Multiple coke oven gas desulfurization MDEA solution pumps are installed in the 200,000 tons/methanol plant, all of which are HG9 single-end face balanced mechanical seals for shaft end sealing. Because the system solution is a desulfurization alkaline solution, it has certain corrosiveness and environmental hazards . The sealing effect of the mechanical seal will directly affect the operation of the pump, and serious environmental pollution accidents will occur. Combined with the daily maintenance of the MDEA solution circulating pump, several factors affecting the sealing effect and the reasonable measures that should be taken are analyzed from the perspective of the internal and external conditions of the mechanical seal.
Cause Analysis of Seal Failure of MDEA Solution Pump
The sealing surface itself will also provide signs of seal failure. For example, when there is vibration, there will be traces of wear on the transmission parts. If the traces are not obvious, it is generally caused by improper assembly. For the graphite ring with poor quality (moving ring), there are more pores inside. When the seal is activated, the carbon particles are easy to fall off, and the sealing surface sticks when the seal is deactivated. The scars on the inner cylindrical surface of the sealing surface are likely to be caused by foreign matter entering the sealing surface or the annular groove on the sealing surface caused by improper installation. Most of them are caused by the deposition of solid particles on the sealing surface. The cracks of the stone stack ring (moving ring) are caused by the expansion of the vibration rubber ring of the transmission part and the internal stress of the graphite ring itself. The surface coking is caused by high temperature. This is common in the high temperature hot oil medium of the oil refinery. The rubber parts are cut and scratched on the surface during installation, which are also common reasons for seal failure. The old fixing screw keyway, spline shaft, sharp shoulder and other signs on the shaft will damage the rubber parts. Here, the following points need to be added to the seal surface wear traces to check the wear traces, which can help analyze the failure
(1) Widened wear: It indicates that there is a serious misalignment of the pump and the reason is:
a, bearing damage
b. Shaft vibration or shaft deformation
c, the shaft is bent
d. Pump cavitation produces vibration;
e. The coupling is misaligned;
f. The pipe is severely deformed
g, the sealing static ring is inclined
(2) The wear scar is narrowed: the wear scar is narrower than the minimum width of the two sealing surfaces, which means that the sealing surface is deformed by the sealing overpressure or temperature
(3) No wear marks: It means that the sealing surface is not bonded. Check whether the compensation mechanism such as the spring is slippery or obstructed.
(4) There are no wear marks on the sealing surface but bright spots on the sealing surface. Bright spots will appear when the sealing surface is warped. The pressure is too high. The cover bolts are not tightened or clamped, or the pump surface is rough. Bright spots can be formed. When the gland with two bolts is used, its rigidity is not enough, and deformation is also a reason for the formation of bright spots. The appearance of this symptom indicates that the seal may leak as soon as the vehicle is driven.
(5) There are cut edges on the sealing surface: this is because the sealing surface is too separated and breaks when being closed. Flashing (vaporization) is a common cause of separation of the sealing surface, especially when there is condensate in the hot water system or fluid, the water expands from the liquid to the gas, which can separate the sealing surface. (The vaporization of the cold medium will also cause it).
Sealed metal parts, such as springs, fixing screws, transmission parts, and metal sleeves, may be the root cause of seal failure. Springs under alternating stress are corroded is its primary problem because metal will corrode rapidly under stress, and stainless steel Springs are susceptible to stress corrosion of chlorides and there are many chlorides in the world. Therefore, foreign departments recommend that stainless steel springs should not be used. It is recommended to use springs of Hastel alloy steel with higher corrosion resistance. In addition, improper assembly causes spring fatigue. Another reason for the failure is that the fixing screws used in the mechanical seal should not be made of hardened materials because heat treatment will reduce the corrosion resistance of the metal, and the softer fixing screws that have not been heat treated can be fastened to the shaft. The vibration and deflection are different. The core will wear the transmission parts. For example, when the sealing surface is stuck when the sealing surface is started, the transmission parts will bend or even be damaged. The heat generated by the friction often aggravates the wear marks that corrode the outer surface of the metal sleeve, which may enter the sleeve from the sealing side. It is caused by solid particles to interfere with the follow-up capability of the seal. It may also be caused by skewness and inconsistency.
Causes of vibration and heating failure of mechanical seal of MDEA solution pump
During the rotation of the equipment, the gap between the dynamic and static ring and the sealing cavity will be too small, and the vibration will be caused by the collision caused by the vibration. Sometimes due to poor corrosion resistance and temperature resistance of the seal end face, insufficient cooling or particle impurities on the end face during installation, vibration and heat generation of the mechanical seal can also be caused.
Causes of the leakage of the mechanical seal medium of the MDEA solution pump
(1) Leakage during static pressure test. Due to the careless installation of the mechanical seal, the end face of the seal is often bruised, deformed, damaged, not cleaned, granular impurities are trapped, or the machine is not compressed due to the loosening of the positioning screw, and the accuracy of the equipment is not enough to make the sealing surface incomplete. Fitting will cause medium leakage. If the sleeve is leaking, the sleeve seal is not compressed or compressed or damaged during assembly.
(2) Periodic or paroxysmal leakage of the rotor assembly of the mechanical seal. Periodic vibration and too much axial movement will cause leakage. The sealing surface of the mechanical seal must have a certain specific pressure so that it can play a sealing role. This requires the mechanical seal. The spring must have a certain amount of compression to give the sealing end surface a thrust to rotate so that the sealing surface produces the specific pressure required by the seal. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft cannot have too much swaying, generally within 025mm. However, in actual design, due to the unreasonable design, the pump shaft often produces a large swaying amount, which affects the mechanical seal. The use of it is very unfavorable.
(3) Frequent leakage of mechanical seals. There are many reasons for the frequent leakage of mechanical seals. The first aspect is the regular leakage caused by the defect of the seal end face. The second aspect is the regular leakage caused by the auxiliary seal ring. The third aspect is the leakage caused by the spring defect. Other aspects include the rotor. Leakage caused by vibration The quality of the transmission tightening and thrust parts is not good or loose, which causes leakage. The leakage caused by the auxiliary mechanism of the mechanical seal. Frequent leakage caused by the problem of the medium.
(4) The vibration of the mechanical seal is too large. The vibration of the mechanical seal is too large, which will eventually lead to the loss of the sealing effect. However, the reason for the large vibration of the mechanical seal is often not only the reason of the mechanical seal itself. Other parts of the pump are also the source of vibration, such as the design of the pump shaft. Reasons for unreasonable processing, insufficient bearing accuracy, poor parallelism of couplings, large radial force and other reasons.
