What are the faults of mechanical seals? How to deal with it?
The mechanical seal failure and treatment are as follows:
First, the performance of mechanical seal failure on the parts:
1. Failure of the sealing end face: wear, hot cracking, deformation, damage (especially non-metallic sealing end faces).
2. Spring failure: relaxation, breakage and corrosion.
3, the failure of the auxiliary seal: assembly failures are falling, cracking, bumping, curling and twisting; non-assembled faults are deformed, hardened, cracked and deteriorated.
Mechanical seal failures manifest as vibration, heat, and wear during operation, and eventually appear as media leaks.
Second, the analysis and treatment of mechanical seal vibration and heat
1. The end face of the moving static ring is rough.
2. The gap between the static and dynamic rings and the sealed cavity is too small, causing collision due to the wobble. Treatment method: increase the inner diameter of the sealing cavity or reduce the outer diameter of the rotating part, at least to ensure a clearance of 0.75mm.
3. The sealing end face has poor corrosion resistance and temperature resistance, and the friction pair is improperly paired. Treatment method: Change the dynamic and static ring material to make it resistant to temperature and corrosion.
4. Insufficient cooling or end face with particulate impurities during installation. Treatment method: increase the diameter of the coolant pipe or increase the hydraulic pressure.
Third, the analysis and treatment of mechanical seal leakage
1. Leak during static pressure test
(1) When the sealing end face is installed, it is bumped, deformed, or damaged.
(2) When the sealing end face is installed, it is not cleaned and has particulate impurities.
(3) The sealing end face is not pressed because the positioning screw is loose or not tightened, and the gland (the static type static ring assembly is the pressure plate).
(4) The accuracy of the machine and equipment is not enough, so that the sealing surface is not completely fitted.
(5) The static and dynamic ring seals are not compressed or the amount of compression is insufficient or damaged.
(6) The direction of the V-ring seal of the static and dynamic ring is reversed.
(7) If it is a bushing leak, the bushing seal is not pressed or the amount of compression is insufficient or damaged when assembled. Treatment method: The inspection and cleaning during assembly should be strengthened, and the assembly should be strictly in accordance with technical requirements.
2. Periodic or paroxysmal leakage
(1) The axial displacement of the rotor assembly is too large. Treatment method: adjust the thrust bearing so that the shaft movement is not more than 0.25mm.
(2) The rotor assembly periodically vibrates. Treatment: Identify the cause and eliminate it.
(3) The pressure inside the sealed chamber often changes greatly. Treatment method: Stabilize the process conditions.
3, frequent leaks
(1) Frequent leakage due to seal end face defects.
a. The amount of spring compression (mechanical seal compression) is too small.
b. The amount of spring compression is too large, and the moving ring of graphite is cracked.
c. The width of the sealing end face is too small, and the sealing effect is poor. Treatment method: increase the width of the sealing end face and increase the spring force accordingly.
d. The floating performance of the compensation seal ring is too bad (the seal ring is too hard or hardened for a long time or the compression amount is too small, and the clearance of the compensation seal ring is too small). Treatment method: the gap of the compensation seal ring is too small, and the gap of the compensation seal ring is increased.
e, joint or joint movement, static ring joint seam leakage (poor installation process, residual deformation; material unevenness; uneven adhesive, deformation).
f, dynamic or static ring damage or cracks.
g, the sealing end face is seriously worn, and the compensation ability disappears.
h, dynamic and static ring sealing end face deformation (the end face is subjected to too much spring force, the friction increases to produce thermal deformation or eccentric wear; the structure of the sealing part is unreasonable, the strength is not enough, the deformation after stress; due to improper processing and other reasons, The sealing parts have residual deformation; the uneven force caused by the installation causes deformation). Solution: Replace the defective or damaged seal ring.
i. The deviation between the end face of the dynamic and static ring seals and the centerline of the shaft is too large, and the deviation of the parallelism between the moving and static ring seal faces is too large. Treatment method: Adjust the sealing end face.
(2) Frequent leakage caused by the auxiliary seal.
a, the material of the sealing ring is wrong, the wear resistance, corrosion resistance, temperature resistance, anti-aging performance is too poor, so that deformation, hardening, cracking, dissolution, etc. occur prematurely.
b. The compression of the O-ring is not correct. When it is too large, it is easy to install; the sealing effect is too small.
c. Install the shaft (or bushing) of the sealing ring, the sealing end cap and the sealing cavity. The surface of the O-ring is burred, the chamfer is not smooth or the corner is not round enough. Treatment method: The burr and the non-smooth chamfer should be properly smoothed, the arc and chamfer should be properly increased, and the smoothing should be smoothed.
d. O-ring seals are broken, cracked, broken, curled or twisted. Treatment method: Do not use gasoline or kerosene to clean the rubber ring; pay attention to the smoothness when assembling the sealing ring.
(3) Leakage due to spring defects.
a, the spring end face is skewed.
b. Multi-spring type mechanical seal, the free height difference between the springs is too large.
(4) Frequent leakage due to other parts, such as poor quality or looseness of the transmission, tightening and thrust parts.
(5) Frequent leakage due to the rotor, such as leakage caused by rotor vibration.
