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1.Axial displacement of the pump shaft
The sealing surfaces of the said mechanical seal must have a certain specific pressure (0.4 ~ 0.6MPa), so as to achieve sealing, and the tightness of the two end faces can be adjusted by springs. In order to ensure the specific pressure value is among this range, it is required that the pump shaft should not have too much displacement (≤0.5mm).
But in actual production, this is often not the case, which is very unfavorable for the use of mechanical seals. In the process of starting the pump, due to the existence of unbalanced axial force, the entire shaft moves to the suction port, causing the mechanical seal to lose its sealing effect.
Commonly used improvement measures are: opening a balance hole on the impeller, and/or installing a balance pipe on the pump body.
2. The deflection of the pump shaft is too large
When the shaft is bent and deformed, the linear displacement of the centroid of the cross-section in the direction perpendicular to the axis is called deflection. The mechanical seal requires uniform force between the two seal end faces. However, sometimes due to unreasonable design of the pump product, the deflection generated at the installation place of the mechanical seal is too large, causing force on the mechanical seal to be uneven, and sealing effect lost. This phenomenon mostly exists in horizontal multistage centrifugal pumps.
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3.No auxiliary flushing system or unreasonable design
The auxiliary flushing system of the mechanical seal is very important, it can effectively protect the sealing surface, and play the role of cooling, lubricating, and washing away debris. However, sometimes its design is unreasonable and the sealing effect cannot be achieved; some auxiliary systems suffer impurities in the flushing fluid, insufficient flow and pressure of the flushing fluid, and unreasonable design of location of flushing port, their correspondent mechanical seals’ sealing effects cannot be achieved，too.
Common measures to avoid the above conditions include：
（1）A reasonably-designed auxiliary flushing system is needed. Its flushing pressure is generally required to be 0.107~0.11MPa higher than the sealing chamber pressure, and the sealing chamber pressure value should be calculated according to the structural form of each pump,the whole system’s pressure and other factors. When the pressure of the shaft seal cavity is very high or even close to the allowed highest limit value of the mechanical seal, the liquid can also be drawn from the sealed cavity to the lower pressure area to take away frictional heat of the mechanical seal.
（2）The pipeline and accessories should be rationally configured as per the operating conditions of each pump. These instruments include coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature meters, etc. In fact, the reliability and life of the mechanical seal depend to a large extent on the configuration of the seal auxiliary system.
In addition,other factors such as insufficient machining accuracy of the pump shaft, shaft sleeve, pump body, and sealing cavity also affect the sealing effect of the mechanical seal and thus should be strictly rectified.
The sealing surfaces of the said mechanical seal must have a certain specific pressure (0.4 ~ 0.6MPa), so as to achieve sealing, and the tightness of the two end faces can be adjusted by springs. In order to ensure the specific pressure value is among this range, it is required that the pump shaft should not have too much displacement (≤0.5mm).
But in actual production, this is often not the case, which is very unfavorable for the use of mechanical seals. In the process of starting the pump, due to the existence of unbalanced axial force, the entire shaft moves to the suction port, causing the mechanical seal to lose its sealing effect.
Commonly used improvement measures are: opening a balance hole on the impeller, and/or installing a balance pipe on the pump body.
2. The deflection of the pump shaft is too large
When the shaft is bent and deformed, the linear displacement of the centroid of the cross-section in the direction perpendicular to the axis is called deflection. The mechanical seal requires uniform force between the two seal end faces. However, sometimes due to unreasonable design of the pump product, the deflection generated at the installation place of the mechanical seal is too large, causing force on the mechanical seal to be uneven, and sealing effect lost. This phenomenon mostly exists in horizontal multistage centrifugal pumps.
3.No auxiliary flushing system or unreasonable design
The auxiliary flushing system of the mechanical seal is very important, it can effectively protect the sealing surface, and play the role of cooling, lubricating, and washing away debris. However, sometimes its design is unreasonable and the sealing effect cannot be achieved; some auxiliary systems suffer impurities in the flushing fluid, insufficient flow and pressure of the flushing fluid, and unreasonable design of location of flushing port, their correspondent mechanical seals’ sealing effects cannot be achieved，too.
Common measures to avoid the above conditions include：
（1）A reasonably-designed auxiliary flushing system is needed. Its flushing pressure is generally required to be 0.107~0.11MPa higher than the sealing chamber pressure, and the sealing chamber pressure value should be calculated according to the structural form of each pump,the whole system’s pressure and other factors. When the pressure of the shaft seal cavity is very high or even close to the allowed highest limit value of the mechanical seal, the liquid can also be drawn from the sealed cavity to the lower pressure area to take away frictional heat of the mechanical seal.
（2）The pipeline and accessories should be rationally configured as per the operating conditions of each pump. These instruments include coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature meters, etc. In fact, the reliability and life of the mechanical seal depend to a large extent on the configuration of the seal auxiliary system.
In addition,other factors such as insufficient machining accuracy of the pump shaft, shaft sleeve, pump body, and sealing cavity also affect the sealing effect of the mechanical seal and thus should be strictly rectified.
