure Life allowed a calculation of the bearing L with appropriate confidence limits and%om this an estimate was made of the continuous load that would account for the failures. Presentation of these calculations persuaded the manufacturer to reexamine the design and it turned out that an incorrect blending profile between feed and extrusion sections on the rotor could give a high enough axial load to account for the failures. The profile was corrected and the failure problem was cured. 2.3 Sulphur attack of floating-bush seals on centrifugal compressors My third example concerns the floating-bush seals on centrifugal hydrocarbon process gas compressors. These normally use mineral oil as a barrier fluid and are lined with white metal to prevent damage at starting. Such seals operate in an environment of the barrier fluid and the process gas. Seal failure occurred on a number of machines through reaction between sulphur in the process gas and copper in the white metal forming copper sulphide that took took up the clearance. Chemical reaction is a function of concentration and temperature. The oil film temperature in the seal ring, which is a function of the rate of shear in the oil film and hence the peripheral velocity, could not be measured. It seemed, however, that a plot of sulphur content against peripheral speed might be of use in defining the bounds of the problem. Fig. 2 gives a summary of experience from a number of machines. The weight of evidence was sufficient to convince the machine manufacturers that there was a genuine problem. Despite the representation of temperature by peripheral
Finally, I should like to look at problems arising from the dynamic instabilities in high-speed rotary machines. The simplistic approach to avoid instability was to specify that machines should not have a natural frequency within 2 10% of the rotor operating speed. With increasing sizes and speeds of machines, and particularly with machines capable of operating over a range of speeds, application of such a simple criterion became unrealistic, more particularly as natural frequencies were normally calculated f o r simply supported rotors with no allowance made for the effect of the dynamic characteristics of the supporting bearing oil films. A change was made to a requirement imposing a limit to the response to specified forcing conditions that were imposed on the test bed. This was clearly an unsatisfactory situation when problems arose on the test bed, or even worse when the machine went into operation, and the problem had to be resolved in an ad hoc way. It was decided that a more detailed understanding was necessary to aid in the solution of problems and in the development of purchasing specifications that would result in problem-free machines. This came to a head with a steam turbine driver on a process gas stream with a design operating speed of 177 rev/s. The difficulty was compounded in this machine in that the rotor was supported on three journal bearings so that the bearing loads were indeterminate; when instability problems arose, there was no obvious solution and in order to maintain production the speed had to be The reduced with a consequent loss in output. University of Leeds was asked to develop a complete rotor dynamic programme, including the dynamic characteristics of the bearing oil films. This involved a theoretical study that could not be tested economically in the laboratory because of the scale. The advantage of the university-industry co-operation was that the theoretical predictions could be validated with measurements taken from operating plant machines. The final programme was of value not only in guiding remedial actions when vibration problems .occurred, but also in allowing a meaningful dialogue between the Company engineers and machine suppliers when new machines were being purchased. 3 CONCLUSIONS The outcome of failure investigations of the
43
blending profile
extrusion
bearings
Fig. 1.
Schematic arrangement of extruder.
100-1OOo
-
10-100
-
\
Approximate sulphur content
O
of gas (PPd 1-10
<1
- \ -
Zone 1
0
\
0
\ 0
0
\ 0
\
x x xx n x I
+
0
Zone 3
\
YX \
1
Peripheral speed of seal ring (m/sec) Fig. 2. Attack of white metal lining of floating-bush seal ring by sulphur in process gas. x o
+
no problem attack of tin-rich white metal, but no attack of leadrich white metal attack of lead-rich white metal
Zone 1. Zone 2. Zone 3.
