Carbon Seal Research Articles

Leakage and wear characteristics of carbon seals for aero-engines

Experimental investigation has been performed to study the leakage and wear characteristics of carbon seal working at high circumferential speed. To expand the scope of application, two newly designed carbon seals were compared: #1 Carbon Seal (CS1) with the inner diameter of 136 mm and including 4 segments, #2 Carbon Seal (CS2) with the inner diameter of 212 mm and including 6 segments. Air leakage tests were firstly conducted in the Medium-speed Seal Test Rig. The pressure ratio changed from 1.04 to 2.02 with the rotating speed varying from 0 to 18300 r/min. Of paramount concern was the durability test, including 300 h running time accumulated by three different working conditions, which was separately implemented on each carbon seal. The morphology variation of the friction surface, wear and leakage were recorded. Results indicated that the leakage monotonously increases with the pressure ratio and decreases with the rotating speed. Comparing with CS1, more typical features exist on the friction surface of CS2, which are generated by more severe wear. Continually, leakage characteristics deteriorate. Furthermore, fitted formula has been educed for the life prediction of carbon seal, which could provide some supports for aero-engine design.

Tribological studies of carbon seal materials at cryogenic temperatures

The results of the tribological properties studies of composite materials at cryogenic temperatures are presented in the present paper. Test equipment has been developed, friction and wear tests have been carried out at a temperature of T = - 196 °C. The test results showed a significant dependence of the composite material wear rate on the load.

Experimental Study on Performances of Carbon Seal and Finger Seal under High-speed and High-pressure Condition

Carbon seal and finger seal are both advanced and used in aero-engine and gas turbine. Compared with the finger seal, the application of carbon seal is longer and its technology is more mature. But the finger seal also has excellent sealing performance, and it is much more economical than carbon seal. Based on a self-developed dynamic sealing test rig, the experimental studies on performances of carbon seal and finger seal under high-speed and high-pressure conditions are carried out in this paper, and the comparison on leakage and durability of the two seals is carried out in-depth to find the possibility that replacing carbon seal with finger seal, which can provide references for the selection of sealing types. The experimental results show that, under the same structural and operating conditions, the durability of carbon seal is better than that of the metal matrix finger seal, but the leakage of finger seal is smaller. Considering the manufacturing costs, the finger seal has potential to replace carbon seal at lower speed. Therefore, developing nonmetallic finger seal is an effective way to promote advanced sealing technology, which combines the flexible structure of finger seal with new material that has high thermal conductivity, low expansion, and low friction.

Numerical Investigation on Windback Seals Used in Aero Engines

Seals are considered one of the most important flow elements in turbomachinery applications. The most traditional and widely known seal is the labyrinth seal but in recent years other types like the brush or carbon seals were introduced since they considerably reduce the sealing air consumption. When seals are used for sealing of aero engine bearing chambers they are subjected to high “bombardment” through oil particles which are present in the bearing chamber. These particles mainly result from the bearings as a consequence of the high rotational speeds. Particularly when carbon or brush seals are used, problems with carbon formation (coking) may arise when oil gets trapped in the very tight gap of these seals. In order to prevent oil migration into the turbomachinery, particularly when the pressure difference over a seal is small or even negligible, significant improvement can be achieved through the introduction of so called windback seals. This seal has a row of static helical teeth (thread) and below this thread a scalloped or smooth shaft section is rotating. Depending on the application, a windback seal can be used alone or as a combination with another seal (carbon, brush or labyrinth seal). A CFD analysis carried out with ANSYS CFX version 11 is presented in this paper with the aim to investigate this seal type. The simulations were performed by assuming a two-phase flow of air and oil in the bearing compartment. Design parameters like seal clearance, thread size, scallop width, were investigated at different operating conditions.

