LaserFace Technology for High Performance Mechanical Seal Applications

Transcription

1 LaserFace Technology for High Performance Mechanical Seal Applications Abstract High levels of reliability and robustness, virtually maintenance free operation at low running costs, reduced complexity of ancillary equipment and low emissions are target specification for the ideal sealing solution or mechanical seal respectively. Conventional mechanical sealing technology however is limited in its ability to meet all of the demands simultaneously. The lubrication regime at the sealing interface is one of the key factors determining the seal performance. To achieve virtually maintenance free operation, sliding contact between the seal surfaces needs to be minimised. The lubrication at the sealing interface therefore needs to be self-generated, sustained for all operational load cycles and controlled to prevent unacceptable high seal leakage. Many of the present lubrication-enhanced seals, however, produce high levels of leakage because the fluid film at the sealing interface is not controlled. Other seals with low levels of leakage often operate in a lubrication regime that limits pressure velocity (pv) performance and operational seal life. Modern high performance seals at high-pressure velocity (pv) operation need to incorporate technologies that generate full fluid film lubrication but also address the issue of seal emission and leakage. LaserFace technology is a new sealing interface technology for mechanical seals sealing liquid like fluids, wet seals. LaserFace is designed to generate a full fluid film at the sealing interface but also includes a mechanism that prevents excessive leakage as a result of the lubrication enhancement. This performance characteristic of LaserFace seals applies to many processes and has already been successfully implemented. Modern high performance seals equipped with LaserFace technology consequently operate at higher performance levels, lower operating costs and increased Mean Time Between Repair (MTBR) intervals. This paper firstly reviews the fundamentals of mechanical seal operation, the drivers of performance reliability in general, before introducing and explaining the principles of LaserFace technology in detail. Typical LaserFace application are discussed. Finally, the incremental benefits of LaserFace are demonstrated by analysing the results of a comparison test between a LaserFace and a conventional seal on a typical lighthydrocarbon duty.

2 1. Introduction Mechanical seals have been extensively developed and have become the most dominant seal technology for rotating equipment in the petrochemical and chemical industries. An ideal mechanical seal would operate reliably, for long periods, without wear, with permanently low friction and extremely low leakage. Reality, however, often differs from the ideal. Real mechanical seals do wear and have a limited life. They can leak appreciably, particularly when the operating conditions pressure, temperature and speed vary with time. To date, most technology solutions employed are passive methods to control the sealing gap with the emphasis on boundary lubrication or hydrostatic fluid film formation. There have also been attempts to introduce hydrodynamic seal face features on a larger scale (hydro-pads, face waviness, hydrodynamic micro-groves or thermo-hydrodynamic circulation grooves) to overcome excessive face friction and wear and with it achieving improvements in seal performance and reliability particularly for high pressure, high velocity application. However, increased fluid film thickness inevitably increases seal leakage/ emissions to unacceptably high levels. The solution to the dilemma, achieving either good face lubrication with high leakage or low emission levels but with limited performance, is a seal face technology that combines hydrodynamic interface lubrication and active leakage control. LaserFace technology is such a technology that promises to deliver not only another additional step towards seal reliability but a significant leap towards the ideal mechanical seal using fluid film control with low friction performance at low levels of seal leakage (emissions). 2. Mechanical Seal Reliability Driver, Sealing Interface Lubrication Although complex analytical methods are required and used in the performance, reliability enhancement of modern high performance seals and the development of new technologies, the fundamental principle of how a mechanical seal operates and the factors that affect reliability, can be described and demonstrated by looking at the simple axial force equilibrium. It is assumed that the arithmetical sum of all axial external loads (hydraulic load from the sealed fluid, spring load and secondary seal friction) is supported by reaction forces at the sealing interface. In this simplistic approach to the problem it applies that the closing force is fully supported by the fluid film but in reality for most mechanical seals the fluid film (hydrostatic pressure and hydrodynamic effect) is not strong enough to support the full closing force and the excess is supported by asperity contact between the sealing rings (figure 1). This lubrication regime described above is called mixed lubrication or if the asperity contact support is predominant boundary lubrication. The sealing gap (h) itself is small, in the order of the surface asperities, and offers very high resistance of radial leakage flow. Unfortunately, the asperity contact that minimises the seal leakage is also the main source of frictional heat and the reason for wear. Frictional heat can cause overheating and in some cases vaporisation of the fluid film, loss of seal face lubrication and severe dry running leading to premature seal failure. A much more complex failure scenario is also caused by asperity where the seal operates in a transient and latent instable mode thermal cycling. Here both effects are observed, sporadic high leakage and accelerated seal wear [1]. 2

