Among the pump failures in refineries and chemical plants, shaft seal failures are more frequent. According to statistics, shaft seal failures of centrifugal pumps account for about 50%, and bearing failures account for about 30%.
(1) Mechanical seal failure phenomenon and its causes
Symptoms
■The leakage is too large or abnormal;
■Power up;
■Overheating, smoking, sounding;
■ Abnormal vibration;
■A large amount of wear products are precipitated.
reason
◆The mechanical seal itself is not good.
◆Improper selection of mechanical seal and poor adaptability.
◆Operating conditions and operation management are not good.
◆The auxiliary device is poor.
(2) Seal failure and its manifestations
▶When the sealed medium passes through the sealing part and causes one of the following situations, the seal is considered to fail:
▶A large amount of media leaks from the sealing system;
▶The pressure of the sealing system is greatly reduced;
▶A large amount of sealing liquid enters the sealing system (such as double mechanical seal).
External characteristics of mechanical seal failure
①The seal continues to leak;
②Seal leakage and sealing ring icing;
③ The seal makes a popping sound when it is working;
④ screaming when the seal is working;
⑤Graphite powder gathers on the outside of the sealing surface;
⑥The sealing life is short.
Specific manifestations of seal failure
When the failed mechanical seal is disassembled and disassembled, it can be found that there are various forms of seal failure, and the common ones are mechanical damage, corrosion damage and thermal damage.
①Mechanical damage
The main form of mechanical damage is wear, including other forms of damage due to insufficient mechanical strength.
(a) Wear damage
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(b) Erosion and cavitation damage
Sealing fluids cause erosive wear on sealing elements. Reason: It occurs in the heavy-duty sealing of the high-pressure cover (above about 30 atmospheres), and the high pressure difference will cause the sealing surface to lift and cause the fluid to erode the sealing surface. Leakage at particularly high peripheral speeds also washes away the bonding material (soft ring) of the sealing surface.
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(c) Bushing wear
Sleeve wear occurs due to relative movement between the O-ring and the sleeve. Reasons: due to machining errors, axial movement and tilting of the ring seat. In addition, the shaft sleeve is worn and the O-ring is damaged due to the solid particles (or other solid particles) in the medium embedded in the O-ring.
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(d) O-ring clogging
O-ring clogging: O-ring clogging due to the accumulation of solid particles (especially fibrous matter) in the product side seal space. In addition, the sealing surfaces cannot maintain tight contact due to the slow deposition of media flash emissions on the atmospheric side.
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(e) Damage due to frictional torque
The wear between the transmission screw and the transmission sleeve generally occurs under frequent start-stop conditions, causing the elastic element to lose its elastic compensation function and even the sealing surface to fail to fit tightly.
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(f) Cracking of metal bellows
The metal bellows is cracked at the welding place due to severe vibration and dry operation, and the fork transmission structure is used to avoid it.
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②Corrosion wear
There are many failures of mechanical seals caused by corrosion, and the causes of corrosion are complicated.
(a) Surface corrosion: Corrosion is the uniform corrosion that occurs on the surface of parts in contact with the medium. The performance is that the weight of the parts is reduced, the strength is lost, the hardness is reduced, and even all of them will be corroded.
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(c) Intergranular corrosion: It generally occurs on the supporting material of the brazed tungsten carbide ring, and occurs on the crystal interface of the metal. Its weight corrosion rate is very small, but it can corrode deeply into the interior of the metal, and it is easy to cause cutting damage due to the notch effect.
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(d) Stress corrosion: Stress corrosion is the appearance of cracks or fractures in metal materials under the combined action of corrosive media and stress. Mechanical seal components such as metal welded bellows, springs, and transmission earrings of transmission sleeves are most likely to fail due to stress corrosion.
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(e) Crevice corrosion: The crevice corrosion caused by the deposition of narrow gaps in the inner hole of the mechanical seal dynamic ring sealing surface is a kind of concentration battery corrosion. Generally, it occurs in the narrow gap where two parts are close (such as the gap between the moving ring and the bushing).
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(f) Battery corrosion: The battery corrosion caused by the potential difference between the silver solder of the sealing ring of the mechanical seal and the chromium steel is due to the potential difference between the two metals in the dielectric and the battery effect. Measures: Try to avoid the combination of two metals with potential difference.
③Heat damage damage
The failure of mechanical seals due to overheating is thermal damage failure. The most common thermal damage failures include end face thermal deformation, thermal cracking, blisters, carbonization, loss of elasticity of elastic components, aging of rubber parts, permanent deformation, turtle Crack and so on.
