The structure and performance characteristics of non-metallic pumps
Non-metal pumps are highly competitive in terms of corrosion resistance and wear resistance, especially the price is relatively economical compared to metal pumps, so non-metal pumps are widely used in chemical, smelting, environmental protection and other industries.
The following factors should be considered when selecting non-metallic pumps: mechanical properties, corrosion resistance and wear resistance, temperature, weight and thermal insulation properties, and flame resistance.
Common non-metallic pumps are: fluoroplastic pumps (polyfluoroethylene propylene F46, fluorine alloy, polyvinylidene fluoride PVF2, etc.), ultra-high molecular weight polyethylene pumps, glass fiber reinforced plastic pumps, and reinforced polypropylene pumps.
Fluoroplastic pump
1, the structure of non-metallic pump
(1) Pump body
The internal cavity of the pump is a spiral flow channel, which is used to collect the liquid thrown from the impeller and lead it to the diffuser tube to the pump outlet. The liquid flow cross-section of the diffuser tube is from small to large, so it can convert part of the kinetic energy of the liquid into static pressure. The pump body has two structures: an integral pump body and a lined pump body. Integral pump body means that the whole pump body is made of non-metallic materials, and is generally only suitable for small-caliber centrifugal pumps. The lining pump body generally refers to the material of the pump body is cast iron, and the lining layer is non-metal. Non-metallic pumps are generally single-stage, volute type, radially split structure.
(2) Impeller
The impellers of non-metallic pumps are divided into three types: open, semi-open and closed. Generally, closed impellers are suitable for conveying media without solid particles, while open and semi-open impellers can also convey media containing solid particles.
There are two types of impellers: integral and lining. The integral impeller has low strength and can only be applied to small pumps. Metal parts are embedded in the lining impeller, so the impeller has high strength.
(3) Seal ring (mouth ring)
A mouth ring is generally provided in metal pumps, but generally not in non-metal pumps. Instead, the axial clearance between the impeller and the pump body is used to adjust the operating clearance.
(4) Shaft and bearing
The impeller is fixed at one end of the pump shaft and a coupling is installed at one end. The pump shaft and the impeller of non-metallic pumps are usually screwed together (Figure 298). The bearings of non-metallic pumps are usually ball bearings, which are lubricated by thin oil.
(5) Shaft seal The shaft seal types of non-metallic pumps include mechanical seal, dynamic seal and no shaft seal structure (
①Mechanical seals Many domestic non-metallic pump factories usually use 152 type (or SB2 type) externally mounted bellows multi-spring single-end mechanical seals. The seal is suitable for sealing cavity pressure 0~0.5MPa, shaft diameter 25~55, temperature <120℃, moving ring material is filled with PTFE, static ring material is alumina ceramic. The seal can be used for more than 30% hydrochloric acid, less than 50% dilute sulfuric acid, more than 98% phosphoric acid and mixed acids of various concentrations. The seal has a long service life, generally greater than 4000 hours. When the working medium is special, the manufacturer can choose other types of mechanical seals (including materials) according to the characteristics of the medium
②Power seal includes auxiliary impeller (or auxiliary blade) seal and parking seal. This type of sealing is often used to transport media containing solid particles. Dynamic sealed pumps need to indicate the maximum allowable inlet pressure to avoid a large amount of leakage when the user uses it at a higher inlet pressure.
③No shaft seal structure refers to a non-metal magnetic drive pump. Magnetic drive pumps can be used to transport various liquids that are not allowed to leak.
Due to the difference in performance between non-metallic materials and metallic materials, the structural design of non-metallic pumps has the following particularities.
(1) The running clearance between the impeller seal and the back vane and the pump body
Because the thermal expansion coefficient of plastic and glass fiber reinforced plastic is 2~6 times larger than that of steel, the running clearance of metal pump is prone to seizure and damage the pump. Excessive clearance reduces the efficiency of the pump and causes the imbalance of the axial force of the pump. Since the radial clearance cannot be adjusted, most of the non-metallic chemical pumps outside the nunnery use axial clearance. In this way, the running clearance can be checked and adjusted without disassembling the pump.
(2) Impeller and shaft connection type
①The metal parts are embedded in the impeller, and the metal parts are connected to the shaft with a key, or the impeller and the shaft are directly connected with a key, and the end of the impeller is protected by an impeller nut. This structure comes from a metal pump, and its reliability is poor due to its many sealing surfaces. After long-term use, the shaft is easily corroded by the medium.
②The metal parts are embedded in the impeller, which is connected with the pump shaft by thread, and the impeller nut is eliminated. This structure can completely protect the shaft to avoid the possibility of the shaft being corroded by the medium. The disadvantage is that the thread processing accuracy is relatively high, and the motor is strictly prohibited to reverse during operation.