3 Figure 1: Mechanical seal axial force equilibrium (closing forces = supporting forces) A common measure to minimise the negative effect of asperities contact is cooling and flushing of the seal (e.g. API 682 recommends for flashing light hydrocarbon seals a minimum temperature margin of 20 C to the vapour temperature). Cooling and flushing are secondary measures, curing the illness but not the disease. Müller and Wallace [2] concluded that running a mechanical seal at high speed in the boundary lubrication regime almost invariably leads to unreliable operation. The solution is to operate without contact and wear by compensating the axial closing force totally by the interface fluid film or pressure respectively. Non-contacting performance can be achieved either by reducing the closing force or increasing the opening force. The amount of reduction possible is, however, limited. Lowering the balance ratio always means that the seal becomes more susceptible to the effects of hang-up and changes in face flatness. Furthermore, the impact of balance ratio on the force equilibrium is pressure dependent. Low pressure application do not benefit in the same way from lowering the balance factor as high pressure ones. The equivalent balance B Equivalent (1) is a good measure to quantify this fact. The equation for B Equivalent shows that for lower differential pressures p the impact of spring face stress (P Spring) and secondary friction (secondary seal, drive mechanism etc.) can be substantial. B Equivalent FFriction PSpring SealfaceAr ea B (1) P Another method to reduce asperities contact is often evident in hydrostatic seals. Here the seal closing force is only slightly reduced but the hydrostatic opening force is increased by the use of a convergent face gap. The face taper is incorporated into the seal rings through precision lapping. The benefit is that the seal itself uses a relatively high balance ratio with a stabilising effect on the sealing gap. However, hydrostatic seals can be very 3

4 sensitive to changes in the face geometry and finish and are therefore limited to a relatively narrow operating band outside which the seal may not perform optimally. In addition, a strongly hydrostatic seal can exhibit static leakage. The most promising method of eliminating asperities contact is by increasing the hydrodynamic fluid pressure. This effectively means that full interface lubrication is established when the seal is in dynamic operation but under static mode the seal faces remain closed with minimal or zero leakage. In contrast to the hydrostatic enhancement, hydrodynamic seals can develop a high degree of face gap stability (film stiffness). Film stiffness means that the seal face develops an increased opening force to counter a forced reduction in film thickness while the hydrodynamic effect is reduced when the film thickness is increased. Hitherto, various methods, including thermo-hydrodynamic circulation grooves [3], hydro pads, various shapes of micro-groove and deliberate introduced tangential waviness [4] have been used to enhance the interface lubrication by the mean of interface hydrodynamics. However, all of these measures inevitably, and sometimes unpredictably, increase the mean thickness of the sealing gap and, hence, increase the leakage of the seal. This increase in leakage with an increase in fluid film thickness can be substantial as the Reynolds equation (2) suggests. The leakage q is a cubic function of the face gap height h. q 3 h p 12 r (2) In reality, designs using hydrodynamic lubrication features as mentioned above very often need to use an increased balance ratio to reduce the inherent excessive face leakage. This effectively and, in contradiction to what has been aimed to achieve in the first place, reduces the film thickness and increases the danger of asperity contact. It becomes clear that a successful approach to increasing seal performance and reliability requires full fluid interface lubrication but also requires active leakage or fluid film control. This realisation lead to the idea of creating, in combination with a lubrication enhancement mechanism, an interface pumping mechanism which returns fluid from the sealing gap to the pressurised fluid space. Together with modern high precision micro machining techniques it progressed into the development and implementation of LaserFace technology. 3. LaserFace Technology Principle of operation LaserFace is a unique seal interface technology that combines full fluid film lubrication with effective seal leakage control. In its current stage of development LaserFace supports application where liquid like processes fluids are sealed. Gas application and application with abrasive process fluids i.e. slurries are not yet supported. The LaserFace groove is applicable within the customary geometrical interface dimensions of proven conventional seals. 4

