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機械密封失效的若干問題分析 Analysis on Several Problems of Mechanical Seal Failure

泵用機械密封種類繁多,型號各異,但泄漏點主要有五處:(l)軸套與軸間的密封;(2)動環與軸套間的密封;(3)動、靜環間密封;(4)對靜環與靜環座間的密封;(5)密封端蓋與泵體間的密封。


1.安裝靜試時泄漏


機械密封安裝調試好后,一般要進行靜試,觀察泄漏量。如泄漏量較小,多為動環或靜環密封圈存在問題;泄漏量較大時,則表明動、靜環摩擦副間存在問題。在初步觀察泄漏量、判斷泄漏部位的基礎上,再手動盤車觀察,若泄漏量無明顯變化則靜、動環密封圈有問題;如盤車時泄漏量有明顯變化則可斷定是動、靜環摩擦副存在問題;如泄漏介質沿軸向噴射,則動環密封圈存在問題居多,泄漏介質向四周噴射或從水冷卻孔中漏出,則多為靜環密封圈失效。此外,泄漏通道也可同時存在,但一般有主次區別,只要觀察細致,熟悉結構,一定能正確判斷。

2.試運轉時出現的泄漏

泵用機械密封經過靜試后,運轉時高速旋轉產生的離心力,會抑制介質的泄漏。因此,試運轉時機械密封泄漏在排除軸間及端蓋密封失效后,基本上都是由于動、靜環摩擦副受破壞所致。引起摩擦副密封失效的因素主要有:(l)操作中,因抽空、氣蝕、憋壓等異常現象,引起較大的軸向力,使動、靜環接觸面分離;(對安裝機械密封時壓縮量過大,導致摩擦副端面嚴重磨損、擦傷;(3)動環密封圈過緊,彈簧無法調整動環的軸向浮動量;(4)靜環密封圈過松,當動環軸向浮動時,靜環脫離靜環座;(5)工作介質中有顆粒狀物質,運轉中進人摩擦副,探傷動、靜環密封端面;(6)設計選型有誤,密封端面比壓偏低或密封材質冷縮性較大等。上述現象在試運轉中經常出現,有時可以通過適當調整靜環座等予以消除,但多數需要重新拆裝,更換密封。

3.由于兩密封端面失去潤滑膜而造成的失效

a)因端面密封載荷的存在,在密封腔缺乏液體時啟動泵而發生干摩擦;

b)介質的低于飽和蒸汽壓力,使得端面液膜發生閃蒸,喪失潤滑;

c)如介質為易揮發性產品,在機械密封冷卻系統出現結垢或阻塞時,由于端面摩擦及旋轉元件攪拌液體產生熱量而使介質的飽和蒸汽壓上升,也造成介質壓力低于其飽和蒸汽壓的狀況。

4.由于腐蝕而引起的機械密封失效

a)密封面點蝕,甚至穿透。

b)由于碳化鎢環與不銹鋼座等焊接,使用中不銹鋼座易產生晶間腐蝕;

c)焊接金屬波紋管、彈簧等在應力與介質腐蝕的共同作用下易發生破裂。

5.由于高溫效應而產生的機械密封失效

a)熱裂是高溫油泵,如油渣泵、回煉油泵、常減壓塔底泵等最常見的失效現象。在密封面處由于干摩擦、冷卻水突然中斷,雜質進入密封面、抽空等情況下,都會導致環面出現徑向裂紋;

b)石墨炭化是使用碳-石墨環時密封失效的主要原因之一。由于在使用中,如果石墨環一旦超過許用溫度(一般在-105~250℃)時,其表面會析出樹脂,摩擦面附近樹脂會發生炭化,當有粘結劑時,會發泡軟化,使密封面泄漏增加,密封失效;

c)輔助密封件(如氟橡膠、乙丙橡膠、全橡膠)在超過許用溫度后,將會迅速老化、龜裂、變硬失彈。現在所使用的柔性石墨耐高溫、耐腐蝕性較好,但其回彈性差。而且易脆裂,安裝時容易損壞。

6.由于密封端面的磨損而造成的密封失效

a)摩擦副所用的材料耐磨性差、摩擦系數大、端面比壓(包括彈簧比壓)過大等,都會縮短機械密封的使用壽命。對常用的材料,按耐磨性排列的次序為:碳化硅-碳石墨、硬質合金-碳石墨、陶瓷-碳石墨、噴涂陶瓷--碳石墨、氮化硅陶瓷--碳石墨、高速鋼--碳石墨、堆焊硬質合金--碳石墨。

b)對于含有固體顆粒介質,密封面進入固體顆粒是導致使密封失效的主要原因。固體顆粒進入摩擦副端面起研磨劑作用,使密封發生劇烈磨損而失效。密封面合理的間隙,以及機械密封的平衡程度,還有密封端面液膜的閃蒸等都是造成端面打開而使固體顆粒進入的主要原因。

c)機械密封的平衡程度β也影響著密封的磨損。一般情況下,平衡程度β=75%左右最適宜。β

7.因安裝、運轉或設備本身所產生的誤差而造成機械密封泄漏

a)由于安裝不良,造成機械密封泄漏。主要表現在以下幾方面:

1)動、靜環接觸表面不平,安裝時碰傷、損壞;

2)動、靜環密封圈尺寸有誤、損壞或未被壓緊;

3)動、靜環表面有異物;

4)動、靜環V型密封圈方向裝反,或安裝時反邊;

5)軸套處泄漏,密封圈未裝或壓緊力不夠;

6)彈簧力不均勻,單彈簧不垂直,多彈簧長短不一;

7)密封腔端面與軸垂直度不夠;

8)軸套上密封圈活動處有腐蝕點。

b)設備在運轉中,機械密封發生泄漏的原因主要有:

1)泵葉輪軸向竄動量超過標準,轉軸發生周期性振動及工藝操作不穩定,密封腔內壓力經常變化等均會導致密封周期性泄漏;

2)摩擦副損傷或變形而不能跑合引起泄漏;

3)密封圈材料選擇不當,溶脹失彈;

4)大彈簧轉向不對;

5)設備運轉時振動太大;

6)動、靜環與軸套間形成水垢使彈簧失彈而不能補償密封面的磨損;

7)密封環發生龜裂等。

c)泵在停一段時間后再啟動時發生泄漏,這主要是因為摩擦副附近介質的凝固、結晶,摩擦副上有水垢、彈簧腐蝕、阻塞而失彈。


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.



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