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1. Introduction The reliability of pumps depends to a large extent on mechanical seals. In petrochemical enterprises, the fluids handled are often flammable and explosive; if a mechanical seal fails and leaks, it not only contaminates the environment and endangers human health but can also lead to serious accidents such as fires and explosions. We start by focusing on the selection of seal structures for mechanical seals used in oil refining pumps under various operating conditions, in order to reduce the failure rate of these seals and improve their reliability. 2. Mechanical seals for high-temperature hot oil pumps. The high-temperature hot oil pumps produced by our factory mainly include bottom-feed pumps, reduced-bottom-feed pumps, and catalytic slurry pumps. These pumps are characterized by high temperatures (370°C–400°C), high oil viscosity, and the presence of impurities at the bottom of the tower. Given these characteristics of high-temperature pumps, we have adopted the YH-609 series mechanical seals. The structural design takes into account the following aspects: 2.1 Rotating metal diaphragm structure – The rotating diaphragm seal can clean itself under the action of centrifugal force, thereby reducing deposits on the outside of the diaphragm and coking on its inside, as well as preventing deformation of the diaphragm due to sudden cooling. The rotation speed of the pump equipment in our factory is generally 2950 r/min, and the linear speed of the mechanical seal is well below 25 m/s; therefore, a rotary design can be used. 2.2 The sealing structure uses a slotted beveled shaft sleeve for compression; this method provides reliable positioning and transmission, is easy to install and remove, and does not damage the shaft. A limit plate is also provided to facilitate adjusting the compression amount of the seal outside the pump. A 45° bevel is provided at one position on the inner diameter of the bellows to distribute stress and extend its lifespan. The auxiliary seal uses flexible graphite instead of synthetic rubber, allowing it to withstand high temperatures of up to 425°C. 2.3 Selection of friction pair materials For the selection of materials for the friction pair, we adopted a \"hard against hard\" configuration; initially, SiC-WC was chosen for both the stationary and moving rings, but after operation for some time, chipping occurred on the moving ring. After switching to YG6-YG6, the equipment has been operating normally. 2.4 Auxiliary measures for mechanical seals include jacket cooling of the seal chamber, self-flushing, and rapid cooling via seal baffle plates. The jacket cooling applied to the shaft seals of high-temperature hot oil pumps in our factory is generally carried out in accordance with the API610 standard. Considering its economic efficiency, positive-pressure self-flushing was adopted for the flushing process; the flushing pressure is generally required to be 0.07–0.1 MPa higher than the pressure in the sealed chamber. The flushing volume is as shown in Table 1: Table 1 Selection of Sealing Flushing Volume. Nominal seal diameter: 48–59, 60–84, 85–95, 95–133; average flushing volume: 0.067, 0.10, 0.13, 0.18. Our factory’s high-temperature hot oil pumps use water as the quenching fluid. The flow rate of this fluid can generally be determined based on the shaft diameter: when the shaft diameter is less than 100 mm, the flow rate is 0.2–2 L/min; when the shaft diameter is greater than 100 mm, the flow rate is 0.5–3 L/min. Given the poor quality and high hardness of the local water, we took measures such as enlarging the inlet and outlet holes for cooling the seal gland and increasing the gap between the baffle plates and the stationary ring, which resulted in a nearly two-fold increase in the flow rate of the quenching fluid. This improved the cooling effect of the seals and effectively prevented scaling on them. 3. Mechanical seals for gasoline and diesel pumps 3.1 Stationary structure of multi-spring mechanical seals The mechanical seals used in such pumps feature a stationary structure (bellows structures are used for pumps operating at temperatures above 200°C). First, avoid the heat generated by high-speed stirring ； Second is to prevent the spring from transmitting torque. 3.2 Selection of friction pair materials. For the selection of materials for the friction pair, we adopt a “WC-C” structure. When selecting auxiliary sealing materials, the following key factors should be considered: a. Good elasticity, with low compressive set ； b. Heat-resistant, does not stick at high temperatures ； c. Compatible with the medium, and does not easily suffer from swelling, hardening, etc ； d. It has a low coefficient of friction and good mechanical properties. Through comparison, we selected fluororubber with higher temperature resistance (temperature range: -20°C to 200°C). 3.3 Auxiliary sealing measures: For the mechanical seal of the light oil pump, we adopted jacket cooling of the sealing chamber and quenching without baffles. 4. Mechanical seals for low-temperature light hydrocarbon pumps 4.1 Multi-spring mechanical seals adopt a stationary structure. Measures such as increasing the length of the tail portion of the non-compensating ring and adopting a secondary rear cover structure are taken to enhance the anti-pumping-out capability of the mechanical seal in light hydrocarbon pumps. 4.2 Adopt a static ring back-cooling measure. For the shaft seals of light hydrocarbon pumps, where the medium is prone to vaporization, we have added back cooling. During normal operation of the shaft seals, it is possible to adjust the temperature at the end face, thereby bringing the phase transition radius closer to the inner diameter and increasing the proportion of the liquid film. When the pump is evacuated, the decrease in end-face temperature leads to a reduction in the flashing pressure, which positively contributes to preventing end-face separation. 4.2 Selection of friction pair materials. For the selection of materials for the friction pair, we adopt a “SiC-C” structure. For the auxiliary sealing material, low-temperature resistant silicone rubber is selected. 4.3 Adopt self-flushing measures. Increase the pressure in the sealing chamber through flushing to prevent dry friction of the seal caused by evacuation. Self-flushing measures not only ensure a high pressure of the medium within the sealing chamber, preventing vaporization of the medium or gas accumulation there, but also allow the friction heat generated by the friction pair to be removed promptly, thereby effectively controlling the temperature rise at the sealing surface, increasing the liquid phase area, and improving the friction conditions. 5. Conclusion: Choosing the mechanical seal design and materials appropriately, as well as making reasonable improvements to the seal, is of great importance for enhancing the reliability of mechanical seals. During this maintenance campaign, we selected and modified all pumps appropriately, and the equipment has been operating normally for the past year since its commissioning.