At present, although domestic enterprises have produced a number of gear pumps suitable for conveying high-viscosity liquids, due to imperfect testing methods, there are still some problems in material selection, leakage and noise prevention. In particular, domestic high-viscosity gear pumps have a large gap with foreign products in terms of efficiency, reliability and service life. Therefore, most of the high-viscosity gear pumps used in China's petroleum and chemical industries still rely on imports.
The development characteristics of high viscosity gear pumps at home and abroad are as follows:
Gear structure
Gears of high-viscosity gear pumps are commonly used with straight teeth, helical teeth, herringbone teeth, and helical teeth. The tooth profiles are mainly involute and arc type. Generally, small gear pumps use involute spur gears, high-temperature gear pumps often use displacement gears, and melt pumps that transport high-viscosity, high-pressure polymer melts use involute helical gears. The gear is integrated with the shaft and its rigidity and reliability are higher than that of the gear pump manufactured separately from the gear and shaft. The gears of foreign low-pressure gear pumps often adopt a square structure, that is, the tooth width of the gear is equal to the diameter of the tip circle. The high-viscosity gear pump used in high-pressure applications has a tooth width smaller than the tip circle diameter. This is to reduce the radial compression area of ​​the gear and reduce the load on the gear and bearing.
Pump body and heating method
In general, the heavier the pump casing of a gear pump, the higher its temperature resistance and compressive strength. The pump body material is often made of ductile iron, or cast aluminum alloy hard mold, or extruded aluminum alloy profile. When the medium to be transported is corrosive, a higher cost stainless steel material can be used. Foreign high-viscosity gear pumps use alloy steel with high nickel and chromium as the pump casing material. This material has better comprehensive performance in terms of strength, reliability and cost. In order to solve the oil trapping phenomenon of the gear pump, a symmetric unloading groove is usually provided on the pump cover, or an asymmetric unloading groove is opened in the direction of the low pressure side, a taper unloading groove is adopted on the liquid suction side, and a rectangular unloading side is arranged on the liquid discharge side. The depth of the groove and relief groove is also deeper than that of the gear pump used in the hydraulic industry.
Due to the high viscosity of the medium transported by the high viscosity gear pump, in order to reduce the flow resistance and improve the liquid absorption capacity of the pump, the medium must be heated or insulated. Heating with a heating element is usually used to heat the viscous liquid evenly. If the temperature fluctuation is not large and the transported high-viscosity liquid is prone to degradation, it is recommended to use fluid heating, especially a gear pump with a large displacement. Fluid heating is divided into built-in and external structures. The so-called built-in type refers to the design of the thermal expansion sleeve inside the gear pump body or the end cover, and the external type connects the thermal jacket and the pump body by bolts. It is necessary to pass steam, heat transfer oil or cooling water into the jacket, depending on the specific conditions of the medium. The built-in type is suitable for occasions where the temperature uniformity of the transport liquid is high, or it is required to uniformly cool the high temperature liquid. When the electric heating method lacks safety or the temperature control is not high, an external structure can be adopted. The internal gear pump of VIKING Company of the United States can control the temperature of the conveying fluid by the jacket of the pump head part, and it can be equipped with an external jacket in high temperature or low temperature environment.
Bearing material, structure and lubrication
Bearings of high-viscosity gear pumps usually use sliding bearings, and a spiral flow path is specially designed on the non-bearing surface of the inner wall of the bearing. The direction of the spiral groove is the same as that of the gear shaft. The outer end of the bearing communicates with the inlet of the pump, and the spiral groove at the inner end of the bearing communicates with the root of the tooth (vacuum portion). When the shaft rotates, the low temperature liquid outside the bearing is sucked into the bearing by the screw action and the pressure difference between the two ends of the bearing. After the bearing is lubricated and cooled, it flows into the teeth which are just disengaged to form a spiral with sufficient lubrication and fast heat dissipation. Self-priming low pressure lubrication system. The advantage of this lubrication method is that the lubricating fluid entering the bearing is all low temperature medium, and the viscous lubricating fluid is easy to form a dynamic pressure oil film with strong bearing capacity. A large amount of lubricating fluid circulation keeps away the heat of the bearing and provides good lubrication and cooling for the bearing. Since there is sufficient liquid to fill the root of the gear that has just been disengaged, the self-priming performance of the gear pump is greatly improved, and the suction phenomenon is avoided, which not only can improve the volumetric efficiency, but also reduce the cavitation and reduce the noise. Sichuan Mechanical Research and Design Institute adopts the immersion of sliding bearings in the medium and forced lubrication through special holes. This technology has been successfully applied in gear pumps in the fields of vinylon, polyester, rubber, resin and fertilizer.
