In many applications, the pump does more than simply transport ambient temperature and pressure water media. Centrifugal pumps become ineffective as viscosity increases, and users need to consider using positive displacement pumps (PD pumps, or positive displacement pumps). When pressure needs to rise, some positive displacement pumps are unsustainable. As the temperature rises, the other pumps will also fail. So when you need more than 35 kg of pressure, or 300 degrees Celsius, or viscosity up to hundreds of thousands or millions of centipoises how to do? Perhaps some pumps have been designed or modified to meet one or both of these requirements, but what if the condition requires the pump to meet all these harsh conditions? This requires high performance external gear pumps designed for these harsh conditions. This pump can handle any or all of these conditions through specially engineered materials, clearances and designs. External gear pump has two same size gear shaft. The drive shaft connects the motor or reducer (via flexible coupling) and drives the other shaft. In heavy duty industrial gear pumps, the gear is usually integral with the shaft (one part) and the journal tolerance is small. The gear shaft is monolithic to withstand high torque loads at high pressures and high viscosities. The four journal bearings are dynamically supported and lubricate the gear shaft with the pumped fluid (see Figure 1). There are three common forms of gear: straight teeth, helical teeth and herringbone teeth. The three forms have their own advantages and disadvantages, have different applications. Straight teeth are the simplest form and are optimally used under high pressure conditions because there is no axial thrust and the delivery efficiency is high. Helical teeth have the least amount of pulsation during delivery and are quieter at higher speeds because the meshing of the teeth is gradual. However, due to the axial thrust, the selection of bearing material may result in limited pressure difference between inlet and outlet and lower viscosity. Because the axial force will push the gear to the bearing face and friction, so only choose the higher hardness of the bearing material or in its end face for special design, in order to cope with this axial thrust. Herringbone teeth are back-to-back helical forms that provide slightly lower pulsation than straight teeth, and axial forces can be balanced. However, the manufacturing cost is high, and assembly / disassembly is difficult because it must be installed in pairs. In high viscosity applications, liquids are easy to cure, or in very large pumps, which is a real drawback. External gear pump operation principle is very simple (see Figure 2). The liquid enters the suction side of the pump, is inhaled by the interdental cavities that are not engaged, and is then entrained within the cavities in the interdental space, reaching the exit end along the outer periphery of the gear shaft. The re-engaged tooth pushes the liquid out of the hole into the back pressure. In theory, the nominal displacement of a positive displacement pump is independent of the pressure. However, volumetric failure or internal leakage is inherent to all types of positive displacement pumps. In order to achieve a high pressure differential and the required nominal flow rate, the gear pump must overcome this internal leakage. There are four ways to leak (see Figure 3): 1) between the gear journal and the bearing, 2) between the gear end face and the bearing face, 3) between the tooth tip and the pump housing, and 4) between the meshing teeth. In order to maximize the pressure bearing capacity of the pump, the clearance between these mating components must be as small as possible to limit internal leakage. However, just narrowing the gap is not as simple as it sounds. Other factors such as temperature, viscosity, and material selection must also be considered. There is not all internal leakage is a bad thing. In gear pumps, some internal leakage is required to lubricate the internal passages and form a fluid film in the sliding bearings to dynamically support the gear shaft. The correct design should be, the internal leakage is 1 to 3% of the flow. Material selection is high temperature industrial pump selection is very important. Gear pump ¾ ¬ commonly used to transport highly corrosive, abrasive or variable fluid. Pump housing, shaft and bearing material must first match with the pumped liquid. The pump design becomes even more complicated when extra heat is taken into account, even considering the thermal expansions of various materials. As mentioned earlier, the smaller the internal gap, the better it is to achieve the highest possible pressure capability. At high temperatures, the pump needs to "expand" within immediate gaps due to the thermal expansion of the components, which is beyond the usual consideration of most general-purpose gear pump manufacturers. Overestimation of material thermal expansion will lead to the pump gap is too loose, but can not produce the required pressure; underestimation of thermal expansion will cause the pump to lock the process temperature. For this reason, pumps designed for high or low temperatures often do not function well at non-design temperatures. For example, if the pump body is 316 stainless steel, the gear shaft is 440B stainless steel, and the bearing is graphite. The 316 stainless steel has an expansion rate of 17x10-6mm / mm / deg C, 440B 11x10-6mm / mm / deg C, and carbon 3.6x10-6mm / mm / deg C. Pump manufacturers must have the ability to calculate pump clearances at high temperatures. Preheating of the pump is necessary to prevent damage to the components by high temperature shocks. Preheat is recommended when the pumped fluid temperature is above 150 ° C. When using a mechanical seal, the pump must be preheated to within 30 ° C of the operating temperature to prevent damage to the sealing surface. Jacket pump can be steam, heat medium and electric heating to preheat. Viscosity is the resistance to fluid flow. The first problem with high viscosity conditions is how to pump fluid. The pump must be turned very slowly to allow the fluid to enter the non-intermeshing cavities, which create a suction that draws the fluid into the pump. The tighter the clearance, the better the seal of the pump and the stronger the suction. Once the fluid enters the pump, the internal gap needs to be properly defined in terms of viscosity. The gap is too small will limit the flow of fluid in the passage, so that the lack of lubrication and bearing overheating; gap is too large, the strength of the liquid film can not support and lubricate the gear shaft, causing journal and bearing direct contact, leading to failure. Another important factor in handling high-viscosity fluids is the high torque of the drive gear, which must be strong enough to transmit the high torque of the drive. Toothed design is very important, too large, the delivery efficiency is not enough, too small can not afford high torque. The torque applied to the gear shaft and the shearing force on the teeth increase with increasing viscosity and differential pressure. When these factors are combined with high temperatures, the design of the gear shaft and toothing becomes extremely important because Metal parts with the temperature coefficient of elasticity decreased. Involving high pressure, high viscosity, high temperature conditions so that users no longer choose to use centrifugal pumps and positive displacement pump (PD pump). And when these conditions become particularly harsh, many other forms of PD pump to use the limit, the only option is the external gear pump. There are many manufacturers of external gear pumps, but few can cope with these conditions. The wise user choice should be that there is a high demand for application verification and there are suppliers that successfully process these records traceable.