Working principle and characteristics of shielded pumps

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Read the article: Working Principle and Characteristics of Shielded Pumps. Thursday, May 29, 2008, 19:47. With the development of the chemical industry and an increasing awareness of environmental and safety issues, water pumps have become an essential part of people’s lives. There are high demands for pumps that are commonly used in daily life, such as oil pumps, chemical pumps, sewage pumps, vacuum pumps, magnetic pumps, diaphragm pumps, centrifugal pumps, self-priming pumps, multi-stage pumps, screw pumps, reciprocating pumps, shielded pumps, drum pumps, level gauges, food pumps, plastic pumps, gear oil pumps, submersible pumps, cleaning machines, pressure testing pumps, and other types of sewage pumps. The requirements for pumps used in the chemical industry are also increasing; in some applications, absolute leak-free performance is required from certain pumps. Such demands have contributed to the development of shielded pump technology. Due to the lack of a shaft seal, shielded pumps can achieve complete leaklessness, which has led to their increasing use in chemical processing plants. 1 Principles and structural features of shielded pumps: In ordinary centrifugal pumps, the drive mechanism involves connecting the impeller shaft of the pump to the motor shaft through a coupling, so that the impeller rotates together with the motor. A shielded pump, on the other hand, is a seal-less pump; both the pump and the drive motor are enclosed within a pressure vessel filled with the medium to be pumped. This pressure vessel has only static seals, and an electric winding set is used to generate a rotating magnetic field that drives the rotor. This design eliminates the rotating shaft sealing mechanism found in conventional centrifugal pumps, thereby achieving complete leaklessness. A canned motor pump combines the pump and the motor together; the rotor of the motor and the impeller of the pump are mounted on the same shaft. A shielding sleeve is used to separate the rotor from the stator of the motor. The rotor rotates within the medium being transported, with power being transmitted to it through the magnetic field of the stator. Furthermore, the manufacturing of shielded pumps is not complex; their hydraulic section can be designed and manufactured according to the structural types commonly used in centrifugal pumps as well as relevant standard specifications. 2 Advantages and disadvantages of shielded pumps 2.1 Advantages of shielded pumps (1) Fully enclosed. There are no dynamic seals in the structure; only static seals are present at the pump’s casing, which enables complete leak-free operation. It is particularly suitable for transporting flammable, explosive, valuable liquids, as well as toxic, corrosive, and radioactive liquids. (2) High safety. Both the rotor and the stator are equipped with shielding sleeves that prevent them from coming into contact with the material; even if these shielding sleeves are damaged, there is no risk of external leakage. (3) Compact structure with low land occupation. The pump and motor are integrated; no centering is required for disassembly or assembly. It has low requirements for the base and foundation, requires little routine maintenance, and results in low maintenance costs. (4) It operates smoothly, with low noise, and requires no lubricating oil. Since there are no rolling bearings or motor fans, no lubricant is required, and it produces low noise. (5) Wide range of applications. It can meet the requirements for various operating conditions such as high temperature, high pressure, low temperature, and high melting point. 2.2 Disadvantages of shielded pumps (1) Since shielded pumps use sliding bearings and are lubricated by the medium being transported, media with poor lubricity are not suitable for transportation using shielded pumps. Generally, the viscosity suitable for shielded pump media is 0.1 to 20 mPa·s. (2) The efficiency of shielded pumps is usually lower than that of sewage pumps with single-end mechanical seals, but it is roughly comparable to that of centrifugal pumps with double-end mechanical seals. (3) Operating for long periods at low flow rates results in lower efficiency of the shielded pump, which leads to heating and vaporization of the liquid, causing the pump to run dry and thus damaging the sliding bearings. 3 Types and application ranges of canned pumps Based on factors such as the temperature, pressure, viscosity of the liquid being transported, and the presence of particles, canned pumps can be classified into the following types: (1) Basic type: The temperature of the medium being transported does not exceed 120°C, and the head does not exceed 150 m. All other types of shielded pumps can be obtained by modifying and improving the basic model. (2) Reverse circulation type: In this type of shielded pump, the flow direction of the liquid used for bearing lubrication, cooling, and motor cooling is exactly the opposite to that in the basic type. Its main feature is that it is less prone to cavitation, making it particularly suitable for transporting liquids that are easy to vaporize, such as liquefied petroleum gas and chloromethane. (3) High-temperature type: The maximum temperature of the medium being transported is generally 350°C, the maximum flow rate is 300 m3/h, and the maximum head is 115 m. It is suitable for high-temperature liquids such as hot oil and hot water. (4) High-melting-point type: The pump and motor are equipped with jackets, which significantly improves the heat resistance of the motor. Suitable for high-melting-point liquids, with a maximum temperature of up to 250°C. Steam or a liquid at a certain temperature can be introduced into the jacket to prevent crystallization of liquids with high melting points. (5) High-pressure type: The casing of a high-pressure type shielded pump is a high-pressure vessel, enabling the pump to withstand very high system pressures. To support the shield casing under internal high pressure, the stator coils can be used to bear the pressure. (6) Self-priming type: When the suction pipe is not filled with liquid, the sewage pump drains the fluid through automatic air extraction, making it suitable for pumping liquid from underground containers. (7) Multi-stage type: Equipped with multiple impellers, suitable for high-head fluid transfer, with a maximum head of up to 400 m. (8) Slurry type: Suitable for transporting liquids mixed with a large amount of slurry. 4 Precautions for selecting shielded pumps: Generally, shielded pumps use a portion of the liquid being transported to cool the motor, and since the gap between the components is very small, the liquid to be transported must be clean. When transporting mixtures of various liquids that produce precipitates, coking, or gels, using a shielded pump (non-slurry type) in such cases may cause blockage of the shielding gap, affecting the pump’s cooling and lubrication and leading to the damage of the graphite bearings and motor. Shielded pumps generally come equipped with circulation cooling pipes; when the ambient temperature is below the freezing point of the liquid being pumped, anti-freezing measures such as insulation sleeves should be used to ensure easy pump startup. Additionally, when starting a shielded pump, it is essential to strictly follow the correct sequence for opening the outlet valve and the inlet valve. When shutting down the pump, first reduce the opening of the outlet valve; once the pump has stopped running, close the inlet valve first and then the outlet valve. In short, the use of shielded pumps ensures complete leaklessness, effectively preventing environmental pollution and material loss. As long as the right model is selected and the operating conditions remain stable, there is almost no need for maintenance during normal operation. Shielded pumps are ideal for transporting flammable, explosive, corrosive, and valuable liquids.