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Analysis of the selection and failure of mechanical seals for high temperature hot water pumps

by:Lepu Seal     2022-01-23

Mechanical seals rely on one or several pairs of end faces that are perpendicular to the shaft for relative sliding under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism. It is composed of static ring (static ring), rotating ring (moving ring), elastic element, spring seat, set screw, rotating ring auxiliary sealing ring and stationary ring auxiliary sealing ring, etc. The anti-rotation pin is fixed on the gland to prevent The stationary ring rotates. Rotating rings and stationary rings are often also referred to as compensating rings or non-compensating rings depending on whether they have axial compensating capabilities. When the working temperature of the mechanical seal exceeds 80 °C, it is used as a high-temperature mechanical seal (some sealing companies take the working temperature> 90 °C as the limit). The dynamic and static ring seats of the mechanical seal for high-temperature hot water pumps are made of metal, which has a small expansion coefficient. Similar to tungsten carbide, it is easy to install the dynamic and static rings. The auxiliary seal adopts PTFE V-shaped seal to ensure the followability of the dynamic and static rings under the working temperature. The tank is flushed, and the structure is simple. Due to the use of a flushing liquid cooler, the auxiliary seal can be guaranteed to be within the allowable working range of PTFE. The problem of high-temperature auxiliary sealing is solved. At present, the mechanical seals for high-temperature hot water pumps at home and abroad generally use contact bellows single-end mechanical seals. The average service life of domestic bellows mechanical seals is about one year, and the imported bellows mechanical seals can reach more than two years. Various measures such as flushing and cooling are adopted to reduce the temperature rise of friction and prevent the failure of the seal caused by coking, thermal cracking, bellows loss, bellows breakage, pump evacuation, and aging of seals. To this end, a large amount of flushing oil and cooling water will inevitably be consumed, resulting in high operating costs (preliminary estimates, the annual operating cost of a hot oil pump for flushing and cooling exceeds 100,000 yuan), large energy consumption, poor sealing reliability, and safety. Low and high maintenance cost.

The selection of mechanical seals for high temperature hot water pumps mainly combines the following aspects:
1. Rotating structure of metal bellows: The rotary bellows seal can clean the bellows itself under the action of rotating centrifugal force, reduce the deposition on the periphery of the bellows and the coking on the inner side, and prevent the deformation of the bellows caused by rapid cooling.
2. The sealing structure adopts the slotted inclined surface to squeeze the shaft sleeve: this kind of positioning transmission is reliable, easy to install and disassemble, and does not damage the shaft, and is equipped with a limit plate, which is convenient for adjusting the compression amount of the seal outside the pump. A 45° bevel is set on one side of the inner diameter of the bellows to disperse the stress and prolong the life of the bellows. The auxiliary seal uses flexible graphite instead of synthetic rubber and can withstand high temperatures up to 425°C.
3. Selection of friction pair materials, a 'hard to hard' structure was adopted for the selection of friction pair materials. At first, SiC-WC was selected as the material for the dynamic and static rings, but after running for a period of time, it was found that the dynamic ring appeared edge gnawing.
Failure analysis of mechanical seals for high temperature hot water pumps:
1. The sealing surface of the moving ring is seriously worn. It may be that the end face specific pressure Pc is too large, so that the PcV value exceeds the allowable [PcV] value. Under the condition that the mechanical seal structure is constant and the pressure of the sealing medium is constant, only the spring specific pressure Psp determines the specific pressure Pc of the end face, so the larger specific pressure of the sealing end face may be one of the reasons for the wear of the sealing face.
2. The cooling water chamber is seriously deposited, the cooling water is insufficient or even cannot be cooled, and the liquid film between the sealing surfaces is vaporized, resulting in increased wear of the sealing surface and aging of the auxiliary sealing ring. The formation of scale is that the temperature of raw water increases after heating, the soluble Ca(HCO3)2 and Mg(HCO3)2 in the water begin to precipitate when heated to about 80 degrees, and Ca(HCO3)2 and Mg(HCO3)2 precipitate. Scale is formed on the inner wall. 3. The temperature of the sealing chamber is too high, and the sealing ring of the moving ring is thermally softened with the shaft sleeve, so that the moving ring cannot float freely in the axial direction to compensate for the wear or displacement of the moving ring, which is the main reason for the large leakage of the shaft seal and the splash reason.
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