Analysis of surface roughness and profile parameters of graphite packing deposition on valve stems and how this influences frictional performance

Transcription

1 Analysis of surface roughness and profile parameters of graphite packing deposition on valve stems and how this influences frictional performance A technical paper presented by James Walker & Co Ltd First presented at Valve World 2012

2 Mark Richardson Product manager compression packing Mark Richardson currently holds the position of compression packing product manager for James Walker & Co. based in the North of England. He is responsible for global marketing, product and business development for the James Walker range of compression packing products. He joined James Walker in 1997 and has held technical roles within the sealing industry for over 15 years. This included 6 years as a senior applications engineer, specialising in valve sealing and fugitive emission applications. Mark graduated from Birmingham University with a BSc in Pure Physics.

3 3 Analysis of surface roughness and profile parameters of graphite packing deposition on valve stems and how this influences frictional performance. Abstract In previous papers we have discussed the effect that stem surface finish has on friction and fugitive emission performance of graphite based control valve packing. Specific stem surface finishes that optimise performance of these parameters have been identified and reported; however the reasons behind this varied performance, specifically friction, and the effect of the resultant graphite boundary layer deposit remains unclear. A step change in frictional performance has previously been observed where the deposition of the graphite takes place at 150 o C. This stem surface finish and the affect the graphite boundary layer deposit has on the original, untreated metallic stem surface requires further investigation. This paper will discuss the equipment, test methods and results when the surface roughness and profile parameters of graphite boundary layer deposits, generated are analysed using non contact multi focus imaging microscope. Trends in this data will be compared with frictional data from valve stems with surface finishes ranging from polished chrome to a rough ground 2.6µm Ra. Conclusions and discussion points will be made relating to the modification of the metal surface and the use of lubrication additives. The effect of elevated temperatures will be investigated along with the influence on attaining common fugitive emissions specifications such as ISO15848, API 622, SHELL SPE and TA Luft VDDI Introduction A well designed and efficiently performing valve can have a significant impact on process profitability, via reduced process costs, ecological factors and safety via fugitive emissions. We have previously reported that whist control valves are perhaps only 5% of a refinery valve population; they are statistically twice as likely to be found leaking in an emission survey, as of course they are continually in operation and thus pose a greater problem than block valves. A substantial component of the friction produced within a linear reciprocating or rotary control valve is generated by the valve stem sealing arrangement. Stem / Seal friction values are influenced by two main factors: materials, and specification driven FE targets. High gland loads used in an attempt to achieve low levels of leakage such as ISO15848, can produce excessive friction. With regard to materials: Graphite seals can generate excessive stick-slip friction producing highly unreliable valve performance. PTFE Seals, In a previous Valve World paper presented by James Walker & Co. Ltd, ref P4052, entitled Control Valves The next Fugitive Emissions Challenge, it was shown that thermal cycling can severely affect the emissions performance of PTFE based packing. This is due to the high coefficient of thermal expansion and contraction of those seals. 3

4 4 Reducing the generated friction and judder caused by a graphite based packing, would allow the desirable attributes of performance under thermal cycling and constant operation to be combined. One way to achieve the required performance is to use high quality braided graphite based packing, with an additional lubrication package and a suitable packing load. The use of graphite as a sealing element relies on the transference of graphite to the stem as a running surface. Little is known about how the graphite deposit effects parameters used to measure stem surface finish or how these subsequently influence frictional and fugitive emissions performance Methodology Stems with surface roughness from Chromed to 2.6µmRa were tested. The graphite coating on the test stems was generated through dynamic testing on the James Walker rising stem fugitive emission test rig fitted with James Walker graphite braided fugitive emissions packing Supagraf Control. The coated stems were then analysed via the Alicona Infinite focus Microscope. This delivered the required surface roughness and profile measurements. The two test apparatus are detailed below: 2.1. Test Apparatus James Walker Rising Stem Fugitive Emissions Test Rig Figure 1 The test rig consists of a simulated valve gland and housing enclosed within a thermal oil chamber. The stem is driven from above by an air cylinder. This produces a rising stem with interchangeable lower sections to allow testing of difference stem diameters and materials. An externally operated oil heater can generate temperatures within the stuffing box from ambient to 200 C. A load cell situated in the stem arrangement generates continuous frictional load data. Load cell data as well as pressure and temperature are all continuously relayed to a PC. Figure 2 The stem and housing sizes used throughout this test programme were ¾ x 1¼ x ¼ (19.05mm x 21.75mm). A 5 ring packing set was fitted in the stuffing box. Load was applied via 2 x M12 socket head cap screws. The rig operates at a full stem stroke of 50mm thus the stem fully passes though the packing set. 4