The material of the bearing is usually made of tool steel and is surface hardened to improve its resistance to gluing. If the conveying medium contains abrasive particles, a very hard bearing material such as ceramics should be used. In recent years, GS-1 PTFE steel composites have been considered as ideal sliding bearing materials. It is composed of a cold-rolled steel sheet (base), sintered spherical porous bronze powder or copper mesh (intermediate layer), and polytetrafluoroethylene (surface layer) three-layer material, which combines the advantages of metal and polytetrafluoroethylene plastic. On the basis of this material, Shanghai Materials Research Institute has developed a SF-type three-layer composite self-lubricating material with better performance. It replaces the bronze powder layer with bronze wire mesh, and the plastic formula of the surface layer has been carefully screened. The bearing material has the advantages of fatigue resistance, high bearing capacity, small friction coefficient and long service life, and is a novel bearing material for improving the technical performance of the gear pump.
Suction port
The suction port of the high-viscosity gear pump is generally large, and a diffusion-shaped suction port is sometimes used to enlarge the volume of the low-pressure zone to reduce the flow rate of the inlet liquid and reduce the suction resistance of the pump. This structure also reduces the radial forces acting on the journals and bearings and extends the service life of high viscosity gear pumps.
Check valve and safety valve
A check valve is installed on the output line of the gear pump, so that the liquid in the system does not flow backward when the pump and the output pipe are inspected; when the gear pump stops with load, a partial vacuum is generated in the output pipe to prevent the pump from being prevented. reverse. A safety valve and other protection devices are also arranged on the outlet line of the high-viscosity gear pump, so that once the outlet passage of the pump is blocked, the safety valve can be opened for pressure relief. The safety valve can be cast in one piece with the pump body or pump cover or it can be assembled separately.
The development characteristics of high viscosity gear pumps at home and abroad are as follows:
Gear structure
Gears of high-viscosity gear pumps are commonly used with straight teeth, helical teeth, herringbone teeth, and helical teeth. The tooth profiles are mainly involute and arc type. Generally, small gear pumps use involute spur gears, high-temperature gear pumps often use displacement gears, and melt pumps that transport high-viscosity, high-pressure polymer melts use involute helical gears. The gear is integrated with the shaft and its rigidity and reliability are higher than that of the gear pump manufactured separately from the gear and shaft. The gears of foreign low-pressure gear pumps often adopt a square structure, that is, the tooth width of the gear is equal to the diameter of the tip circle. The high-viscosity gear pump used in high-pressure applications has a tooth width smaller than the tip circle diameter. This is to reduce the radial compression area of ​​the gear and reduce the load on the gear and bearing.
Pump body and heating method
In general, the heavier the pump casing of a gear pump, the higher its temperature resistance and compressive strength. The pump body material is often made of ductile iron, or cast aluminum alloy hard mold, or extruded aluminum alloy profile. When the medium to be transported is corrosive, a higher cost stainless steel material can be used. Foreign high-viscosity gear pumps use alloy steel with high nickel and chromium as the pump casing material. This material has better comprehensive performance in terms of strength, reliability and cost. In order to solve the oil trapping phenomenon of the gear pump, a symmetric unloading groove is usually provided on the pump cover, or an asymmetric unloading groove is opened in the direction of the low pressure side, a taper unloading groove is adopted on the liquid suction side, and a rectangular unloading side is arranged on the liquid discharge side. The depth of the groove and relief groove is also deeper than that of the gear pump used in the hydraulic industry.