Theoretical Analysis of the Static Characteristics of the Carbon Segmented Seal

The segmented carbon seal is regularly used for sealing bearing chambers of aeronautical turboengines or as part of a buffer seal in space turbopumps. The seal operates with contaminated air or with an inert gas and is made of many identical carbon segments (generally three or six) with reciprocally overlapping ends. The segments are pressed against the rotor by the pressure difference between the upstream and the downstream chambers and by a circumferential (garter) spring. The pressure difference and an axial spring press the segments also against the stator. The inner cylindrical surface of each segment is provided with pads that create an aerodynamic lift proportional to the rotor speed. Following this lift force, the segments of the seal are pushed away from the rotor and the seal opens. The contact between the rotor and the segments is lost, and an axial leakage path is thus created. Although it was developed since long, a model for calculating the characteristics of the segmented seal is completely absent from the scientific literature. The goal of the present work is to fill this gap at least for the static characteristics (leakage and torque). The analysis is carried out for a single segment of the seal by supposing that all the segments have the same characteristics. Each segment has a planar motion (i.e., three degrees-of-freedom (3DOF)), and therefore the film thickness under each pad is not uniform. Given the stationary operating conditions (pressure difference and rotation speed), the present model calculates the equilibrium position of each segment on the bases of the lift and the friction force acting on the pads, the friction forces acting on the nose of the seal, and the radial and axial springs. Once found the static equilibrium position, the leakage and the torque of the seal are calculated. A parametric study enlightens the importance of the pad waviness, the pocket depth, and the spring forces on the characteristics of the segmented seal.

Microwave synthesis of Li3V2(PO4)3/C as positive-electrode materials for rechargeable lithium batteries

The paper reports a microwave irradiation method to rapidly synthesize Li3V2(PO4)3/C materials as cathode for lithium ion batteries by the self-assembly microwave reaction oven with carbon seal reactor, using LiH2PO4, V2O5 and sucrose as raw materials. Sucrose was used to be reducer and carbon source. Thermogravimetric (TG) analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to characterize its structure and morphology. Electrochemical properties of the Li3V2(PO4)3/C materials were studied by cyclic voltammetry (CV) and charge–discharge cycling performance. The results showed that the diffraction peaks of the sample correspond to a single-phase, and can be indexed as monoclinic structure with a space group of P21/n. An electrochemical test showed that Li3V2(PO4)3/C demonstrated an excellent electrochemical capacity of 138mAhg−1 at 0.2C rate and 124.1mAhg−1 at 5C rate with stable cycle ability.

Non-Contacting Carbon Seal for High-Temperature and Large-Diameter Turbomachinery Applications

Non-contacting circumferential segmented hydrodynamic carbon seals have been used in aviation engines to seal lubrication oil from air flow path in bearing locations. Its tight clearance and long life from non-contacting operation are attractive to large gas turbine and steam turbine where efficiency is critical. In this paper, development of segmented carbon seals for large turbomachine is presented. The seal is targeting locations where temperature is over 500C (930F) and rotor diameter is over 0.38 meter (15 inches). Comparing to current circumferential segmented carbon seal technology, the new seal is capable of running 260C (480F) hotter and it is 3 times larger.Three sets of test seals were made with newly developed manufacture process. The test seals are 12 inches in diameter, made of three different carbon grades that were down-selected from a dozen candidates having potential of enduring high temperature of air and superheated steam. A test rig was set up to test one pair of seals at a time. One of the seals is facing high-temperature steam, while the other one is exposed to hot air.

Carbon seal materials with superior mechanical properties and fine-grained structure fabricated by a new process

Carbon seal material was prepared using a new liquid mixing process of the raw materials, which had not only lowered open porosity but also provided excellent mechanical properties, especially higher compressive strength (210 MPa) compared with the material prepared by the conventional method. Scanning electron microscopy (SEM) results show that such material has a fine-grained structure as well as little pore diameter. The influence of the manufacturing procedure of the materials on the performance and microstructure is investigated. In addition, correlations between properties and microstructure are also discussed.

Load-Induced Reduction of Real Contact Area in Carbon Face Seals Operating Under Boundary Lubrication Conditions

The topography of carbon seal faces tends to distort under compression, resulting in an increase in the real mechanical contact area between the seal faces. This reduces the effective area between the seal faces subjected to full hydraulic pressures, and consequently increases the mechanical contact forces. The electrical resistance in the junction between the carbon seal face and a lapped steel mating ring was used to estimate the real contact area between the two materials. The estimated reduced contact area was then utilized to compute mechanical contact forces. The computed values were cross-checked against mechanical contact forces derived from torque measurements. Finally, a semi-empirical algorithm is presented, allowing the effect of an increased mechanical contact area to be computed for a typical carbon insert with a modulus of elasticity in the range of 2 × 1010 N/M2.