No problem Use copper-free white metal (lead-rich) Redesign seal to reduce temperature
I
44 t y p e s d e s c r i b e d c a n r a n g e from s o l u t i o n s b a s e d on a n u n d e r s t a n d i n g of t h e mechanisms i n v o l v e d (Examples 2.1 and 2.2), t h r o u g h a c r u d e g r a p h i c a l p r e s e n t a t i o n of t h e bounds o f t h e problem area u s i n g p a r a m e t e r s b a s e d on a t h e o r e t i c a l a p p r e c i a t i o n of t h e s i g n i f i c a n t v a r i a b l e s i n v o l v e d (Example 2.3) t o a r e s e a r c h programme aimed a t a complete u n d e r s t a n d i n g and d e f i n i t i o n o f t h e problem (Example 2.4). The f i n a l l i n k i n t h e c h a i n is communication. T h i s can b e e f f e c t e d i n a number of ways. For a one-off problem it may end w i t h r e c t i f i c a t i o n i n c o - o p e r a t i o n w i t h t h e manufacturer. Where t h e f i n d i n g s are of wider a p p l i c a t i o n t h e y c a n be i n c o r p o r a t e d i n a Company Purchase S p e c i f i c a t i o n t o p r e v e n t f u t u r e r e p e t i t i o n of t h e same problem, b u t , i n a d d i t i o n , p u b l i c a t i o n of a p a p e r i n a j o u r n a l w i t h a r e f e r e e i n g p r o c e d u r e is o f v a l u e i n making t h e findings available. I n t h i s c o n t e x t encouragement s h o u l d b e g i v e n t o t h e a c c e p t a n c e o f case h i s t o r y s t u d i e s , even i f t h e s e l a c k some o f t h e t h e o r e t i c a l background t h a t t e n d s t o b e a r e q u i r e m e n t of many l e a r n e d societies b e f o r e Not o n l y does papers can be considered. p u b l i c a t i o n a l l o w t h e d a t a t o be u s e d i n d e s i g n g u i d e s , s u c h as t h e ESDU Data Items (ESDU I n t e r n a t i o n a l L t d . , London), b u t t h e f a c t t h a t p u b l i c a t i o n h a s been a c c e p t e d l e n d s weight t o a one-to-one d i s c u s s i o n w i t h a s u p p l i e r and a l l o w s understandingof t h e r a t i o n a l e behind t h e i n c o r p o r a t i o n of a d d i t i o n a l r e q u i r e m e n t s i n purchase s p e c i f i c a t i o n s .
4
THE FUTURE
These examples have been chosen from a v e r y wide r a n g e o f p a s t problems t h a t have been satisfacto r i l y s o l v e d and are i n t e n d e d t o i l l u s t r a t e t h e approach t a k e n by one u s e r . Inevitably t h i s h a s been a r e a c t i v e one and i n a forward l o o k i n g meeting o f t h i s t y p e it is p r o b a b l y o f more i n t e r e s t t o i d e n t i f y p o s s i b l e f u t u r e problem areas. T h i s n e c e s s a r i l y means a c e r t a i n d e g r e e o f s p e c u l a t i o n , o t h e r w i s e t h e problems would have a l r e a d y been i d e n t i f i e d and h o p e f u l l y Areas t h a t I f e e l are l i k e l y t o b e o f solved. c o n c e r n i n t h e t r i b o l o g y o f machines are s e a l i n g , a r a t h e r b e t t e r understanding o f t h e influence o f f o u n d a t i o n dynamics on t h e i n s t a b i l i t y of machines, t h e need t o i d e n t i f y t h e e f f e c t o f s o l i d p a r t i c l e s on t h e performance o f l u b r i c a t e d components i n o r d e r t h a t more r e a l i s t i c filtra t i o n r e q u i r e m e n t s can be s p e c i f i e d . The p r e s e n t r a t h e r a d hoc t r e a t m e n t of t h e s e s u b j e c t s r e f l e c t s a l a c k of s a t i s f a c t o r y u n d e r s t a n d i n g and t h e p o t e n t i a l f o r f u t u r e problems.
Paper Il(iv)
Tribologicaldesign-The process industries J. D. Summers-Smith
In general, the process industry does not design its own machines, but has to rely on what is available from machine manufacturers. It is important that the results of operating experience are fed back to machine designers in order that enhanced reliability of future designs is obtained. Examples, based on tribological machine elements, are given to illustrate different ways in which operating problems can be addressed and the experience made available for incorporation in future designs.
