机械密封失效的若干问题分析 Analysis on Several Problems of Mechanical Seal Failure

泵用机械密封种类繁多，型号各异，但泄漏点主要有五处：（l）轴套与轴间的密封；（2）动环与轴套间的密封；（3）动、静环间密封；（4）对静环与静环座间的密封；（5）密封端盖与泵体间的密封。

1.安装静试时泄漏

机械密封安装调试好后，一般要进行静试，观察泄漏量。如泄漏量较小，多为动环或静环密封圈存在问题；泄漏量较大时，则表明动、静环摩擦副间存在问题。在初步观察泄漏量、判断泄漏部位的基础上，再手动盘车观察，若泄漏量无明显变化则静、动环密封圈有问题；如盘车时泄漏量有明显变化则可断定是动、静环摩擦副存在问题；如泄漏介质沿轴向喷射，则动环密封圈存在问题居多，泄漏介质向四周喷射或从水冷却孔中漏出，则多为静环密封圈失效。此外，泄漏通道也可同时存在，但一般有主次区别，只要观察细致，熟悉结构，一定能正确判断。

2.试运转时出现的泄漏

泵用机械密封经过静试后，运转时高速旋转产生的离心力，会抑制介质的泄漏。因此，试运转时机械密封泄漏在排除轴间及端盖密封失效后，基本上都是由于动、静环摩擦副受破坏所致。引起摩擦副密封失效的因素主要有：（l）操作中，因抽空、气蚀、憋压等异常现象，引起较大的轴向力，使动、静环接触面分离；（对安装机械密封时压缩量过大，导致摩擦副端面严重磨损、擦伤；（3）动环密封圈过紧，弹簧无法调整动环的轴向浮动量；（4）静环密封圈过松，当动环轴向浮动时，静环脱离静环座；（5）工作介质中有颗粒状物质，运转中进人摩擦副，探伤动、静环密封端面；（6）设计选型有误，密封端面比压偏低或密封材质冷缩性较大等。上述现象在试运转中经常出现，有时可以通过适当调整静环座等予以消除，但多数需要重新拆装，更换密封。

3.由于两密封端面失去润滑膜而造成的失效

a）因端面密封载荷的存在，在密封腔缺乏液体时启动泵而发生干摩擦；

b）介质的低于饱和蒸汽压力，使得端面液膜发生闪蒸，丧失润滑；

c）如介质为易挥发性产品，在机械密封冷却系统出现结垢或阻塞时，由于端面摩擦及旋转元件搅拌液体产生热量而使介质的饱和蒸汽压上升，也造成介质压力低于其饱和蒸汽压的状况。

4.由于腐蚀而引起的机械密封失效

a）密封面点蚀，甚至穿透。

b）由于碳化钨环与不锈钢座等焊接，使用中不锈钢座易产生晶间腐蚀；

c）焊接金属波纹管、弹簧等在应力与介质腐蚀的共同作用下易发生破裂。

5.由于高温效应而产生的机械密封失效

a）热裂是高温油泵，如油渣泵、回炼油泵、常减压塔底泵等最常见的失效现象。在密封面处由于干摩擦、冷却水突然中断，杂质进入密封面、抽空等情况下，都会导致环面出现径向裂纹；

b）石墨炭化是使用碳-石墨环时密封失效的主要原因之一。由于在使用中，如果石墨环一旦超过许用温度（一般在-105～250℃）时，其表面会析出树脂，摩擦面附近树脂会发生炭化，当有粘结剂时，会发泡软化，使密封面泄漏增加，密封失效；

c）辅助密封件（如氟橡胶、乙丙橡胶、全橡胶）在超过许用温度后，将会迅速老化、龟裂、变硬失弹。现在所使用的柔性石墨耐高温、耐腐蚀性较好，但其回弹性差。而且易脆裂，安装时容易损坏。

