Selection of pumps for chemical applications

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When selecting a pump, it is necessary to take into account the characteristics of different pumps. Pumps are classified into two main categories based on the principle by which they act on liquids: vane pumps and positive displacement pumps. Vane pumps draw in or discharge liquid through the centrifugal force generated by the rotating vanes inside the pump. Positive displacement pumps draw in or force out liquid by means of the compressive action generated by the reciprocating or rotating motion of the pump’s piston or rotor. Vane pumps are classified into centrifugal pumps, axial flow pumps, and vortex pumps based on the structure of the vanes inside the pump. Positive displacement pumps are further divided into piston (plunger) pumps and rotary pumps. 4.1.1 For centrifugal pumps, the viscosity of the liquid at the delivery temperature should not exceed 650 mm2/S; otherwise, the pump’s efficiency will decrease significantly. (When the viscosity is greater than 650 mm2/S, the performance of centrifugal pumps declines significantly, and they are generally not used for such applications. However, since centrifugal pumps deliver fluid without pulsations, do not require safety valves, and allow for easy flow regulation, they are often employed in chemical manufacturing to transport liquids with a viscosity of up to 1000 mm2/S.) The flow rate is high, while the head is relatively low. The amount of gas dissolved or entrained in the liquid should not exceed 5% (by volume). When the liquid contains solid particles, special centrifugal pumps (such as slurry pumps) are advisable. If a large variation in flow rate with little change in head is required, a centrifugal pump with a flat flow-rate–head curve is suitable; if a small variation in flow rate with a large change in head is needed, a centrifugal pump with a steep flow-rate–head curve is appropriate. 4.1.2 For positive-displacement pumps, when the viscosity of the liquid at the transfer temperature is greater than 650 mm2/S, and the flow rate is low while the head is relatively high, reciprocating pumps are suitable. Gases dissolved in or entrained in the liquid allow a concentration of slightly more than 5% (by volume). When precise measurement of the liquid is required, a plunger-type metering pump can be used; when absolute leakage prevention is necessary, a diaphragm metering pump is suitable. Gear pumps and triple-screw pumps should not be used for liquids with poor lubricity properties, as reciprocating pumps are a better choice in such cases. For applications with low flow rates, low temperatures, and stable pressure requirements, rotary pumps or twin-screw pumps are suitable choices. Type Applicable viscosity range (mm2/s): Blade pumps, Centrifugal pumps – <150; Vortex pumps – <37.5; Positive displacement pumps: Reciprocating pumps – <850; Metering pumps – <800; Rotary piston pumps – 200–10,000; Single-screw pumps – 10–560,000; Twin-screw pumps – 0.6–100,000; Three-screw pumps – 21–600; Gear pumps – <2,200. 4.2 The selection of a pump should be initially determined based on the flow rate and head required by the installation, as well as the classification and applicable range of the pumps. Since centrifugal pumps have a simple structure, produce no pulsations in the fluid flow, and allow for easy flow regulation, they should be preferred whenever possible, except in situations where they are not suitable. Once the pump selection is determined, the pump series and material can be chosen based on the parameters of the process unit and the properties of the medium. Then, the specific model (specifications) of the pump is determined based on the samples and relevant technical documents provided by the pump manufacturer. 4.3 When a pump designed for special media is used to transport gas-containing liquids, the pump’s flow rate, head, and efficiency all decrease. The higher the air content, the faster the efficiency declines. As the gas content increases, the pump generates additional noise and vibration; in severe cases, this can lead to increased corrosion or even flow interruption and shaft failure. To ensure the reliable operation of the pump, measures can be taken to reduce the gas content in the liquid: 4.3.1 The design of the liquid suction tank and the arrangement of the pump’s suction pipes should ensure that each pump in parallel can draw in an equal amount of liquid. The pump’s suction inlet should have a certain depth of submersion within the tank, as well as a certain height above the bottom of the tank. 4.3.2 The inlet pipe, return pipe, and waste liquid collection pipe of the liquid absorption tank should be kept away from the pump’s suction port to prevent bubbles from being drawn into the pump before they have dissipated. At the same time, the suction tube should not be placed in the center of the tank, nor too close to the tank walls; it should generally be at a distance of more than 1 unit from the tank walls. 5D, to avoid creating vortices or vacuum. Ensure that the pipe joints are properly sealed to prevent air from leaking in. When arranging the inhalation pipeline, areas where air pockets may form should be avoided. 4.3.3 When transporting liquids containing solid particles, the solid particles suspended in the liquid cannot absorb, store, or transfer energy like the liquid does, nor can they transfer kinetic energy to the liquid. The presence of solid particles results in lower pump head and efficiency compared to transporting clean water. 4.3.4 When transporting volatile liquids, the main consideration is the impact of the properties of these liquids on the pump: 4.3.4.1 High pressure at the pump inlet; 4.3.4.2 The vaporization pressure changes significantly with temperature. 4.3.43 Strict requirements for the shaft seal of the pump, and the impact of the pump’s suction pressure on the vaporization of the material. 4.3.5 For transporting liquids that must not leak, sealless pumps (magnetic drive pumps and shielded pumps) or mechanically sealed pumps equipped with leakage collection and alarm devices should be used. Comparison between magnetically driven pumps and shielded pumps:
Item | Magnetically Driven Pump | Shielded Pump
--- | --- | ---
Thickness of isolation sleeve (or shield) | 3 times that of the shielded pump’s isolation sleeve |
Consequences of damage to the isolation sleeve (or shield) | Fluid leaks into the atmosphere | There is a second leak-proof layer (motor casing) that prevents fluid from leaking into the atmosphere, but this can damage the motor stator
Efficiency | Slightly lower | Slightly higher
Remote operation | Not available currently | Possible
Requirements for manufacturing technology and equipment | Lower | Higher
Driver | Standard motor or turbine | Specialized motor
Noise | Slightly higher (motor with fan) | Slightly lower (motor without fan)
Axial length | Longer | Shorter
Coupling | Coupling is present; alignment is required | No coupling
Bearing wear monitor | Still in the experimental stage | Available
Price | Basically similar | Basically similar
Ease of routine maintenance | Easy | Difficult
Application range (power in KW/temperature in °C/pressure in MPA) | Good performance | Good performance; suitable for normal pressure, room temperature conditions with clean fluids that do not easily vaporize | Generally not suitable; specialized models are available
Fluids containing solid particles | Generally not suitable; especially not suitable for fluids containing iron particles | There are models designed for fluids with solid particles (external cleaning fluid is required)
High-melting-point, crystalline fluids | Generally not suitable | Specialized models are available
Highly corrosive fluids | Good performance | Not suitable (due to limitations of the shielding sleeve)
4.3.6 For transporting corrosive fluids, corrosion-resistant pumps should be used. These pumps feature corrosion-resistant materials in their flow-through components, while other non-corrosion-resistant parts, such as brackets, need to be protected from corrosion. The clearance of the sealing ring (flange ring) is larger than that of the water pump, to prevent operation at low flow rates and thus avoid increased corrosion due to rising liquid temperature. When stopping the pump, the suction valve should be closed promptly, or a shutdown seal should be used to prevent the medium from leaking out of the pump. For commonly used metal pumps, the materials used for their flow-through components include ordinary cast iron, high-silicon cast iron, stainless steel, high-alloy steel, titanium and its alloys, etc. Different materials can be selected depending on the properties of the medium and the temperature range. High-alloy steels, titanium, and their alloys are expensive, and their use should generally be avoided unless it is absolutely necessary. Metal pumps generally have better temperature resistance, pressure resistance, and operational stability than non-metallic pumps. The materials used for the flow-through components of non-metallic pumps include polyvinyl chloride, fiberglass-reinforced plastic, polypropylene, F46, fluorocarbon alloys, PVDF, ultra-high molecular weight polyethylene, graphite, ceramics, and glass-lined materials. The material should also be selected based on the properties of the medium and the temperature range. Generally, non-metals have lower temperature and pressure resistance compared to metals. Therefore, it is commonly used in applications with low flow rates, as well as those with low temperatures and operating pressures. 4.3.7 Selection of vacuum pumps: Vacuum pumps are used to achieve a vacuum level lower than atmospheric pressure. They are typically classified based on the degree of vacuum or the pumping speed. They can generally be divided into vane types, positive-displacement types (piston and screw types), and water-ring types. There are several indicators that can reflect the performance of a vacuum pump: A. Vacuum level: expressed in terms of absolute pressure P, with units of Kpa, Torr, and mmHg (1 atmosphere = 101.325 Kpa = 1 Torr = 760 mmHg = 1.01325 bar). B. Pumping speed: refers to the volume of gas drawn into the vacuum pump’s inlet per unit of time (i.e., the volumetric flow rate under normal temperature and pressure conditions). Per cubic unit: cubic meters per hour, liters per second. The pumping speed of a vacuum pump is related to the suction pressure; the higher the suction pressure, the greater the pumping speed. C. Ultimate vacuum: refers to the stable lowest pressure value that can be achieved by the vacuum pump during pumping, also known as the maximum degree of vacuum. D. Selection of various pumps: Mechanical pumps and piston pumps are suitable only for applications with low vacuum levels and little water vapor present; for example, W-type reciprocating vacuum pumps. Vane pumps are used in situations where a large volume of gas needs to be evacuated, and an auxiliary pump is necessary in such cases. Rotary vane vacuum pumps have a relatively low pumping speed. Molecular pumps are suitable for applications requiring high precision. Water ring pumps are appropriate for low-vacuum applications, such as those involving the removal of water vapor. Jet pumps, including water jet pumps, use water under pressure of 0.2–0.3 MPa as a driving force; they are suitable for removing water vapor and condensable gases. They function by utilizing the principle of conversion between static pressure energy and dynamic pressure energy during fluid flow. Steam jet pumps can handle various gases, and different numbers of stages can be selected according to specific requirements. Oil diffusion pumps are suitable for high vacuum levels, as are molecular centrifugal pumps. This post was last edited by hmwangsh on 2007-1-31 08:37