5 Inlet Groove Return Flow Return Groove Inflow Drag Flow Figure 2 and 3: LaserFace groove pattern with suggested flow vectors (left), mechanical seal with LaserFace mating ring (right) The LaserFace face pattern combines a pair of symmetrical precision-machined microgrooves (typically a few micrometres deep) each of which performs a different task (figure 2 and 3). The square inlet-groove is connected to the pressurised fluid area and through it, the process fluid can penetrate deeply into the sealing interface. The fluid within it is dragged in tangential direction by the sliding counter face. When the fluid reaches the edge of the inlet-groove it generates a strong hydrodynamic pressure that lifts the seal faces, thus stabilising the sealing gap. Full fluid film lubrication can be achieved at quite low shaft speeds as shown in figure 3. This hydrodynamic generated fluid film guaranties low friction and low wear performance. Even at high velocities the LaserFace lubrication enhanced seal operates with less interface friction due to an increased face gap. Figure 3: Frictional Torque vs. shaft speed for plain-face and LaserFace mechanical seal (Comparison Test results; Water Service [5]) LaserFace Seal Torque Plain Face Seal Torque LaserFace Seal (Equivalent Coefficient of Friction) Plain Face Seal (Equivalent Coefficient of Friction) [Nm] Shaft Revolution [1/min] 5

6 At this point, however, with increased gap the radial leakage flow resistance of the sealing gap is vastly reduced and leakage levels would usually increase substantially as the Reynolds equation suggests. With the LaserFace seal however, the semi-circular grooves, also known as return-grooves, control the amount of leakage passing through the sealing gap. The return grooves are located within the sealing interface and are not connected to the pressurised fluid area, thus the groove collects the excessive fluid film and guides it along the trailing edges to the discharge end of the groove. The discharge end is very close to the high pressure edge of the sealing gap where the fluid pressure inside the groove is at a maximum and much higher than the sealed pressurised fluid (figure 4). Following the path of least resistance, a great deal of the fluid passes from the recess towards the pressurised fluid outside the sealing gap. In addition, cavitation or vaporisation of the fluid film in the expansion zones of the return-grooves block radial leakage flow from the outer seal interface diameter. The efficiency of the return pumping mechanism results from the combination of shape, position and depth of the return groove [2]. Example: Water, P = 1.7 MPa Inlet Groove with Hydrodynamic Pressure Region Return Groove with Discharge Pressure Region Figure 4: Typical sealing interface pressure distribution of a LaserFace seal (FEA/ CFD prediction with CSTEDY) With LaserFace the combination of hydrodynamic grooves and fluid film re-circulation grooves (returngrooves) gives the benefits of a seal operating with hydrodynamic lubrication and boundary lubrication, in other words, low friction and wear but together with low static and dynamic leakages. Figure 5 underlines this statement. In figure 5 the two important performance measures for a mechanical seal, leakage and face friction, are compared for a standard plain face seal, the same seal with a conventional hydrodynamic face groove (inlet groove) and a seal equipped with the LaserFace return groove. The bar chart on the left illustrates the comparison using good lubricating fluids and the one on the right for a more difficult duty with a flashing, poorly lubricating liquid. The performance figures are presented in relative values where 100% is always the highest recorded value from the three seal configurations. 6