Counterrotating intershaft seals for advanced engines

Feasibility for application of a counterrotating intershaft carbon seal to high thrust-to-weight military gasturbine engines was demonstrated through a series of rig tests. The noncontacting high-speed (800 ft/s) seal incorporating hydrodynamic lift geometry with spiral grooves in the seal plates similar to the one designed by NASA-Lewis Research Center. Seal air leakage and carbon wear rates were determined for each of two configurations in rig screening evaluations of 10-h duration. One seal assembly was subjected to an additional 50-h endurance testing. For both configurations, seal air leakage was approximately one-third the leakage that would occur with a conventional labyrinth seal. The wear rate was low for the configuration tested a total of 60 h.

Comparison of Surface Damage Caused by Sliding Wear and Cavitation Erosion on Mechanical Face Seals

To clarify the dangerous surface damage in mechanical seals, both sliding wear and vibratory cavitation erosion tests were performed on sintered carbon materials. The differences in damage between cavitation erosion and sliding wear were investigated through observation of surface topographies and volume losses. Roughness, Ra, of worn surface is rather small, about 1 μm in the worst case, and is independent of the worn depth. In cavitation erosion, however, Ra of eroded surface increases rapidly and thereafter attains a large steady value (3.6 μm- 11 μm for various carbon materials) with increasing eroded depth regardless of the cavitation intensity. The sliding surfaces of actual operating mechanical seals were compared with our test results and thus it was concluded that the severe damage in the mechanical seals is caused by the concentrated repetition of cavitation erosion. Furthermore, the wear and cavitation erosion resistance were compared among the various carbon seal materials. From these results, it has become clear that the cavitation erosion test is important to find out the quality of mechanical seal materials.

Experimental Evaluation of a Mixed Friction Hydrostatic Mechanical Face Seal Model Considering Radial Taper, Thermal Taper, and Wear

In this paper the results of experimental investigations of the effects of radial taper on mechanical face seals are presented and compared to theory. The previously published theory considers the effects of thermal taper caused by a temperature gradient in the seal rings; mixed friction in the case where load support is shared between hydrostatic support and partial contact of the seal faces; surface roughness, which affects both load sharing and leakage; and wear, which alters the radial profile. Fifteen tests were run using a 100 mm diameter carbon versus tungsten carbide seal at 1800 rpm and 3.45 MPa in water. Test duration was up to 100 hr. Varying amounts of radial taper were used. Tests were run at balance ratios of 1.00 and 0.75. Initial and final surface profiles were recorded. Seal torque, leakage, and face temperatures were recorded as functions of time. Results show that theory predicts initial torque and leakage as functions of initial taper quite well, given knowledge of seal surface characteristics. Predicted equilibrium thermal taper as a function of torque for a balance ratio of 1.0 is good. For a seal having a balance ratio of 0.75, predicted equilibrium thermal rotation shows some agreement but more experimental data are needed. The results of 1.00 balance ratio tests suggest that after a long period of operation, any initial taper will be worn away and the seal would continue to operate as a parallel face seal. Results from long-term tests indicate that the wear coefficient is not a constant. While the experimental results support the basic concepts of the model, the results show where further work must be done to better understand the role of surface roughness and wear processes in mechanical face seals.

Mainshaft Seals for Small Gas Turbine Engines

An experimental evaluation of mainshaft seals for small gas turbine engines was conducted with shaft speeds to 213 m/s (700 ft/sec), air pressures to 148 N/cm2 abs. (215 psia), and air temperatures to 412 K (282 F). A radial face seal incorporating self-acting geometry for lift augmentation was evaluated. In addition, three conventional carbon seal types (face, circumferential segmented, and rotating ring) were run for comparison. Test results indicated that the conventional seals used in this evaluation may not be satisfactory in future advanced engines because of excessive air leakage. On the other hand, the self-acting face seal was shown to have the potential capability of limiting leakages to one-half that of the conventional face seals and one-fifth that of conventional ring seals. A 150 hour endurance test of the self-acting face seal was conducted at speeds to 145 m/s (475 ft/sec), air pressures to 124 N/cm2 abs. (180 psia), and air temperatures to 408 K (275 F). The seal wear was not measurable. ...

Wear and friction of impregnated carbon seal materials in liquid nitrogen and hydrogen : D. W. Wisander and R. L. Johnson. (NASA). Cryogenic Eng. Conf., Boulder, Colo., 1960, 12 pp. and illus

Wear and friction of impregnated carbon seal materials in liquid nitrogen and hydrogen : D. W. Wisander and R. L. Johnson. (NASA). Cryogenic Eng. Conf., Boulder, Colo., 1960, 12 pp. and illus