1 PHILOSOPHY I speak from experience in the chemical process industry and, in particular, with machines problems in that industry. In general, apart from a few highly specialised cases, the process industry does not design its own machines, but has to rely on purchasing what is available in the market place, influencing this as far as possible to achieve cost effective reliability. It has to be emphasised that in the present competitive climate in the field of process chemicals, the user is probably more interested in predictability of performance, rather than reliability per se, so that he can select equipment giving the lowest life cycle cost per unit of product, balancing initial capital cost against maintenance costs, downtime costs, particularly unscheduled downtime costs. In the context of this symposium I want to focus primarily on machine elements and the impact they have on machine performance, not on performance itself nor on other aspects such as strength and corrosion that have to be equally addressed. The user is in the unique position that he alone can obtain experience of operation under real conditions, where machines are subject to process upsets, operator malpractice, gradual deterioration in service, and other factors, probably unforeseen, outside those specified in the design, rather than the more rigorously controlled conditions of the test bed. In many cases it is not even possible to match the service conditions on the test bed: for example, it may not be practicable to use the actual process fluid or to be able to realise the dynamic foundation conditions that a machine mounted on a marine platform will be subject to. My starting point is failure, using this in the broadest sense to include not only breakdown, but also the need to restrict output to avoid breakdown. The user has a number of priorities: the over-riding need for a 'quick-fix' in order to get back on line as quickly as possible and minimise production loss; an understanding of the problem in order to prevent its recurrence and, if necessary, to ensure that the manufacturer incorporates the experience in new designs, though in the urgency of the production
situation some discipline has to be exercised if adequate time and effort are to be devoted to the last two of these items. Failure analysis should involve the active cooperation of the machine designer, who in many cases has, first of all, to be persuaded that there is a genuine problem. It is a matter of fact that the normal immediate reaction of a machine manufacturer is that any failure is an operational effect, not a matter of the design or manufacture of the machine. A real understanding of the machine and the functioning of the machine elements by the user is crucial in developing the necessary degree of understanding between the user and the designer. 2 EXAMPLES
I should like to give from my experience some examples that bring out the above points. 2.1 Boundary lubrication Pn non-oxidising environments This example concerns two 2-stage, double-acting reciprocating compressors supplied by an internationally known manufacturer. The vast majority of machines are lubricated in an environment of atmospheric air. This machine, however, was to handle very pure nitrogen ( < 2ppm 02, < 2ppm H20) so that lubrication of the piston rings had to take place in a non-oxidising atmosphere. The manufacturer's experience was largely in the field of air compressors, but he expected no difficulty with nitrogen, a gas with very similar thermodynamic properties to air. Both machines, however, suffered very severe cylinder scuffing during the initial period of commissioning. The need to ensure adequate boundary lubrication of the piston rings at the ends of the stroke had been overlooked. Boundary lubrication, whether by the natural products found in hydrocarbon oils or by incorporated anti-wear or ep additives requires an oxide film on the contacting surfaces. These machines not only operated in a completely non-oxidising atmosphere, but to ensure purity of the gas the crankcase, from
which the lubricant was fed to the cylinders, In normal was also blanketted with nitrogen. circumstances any oxide film that is removed under boundary conditions is rapidly replaced In this case, from the atmospheric moisture. however, there was insufficient oxygen present, boundary lubrication broke down and the cylinders scuffed. The solution was simple once the mechanism of failure had been identified: the cylinder lubricant was supplied from outside the machine where it was in contact with air and the normal dissolved moisture was sufficient to maintain the oxide film on the cylinder wall.
