ANALYSIS AND SOLUTION OF MECHANICAL SEAL LEAKAGE
Mechanical pump is as a fluid process machinery in the chemical plant, and mechanical seal has been used in the mechanical pump in recent years in order to prevent the pump from leakage, which the effect is very good. In this paper we mainly introduce importance of mechanical seal, basic structure of mechanical seal, working principle of mechanical seal and failure analysis and solutions of mechanical seal.
leakage of mechanical seal cause solution
1. Importance of mechanical seal during chemical fluid process machinery pump
During the process of chemical production, mechanical pump is used to convey the fluid medium to ensure normal operation of continuous industrial production. Once the normal operation can not be conducted by mechanical pump, the whole fluid process will be interrupted, also the production of following system will be seriously affected. In accordance with the preliminary statistics of equipment of a factory, pump damage, the frequency of maintenance and seal failure ( mainly mechanical seal failure ) account for overall of 40~50%, therefore the reliability and long-term effectiveness of mechanical seal is very important to the long-term and reliable operation of the mechanical pump.
2. Basic structure and working principle of mechanical seal
Mechanical seal is also called end face seal or shaft seal, which is a preventing fluid leakage device. It consists of at least one pair of end faces perpendicular to the rotating axial line, which can keep fitting and relative sliding under the action of fluid pressure and elastic force ( magnetic force )of compensation mechanism and the cooperation of secondary seal. Its essence is to use the end face sealing which isn’t easy to leak instead of the axial sealing which is easy to leak. Mechanical seal is two smooth and flat end faces perpendicular to the shaft (axial line), which is composed of rotary ring and the stationary ring. A proper pressing force is formed between the rotary end faces by the action of the elastic elements ( spring and bellows ) and the sealing medium pressure, which keep the two end faces tightly fitting and maintain a thin and stable fluid film between the end faces to achieve good lubrication and sealing.
There are four parts of basic structure of mechanical seal:
(1) Sealing end faces composed of rotary ring and static ring, or called friction pairs;
(2) Buffer compensation mechanism composed of elastic elements as main parts, which keep the sealing end faces tightly fitting;
(3) Secondary seal, consisting of O rings, gaskets of rotary ring and statinary ring.
(4) Transmission mechanism which make the rotary ring rotate along with the shaft.
General mechanical seal structure is shown in the figure as following:
1. Stationary Ring 2. Rotary Ring 3. drive pin 4. Spring 5. Retainer
6. Set Screw 7. Drive Screw 8. O ring of Rotary Ring 9. O ring of Stationary Ring
10. Anti-rotating pin 11. Glland 12. Pushing Ring 13. Shaft Sleeve Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ、Ⅵ leakage point
Basic principle of mechanical seal
The rotary ring (item 2) is put into motion by the shaft through the drive collar ( composed of items 4,5,6 and 7 etc.) and pushing ring (item 12), while the stationary ring (item 1) is fixed, the leakage of media is prevented by the close fitting between the sealing surfaces relying on the media pressure and spring force. After friction pairs are worn,the compensation is carried out under the push of spring ( item 4) . The leakage of media between rotary ring and shaft is prevented by O ring ( item 8) on the rotary ring; also the leakage of media between stationary ring and gland ( item 11) is prevented by O ring (item 9) on the stationary ring.
3. Several leakage points of mechanical seal
The position of mechanical seal in the entire equipment is small, but it is very easy to be failure. There are six possible leakage channel i.e. I, II, III, IV, V, VI shown in the figure.
(1). The seal leakage point between the end faces of rotary ring and stationary ring
( Channel I in the figure) is the only one rotary sealing point of mechanical seal).
Channel I is the rotary seal composed of the end faces of rotary ring and stationary ring which fit together to conduct the relative sliding. It is primary seal in the mechanical seal, so the processing requirements of end faces are very high, at the same time, the unit area pressure exerted on the end face must be strictly controlled in order to keep necessary lubricating film between the end faces of seal. A stable lubricating film can not be easily formed if the pressure is too big, which will accelerate the wear of end faces; the leakage will increase if the pressure is too small. Therefore, during the design and installation of mechanical seal, the value of unit area pressure must be guaranteed within the most appropriate range, i.e. selecting of reasonable compression ratio to obtain a good sealing performance and long life.
