Explanation and solutions to common centrifugal pump mechanical seal leakage situations

Abstract: The causes of leakage of general-purpose centrifugal pump mechanical seals are analyzed, and various leakage phenomena and possible parts that may cause leakage are explained. At the same time, possible problems that may occur during the maintenance of general-purpose centrifugal pump mechanical seals are explained. A brief description.
1. Introduction to structural principles
Mechanical seal is a contact dynamic seal for rotating shafts. Under the action of fluid medium and elastic elements, two sealing end faces perpendicular to the axis center line are in close contact with each other and rotate relative to each other, thereby achieving sealing requirements.
There are many types of general centrifugal pump mechanical seals with different models, but their leakage points are basically at 6 places: ① The end faces of the moving and static rings; ② The auxiliary seals of the static ring and the static ring box; ③ The moving ring and the shaft The auxiliary seal of the sleeve; ④ the seal between the static ring box and the sealed pump body; ⑤ the seal between the shaft sleeve and the pump shaft; ⑥ the matching part of the moving ring inlaid structure.
Its main structure is shown in Figure 1.
Figure 1 Schematic diagram of mechanical seal structure
1. Bushing 2, sealing gasket 3, spring seat 4, spring 5, push ring 6, moving ring O-ring 7, retaining ring 8, moving ring 9, stationary ring O-ring 10, stationary ring 11, sealing packing 12 , anti-rotation pin 13, static ring seat 14, static ring seat seal 15, locking screw 16, pump shaft
2. Mechanical seal failure symptoms
(1) Failure of sealing end face: wear, thermal cracking, deformation, damage (especially non-metallic sealing end face). (2) Spring failure: relaxation, breakage and corrosion. (3) Failure of the auxiliary sealing ring: assembly failures include falling pieces, cracks, bruises, curling and distortion; non-assembly failures include deformation, hardening, cracking and deterioration.
Failures of mechanical seals are concentrated in the form of vibration, heat, wear and tear during operation, and eventually appear in the form of leakage of the medium.
3. Cause analysis and treatment of mechanical seal leakage
Generally, mechanical seals for pumps must undergo static and dynamic tests after installation to confirm that the mechanical seal is installed correctly. When leakage is found, it can be repaired in time. In addition, leakage may suddenly occur during normal operation. At this time, a comprehensive analysis can be conducted according to the situation to confirm the real cause of the mechanical seal leakage for easy solution. The three situations of leakage, periodic or paroxysmal leakage and regular leakage during the static pressure test are explained below.
3.1 Leakage during static pressure test
a. The sealing end faces are bruised, deformed, and damaged during installation; b. Granular impurities are caught during installation between the sealing end faces; c. The sealing end faces are loose or not tightened due to the positioning screws, and the gland (the static ring component of the static type is a pressure plate) There is no compression; d. The precision of the machine and equipment is not enough, so that the sealing end faces do not fit together; e. The sealing surfaces of the dynamic and static rings are not pressed tightly or the compression amount is insufficient or damaged; f. The “V” shaped sealing rings of the dynamic and static rings are installed in the opposite direction; g If the shaft sleeve leaks, it means that the shaft sleeve sealing ring was not compressed or the compression amount was insufficient or damaged during assembly.
Treatment: Strengthen inspection and cleaning during assembly; assemble strictly according to technical requirements.
3.2 Periodic or paroxysmal leakage
a. The axial movement of the rotor assembly is too large.
Treatment: Adjust the thrust bearing so that the axial movement of the shaft is not greater than 0.125mm.
b. The rotor assembly vibrates periodically.
Treatment: Find out the cause and eliminate it.
c. The pressure in the sealing chamber often changes significantly.
Treatment: Stabilize process operating conditions.
3.3 Frequent leaks
3.3.1 Frequent leakage due to defects in the sealing end face
a. The spring compression amount (mechanical seal compression amount) is too small. b. The spring compression is too large and the graphite moving ring is cracked. c. The width of the sealing end face is too small.
Treatment: Increase the width of the sealing end face and increase the spring force accordingly.
a. The floating performance of the compensation sealing ring is too poor (the sealing ring is too hard or hardened or the compression amount is too large, and the gap of the compensation sealing ring is too small).
Solution: The gap of the compensation seal ring is too small, increase the gap of the compensation seal ring.
b. Leakage at the joint between diamond-inlaid or bonded dynamic and static rings (poor inlaying process, residual deformation; uneven material; deformation of adhesive). c. Damage or cracks in dynamic and static rings. d. The sealing end face is worn and the compensation ability disappears. e. Deformation of the dynamic and static ring seal end faces (the spring force on the end face is too great, and the massage heat is too great, resulting in thermal deformation; the sealing parts have unreasonable structure, insufficient strength, and are deformed by force; due to processing and other reasons, the sealing parts have Residual deformation; deformation caused by uneven force during installation).
Treatment: Replace the defective or damaged sealing ring.
f. The vertical deviation between the dynamic and stationary ring sealing end surfaces and the shaft centerline is too large, and the relative parallelism difference between the dynamic and stationary ring sealing surfaces is too large.
Treatment: Adjust the sealing end face.
3.3.2 Frequent leakage caused by auxiliary sealing ring
a. The sealing ring is made of the wrong material, which is wear-resistant, corrosion-resistant, temperature-resistant, and has poor aging resistance, resulting in premature deformation, hardening, cracking, melting, etc. b. The compression amount of the “O”-shaped sealing ring is wrong. If it is too large, it will be easily damaged. If it is too small, the sealing effect will be poor. c. The shaft (or sleeve), sealing end cover and sealing cavity where the sealing ring is installed have burrs and chamfers on the surface where the “O” sealing ring is pushed, or the corner chamfering is not large enough.