speed, and also the lack of definition of the sulphur content of the gas, both in chemical nature and amount (available only to the nearest order of magnitude), it was possible to define The use of a a boundary to the problem. lead-rich, copper-free white metal as a seal ring lining gave some relief, but at the most severe conditions even lead was attacked to form lead sulphide deposits and in these cases a seal redesign was required to reduce the temperature. Fig. 2. shows how three zones were defined that could be used in the purchase specification of new machines. 2 . 4 Bearing influenced rotor dynamics
2.2 Load calculation f o r rolling bearings
My second problem involved failures of roller bearings on the rotors of two plastics extruders (Fig. 1). Examination showed the bearings were failing by fatigue. According to the manufacturer the bearings were operating well within their rated capacity and it was suggested that failure was caused by overload arising from starting without preheating the plactics granules as recommended. In these conditions it was estimated that it would take 15 minutes operation before frictional heating produced the required temperature. A simple calculation, however, showed that operation for 15 minutes per day at the overload trip current of the motor was well within the capacity of the bearings. A Weibull plot of the time to failure Life allowed a calculation of the bearing L with appropriate confidence limits and%om this an estimate was made of the continuous load that would account for the failures. Presentation of these calculations persuaded the manufacturer to reexamine the design and it turned out that an incorrect blending profile between feed and extrusion sections on the rotor could give a high enough axial load to account for the failures. The profile was corrected and the failure problem was cured. 2.3 Sulphur attack of floating-bush seals on centrifugal compressors My third example concerns the floating-bush seals on centrifugal hydrocarbon process gas compressors. These normally use mineral oil as a barrier fluid and are lined with white metal to prevent damage at starting. Such seals operate in an environment of the barrier fluid and the process gas. Seal failure occurred on a number of machines through reaction between sulphur in the process gas and copper in the white metal forming copper sulphide that took took up the clearance. Chemical reaction is a function of concentration and temperature. The oil film temperature in the seal ring, which is a function of the rate of shear in the oil film and hence the peripheral velocity, could not be measured. It seemed, however, that a plot of sulphur content against peripheral speed might be of use in defining the bounds of the problem. Fig. 2 gives a summary of experience from a number of machines. The weight of evidence was sufficient to convince the machine manufacturers that there was a genuine problem. Despite the representation of temperature by peripheral
Finally, I should like to look at problems arising from the dynamic instabilities in high-speed rotary machines. The simplistic approach to avoid instability was to specify that machines should not have a natural frequency within 2 10% of the rotor operating speed. With increasing sizes and speeds of machines, and particularly with machines capable of operating over a range of speeds, application of such a simple criterion became unrealistic, more particularly as natural frequencies were normally calculated f o r simply supported rotors with no allowance made for the effect of the dynamic characteristics of the supporting bearing oil films. A change was made to a requirement imposing a limit to the response to specified forcing conditions that were imposed on the test bed. This was clearly an unsatisfactory situation when problems arose on the test bed, or even worse when the machine went into operation, and the problem had to be resolved in an ad hoc way. It was decided that a more detailed understanding was necessary to aid in the solution of problems and in the development of purchasing specifications that would result in problem-free machines. This came to a head with a steam turbine driver on a process gas stream with a design operating speed of 177 rev/s. The difficulty was compounded in this machine in that the rotor was supported on three journal bearings so that the bearing loads were indeterminate; when instability problems arose, there was no obvious solution and in order to maintain production the speed had to be The reduced with a consequent loss in output. University of Leeds was asked to develop a complete rotor dynamic programme, including the dynamic characteristics of the bearing oil films. This involved a theoretical study that could not be tested economically in the laboratory because of the scale. The advantage of the university-industry co-operation was that the theoretical predictions could be validated with measurements taken from operating plant machines. The final programme was of value not only in guiding remedial actions when vibration problems .occurred, but also in allowing a meaningful dialogue between the Company engineers and machine suppliers when new machines were being purchased. 3 CONCLUSIONS The outcome of failure investigations of the
43
blending profile
extrusion
bearings
Fig. 1.
Schematic arrangement of extruder.
100-1OOo
-
10-100
-
\
Approximate sulphur content
O
of gas (PPd 1-10
<1
- \ -
Zone 1
0
\
0
\ 0
0
\ 0
\
x x xx n x I
+
0
Zone 3
\
YX \
1
Peripheral speed of seal ring (m/sec) Fig. 2. Attack of white metal lining of floating-bush seal ring by sulphur in process gas. x o
+
no problem attack of tin-rich white metal, but no attack of leadrich white metal attack of lead-rich white metal
Zone 1. Zone 2. Zone 3.
