There are many reasons that can cause the failure of mechanical seals in pumps, such as wear, corrosion, high temperature, installation, operation or errors in equipment itself. In this article, we will focus on the impact of temperature on pump mechanical seal parts.
Thermal cracking is one of the most common failure modes for pump mechanical seal parts, such as those used in residual oil pumps, coking cycle oil pumps, and vacuum tower bottom pumps. Dry friction, sudden interruption of cooling water, impurities entering the sealing surface, and evacuation can all cause radial cracks on the sealing surface.
Graphite carbonization is one of the main reasons for the failure of mechanical seals that use carbon-graphite rings. When the graphite ring exceeds the allowable temperature (generally between -105℃ and 250℃), resin will precipitate on the surface, and carbonization will occur near the friction surface. When there is a binder, it will foam and soften, resulting in increased leakage and seal failure.
Auxiliary seals (such as fluororubber, ethylene-propylene rubber, and all-rubber) will quickly age, crack, harden, and lose elasticity after exceeding the durability temperature. Flexible graphite is now used for its good high-temperature resistance and corrosion resistance, but its rebound is poor and it is easy to become brittle and crack, which can cause damage during installation.
The poor welding quality of the mechanical seals parts device piping system will seriously affect the safety of the feedwater pump. The alarm temperature of the mechanical seal liquid temperature is set to 80℃, and the pump will shut down at 90℃. Leakage of the mechanical seal device piping system will cause a rapid increase in the mechanical seal liquid temperature, and the cooling of the dynamic and static rings and the fitting surface will not be good, causing overheating and damage to the dynamic and static rings.
The maintenance of pump mechanical seal parts is more complicated. Because the assembled pump mechanical seal part must be leak-proof, it is necessary to check the technical dimensions of the pump mechanical seal parts.
Check the length of the anti-rotation pin and the depth of the pin hole, to avoid the pin being too long and the static ring cannot be assembled in place. This situation will damage the pump mechanical seal parts.
Measure whether the compensating spring length has changed. The change in the function of the spring will indirectly affect the surface pressure of the sealing end face. In some cases, the spring will elongate after a long time of operation, and the compensating spring on the dynamic ring will also deform due to centrifugal force.
Measure the dimensions of the sealing surfaces of the dynamic and static rings. It is used to verify the radial width of the dynamic ring. When using different friction materials, the radial width of the hard material friction surface should be 1-3mm larger than that of the soft material. Otherwise, the sharp corners of the hard material end face will be embedded in the end face of the soft material.
Check the surface pressure of the pump mechanical seal parts. If the surface pressure is too high, it will cause friction and heat on the friction surface of the mechanical seal, slow down the wear of the end face, and reduce the friction power; if it is too small, it is easy to leak. The surface pressure is determined during design, and we can only rely on the measurement of the tightening force of the pump mechanical seal parts during assembly. The usual method is to measure the vertical distance from the static ring end face to the cover end face after it is installed, and then measure the vertical distance from the dynamic ring end face to the cover end face. The difference between the two is the tightening force of the pump mechanical seal parts.
Check the clearance between the dynamic ring, static ring, and the shaft or sleeve. The inner diameter of the static ring is sometimes 1-2mm larger than the shaft diameter. For the dynamic ring, to ensure the floating performance, the inner diameter should be 0.5-1mm larger than the shaft diameter, to compensate for the shaft vibration and deviation. However, the clearance cannot be too large, otherwise, the sealing ring of the dynamic ring will be stuck, resulting in the failure of the pump mechanical seal parts function.