(3) The thickness of the lining layer of the pump body and its fixation are in accordance with the standards of foreign pump companies. The thickness of the lining layer is generally 3~5mm. The lining layer of some domestic manufacturers is too thin (about 1.52mm), which makes local lining defects prone to The allowable use temperature and service life of the pump have decreased, and the possibility of the lining layer falling off the base metal of the pump body and back cover has also increased.
The fixing method of the lining layer can adopt the method of arranging a dense annular dovetail groove on the base metal, so that the lining layer is fixed firmly and the machining volume is small. The method of arranging denser small holes (or threaded holes) on the base metal can also be used. The fixing of the lining layer is not as firm as the dovetail groove, and the machining volume is larger.
(4) Axial force balance
There are mostly metal parts embedded in the impeller of the plastic pump, and it is generally not suitable to balance the axial force with the balance hole. The back blade is widely used in China to balance the axial force of the impeller.
2. Performance characteristics of non-metallic pumps
The structure and material of non-metal pumps are quite different from those of metal pumps, so their performance characteristics are also quite different
(1) The pump body has low pressure resistance
Because the mechanical properties of non-metallic materials, such as tensile strength, bending strength, impact toughness and hardness, are far lower than metal materials, the pressure resistance of non-metallic pumps is lower
The pressure resistance of the general non-metallic pump shell is ≤1.6MPa. Non-metallic materials can be reinforced with glass fiber or carbon fiber to improve their mechanical strength. For example, the tensile strength of polypropylene is 30~38MPa, and the bending strength is 38~-5MPa. After glass fiber is reinforced, the tensile strength can reach 60~80MPa. The bending strength is 80~10MPa.
(2) Allowable use temperature The allowable maximum use temperature of non-metallic pumps is much lower than that of metal pumps. The temperature resistance range is limited by many factors. One is the decrease in the strength of the material when the temperature rises (some plastics appear brittle at low temperatures); the second is that the thermal expansion coefficient of the plastic is 2-6 times that of cast iron. When the operating temperature is higher, the operating gap If it becomes smaller, the pump cannot operate safely, or the lining layer and the cast iron shell cannot fit well; the third is that some plastics are prone to "creep" and the size of the parts changes during continuous operation at higher temperatures. Therefore, the allowable use temperature of non-metallic pumps is related to material properties, structural design and other factors. The heat resistance of the material should not be mistaken for the allowable use temperature of the pump. For example, the maximum working temperature of PTFE is about 237°C. The pump manufacturer should correctly specify the allowable operating temperature of non-metallic pumps on the sample according to the material variety and structure design, and considering the long-term continuous operation and reliability of the pump.
(3) Minimum continuous flow
In order to avoid the non-metallic pump working at a small flow rate, the temperature of the medium will increase and the pump will be damaged. Generally, the minimum continuous flow rate of the non-metallic pump is 30% of the rated flow. That is, the pump is not allowed to work at less than 30% of the rated flow
(4) Corrosion resistance
Non-metallic materials have excellent corrosion resistance compared with metals, and there are no corrosion forms such as crevice corrosion, pitting corrosion, selective corrosion and galvanic corrosion that appear in metals. However, when plastic is subjected to tensile stress in certain corrosive media, stress cracking is easy to occur, which is similar to the stress corrosion of metal. Plastics and rubber will swell or dissolve in some organic solvents, so attention should be paid when using them. Table 2-50 lists the corrosion resistance of commonly used non-metallic materials.
(5) Abrasion resistance
Non-metallic materials with good wear resistance include industrial ceramics, wear-resistant rubber and new high-performance engineering plastics (ultra-high molecular weight polyethylene, polyvinylidene fluoride, etc.). Industrial ceramics (corundum ceramics, silicon nitride ceramics, etc.) have the best wear resistance (its Mohs hardness is 9), but the disadvantage is that they are fragile and cracked. The wear resistance of rubber is very good. Rubber lined pumps are the most widely used in conveying particle-containing media. The disadvantages are that they are easy to age and the use temperature is low, generally below 100°C. Ultra-high molecular weight polyethylene (UHMWPE) and polyvinylidene fluoride (PVDF) also have good abrasion resistance (dynamic friction coefficient of 01~0.22), especially UHMWPE is known as the king of wear resistance in engineering plastics, and can be used as a Abrasion-resistant pump material required by abrasion, but high processing requirements. It is generally believed that thermoplastics have good abrasion resistance and thermosetting plastics (phenolic glass fiber reinforced plastics) have poor abrasion resistance, but it should be noted that as the temperature increases, the abrasion resistance will decrease.