6.由于密封端面的磨损而造成的密封失效

a）摩擦副所用的材料耐磨性差、摩擦系数大、端面比压（包括弹簧比压）过大等,都会缩短机械密封的使用寿命。对常用的材料，按耐磨性排列的次序为：碳化硅-碳石墨、硬质合金-碳石墨、陶瓷-碳石墨、喷涂陶瓷--碳石墨、氮化硅陶瓷--碳石墨、高速钢--碳石墨、堆焊硬质合金--碳石墨。

b）对于含有固体颗粒介质，密封面进入固体颗粒是导致使密封失效的主要原因。固体颗粒进入摩擦副端面起研磨剂作用，使密封发生剧烈磨损而失效。密封面合理的间隙，以及机械密封的平衡程度，还有密封端面液膜的闪蒸等都是造成端面打开而使固体颗粒进入的主要原因。

c）机械密封的平衡程度β也影响着密封的磨损。一般情况下，平衡程度β=75%左右最适宜。β

7.因安装、运转或设备本身所产生的误差而造成机械密封泄漏

a）由于安装不良，造成机械密封泄漏。主要表现在以下几方面：

1）动、静环接触表面不平，安装时碰伤、损坏；

2）动、静环密封圈尺寸有误、损坏或未被压紧；

3）动、静环表面有异物；

4）动、静环V型密封圈方向装反，或安装时反边；

5）轴套处泄漏，密封圈未装或压紧力不够；

6）弹簧力不均匀，单弹簧不垂直，多弹簧长短不一；

7）密封腔端面与轴垂直度不够；

8）轴套上密封圈活动处有腐蚀点。

b）设备在运转中，机械密封发生泄漏的原因主要有：

1）泵叶轮轴向窜动量超过标准，转轴发生周期性振动及工艺操作不稳定，密封腔内压力经常变化等均会导致密封周期性泄漏；

2）摩擦副损伤或变形而不能跑合引起泄漏；

3）密封圈材料选择不当，溶胀失弹；

4）大弹簧转向不对；

5）设备运转时振动太大；

6）动、静环与轴套间形成水垢使弹簧失弹而不能补偿密封面的磨损；

7）密封环发生龟裂等。

c）泵在停一段时间后再启动时发生泄漏，这主要是因为摩擦副附近介质的凝固、结晶，摩擦副上有水垢、弹簧腐蚀、阻塞而失弹。

There are many types of mechanical seals for pumps, with different models, but there are mainly five leakage points: (l) the seal between the shaft sleeve and the shaft; (2) the seal between the moving ring and the shaft sleeve; (3) the seal between the moving and static rings ; (4) The seal between the static ring and the static ring seat; (5) The seal between the sealing end cover and the pump body.

1. Leakage during installation static test

After the mechanical seal is installed and debugged, a static test is generally required to observe the leakage. If the amount of leakage is small, there is usually a problem with the moving or static ring seals; when the amount of leakage is large, it indicates that there is a problem between the moving and static ring friction pairs. On the basis of preliminary observation of the leakage and judgment of the leakage location, the manual cranking observation is performed. If there is no obvious change in the leakage, the static and dynamic ring seals are defective; if the leakage changes significantly during cranking, it can be concluded that the leakage is dynamic or dynamic. There are problems with the static ring friction pair; if the leakage medium is sprayed along the axial direction, there are mostly problems with the moving ring seal. If the leakage medium is sprayed around or leaks from the water cooling hole, the static ring seal is mostly ineffective. In addition, leakage channels can also exist at the same time, but there are generally primary and secondary differences. As long as you observe carefully and are familiar with the structure, you will be able to make a correct judgment.

2. Leakage during trial operation

After the mechanical seal for the pump has undergone static testing, the centrifugal force generated by the high-speed rotation during operation will inhibit the leakage of the medium. Therefore, the mechanical seal leakage during the trial operation is basically caused by the damage of the dynamic and static ring friction pairs after the failure of the shaft and the end cover seal is eliminated. The main factors that cause the failure of the friction pair seal are: (l) During operation, due to abnormal phenomena such as evacuation, cavitation, pressure holding, etc., a large axial force is caused to separate the contact surfaces of the dynamic and static rings; (for installing mechanical seals) When the compression is too large, the end face of the friction pair will be severely worn and scratched; (3) The moving ring sealing ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring; (4) The static ring sealing ring is too loose, when the moving ring When floating in the axial direction, the static ring separates from the static ring seat; (5) There are granular materials in the working medium, and the friction pair is entered during operation, and the sealing end faces of the moving and static ring are detected; (6) The design selection is wrong, and the sealing end face ratio Low pressure or large cold shrinkage of the sealing material, etc. The above phenomenon often occurs in trial operation, and sometimes it can be eliminated by adjusting the static ring seat, but most of them need to be re-disassembled and replaced.