(2). The seal leakage point between rotary ring and shaft or shaft sleeve
(Channel VI shown in the figure) belongs to static sealing point with O ring or PTFE etc.
(3). The seal leakage point between stationary ring and stationary ring seat (gland)
(Channel II shown in the figure )belongs to static sealing point with O ring, aluminum gasket or PTFE ring etc.
(4). The seal leakage point between gland and pump seal chamber
( Channel IV shown in the figure ) belongs to static sealing point with asbestos gasket, O ring, graphite gasket, aluminum gasket or PTFE ring etc.
(5). The leakage point between shaft sleeve and shaft
(ChannelＶshown in the figure ) belongs to static sealing point with graphite gasket, aluminum gasket, O ring etc.
(6). The leakage point between rotary ring and rotary ring seat
(Channel III shown in the figure ) belongs to static sealing point ensured with seal ring and the fitting during manufacture, also the form of surfacing welding is designed for rotary ring to achieve no leakage.
4. Analysis and solution of mechanical seal leakage
(1). Two cases of of elastic elements failure
a. Spring or bellows rupture ( or welding joint of the bellows crack ): when evacuation and big vibration will occur under unstable running of pump, and fatigue or even broken phenomenon of the elastic elements will occur under long-term action of alternating load; also not strong welding, no heat treatment or unperfect heat treatment process etc. Usually elastic elements should be replaced and the faults such as evacuation and big vibration should be removed for pump.
b. Elastic elements failure: this mainly occurs in high temperature and heat medium during pump running, sometimes also occurs in low temperature medium. On the one hand, elastic elements are failure due to the scaling deposition in the internal void, this problem can be solved with adding seal oil flushing or using soft water; also the rotary type structure can be designed for the bellows, and rotary bellows mechanical seal has centrifugal effect of self cleaning, so scaling deposition can be solved. One the other hand, taking the mechanical bellows seal as an example, the main problem is bellows failure under high temperature condition. For example, this failure often occurs in high temperature oil pump and hot water pump during running. The main cause of the failure through analysis are due to the following factors: the bellows material, the welding technology of wave profile of convolution and post welding heat treatment. The problem of failure of bellows has been solved basically through material selection and wave profile design, especially the selection of material of convolution is AM350, Hastelloy B and Hastelloy C, Inconel–718 and nikel-chromium alloy with corrosion resistance and high temperature resistance.
Mechanical seal type, bellows material and matching pairs of rotary ring and stationary ring should be selected reasonably, and this problem has been solved through the current technology; to add the seal oil to mechanical seal for flushing, it can not only reduce the temperature of mechanical seal but also prevent the occurrence of coking and scaling in the process.
(2). The failure of rotary ring and static ring is mainly the following aspects:
a. Shrink structure of seal ring is easy to loosen and fall off under the high temperature ( hot oil and hot water etc.) situation. In view of this situation, the whole structure or surfacing welding hard alloy can be used to solve; if the shrink structure must be used for some matching pairs, one is to select appropriate amount of interference of shrink; two is to use sealant to bond; three is to use materials such as Carp 42,Ti12 etc. As face holder. Before assembling, should check if there are defects and loose.
b. Crack or fracture of rotary ring and stationary ring.
Preheating of mechanical pump shall be fast under high temperature medium environment, the leakage phenomenon shall occur during the operation of equipment when the stress crack shall be generated under rapid heating of the rotary ring ( hard ring ) of mechanical seal; in addition, the phenomenon of evacuation and vibration shall occur during the abnormal operation of mechanical pump, under this working condition, carbon ring of rotary ring and stationary ring in mechanical seal shall be out of face holder due to evacuation and can`t return to the original position, which shall be broken during the operation of pump, resulting in the failure of shaft seal.