No problem Use copper-free white metal (lead-rich) Redesign seal to reduce temperature
I
44 t y p e s d e s c r i b e d c a n r a n g e from s o l u t i o n s b a s e d on a n u n d e r s t a n d i n g of t h e mechanisms i n v o l v e d (Examples 2.1 and 2.2), t h r o u g h a c r u d e g r a p h i c a l p r e s e n t a t i o n of t h e bounds o f t h e problem area u s i n g p a r a m e t e r s b a s e d on a t h e o r e t i c a l a p p r e c i a t i o n of t h e s i g n i f i c a n t v a r i a b l e s i n v o l v e d (Example 2.3) t o a r e s e a r c h programme aimed a t a complete u n d e r s t a n d i n g and d e f i n i t i o n o f t h e problem (Example 2.4). The f i n a l l i n k i n t h e c h a i n is communication. T h i s can b e e f f e c t e d i n a number of ways. For a one-off problem it may end w i t h r e c t i f i c a t i o n i n c o - o p e r a t i o n w i t h t h e manufacturer. Where t h e f i n d i n g s are of wider a p p l i c a t i o n t h e y c a n be i n c o r p o r a t e d i n a Company Purchase S p e c i f i c a t i o n t o p r e v e n t f u t u r e r e p e t i t i o n of t h e same problem, b u t , i n a d d i t i o n , p u b l i c a t i o n of a p a p e r i n a j o u r n a l w i t h a r e f e r e e i n g p r o c e d u r e is o f v a l u e i n making t h e findings available. I n t h i s c o n t e x t encouragement s h o u l d b e g i v e n t o t h e a c c e p t a n c e o f case h i s t o r y s t u d i e s , even i f t h e s e l a c k some o f t h e t h e o r e t i c a l background t h a t t e n d s t o b e a r e q u i r e m e n t of many l e a r n e d societies b e f o r e Not o n l y does papers can be considered. p u b l i c a t i o n a l l o w t h e d a t a t o be u s e d i n d e s i g n g u i d e s , s u c h as t h e ESDU Data Items (ESDU I n t e r n a t i o n a l L t d . , London), b u t t h e f a c t t h a t p u b l i c a t i o n h a s been a c c e p t e d l e n d s weight t o a one-to-one d i s c u s s i o n w i t h a s u p p l i e r and a l l o w s understandingof t h e r a t i o n a l e behind t h e i n c o r p o r a t i o n of a d d i t i o n a l r e q u i r e m e n t s i n purchase s p e c i f i c a t i o n s .
4
THE FUTURE
These examples have been chosen from a v e r y wide r a n g e o f p a s t problems t h a t have been satisfacto r i l y s o l v e d and are i n t e n d e d t o i l l u s t r a t e t h e approach t a k e n by one u s e r . Inevitably t h i s h a s been a r e a c t i v e one and i n a forward l o o k i n g meeting o f t h i s t y p e it is p r o b a b l y o f more i n t e r e s t t o i d e n t i f y p o s s i b l e f u t u r e problem areas. T h i s n e c e s s a r i l y means a c e r t a i n d e g r e e o f s p e c u l a t i o n , o t h e r w i s e t h e problems would have a l r e a d y been i d e n t i f i e d and h o p e f u l l y Areas t h a t I f e e l are l i k e l y t o b e o f solved. c o n c e r n i n t h e t r i b o l o g y o f machines are s e a l i n g , a r a t h e r b e t t e r understanding o f t h e influence o f f o u n d a t i o n dynamics on t h e i n s t a b i l i t y of machines, t h e need t o i d e n t i f y t h e e f f e c t o f s o l i d p a r t i c l e s on t h e performance o f l u b r i c a t e d components i n o r d e r t h a t more r e a l i s t i c filtra t i o n r e q u i r e m e n t s can be s p e c i f i e d . The p r e s e n t r a t h e r a d hoc t r e a t m e n t of t h e s e s u b j e c t s r e f l e c t s a l a c k of s a t i s f a c t o r y u n d e r s t a n d i n g and t h e p o t e n t i a l f o r f u t u r e problems.