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Failure analysis

(XNUMX) The friction marks are larger than the width of the end face

  1. The vibration of the pump is too large when the pump is running (causing radial and axial vibrations during the operation of the moving ring, large changes in the thickness of the liquid film, and sometimes the sealing surface is pushed open, resulting in increased leakage).

  2. The narrow ring is not concentric with the shaft (the gap between the stop of the static ring gland is too large and it sinks due to its own weight).

(XNUMX) The friction marks are smaller than the width of the end face

  1. The inner edge of the narrow ring is high, the outer edge is low, the end face gap is convergent, the liquid film pressure increases, and the P ratio decreases, causing leakage (if the lubrication conditions are good, it will not be easy to run and return to normal for a long time).

  2. The outer edge of the narrow ring is high, the inner edge is low, the end face gap is horn-shaped, the pressure of the liquid film decreases, and the P ratio increases (if the lubrication condition is good, it can run faster and can be used, but the lubrication condition is poor, which will increase wear Will soon expire).

  3. The friction marks are discontinuous, or partial contact, and sometimes a large semicircle (commonly known as a horseshoe shape) appears to cause leakage (the hard ring is not flat, and it may be thermal stress deformation).

(XNUMX) No friction marks on the hard ring

  1. The transmission device and the shaft (or sleeve) slip (such as the top wire is loose under high temperature)

  2. The end face is not pasted and the installation is wrong

  3. The inlay ring loosens under high temperature

(XNUMX) Uniform ring grooves appear on the graphite end surface

  1. The liquid film on the end surface vaporizes, the working temperature of the medium is high, and the friction heat generates vaporization.

  2. The high temperature of the end surface reduces the performance of resin-impregnated graphite.

  3. Non-balanced mechanical seal, large K, high PV value, a large amount of friction heat produces vaporization.

  4. Poor lubricity of the medium (liquid hydrocarbon, hot water).

  5. Evacuation and cavitation, causing dry friction or semi-dry friction, making the grooves deeper

(XNUMX) A ring-shaped deep groove is formed in the middle of the graphite end face

  The end face is overheated and overloaded (prone to occur on the unbalanced mechanical seal of the diesel pump with higher temperature), the graphite tears and attaches to the hard ring to wear the graphite, and adhesive wear occurs.

(XNUMX) Wear of graphite inner edge

  The high-temperature pump is sealed, and the gland is filled with cooling water. The cooling water scales at high temperatures, and the graphite inner edge is worn away. At the same time, the moving ring is blocked without compensation. (Change to softened water or low-pressure steam)

(XNUMX) The graphite ring steps are all worn off prematurely

  1. Poor lubricity of the medium (such as butene, etc.), non-balanced mechanical seal, no flushing or cooling.

  2. The graphite is of poor quality, coarse in texture and not wear-resistant.

(XNUMX) There is a gap on the outer edge of the graphite ring

  The flushing volume is too large, the flushing hole is facing the graphite, and the graphite ring is eroded and chipped.

(XNUMX) Graphite ring fracture

  1. Light hydrocarbon medium

  The diameter is large, there is no auxiliary facilities such as washing, the PV value is large, the frictional heat is not easy to lose, and the vaporization between the sealing surfaces increases the temperature of the graphite ring.Due to the good heat transfer of graphite, the auxiliary sealing ring heats up and expands (PTFE), and the extrusion force is large.At the same time, expansion thermal stress exists near the end face of the seal.The action of two forces causes the graphite to break.

  2. Balanced seal. Due to installation errors, the step of the shaft sleeve is against the step of the moving ring, and the axial force is too large, causing the graphite to break.

(1) Corrosion pits appear in the graphite ring (size ranging from 5 to XNUMXmm, irregular shape)

  It mostly occurs on pumps with high-viscosity media (such as residual oil at about 100°C), which is in a sliding state due to extrusion stress on the surface.The graphite surface is internally subjected to shear stress.The greater the viscosity, the greater the friction, and the greater the shear stress. When the strength of graphite is exceeded, the surface will peel off and pits will appear.