(6) Frequent leakage due to mechanical seal auxiliary mechanism, such as flushing coolant flow rate is too small or too large; pressure is too small or too large; liquid injection direction or position is wrong; poor injection quality, impurities.
(7) Frequent leakage due to problems with the medium.
a. The medium contains suspended particles or crystals. Due to accumulation for a long time, it is blocked between the moving ring and the shaft, between the springs, between the spring and the spring seat, so that the compensation seal ring can not float and lose the compensation buffering effect.
b. The suspended particles or crystals in the medium are blocked between the sealing end faces, so that the sealing end faces are not well fitted and wear quickly. Treatment method: Before starting the vehicle, first open the flushing and cooling valve, then drive and drive the vehicle after a period of time, and then open the large flushing coolant; properly increase the inlet temperature of the medium; improve the effect of medium filtration and separation.
Correct selection and use of mechanical seals for solid-liquid two-phase flow pumps
Due to the presence of abrasive particulate media in solid-liquid two-phase flow, the use of general mechanical seals often constitutes the following five hazards:
1 The end face of the seal is intensified. The sealing surface enters the end surface due to particle leakage, which acts as an abrasive and accelerates the wear of the sealing surface.
2 The media side particles are clogged. Due to the accumulation of particles, bridging hinders the movement of the spring, the pin and the auxiliary sealing ring, resulting in a decrease in the followability and floatability of the compensating ring.
3 Atmospheric side particles are clogged. Due to the mechanical seal of the conventional design, the gap between the inner diameter of the sealing surface and the shaft (or the bushing) is small, and the leaked solid particles cannot be discharged in time, which is easy to accumulate and block, hindering the movement of the auxiliary sealing ring, thereby causing the sealing failure.
4 abrasion. Refers to the localized bite and tear of the surface of the sealing element due to the action of abrasive particles. It usually occurs when using softer steel or graphite materials due to the impact of flushing water or sealant. In the case of granular media, it occurs more seriously;
5 wear of the transmission components. Since the components of the drive pin are in the granular medium, the wear of the particles is exacerbated by the grinding action of the particles during the movement.
When selecting the type, the mechanical seal product should avoid the action of the particles as much as possible, and the five kinds of failures should not be generated. There are two major ways to solve the problem of the granular medium by the mechanical seal: one is to set some additional internal structure or auxiliary for the mechanical seal. Measures (such as spiral seals, lip seals, seals, flushing water, tanks or tanks to establish liquid barriers to prevent particle build-up, etc.), or external devices (such as cyclone-solid-liquid separators, magnetic filters, etc.), to avoid the above five failures Appears to maintain a good working condition of the mechanical seal. This approach can be used for important equipment in more important situations. However, due to space constraints, or because the auxiliary facilities are too expensive, and because some occasions, materials are not allowed to have liquid sealing or flushing water into the product, the choice should be to design a new type of mechanical sealing structure, which can directly Used in granular media to meet the sealing requirements of the production process.
In order to achieve the purpose of reliable sealing, long life, simple structure, convenient assembly and disassembly, easy adjustment and low cost, the specific method is as follows: The spring and the auxiliary sealing ring are used in combination. The main advantage is that it has high elasticity and the spring is not in contact with the medium, avoiding the problem of being easily blocked by particles.
In order to ensure that the friction pair has wear resistance and abrasion resistance in the granular medium, the hardness of the friction pair material must be higher than the hardness of the abrasive particles. Hard and hard pairs are usually available, and the material can be tungsten carbide or silicon carbide. Compared with tungsten carbide, silicon carbide has higher hardness, better thermal conductivity, better chemical stability, and self-lubricating, but at a higher cost. According to the research of AI GoLubiev (former Soviet Union) and others on the mechanism of high hardness friction pair wear in abrasive granular media, the width of the friction pair should be wider than that of the general mechanical seal to obtain a higher service life. The widths of the moving and stationary rings are equal, which helps to prevent the particles from wearing on the sealing end face, and at the same time, has sufficient area to avoid large mismatch. Therefore, it is possible to accommodate radial and axial runouts that are much larger than the general mechanical seal end faces.
The mechanical seal of the mixed flow pump should be designed as an inflow type. The granular medium is on the outside of the seal ring. The action of centrifugal force and inertial force causes the particles and impurities to move outward and throw away from the sealing surface. Unlike general mechanical seals, the gap between the bushing and the seal ring should be large. When there is material leakage, it can be discharged in time to avoid particle accumulation and blockage. The design of the sealed chamber must have a suitable space to allow the material of the sealed chamber to flow without depositing deposits and to easily cool and lubricate the seal. In order to reduce the influence of the medium pressure in the pump on the specific pressure of the sealing end face, a balanced mechanical seal structure is adopted.
End face specific pressure is one of the most important factors affecting sealing performance and service life. In order to prevent the particulate medium from entering the sealing end face, the leakage amount is increased, the end face wear is intensified, and the sealing failure is caused, so that the end face specific pressure is larger than the general one. However, if the end face is too large, the friction surface will be heated and the wear will be aggravated, and the power consumption will increase. When designing, the specific pressure of the end face is about 0.3 MPa. (Source: Hebei Innovation Sealing Material Co., Ltd.)
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