Frictional characteristics of impregnated graphite with different graphitization degree versus chromium stainless steel under varying PV values
Introduction
As a sort of shaft end seal suitable for complex working conditions, mechanical seal is widely used in aerospace turbo pump [1]. In general, mechanical seal consists of two mainly flat rings, a stationary ring (softer ring) and a rotational ring (harder ring), for the purpose of separating pressurized fluid from the atmosphere. Meanwhile, it is a significant factor to consider when choosing a proper stationary ring to improve frictional characteristics and extend the service life of the seal [2]. The graphite material exhibits excellent self-lubricating properties, chemical inertness and cheap affordability, which makes it suitable as the raw material to manufacture stationary ring for industrial applications [3,4]. However, such inherent characteristics of the non-impregnated graphite as insufficient hardness, low wear resistance and large porosity [5], restrict its practical application. Subsequently, the impregnated graphite [6,7], which was intended to replace the non-impregnated graphite, was taken as the material to manufacture stationary seal ring for its capability of improving bearing capacity and impermeability. Previously, based on experimental results, Lancaster [8] and Giltrow et al. [9] came to the conclusion that the degree of graphitization, referred to as the quantity of graphite crystalline growth upon heating of carbon-containing materials, could have impact on the formation of transfer film onto the mating ring, thus affecting the friction and wear properties. Recently, Jia et al. [10] reported that the graphitization degree was associated with the firing temperature of the raw graphite, which could affect the frictional characteristics of mechanical seal. They conducted research into the friction and wear properties exhibited by three kinds of impregnated graphite with varying degrees of graphitization under dry and corrosive environments. As demonstrated by the results, the coefficient of friction and the rate of wear were on the rise with the increasing degree of graphitization and the impregnated graphite with a medium degree of graphitization was more suitable to be used under corrosive conditions. However, other factors affecting the degree of graphitization, for example, cooling rate, the temperature of the impregnation process, the curing temperature of the resin and so on, were not considered for discussion.
In the design of mechanical seal, PV values are generally regarded as the basis for theoretical design and the composite measurement values used to assess the performance of a seal pair [5,11,12]. Thus, PV values play a crucial role in improving the frictional characteristics of a mechanical seal. Up to now, plenty of experimental studies have been performed to demonstrate that the PV-values of graphite-carbon mechanical seals could exert a significant influence on the frictional characteristics of a mechanical seal. Sato et al. [13] demonstrated that both thermal shock resistance and thermal shock fracture toughness could be affected by the PV value, which had impact on the amount of leakage and wear for mechanical seals. Khurshudov et al. [14] performed study on the unlubricated sliding contact of ceramic materials and amorphous carbon under different sliding velocities, which led to the discovery that the main wear mechanism was abrasion at low sliding velocities but the high friction heat was responsible for the failure at high sliding velocities. As discovered by Goilkar et al. [15], the combination of balance ratio and steam pressure could make difference to the wear rates and friction losses. Under the condition of a higher balance ratio, the frictional torque between the two faces was directly proportional to the steam pressure. However, the steam pressure exerted no obvious effect on frictional torque under the condition of a low balance ratio. Jones [16] found out that the capability of PV values was determined by the development of a graphite contact film formed on the sealing surface of the hard mating ring, which affected the performance of the seal in his study.
Moreover, it has been reported that face temperature determined the frictional characteristics and life cycle of a mechanical seal [[17], [18], [19], [20], [21], [22]]. Besides, Lancaster et al. [23] confirmed that face temperature would be subject to the influence of the PV value and the material used to produce stationary rings. Generally speaking, in practical applications, the sealed medium in a seal box has a specific pressure and temperature. The viscous and solid friction heat between the two faces and stirring heat of the rotational ring will lead to a significant rise in face temperature, which could cause such problems as deformation of the end faces, hot cracking and carbonization, thus reducing the service life of the seal [24]. However, there are only a few reports on the variation of face temperature with different seal pressures or different rotational speeds and the influence of face temperature on frictional characteristics between the end faces. Moreover, it has been found that the service life of the phenolic resin impregnated graphite ring with varying degrees of graphitization used in high speed turbo pumps is different in practice. However, the frictional failure mechanism remains unclear at present and there is no relevant literature available for reference. Therefore, the objective of this work is to explore the effect of face temperature under different PV values on the frictional characteristics of impregnated phenolic resin graphite rings with varying degrees of graphitization.
Section snippets
Mechanical seal test rig
The test rig for mechanical seals consists of five parts, including a bearing system, a sealing medium circulation system, a frequency conversion drive system, a data acquisition system, as well as a sealing chamber and mechanical seal assembly. A schematic diagram of the test rig is shown in Fig. 1. To be more specific, the bearing system comprised of air compressor, dryer, filter, pressure regulating valve, spray lubricator, temperature monitoring and vibration monitoring is capable to
Effect of rotational speed on face temperature
Fig. 3(a)-(c) illustrate the evolution of face temperature of a stationary ring made of phenolic resin impregnated graphite with varying degrees of graphitization as a function of time under different rotational speeds and a specified fluid pressure value of 1.5 MPa. Fig. 3(d) presents the temperature increment Δt (which equals the temperature reached steady state minus initial temperature in the seal chamber) under varying rotational speeds.
In general, the overall trend of each curve in Fig. 3
Conclusions
In this study, an investigation was conducted into the frictional characteristics of phenolic resin impregnated graphite stationary ring with varying degrees of graphitization versus chromium stainless steel rotational ring under varying PV values. The conclusions drawn from the study are as follows:
- 1.
The PV value of mechanical seal has impact on the temperature fluctuation value, the temperature increment and the time required to reach steady state of impregnated graphite stationary rings with
Declaration of competing Interest
None.
Declaration of competing Interest
None.
Acknowledgments
The research work is supported by the National Key R&D Program of China (2018YFB2000800) and the National Natural Science Foundation of China (U1737202, 51605436).
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