(XNUMX) Burns and cracks on the surface of cemented carbide

  The annular grooves on the sealing surface are the result of the vaporization of the sealing surface (the sealing cavity is still liquid). Then, the surface burns and thermal cracks are caused by the liquid in the sealing cavity that has been vaporized or evacuated, that is, the friction pair is dry or semi-fluid. In dry friction, the surface temperature rises sharply, and the friction pair is overheated. Once the liquid reappears, the friction pair is rapidly cooled, resulting in a large temperature stress.For materials with good thermal conductivity and high strength (such as WC), there are traces of buffing and discoloration.Sometimes cracks appear on the sealing surface, and radial cracks appear on the surface of materials with poor thermal conductivity and low strength (surfacing).No matter which material, there will be ring grooves on the ring surface.

(XNUMX) Introduction to other faults

  1. The mounting ring is loose

  In high temperature pumps (or occasions with high PV value) the mounting ring is easy to loosen and cause leakage.If the medium is heavy oil, residual oil, etc., there are oil stains between the ring and the seat, but the wear is small (due to good lubricity), if the medium is gasoline, the liquid is light, there is abrasion between the ring and the seat, but there is no oil stain.

  2. Failure mode of auxiliary sealing ring

  (1) The mounting ring is loose and the end face is worn.

  (2) 4FV ring lip curling (cavitation in the sealed cavity.)

  (3) 4F changed from white to grayish yellow, and the two pieces were stuck together; the rubber cracked and stuck to the sealing ring. (The pump is evacuated, caused by dry friction on the sealing surface).

  (4) The sealing ring that only relies on the radial seal will lose its elasticity due to the aging of the rubber during use, the 4F cold flow will deform, and the loss of interference will cause leakage.

  3. Failure of the transmission mechanism

  Sealed transmission parts are often set screws, transmission screws, transmission pins, flanges, forks, or even a single large spring. Their failures are often:
  (1) High temperature makes it loose;
  (2) The cavitation of the pump or the installation of the non-compensation ring is inclined, causing vibration, shock and abrasion;
  (3) The end-to-face specific pressure is too high or frequently fluctuates greatly;
  (4) Improper assembly (easy to bite) causes high friction coefficient and poor lubricity;
  (5) Frequent opening and stopping or excessive force.

  4. The spring is broken

  (1) Improper design (spring index is too small, c=D/d should be ≥4 (5~8)), material defects, poor manufacturing, fatigue crack growth.

  (2) Stress, corrosion and fracture.

  (3) Overload fracture.

  5. There are pits on the shaft sleeve (by the compensation ring seal)

  (1) Cavitation of the medium in the pump
  (2) The installation of the non-compensation ring is inclined.These two reasons can cause vibration, and the axial vibration frequency of the seal ring is too high to cause pitting in the shaft sleeve.

  6. Failure of welded metal bellows

  (1) Missing bomb at high temperature, rupturing under high pressure

  (2) Coking and carbon deposition between wave plates, blocking and losing bomb (cannot be used in pumps with coking, carbon deposition and crystal precipitation)

  (3) The externally mounted type has a small elasticity, and the end face is opened

(XNUMX) Failure of pump evacuation seal

  Pump evacuation refers to cavitation in the impeller or no liquid in the pump, as well as leakage of gas or liquid vaporization.At this time, the seal is in a dry or semi-dry friction state.