5 Alicona Standard Infinite Focus Microscope The Alicona Standard Infinite Focus Microscope is a non contact 3D measurement device for measuring surface roughness parameters and surface forms simultaneously. Multiple image fields are stitched together to form a composite image (Figure 4) Scans are taken in the z axis but only images in focus from each image plane are used. This allows an area of a curved surface such as the test stems to be measured. As the measurement is non contact and a coordinate measurement system is used all results are traceable and ensure repeatable accuracy. Figure 3 Image courtesy of Alicona Imaging GmbH 2.2. Testing Methodology and Regime Each test stem was subjected to 1000 stem cycles on the James Walker rising stem fugitive emissions test rig. The packing was fitted in accordance with the James Walker fitting procedure, and a packing load of 70MPa was applied. No leakage measurements were taken throughout the test as these results for each stem finish have already been published in a paper entitled, How stem finish affects friction and fugitive emissions performance with graphite based control valve packing. (Valve World 2010) This paper showed that during dynamic tests carried out at ambient and 150 o C temperatures, in combination with specific stem finishes, produces a step change in frictional performance. These stems were then analysed by Sheffield Hallam University (UK) using the Alicona Standard Infinite Focus Microscope. The completed image of the surface is then mathematically interrogated to produce the required surface finish and profile parameters. Figure 4 shows a typical trace image generated from the Alicona Standard Infinite Focus Microscope. The parameters measured and recorded during this investigation are shown below in Figure 5. Figure 4 Figure 5 5

6 6 For the purpose of this investigation Ra, Rz, Rmax and Rsk surface parameters were considered. These were chosen as they were believed that the attributes associated with these parameters are the most probable causes of a change in friction. 3 separate scans of each section were taken and the results averaged. To aid understanding the 4 parameters are defined in Figure 6. Figure 6 Parameter Definition Typical Use Ra Rzi Rz Rmax Rsk Roughness Average: Arithmetic average of the absolute values of the roughness profile coordinates Single Roughness Depth: is the vertical distance between the highest peak and lowest valley within a sampling length. Mean Roughness Depth: is the arithmetic mean value of the single roughness depths Rzi of consecutive sampling depths. Maximum Roughness Depth: is the largest depth within the evaluation length. Taken from DIN EN ISO 4287: ASTM B46.1 Skewness: is the measure of the asymmetry of the amplitude density curve. Typically used to describe the roughness of machined surfaces. It is useful for detecting general variations in overall profile height characteristics and for monitoring an established manufacturing process. Used in the evaluation of surface texture on limitedaccess surfaces such as small valve seats and the floors and walls of grooves, particularly where the presence of high peaks or deep valleys will impact on functionality. Describes load carrying capacity, porosity, and characteristics of nonconventional machining processes. Negative skew is a indication of a good bearing surface. 3 specific areas of each graphite deposit was analysed and compared. DIRECTION OF STEM MOVEMENT 2 1 Figure 7 3 6