Due to the high viscosity of the medium transported by the high viscosity gear pump, in order to reduce the flow resistance and improve the liquid absorption capacity of the pump, the medium must be heated or insulated. Heating with a heating element is usually used to heat the viscous liquid evenly. If the temperature fluctuation is not large and the transported high-viscosity liquid is prone to degradation, it is recommended to use fluid heating, especially a gear pump with a large displacement. Fluid heating is divided into built-in and external structures. The so-called built-in type refers to the design of the thermal expansion sleeve inside the gear pump body or the end cover, and the external type connects the thermal jacket and the pump body by bolts. It is necessary to pass steam, heat transfer oil or cooling water into the jacket, depending on the specific conditions of the medium. The built-in type is suitable for occasions where the temperature uniformity of the transport liquid is high, or it is required to uniformly cool the high temperature liquid. When the electric heating method lacks safety or the temperature control is not high, an external structure can be adopted. The internal gear pump of VIKING Company of the United States can control the temperature of the conveying fluid by the jacket of the pump head part, and it can be equipped with an external jacket in high temperature or low temperature environment.
Bearing material, structure and lubrication
Bearings of high-viscosity gear pumps usually use sliding bearings, and a spiral flow path is specially designed on the non-bearing surface of the inner wall of the bearing. The direction of the spiral groove is the same as that of the gear shaft. The outer end of the bearing communicates with the inlet of the pump, and the spiral groove at the inner end of the bearing communicates with the root of the tooth (vacuum portion). When the shaft rotates, the low temperature liquid outside the bearing is sucked into the bearing by the screw action and the pressure difference between the two ends of the bearing. After the bearing is lubricated and cooled, it flows into the teeth which are just disengaged to form a spiral with sufficient lubrication and fast heat dissipation. Self-priming low pressure lubrication system. The advantage of this lubrication method is that the lubricating fluid entering the bearing is all low temperature medium, and the viscous lubricating fluid is easy to form a dynamic pressure oil film with strong bearing capacity. A large amount of lubricating fluid circulation keeps away the heat of the bearing and provides good lubrication and cooling for the bearing. Since there is sufficient liquid to fill the root of the gear that has just been disengaged, the self-priming performance of the gear pump is greatly improved, and the suction phenomenon is avoided, which not only can improve the volumetric efficiency, but also reduce the cavitation and reduce the noise. Sichuan Mechanical Research and Design Institute adopts the immersion of sliding bearings in the medium and forced lubrication through special holes. This technology has been successfully applied in gear pumps in the fields of vinylon, polyester, rubber, resin and fertilizer.
The material of the bearing is usually made of tool steel and is surface hardened to improve its resistance to gluing. If the conveying medium contains abrasive particles, a very hard bearing material such as ceramics should be used. In recent years, GS-1 PTFE steel composites have been considered as ideal sliding bearing materials. It is composed of a cold-rolled steel sheet (base), sintered spherical porous bronze powder or copper mesh (intermediate layer), and polytetrafluoroethylene (surface layer) three-layer material, which combines the advantages of metal and polytetrafluoroethylene plastic. On the basis of this material, Shanghai Materials Research Institute has developed a SF-type three-layer composite self-lubricating material with better performance. It replaces the bronze powder layer with bronze wire mesh, and the plastic formula of the surface layer has been carefully screened. The bearing material has the advantages of fatigue resistance, high bearing capacity, small friction coefficient and long service life, and is a novel bearing material for improving the technical performance of the gear pump.
Suction port
The suction port of the high-viscosity gear pump is generally large, and a diffusion-shaped suction port is sometimes used to enlarge the volume of the low-pressure zone to reduce the flow rate of the inlet liquid and reduce the suction resistance of the pump. This structure also reduces the radial forces acting on the journals and bearings and extends the service life of high viscosity gear pumps.
Check valve and safety valve
A check valve is installed on the output line of the gear pump, so that the liquid in the system does not flow backward when the pump and the output pipe are inspected; when the gear pump stops with load, a partial vacuum is generated in the output pipe to prevent the pump from being prevented. reverse. A safety valve and other protection devices are also arranged on the outlet line of the high-viscosity gear pump, so that once the outlet passage of the pump is blocked, the safety valve can be opened for pressure relief. The safety valve can be cast in one piece with the pump body or pump cover or it can be assembled separately.
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