  1. Visually detectable ring grooves appear on the surface of the graphite ring

  2. Radial cracks appear on the surface of the hard ring (surfacing or spraying)

  3. Ring grooves appear on the hard ring (WC), and sometimes radial cracks or breaks appear

  4. The auxiliary sealing ring is deteriorated (the rubber is aging, cracked, 4F turns grayish yellow, and the two pieces are bonded together in severe cases)

  5. The hot-loading ring falls off

(XNUMX) Cavitation in the sealed cavity and seal failure phenomenon

  Cavitation in the seal cavity refers to the fierce hydraulic shock around the mechanical seal, which drives the seal to undergo rapid axial reciprocating motion, which causes the seal to be severely damaged, which is much more serious than the pump evacuation.Its failure phenomenon includes the phenomenon caused by the pump evacuating, in addition, there are the following phenomena:

  1. The static ring is out of position or the anti-rotation pin bends

  2. 4F-V ring lip curling

  3. The moving ring is drawn into the transmission seat and jammed, and the spring loses its effect.

  4. The brittle material (such as SiC) in the friction pair will peel off due to knocking

(XNUMX) Failure phenomenon of vaporization on the end face of the seal

  The end surface vaporization is due to the high PV value, or the high temperature pump has no auxiliary facilities, or the flushing is interrupted (the filter is blocked), and the cooling jacket water of the high temperature pump is fouled, the cooling capacity is reduced, and the pressure in the sealed cavity is reduced by the pressure drop at the pump inlet. The increase in the content of light components in the light hydrocarbon pump may cause end surface vaporization.The sealed cavity is still full of liquid, no cavitation occurs, and no pump evacuation occurs. Its failure phenomena are:

  1. The end face of the graphite ring has uniform and fine ring grooves

  2. Grooves also appear in hard ring and surfacing welding

  3. The surface of the WC ring is partially polished, and grooves sometimes appear in hot water and light hydrocarbon media.

  4. A small piece of graphite is bonded to the surface of the hard ring, and the small fast graphite grinds the surface of the graphite ring into a deep groove.

(XNUMX) Failure phenomenon of excessive pump vibration

  Excessive vibration of the pump has no obvious failure phenomenon, but it has a great impact on the reliability and service life of the seal, and its degree depends on the degree of vibration.

  1. The sealing work is unstable, sometimes the two ends are open (especially the balanced mechanical seal), the leakage is large, and the service life is shortened.

  2. Mechanical impurities are easy to enter the end surface and aggravate wear.

  3. The friction trace is larger than the width of the end face.

  4. The shaft sleeve and auxiliary ring wear faster.

  5. The transmission mechanism is easy to damage and fail.

(XNUMX) Failure phenomenon without flushing

  1. The frictional heat cannot be conducted away in time, the temperature of the friction pair is high, the sealing end face is easy to vaporize, the work is unstable, and it is easy to fail;

  2. Mechanical impurities in the sealed cavity are easy to enter the end surface, and the end surface has grooves and becomes invalid;

  3. When the pump impeller is cavitation or the pump is evacuated, it is easy to damage the seal;

  4. When a small amount of vaporization in the sealed cavity, it is easy to develop into cavitation, which will seriously damage the seal (if there is injection flushing, it can be avoided);

  5. Impurities accumulate around the spring, block and lose elasticity;

  6. Some spare pumps do not run for a long time, and the sealed cavity is blocked, causing the cranking to not move.

(XNUMX) Matters needing attention in the selection of moving ring materials

  1. The thermal conductivity is higher than the static ring (the moving ring has rotating heat), which is conducive to heat dissipation, otherwise the end surface temperature is high and the service life is short;

  2. Do not spray ceramics as far as possible (poor thermal conductivity and easy to crack) in the occasions where the corrosion is small, especially in liquid hydrocarbons.

  3. In the sealing without flushing, try not to choose surfacing and low wear resistance materials, because the surface is prone to grooves and the service life is short;

  4. The high temperature pump is not well sealed for flushing, try not to heat the ring, that is to use, choose the material with the same line expansion coefficient;

  5. Cobalt-based WC has poor corrosion resistance and short life in acidic media;

  6. For the seal with a larger diameter (φ90), the moving ring driven by the flange does not use WC, but instead uses graphite or SiC (small wear).

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