7 7 Position Description Figure 8 1 Uncoated No graphite deposit 2 3 Transition Stationary/Dynamic area In this area the stem deposit is formed in the transition from the stem moving in one direction to the opposite. As there is dwell between these two movements there will also be a stationary period. Dynamic In this area the graphite deposit is formed entirely via dynamic movement. The result of this work was surface roughness and profile data corresponding to graphite deposits created through reciprocating contact on varying stem surface finishes, at two temperatures, ambient and 150 o C Results 3.1. Results Table Figure Ra Figure 10 Figure 11 CR

8 Rz Figure 12 Figure 13 CR Rmax Figure 14 Figure 15 CR

9 Rsk Figure 16 Figure 17 CR Results Analysis Figure 18 is an extract from the paper entitled, How stem finish affects friction and fugitive emissions performance with graphite based control valve packing. The step change in frictional performance in the second cluster of results takes place after thermal cycle from ambient temperature to 150 o C. Figures will be analysed, as a change in these parameters may indicate a change in friction. FRICTIONAL STEP CHANGE Figure Ra Values Both at ambient and elevated temperatures, the effects on Ra values are similar for any given stem surface finish. There is very little variation in the uncoated results, with the exception of the 2.6µmRa stem. 9

10 10 In all instances the transition position shows a reduction which then recovers in the dynamic position. This would suggest that a thicker layer of graphite is deposited at the ends when the stem is accelerating/decelerating or static as opposed to the dynamic area, where the movement of the stem would appear to wipe off and reduce the graphite deposit. This is best demonstrated in images 19 and µmRa 150 o C Dynamic Image Figure µmRa 150 o C Transition Image Figure 20 Although very difficult to measure it can be assumed that more graphite is deposited on the surface as Figure 20 is darker than Figure 19. This increased graphite thickness would smooth the surface topography and reducing the Ra value. The reduction in friction shown in figure 18 could result in a significant change in the Ra values. Comparisons of results at ambient temperatures to 150oC do not show this change. The stems used were specified to be surface finishes of Chromed, 0.2, 0.4, 0.6, 0.8 and 2.6µmRa. Comparison of these nominal figures against the values generated from the uncoated areas show major differences. Nominal Ra Value (µmra) Measured uncoated Ra Value (µmra) Ambient 150 o C Chromed (smooth) Figure 21 Experimental variations aside, these figures do show that valve stems supplied may not always match the initial specification Rz Values Results at ambient temperature display a logical trend of reduction as the nominal stem surface finish also reduces. The addition of temperature produces more variable results, with some significantly higher than corresponding values at ambient temperature. It was predicted that these figures would be lower, than those at ambient temperature, as the friction is lower. This results are the opposite of this in many cases Rmax Values The results for Rmax mirror the logical trend of Rz at ambient temperature; however the results produced are far more variable at 150 o C. As with the Ra and Rz results there is no evidence of a reduction in the Rmax value at ambient or elevated temperatures, which can be linked to reduced friction. 10

11 Rsk There was very little impact of the graphite deposition on the Rsk values other than an increase on the Chromed finished stem. The Rsk value is inversely proportional to the effectiveness of the valve stem as a bearing surface. The increased result with the Chromed finish actually means that this is a worse bearing surface, and also may indicate increased friction Discussions 5.1. Graphite Clusters From the above results, it is apparent that although there are some changes of the surface roughness and profile parameters of valve stems due to graphite deposition; there is no clear correlation between these and the previously observed reduction in friction, at elevated temperatures. If surface roughness parameters are not indicating a change in friction, what other influencing factors could cause the reduced. With further analysis of the highly magnified images of the graphite deposits, there do appear to be some macro changes taking place. Figures show the graphite deposition at 150 o C and Ambient temperatures, on two stems of differing nominal surface finishes. 2.6µmRa Cold Dynamic position Figure µmRa 150 o C Dynamic position Figure 23 Figures 22 and 23 show that the graphite deposition on the surface is inconsistent and patchy. 0.4µmRa Cold Dynamic Graphite Cluster Figure µmRa 150 o C Dynamic Figure 25 11