7 Figure 5: Mechanical seal performance comparison with conventional seal face technology and LaserFace The standard seal operates for both process liquids with very low leakage but generates an appreciable amount of face heat due to asperity contact. When the seal is equipped with a simple hydrodynamic feature the operational gap is increased and asperity contact is prevented. The amount of face friction is a direct result of viscous shear from the sealing interface fluid film. This explains the much larger reduction in friction for the propane duty. However, the results also demonstrate that hydrodynamic face lubrication causes a significant increase in seal leakage. The LaserFace seal on the other hand achieves full hydrodynamic interface lubrication at low leakage levels. 4. Typical LaserFace Application Applications that benefit most from LaserFace technology include cases where the sealed fluid gives very poor interface lubrication, which results in high asperity contact and seal wear. Such difficult services are, for example flashing light hydrocarbon services and hot water. The 48XP seal (figure 6) is a new light hydrocarbon seal where the key feature LaserFace allows the seal to operate reliably as a single or un-pressurised tandem seal with such low temperature margins that could only have been sealed in the past using double seals. For example, API 682 recommends a minimum temperature margin of 20 degree Kelvin or 30% pressure margin between the sealed pressure and the vapour pressure of the sealed fluid at its prevailing temperature. This means effectively that most flushing light hydrocarbon seals need to be either substantially cooled or replaced by pressurised dual seals in order to achieve good MTBR s (Mean Time Between Repair). The T48XP seals do not require this, however, but allow operation to within 3 degrees Kelvin of the Saturation Vapour Pressure (SVP) temperature. It has been found that, even when sufficient API recommended temperature margins are present, the LaserFace equipped T48XP seal exceeds the life of its conventional counterparts by a significant margin as the hydrodynamic face lubrication guaranties lower wear performance. Section 5 will illustrate the benefits in a performance comparison between a 48XP seal with conventional plain-face mating ring and the LaserFace mating ring. 7

8 Figure 6 (left): T48XP LaserFace seal for flashing light hydrocarbon application (seal shown together with noncontacting containment seal) Figure 7 (right): The T8600 high performance seal for critical high pressure offshore and pipeline application. Besides the use of LaserFace in single and un-pressurised tandem seals for poor lubricating liquids, LaserFace is also used in high performance pressurised dual seals (T8600, figure 7) where, in contrast to the T48XP type services, the seal interface is lubricated by the pressurised barrier liquid (frequently oil). Here the seal benefits from LaserFace by an increased film stiffness allowing higher face closing forces to be used for greater seal stability (reduced hang-up, increased axial damping effects, etc.) [6]. Figure 8 shows the excess load capacity at the sealing interface for a 2 seal at 10 bar barrier oil pressure. Positive load capacity means that the seal face is fully supported by the fluid film (non-contacting performance) with an excess of supporting force (calculated near surface asperities). Negative load capacity means that the closing force is partially supported by asperity contact and the seal will operate in the mixed lubrication regime. One can see that the plain face seal operates at low pressure (minimal hydrostatics) and speeds in contacting mode. Figure 8: Fluid Film Load Support LaserFace vs. Conventional Seal 3500 Excess Load Capacity [N] Plain Mating Ring 3 Light Bands Convex Plain Mating Ring Flat Lapped LaserFace Mating Ring Flat Lapped Shaft Revolution [1/min]

9 Only at high speed the hydrodynamic effect provides enough load support. However, even at high speed the load excess preventing asperity contact is relatively small. The LaserFace seal on the other hand develops already at low shaft speeds an incrementally stronger fluid film and excess load capacity. Test and field trial results confirmed that the load capacity of the LaserFace seals could even overcome temporary dynamic reverse pressure situations in case of a loss of barrier pressure or in the event of high process pressure spikes. In those cases LaserFace helped to improve seal and application reliability. 5. Case Study The benefit of LaserFace technology is best demonstrated in a comparison test of two identical seals, except that one is equipped with a LaserFace mating ring and the other with the conventional plain face mating ring. The test duty is liquid propane at 21 bar g, at a pumping temperature of 56 C and an API flush plan 11 set at 10 l/min flush flow. The flush temperature margin is only 6 degrees Kelvin to the SVP. Knowing that API 682 would recommend a reduction in flush temperature of 14 degree Kelvin to 42 C, this test duty was deliberately chosen as a more praxis-near example of a refinery duty where temperature margins at suction pressure are minimal and the option of cooling or increasing the box pressure is limited. Figure 9: Performance comparison plot of LaserFace seal vs. conventional seal 9

