端面宽度对机械密封极限p_(c)v(pressure×velocity)值的影响

Effects of End Face Width of Mechanical Seals on the Limit p_(c)v(pressure×velocity) Value

端面宽度是机械密封的重要设计参数,为了研究端面宽度对机械密封极限p_(c)v值(端面比压p_(c)×端面平均线速度v)的影响,本文作者选取浸呋喃树脂石墨与微孔常压烧结碳化硅作为密封摩擦副,采用定速度变载荷和变速度定载荷两种不同测试方法获取了五种端面宽度摩擦副的极限p_(c)v值,分析了不同端面宽度摩擦副在不同测试方法下的摩擦系数时变特性和密封介质温度时变特性,并通过扫描电子显微镜和三维激光形貌仪分析了摩擦副的表面磨损特征.结果表明:在相同测试方法下,与较宽端面摩擦副相比,较窄端面摩擦副的极限p_(c)v值较高并且密封失效时摩擦系数较小而密封介质温度较高;在给定端面宽度条件下采用定速度变载荷方法测试时,较高速度15.68 m/s对应摩擦副的极限p_(c)v值相对于较低速度7.84 m/s时较高,其中极限p_(c)值较低;达到极限p_(c)v值时,摩擦副表面的磨损机理主要为黏着磨损,其诱因在于端面摩擦扭矩的突增引起端面过热,导致两密封端面因液膜汽化加剧发生干摩擦.定速度变载荷和变速度定载荷两种方法均可用于测试机械密封极限p_(c)v值,但采用变速度定载荷方法测试时密封失效对应的摩擦系数突增幅度相对较不明显,失效的现象较难捕捉,石墨表面被破坏的程度也较轻.在满足材料强度的要求下,减小端面宽度,有效避免端面过热,有利于机械密封达到更大的极限p_(c)v值.本文为机械密封极限p_(c)v值的测量以及端面宽度的设计和密封的延寿方法提供了指导.

End face width is a significant design parameter of mechanical seal,which numerical value is an important factor affecting end face specific pressure and tribological characteristics.The furan resin-impregnated graphite and microporous atmospheric sintered silicon carbide were adopted as the sealing friction pairs to investigate the influence of end face width of mechanical seal on the limit p_(c)v value(i.e.the product of specific pressure p_(c)and average linear velocity v).The limit p_(c)v values of friction pairs with five different values of the end-face width were obtained by means of variable load with a constant speed(namely PV method)and variable speed with a constant load(namely VP method)in water medium environment.According to the PV method,two rotational speeds of 3000 r/min and 6000 r/min were selected,corresponding to the average linear velocity of the sealing end face were 7.84 m/s and 15.68 m/s,and which were defined as PV3000 method and PV6000 method respectively.The sudden increase of friction coefficient was used as the basis for judging seal failure.The time-varying characteristics of friction coefficient and sealing medium temperature with different widths of end faces were analyzed with the above three methods.The surface wear morphologies of the friction pairs were measured with scanning electron microscope and three-dimensional(3-D)measuring laser microscope,and the failure mechanism when the seal reached the limit p_(c)v value was revealed.The results showed that the friction pair with a narrower end face can reach a greater limit p_(c)v value with the same measuring method,corresponding to a smaller friction coefficient and a greater temperature rise in the sealing medium until the failure of the seal.Under the condition of given end-face width,the limit p_(c)v value measured by VP method was similar to that measured by PV3000 method,but obviously smaller than that measured by PV6000 method.Under the condition of given end-face width and the PV method,the friction pairs with the speed of 15.68 m/s had a higher limit p_(c)v value and a lower limit p_(c)value than those with the speed of 7.84 m/s.The reason was that the increase of friction linear velocity intensified the friction heat generation of end face,which was not conducive to the friction pair reaching a larger endface specific pressure.When the limit p_(c)v value was reached,the sealing surfaces of graphite rings with five different values of end face width all had the phenomenon of large-area material peeling off,and the main wear mechanism of the friction pairs was adhesive wear,which was mainly due to the overheating of the end faces and the vaporization of the liquid film caused by the sudden increase of friction torque.Both the PV and VP method can be used to determine the limit p_(c)v value of the mechanical seal.Compared with the PV method,the sudden increase in friction coefficient corresponding to seal failure was relatively insignificant,and the damage to the graphite surface was weak through the VP method.Therefore,it was recommended to adopt the means of variable load with a constant speed,and the average linear velocity of the sealing end face was recommended to be 7.84 m/s.On the premise of meeting the requirements of material strength,the reduction in the end face width can effectively avoid overheating of the end faces,which was beneficial for the mechanical seal to reach a greater limit p_(c)v value.It provided a theoretical and experimental basis for designing the end-face width and prolonging the service life of mechanical seals in the future.