1 E N G I N E E R I N G - gear Pump Basics EXTERNAL gear PUMP PRINCIPLE OF OPERATION & ADVANTAGES. IDLER. gear . IDLER. SHAFT. INLET OUTLET OUTLET INLET. DRIVE. SHAFT. DRIVE. gear . HOUSING. Counter-Clockwise Rotation Clockwise Rotation of Drive gear of Drive gear DIRECTION. OF FLOW. Figure 1: Cross-sectional views of external gear pump demonstrating operating principle. The external gear pump is a positive of the gears on the discharge side of the The tight clearances of the working parts displacement (PD) type of pump pump forces the liquid out of the pump inside a gear pump are what enable it generally used for the transfer and and into the discharge piping.
2 To effectively pump liquids against high metering of liquids. The pump is so pressure. Low viscosity fluids such as named because it has two gears that Figure 1 also shows that the direction alcohols and other solvents have more are side-by-side or external to each of rotation of the drive gear determines of a tendency to slip thru these tight other. (This nomenclature differen- the direction of flow thru the pump, and spaces from the higher-pressure tiates it from an internal gear pump, which side of the pump is the inlet and discharge side of the pump back to the which has one gear positioned inside which side is the outlet.)
3 If the direction lower-pressure suction side of the pump. the other.) The gear pump is a of rotation of the motor (and therefore The phenomenon of slip causes precision machine with extremely the drive gear ) is reversed, the direction a reduction in flow rate and pump tight fits and tolerances, and is of flow thru the pump will also reverse. efficiency. Slip depends on the capable of working against high This bi-directional flow characteristic is magnitude of the differential pressure differential pressures. one of the many advantages inherent ( , the difference between the to gear pumps .
4 Discharge and suction pressures), the The working principle of the external viscosity of the liquid pumped and the gear pump is illustrated in Figure 1. Another important advantage of the working clearances inside the particular A drive gear (that is driven by a gear pump is its self-priming capability. pump that is used. Slip increases with motor) rotates an idler gear in the gear pumps are capable of self-priming decreasing viscosity, increasing opposite direction. When the gears because the rotating gears evacuate differential pressure and increasing rotate, the liquid, which is trapped in air in the suction line.
5 This produces a gear -housing clearances, and is usually the gear teeth spaces between the partial vacuum that allows the atmos- measured as a percent decrease from housing bore and the outside of the pheric pressure to force the liquid into ideal flow ( , flow with zero slip). For gears, is transferred from the inlet side the inlet side of the pump. This ability of fluid viscosities greater than about 50-100. of the pump to the outlet side. It is the gear pump makes it an ideal choice cP (depending on the particular pump), important to note that the pumped when the application requires that the the slip is minor, but it still depends on the liquid moves around the gears and pump be located above the liquid level, differential pressure.
6 This behavior is not between the gears. The rotating and the liquid must be lifted to the shown in Figure 2, which compares a gears continue to deliver a fresh pump. Because a gear pump cannot typical gear pump's performance curve supply of liquid from the suction (inlet) create a perfect vacuum, the total lift for a thin fluid (such as water with a side of the pump to the discharge (including pipe friction losses) should not viscosity of about 1 cP at room temper- (outlet) side of the pump, with exceed about PSI, or about one-half ature) with that of a moderately viscous virtually no pulsations.
7 The meshing of the atmospheric pressure. fluid (such as a particular oil with a viscosity of 100 cP). 1 tel..0777 fax..1847 w w w. l i q u i f l o . c o m E N G I N E E R I N G - gear Pump Basics EXTERNAL gear PUMP PRINCIPLE OF OPERATION & ADVANTAGES (continued). 1 cP Fluid (Water) 100 cP Fluid (Oil). Figure 2: Performance curves for a typical external gear pump showing slip as a function of viscosity and differential pressure. The Flow vs. Pressure curves for the thin gear pumps properly designed and fluid have high slopes, which indicate engineered can offer many advantages.
8 Significant reductions in flow rate with These include compactness, simplicity of increasing differential pressure ( , high design, easy serviceability, bi-directional slip). The curves for the 100 cP fluid are flow capability, ability to self-prime, almost level, which indicate nearly pulseless flow, low NPSHR (net positive constant flow rates with increasing suction head required), high MTBM. differential pressure ( , almost zero slip). (mean time between maintenance), The close tolerances and tight spaces high-pressure and high-temperature inside the gear pump also limit liquids capability, precise and accurate containing abrasives from being metering, and availability in multiple seal pumped.
9 This is because the abrasive configurations or sealless mag-drives. particles can work their way into these Liquiflo has over 35 years experience tight spaces and cause accelerated in designing and manufacturing quality wear, and this can rapidly reduce pump high-alloy gear pumps for the chemical performance. The resulting wear rate processing industry, and extensive from pumping abrasives is dependent experience in pumping acids, caustics, upon the hardness, size, and concen- solvents, polymers and other types of tration of the particles, and the operat- chemicals.
10 Liquiflo's experienced ing speed of the pump. The wear rate engineers are available to assist you of a pump can also be adversely with your special chemical pumping affected by pumping thin fluids because applications. they have poor lubricating properties. For this reason, more care must be taken when making material selections for the internal components of the gear pump. Special materials are available to increase lubrication (such as carbon graphite) and resist wear when pumping extremely thin liquids or liquids contain- ing abrasives.