10 Figure 9 shows the recorded pressure but, more interestingly, the temperatures recorded throughout the 200- hour test duration. The performance plot also indicates the vapour temperature of 62 C. The top graph shows the performance of the seal with LaserFace and the bottom graph shows the performance without. Sporadic leakage recordings throughout the test confirmed emission levels below 200 ppm for both seals. The test readings (figure 9) clearly illustrate the benefit of LaserFace. Although both seals show only a very small temperature rise between flush inlet T1 and outlet temperature T2, which would suggest good face lubrication and low friction performance, the actual performance of the seals is indeed significantly different. Measuring the temperature rise of the mating ring back-face with temperature probe T3 reveals a completely different picture of the true seal behaviour. The conventional seal shows very high and erratic temperatures. The mean face temperature was above the vapour temperature throughout the test duration suggesting that the seal was subjected to dry running (operating in the boundary lubrication regime). In addition, the occasional very high temperature spikes indicate that the excessive component temperatures caused instant and complete flashing of the liquid around the seal, covering the components in produced vapour. At this point the heat transfer was dramatically reduced causing overheating of the mating ring until such time that the flush liquid could displace the vapour blanket that had been produced. The LaserFace equipped seal however shows a completely different picture. The mating ring temperature T3 is constantly very low, almost identical to the flush inlet temperature. This suggests very low friction performance with no or negligible small asperity contact. As a matter of fact, the heat generated by the LaserFace seal was so small that the cooling effect from the air around the ID of the mating ring reduced the back-face temperature by 0.5 C to the flush inlet temperature T1. The post-test component inspection confirmed what the temperature readings already indicated. The conventional seal shows a high degree of component wear and blistering or chipping at the carbon interface inner edge (figure 10) due to very high asperity contact. The LaserFace seal components are almost as new with no measurable wear (figure 11). Only minor scratches on the mating ring indicate that the components have been used on test. Figure 10, 11: Post-test component condition of conventional seal (left) and LaserFace (right) 10

11 6. Conclusion LaserFace is the first commercially available seal interface technology that provides low friction and wear performance at low levels of leakage through fluid film control. The benefits of LaserFace have been demonstrated in direct comparison tests between conventional and LaserFace seals. The benefits of LaserFace have also been proven in field application for light hydrocarbons, hot water and in high duty offshore applications. LaserFace introduces a significant enhancement in seal performance, operational stability and seal life. LaserFace seals will therefore positively impact not only on the direct running costs of the seal (e.g. reduced cooling and energy costs) but more significantly on the application availability and reliability by reduced outages and increased Mean Time Between Repair intervals. Because LaserFace is geometrically compatible with proven conventional mechanical seals, it offers efficient upgrade options for seals currently in operation. Further refinement of the technology may extend the use of LaserFace from liquid based process fluids into full vapour operation in the future. 7. Literature [1] Parmar, A. (1992): Thermal Cycling In Mechanical Seals Causes, Prediction, Prevention. Fluid Sealing, BHR Group Ltd. 13 th International Sealing Conference on Fluid Sealing, Belgium, pp Dordrecht: Kluwer Academic Publishers [2] Müller, H.K., Wallace, N.M. (1994): Development of Low Friction, Low Leakage Mechanical Seals using Laser Technology. Texas A&M, 11 th Pump User Symposium, USA. [3] Mayer, E. (1982): Mechanical Seals. London, England: Butterworths & Co. [4] Lebeck A.O. (1991): Principles and Design of Mechanical Face Seals. New York, USA: John Wiley. [5] Müller, H.K., Schefzik, C., Wallace, N.M., Evans, J.G. (1997): LaserFace Sealing Technology: Analysis and Application. BHRG, 15 th International Conference, Fluid Sealing. Netherlands. [6] Morton, J.L., Evans, J.G. (2003), Developments in high performance seal designs for critical high pressure offshore and pipeline applications. Texas A&M, 19 th Pump User Symposium. USA. Paper first presented in 2004 at the Pump Users International Forum in Karlsruhe by Klaus-Dieter Meck, now Core Technology Manager Global Research and Development, John Crane 11