Treatment: Burrs and rough chamfers should be trimmed and smoothed appropriately, and the arcs and chamfers should be appropriately enlarged and trimmed and smoothed.
d. The “O”-shaped sealing ring has fallen off, cracked, damaged, curled and twisted.
Treatment: Pay attention to assembly matters.
3.3.3 Leakage due to spring defects
a. The spring end face is deflected. b. For multi-spring mechanical seals, the free height difference between springs is too large.
3.3.4 Regular leakage due to other parts
Such as leakage caused by poor quality or loose transmission, tightening and thrust parts.
3.3.5 Regular leakage due to rotor
Such as leakage caused by rotor vibration.
3.3.6 Frequent leakage due to mechanical seal auxiliary mechanism
The flow rate of the flushing coolant is too large or too small, and the pressure is too large or too small; pay attention to the wrong direction and position; pay attention to poor quality and impurities.
3.3.7 Frequent leaks caused by media problems
a. There are suspended particles or crystals in the medium that have accumulated for a long time and become blocked between the moving ring and the shaft and between the springs. Between the spring and the spring seat, the compensation sealing ring cannot float and loses the compensation buffering effect. b. Suspended particles or crystals in the medium are blocked between the sealing end faces, causing rapid wear of the sealing end faces.
Treatment: Before driving, open the flushing coolant valve and wait for a period of time before turning and driving; for example, increase the flushing coolant; appropriately increase the medium inlet temperature; improve the media filtration and separation effects.
4. Several issues that need attention during maintenance
In actual maintenance work, it is often easy to have some misunderstandings, mainly due to deviations in the understanding of the sealing principle of some ancillary parts of the mechanical seal. Here are some brief explanations on this issue.
4.1 The greater the spring compression, the better the sealing effect.
In fact, if the spring compression is too large, it will cause rapid wear of the sealing friction pair, and it may be burned out in an instant. Excessive compression causes permanent plastic deformation of the spring and loses the ability to adjust the end face of the moving ring, thus causing seal failure.
4.2 The tighter the dynamic ring seal, the better
In fact, too tight a dynamic ring seal is harmful rather than helpful. First, it aggravates the wear between the seal ring and the sleeve, causing premature leakage; second, it increases the resistance to axial adjustment and movement of the moving ring, making it impossible to adjust in a timely manner when working conditions change frequently; third, the spring is easily damaged due to excessive fatigue; The fourth is to deform the dynamic ring sealing ring and affect the sealing effect.
4.3 The tighter the static ring seal, the better
The static ring sealing ring is basically in a static state. A relatively tight sealing ring will have a better sealing effect, but being too tight is also harmful. First, it causes excessive deformation of the static ring, which affects the sealing effect; second, the static ring is mostly made of graphite, which is generally brittle and can easily cause fragmentation due to excessive stress; third, it is difficult to install and disassemble, and the static ring is easily damaged. The selection of the installation form of the static ring should fully consider the floating ability under the corresponding working conditions. After installation, it can automatically follow the moving ring surface and fit with it. If the sealing ring of the static ring is too tight and the static ring cannot float after installation, then It will cause eccentric wear and premature failure.
4.4 The tighter the impeller locking nut, the better
Among mechanical seal leaks, leakage between the sleeve and the shaft (inter-shaft leakage) is relatively common. It is generally believed that inter-shaft leakage means that the impeller lock nut is not locked. In fact, there are many factors leading to inter-shaft leakage, such as failure and offset of the inter-shaft gasket, impurities in the shaft, and large gaps between the shaft and the sleeve. Shape and position errors, damaged contact surfaces, gaps between components on the shaft, too long threads on the shaft head, etc. will all lead to leakage between shafts. Over-tightening the lock nut will only cause premature failure of the intershaft washers. In fact, moderately tightening the locking nut can make the inter-shaft pad always maintain a certain compression elasticity. During operation, the locking nut will automatically lock in time, so that the shaft is always in a good sealing state.
4.5 New ones are better than old ones
Relatively speaking, the effect of using a new mechanical seal is better than that of the old one. However, if the quality or material of the new mechanical seal is improperly selected, or the fit size error is large, the sealing effect will be affected. In polymeric and permeable media, if there is no excessive wear of the static ring, it is better not to replace it. Because the static ring is in a static state for a long time in the static ring seat, the polymer and impurities are deposited into one body, which plays a better sealing role. In fact, in many cases, there is no problem with the mechanical seal. As long as appropriate adjustments and treatments are made, the mechanical seal can continue to be used. In this way, you can save costs, reduce workload, and ensure the sealing effect, which is a multi-purpose thing.
4.6 It is better to dismantle and repair than not to dismantle
Once the mechanical seal leaks, it is not completely correct to rush to dismantle and repair it. This approach should be analyzed based on the specific situation. In fact, sometimes the mechanical seal is not damaged, and leakage can be eliminated by adjusting the working conditions or adjusting the seal appropriately. This avoids the waste of parts and the useless work of maintenance personnel. During maintenance, correct judgment should be made based on the actual situation, so as to take targeted maintenance. According to the development trend of seals, many seals are now designed as cartridges, which effectively prevents many of the above leaks.