Sealless pump: canned motor pump

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1. Concept of shielded pumps: Shielded pumps, also known as sealless pumps, are a type of centrifugal pump. They are developed by integrating the pump’s impeller with the motor rotor, and these components are placed within the same sealed housing. Non-magnetic metal sheets are installed on the surface of the stator assembly and on the outer surface of the rotor assembly to completely isolate it from the liquid, thereby creating a shield that ensures full sealing with no leaks. 2. Classification of canned motor pumps
2.1 Basic type canned motor pumps
The basic type is the most commonly used type in practical applications. It is used for pumping media that are not prone to vaporization, whose temperature does not exceed the specified maximum inlet temperature of the motor, and which contain no particles and have a relatively low melting point. The basic type further includes the inner-loop basic type and the outer-loop basic type. 2.2 Reverse-circulation type shielded pump: The reverse-circulation type is suitable for transporting easily vaporizable liquids. Its characteristic is that the lubrication and cooling path is opposite to that of the basic type, hence it is called the reverse-cycle type. This type of pump does not require a vent valve; its reverse circulation circuit allows the coolant in the motor to be discharged into the gas phase area of the liquid inlet tank, thereby ensuring that no gas remains within the pump and motor chambers. This prevents issues related to bearing lubrication and motor cooling. Such a design is suitable for transporting low-temperature liquefied gases, such as LPG and LNG, which are liquids with low density and high volatility. When a counter-circulation type shielded pump is used for liquid transfer, in what direction does the liquid flow? When a counter-circulation type shielded pump transports liquid, the flow direction of the liquid is divided into three parts: the main flow part and two circulation liquid parts. (1) In the main fluid stream, as the liquid enters through the pump inlet, it moves rapidly due to the rotation of the impeller, and under the effect of centrifugal force, the main stream is discharged from the pump outlet. (2) A portion of the liquid in the circulating fluid stream, as the main flow is discharged from the impeller outlet, enters the gap between the stator and rotor shields through the small holes in the end cover. From there, it moves into the gap between the rear bearing and the shaft sleeve, and then flows toward the reverse circulation pipe on the end cover, returning to the liquid inlet tank. (3) Another portion of the liquid passes through the impeller via the gap between the front cover and the impeller hub, and returns to the inlet. Why can’t the liquid in a counter-circulation shielded pump vaporize? The temperature of the liquid rises due to the heat generated by the friction between the shaft and the bearings, as well as the heat from the motor. When this heated liquid returns to the inlet of the impeller, vaporization of the liquid occurs, leading to cavitation. This results in vibrations, noise, reduced flow rate, and insufficient head in the pump, preventing it from functioning properly. In contrast, in a counterflow pump, the heated liquid does not enter the suction side of the pump but instead returns to the liquid reservoir, thereby avoiding the occurrence of cavitation. 2.3 High-temperature separation type shielded pump: The high-temperature separation type is suitable for transporting high-temperature liquids such as heat transfer oil and hot water. The high-temperature liquid mentioned here generally refers to those with a temperature higher than the maximum allowable temperature for the liquid entering the motor, but lower than 350°C (450°C in the case of specially designed units). All high-temperature separation types are equipped with a connecting element (also known as a heat shield), a heat exchanger, and a cooling jacket. Even though the temperature of the liquid being transported is higher than the specified maximum temperature for liquid entering the motor, the heat shield of the connecting element along with the cooling effect of the heat exchanger ensure that the temperature of the liquid entering the motor chamber remains below that specified maximum value. The cooling of this shielded electric pump motor and the lubrication of its bearings are achieved through forced circulation by a secondary impeller installed at the rear of the rotor. What is the direction of fluid flow in the liquid transport by high-temperature shielded pumps? (1) The main flow direction of the liquid is from the pump’s suction inlet into the pump chamber, where the high-speed rotation of the impeller generates centrifugal force. Driven by centrifugal force, it is discharged from the impeller’s vanes to the pump’s outlet. (2) A portion of this circulating liquid is transported forward by the secondary impeller: through the gap between the stator shield sleeve and the rotor shield sleeve, through the gap between the front bearing and the front shield sleeve, through the heat exchanger, through the central hole in the rear end cover, through the gap between the rear bearing and the rear shaft sleeve, through the central hole at the pump shaft end, and then to the shield sleeve. It undergoes back-and-forth cycles, forming a low-temperature circulation system. (3) Another portion of the circulating fluid is forcibly pumped by the auxiliary impeller; this fluid enters the central hole at the shaft end through the gap between the rear bearing and the rear sleeve, and the high pressure generated by the impeller is transmitted into the motor chamber via the inclined small holes in the connecting element, thereby preventing cavitation from occurring in the area around the auxiliary impeller. 2.4 Slurry-separating shielded pumps are used for transporting liquids containing a small amount of soft particles. Their characteristic is that the circulating fluid for the motor is clean mother liquor injected into a rear filling tank; the liquid in the motor chamber and the pump chamber are separated by a mechanical seal. Generally, pressure is applied to keep the pressure in the filling tank higher than the pressure at the pump inlet, thereby preventing particles from entering the motor chamber. In addition, depending on the properties of the medium, there are also shielding pumps such as the ultra-high heat resistance basic type, self-priming type, and high melting point type.