12 12 Although there is some inconsistent graphite deposition in the ambient temperatures Figure 25 shows that at 150 o C the deposition is reduced and consistent. When this analysis is carried on the remaining stems, the critical surface finish appears to be 0.8 µmra. Below this surface finish the graphite deposit is changed, from inconsistent with a number of graphite clusters, to regular and a reduced number of clusters. It is the writer s belief that the presence of the inconsistent deposition of graphite will cause subsequent inconsistent friction and consequently stick-slip movement of the stem. The reason for this improved consistency of the graphite deposition at elevated temperatures remains unclear. However given that graphite will be unaffected structurally by the change in temperature from ambient to 150 o C, it can be presumed that the effect is due to a change in the lubrication additives, such as greases and/or PTFE. This change is most likely to be a reduction in the viscosity. It is commonly accepted that pure graphite when used in a dynamic application may produce balling, or the locally increased deposit of graphite. With the valve packing used in these tests, the graphite deposit is probably a homogeneous paste consisting of graphite and elements of the lubrication package. As the temperature increases the viscosity of these elements drops reducing the propensity of the graphite to ball or cluster. Through dynamic stem movement, the deposits with reduced viscosity would be now smeared and dissipated, into a more uniform layer. The suggestion above does not explain the relationship with graphite clustering and stem surface finish. It has been shown that this reduction in graphite clusters only takes place below 0.8µmRa. One possible reason is simply the larger peaks and troughs on the topography of the valve stems with rougher surface finishes have increased friction with the graphite deposit, and limit this smearing action. Further analysis is required on the chemical composition of these graphite clusters including the % ratios of graphite to lubricants, as this may also help to explain this effect Residual Manufacturing Marks The stems used for this paper were manufactured to a nominal stem surface finish through a traverse grinding process. On the image below you can see the regular markings that are indicative of this type of process. Traverse grinding marks Figure 26 Turning marks Turning marks Contrast and brightness have been changed to accentuate manufacturing marks. There are also some additional marks that cannot be attributed to the movement of the packing against the stem. It is likely that the initial manufacturing process of the stems included turning on a lathe to produce the approximate diameter required. These marks appear at regular intervals consistent with this type of machining. The change in spacing and angle of these marks may be due to changes in direction and speed of the lathe. Although they do not appear to have influenced the results of the surface roughness and profile measurements taken it is also apparent that spiral tracking of leakage via these residual turning marks may still be possible. 12

13 Conclusion In the analysis of surface roughness and profile parameters of graphite deposits, it was found that there were no trends or correlations in the data that would that could be identified with the reduction in friction at elevated temperatures, as observed in previous tests. Generation of these deposits at 150 o C did show some reduction, however the overriding consequence was that the surface roughness values became more erratic and variable. The measurements of roughness parameters carried out on evaluation lengths of the magnitude of 3-4mm. To understand the causes in the reduction in friction, this paper also considered larger scale surface deposit changes. Evidence was found that a combination of stem surface finish and temperature can cause the formation of graphite clusters, and inconsistent graphite deposit. Inconsistent graphite deposit will result in stick-slip dynamic movement and higher friction. In stems with a nominal surface roughness >0.8 µmra the graphite deposits generated at 150oC displayed a more regular and even graphite deposit. The graphite deposit in these instances showed a significantly reduced number of graphite clusters, the consequence of this is significantly reduced friction. This suggested step change in performance matches the step change relationship between stem surface finish / temperature and friction observed in previous tests. The above work suggests that selecting a packing that meets fugitive emissions standards such as API622, TA Luft VDI 2440 and ISO 15848, is now not enough. By selection of the correct packing, stem surface finish and including a thermal cycle within the installation regime, it is possible to combine exceptional fugitive emissions and frictional performance. This result could help the production of more efficient and lower cost valve/actuator systems. 13

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