Classification of pumps and fans, characteristics of various pumps and fans (locations of use)

Performance characteristics and selection of canned pumps: With the development of the chemical industry and increasing awareness of environmental and safety issues, the demands on pumps used in chemical applications are also rising. In some cases, there is a requirement for absolutely zero leakage from certain pumps, and it is this demand that has driven the development of canned 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 electrical 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 formats commonly used in centrifugal pumps as well as relevant standard specifications. 2 Advantages and disadvantages of canned pumps 2.1 Advantages of canned 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 prevention. This makes it 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 lubricant. Due to the absence of rolling bearings and 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 the medium in shielded pumps is 0.1 to 20 mPa·s. (2) The efficiency of shielded pumps is usually lower than that of centrifugal pumps with single-end mechanical seals, but it is roughly comparable to that of centrifugal pumps with double-end mechanical seals. (3) When operating for long periods at low flow rates, the lower efficiency of shielded pumps leads to heating and vaporization of the liquid, resulting in dry running of the pump and thus damage to the sliding bearings. 3 Types and application ranges of canned pumps Depending 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 is not more than 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 of 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 heat transfer oils 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 pump drains the fluid through automatic vacuum creation, making it suitable for extracting liquid from underground containers. (7) Multi-stage type: Equipped with multiple impellers, suitable for high-head fluid transport, 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 may produce precipitates, coking, or gels, the use of a shielded pump (non-slurry type) in such cases can lead to blockage of the shielding gap, affecting the pump’s cooling and lubrication and resulting in damage to 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 insulated pipes should be employed 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 summary, 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 little need for maintenance during normal operation. Shielded pumps are ideal for transporting flammable, explosive, corrosive, and valuable liquids.

Since shielded pumps can achieve complete leaklessness, why is there then a slurry type? Slurry doesn’t require complete leaklessness, right? Let’s discuss it