c. Excessive wear of matching pairs
Stationary ring of matching pairs shall be worn seriously due to long-term operation of mechanical seal or big compression ratio of elastic elements and big specific pressure between end faces, thus the compression ratio shall be insufficient, which shall result in the failure of mechanical seal. Above two cases can be used with replacing of matching pairs and adjusting compression appropriately. In addition, anti-evacuation mechanical seal design or adjustment of process can be used for mechanical seal with easy evacuation, so that the mechanical pump can work in a normal working condition.
d. End faces failure under good matching pairs
Mechanical seal leakage shall occur due to the limited compensation of elastic element of mechanical seal under the spindle bending and big vibration of mechanical pump. The cause of shaft bending and big vibration of mechanical pump must be eliminated,so leakage can be eliminated. The method we usually use is to straighten the shaft or replace the shaft. In another condition, the specific pressure of end faces of mechanical seal is less than the specific pressure of end faces of mechanical seal working in the normal condition when the compression ratio of mechanical seal is too small, the leakage of end faces of mechanical seal shall occur, this problem can be solved to properly increase the compression ratio of elastic element.
(3). Failure of seal ring of rotary or stationary and shaft sleeve as following:
a. Expansion and aging
The rubber shall expand and cause the volume change when rubber material of seal ring and working medium can not properly work together, as a result, the excessive heat shall be generated, which shall make the material aging. Therefore, some good heat-resistant materials such as fluorine rubber (viton) shall be used in the medium or high temperature working situation. The phenomenon of temporary hardening shall occur when seal ring encounter cold working condition, this is the one of reasons for fracture, and this kind of hardening shall resume when the temperature returns to the normal temperature. So the cold-resistant materials should be used in the mechanical pump which deliver low temperature medium or work in low temperature situation. In addition, O ring is a kind of vulnerable part, it shall be aging slowly under the action of the oxygen of air and the natural sunshine, the place with high temperature, sunlight, humidity and air circulation should be avoided for storage, they can be put into some moisture-proof bags such as polyethylene or foil bags, and then put in cool place for storage.
b. The influence of O ring and shaft ( or shaft sleeve ) or O ring embedded groove
According to the information, if O ring used for fixed ( static )seal is made of general rubber material, a larger deformation shall be formed easily when compression amount is more than 40%, therefore compression amount should be as small as possible; while if it is too small, or the size of installation structure is slight different, there shall be no compression in the local area, which shall cause leakage. Compression amount of O ring is usually controlled of 10%~20% thickness in engineering application. During the installation of mechanical seal, if the shaft sleeve becomes oval due to deformation, the radial size of shaft shall be small or the radial shape of shaft shall be oval due to the wear, compression amount of O ring shall be insufficient, which shall cause leakage too.
In addition, if the processing for the shaft sleeve and the embedded groove of shaft sleeve or seal gland and the sealing groove of seal chamber is too deep, it shall cause insufficient compression amount of O ring, which shall also cause leakage.
(4). The leakage of mechanical seal due to other situation
a. The leakage due to the assembling quality
Low assembling level of worker, concentricity and deviation of verticality between the shaft and the two end faces of matching pairs and the adjustment of compression ratio can influence the operation of mechanical seal when assembling, theses shall cause the leakage in different degree.
b. The failure of mechanical seal due to other damaged parts of pump
① Bearing damage: if the bearing is burned and cage is damaged during the
operation of pump, the spindle shall lose balance and shift in the
chamber of pump; the abnormal sound and excessive vibration shall occur,
also the leakage of mechanical seal shall be caused.
② Loosened impeller: if the back cap of impeller loose due to the reverse rotation of pump or other reasons, the large vibration of pump shall occur, and the bearing shall be damaged easily, the leakage of mechanical seal shall be caused easily due to the operation of rotary ring and static ring under the circulation driving force.
c. Damaged dynamic balance: some mechanical problems such as the serious eccentric wear of impeller and the shaft bending etc. can cause mechanical seal leakage too.
The causes of failure of mechanical seal are various from above analysis, the corresponding solutions should be used for the different failures. In practice, the leakage phenomenon must be carefully observed and analyzed to judge the accurate failure position and find out the failure cause and solve them, thus the maintenance of pump and long-term operation of pump shall be guaranteed, and also the normal operation of chemical production shall be guaranteed, and the maximum economic benefit can be achieved finally.