3. Failure caused by the loss of lubricating film on the two seal end faces

a) Dry friction occurs when the pump is started when the seal cavity lacks liquid due to the existence of the end face seal load;

b) The pressure of the medium is lower than the saturated vapor pressure, causing the liquid film on the end surface to flash and lose lubrication;

c) If the medium is a volatile product, when the mechanical seal cooling system is fouled or blocked, the saturated vapor pressure of the medium will rise due to the friction of the end face and the heat generated by the rotating element stirring the liquid, and the pressure of the medium will also be lower than its saturated steam. Pressure.

4. Mechanical seal failure due to corrosion

a) Pitting or even penetration of the sealing surface.

b) Because the tungsten carbide ring is welded to the stainless steel seat, the stainless steel seat is prone to intergranular corrosion during use;

c) Welded metal bellows, springs, etc. are prone to rupture under the combined action of stress and medium corrosion.

5. Mechanical seal failure due to high temperature effect

a) Thermal cracking is the most common failure phenomenon of high-temperature oil pumps, such as sludge pumps, refining pumps, and atmospheric and vacuum tower bottom pumps. In the case of dry friction at the sealing surface, sudden interruption of cooling water, impurities entering the sealing surface, evacuation, etc., radial cracks will appear on the ring surface;

b) Graphite carbonization is one of the main reasons for seal failure when using carbon-graphite rings. In use, if the graphite ring exceeds the allowable temperature (usually -105～250℃), resin will precipitate on its surface, and the resin near the friction surface will be carbonized. When there is a binder, it will foam and soften. Increase the leakage of the sealing surface and seal failure;

c) Auxiliary seals (such as fluorine rubber, ethylene propylene rubber, all rubber) will quickly age, crack, harden and lose elasticity after exceeding the allowable temperature. The flexible graphite used now has good high temperature resistance and corrosion resistance, but its resilience is poor. Moreover, it is brittle and easily damaged during installation.

6. Seal failure caused by the wear of the seal end face

a) The material used in the friction pair has poor wear resistance, large friction coefficient, and excessive end-face specific pressure (including spring specific pressure), which will shorten the service life of the mechanical seal. For commonly used materials, the order of wear resistance is: silicon carbide-carbon graphite, cemented carbide-carbon graphite, ceramics-carbon graphite, spray ceramics-carbon graphite, silicon nitride ceramics-carbon graphite, high-speed steel --Carbon graphite, hardfacing hard alloy--Carbon graphite.

b) For media containing solid particles, the ingress of solid particles on the sealing surface is the main reason for the failure of the seal. The solid particles enter the end face of the friction pair to act as an abrasive, causing the seal to be severely worn and fail. The reasonable clearance of the sealing surface, the balance degree of the mechanical seal, and the flashing of the liquid film on the sealing end surface are the main reasons that the end surface opens and solid particles enter.

c) The balance β of the mechanical seal also affects the wear of the seal. In general, the degree of balance β=75% is the most appropriate. β

7. Leakage of mechanical seal caused by errors in installation, operation or equipment itself

a) The mechanical seal leaks due to poor installation. Mainly manifested in the following aspects:

1) The contact surface of the moving and static rings is not flat, which is bumped or damaged during installation;

2) The size of the dynamic and static ring seals is wrong, damaged or not compressed;

3) There are foreign objects on the surface of the moving and static rings;

4) The direction of the dynamic and static V-shaped sealing rings is reversed, or the opposite side is installed when installing;

5) Leakage at the shaft sleeve, the sealing ring is not installed or the pressing force is insufficient;

6) The spring force is uneven, the single spring is not vertical, and the length of multiple springs is different;

7) The end face of the sealed cavity is not perpendicular to the shaft;

8) There are corrosion spots in the movable part of the sealing ring on the shaft sleeve.

b) When the equipment is in operation, the main reasons for the leakage of the mechanical seal are:

1) The axial movement of the pump impeller exceeds the standard, periodic vibration of the rotating shaft, unstable process operation, frequent changes in the pressure in the seal cavity, etc. will cause periodic leakage of the seal;

2) The friction pair is damaged or deformed and cannot run-in causing leakage;

3) Improper selection of sealing ring material, swelling and losing elasticity;

4) The big spring turns incorrectly;

5) The vibration is too large when the equipment is running;

6) The scale formed between the moving and static rings and the shaft sleeve causes the spring to lose elasticity and cannot compensate for the wear of the sealing surface;

7) The sealing ring is cracked.

c) Leakage occurs when the pump is restarted after being stopped for a period of time. This is mainly due to the solidification and crystallization of the medium near the friction pair, the scale on the friction pair, the corrosion of the spring, the blockage and the loss of elasticity.

*以上为网络转载