Gear pumps are often used in pumping relatively viscous liquids, such as some viscous liquid hydrocarbons, liquid fuels, lubrication oil pumping in machinery packages, hydraulic units and fluid power transfer units. Gear pumps are the most popular type of positive displacement pump. Small gear pumps usually operate at a speed between 1,700 rpm and 4,500 rpm, and larger models most often operate at speeds below 1,000 rpm.
A gear pump produces flow by carrying fluid between the teeth of two meshing gears. The chambers formed between adjacent gear teeth are enclosed by the pump housing and side plates, also called wear or pressure plates. A partial vacuum is created at the pump suction; fluid flows in to fill the space and is carried around the discharge of the gears. As the teeth mesh at the discharge end, the fluid is forced out. Volumetric efficiencies of gear pumps run as high as 91 percent.
Gear pumps have close tolerances and shaft support, usually on both sides of the gears. This allows them to run to pressures beyond 200 bar gauge (Barg), making them well suited for use in high-pressure applications. With bearings in the liquid and tight tolerances, gear pumps are not usually well suited to handle abrasive or extremely high temperature applications.
Tighter internal clearances provide for a reliable measure of liquid passing through a pump and for greater flow control. Because of this, gear pumps might be employed for some precise transfer and metering applications.
General Notes on Gear Units
During the past few decades, a large number of pump concepts have emerged, and the selection of an appropriate pump for a specific viscous liquid application has become a major consideration. In general, a specific pump can be operated efficiently for one application but might be inappropriate for others. To aid the selection and design of pumps, different charts and tables have been developed to illustrate the efficiencies and performance of various pump types as a function of the specific speed and other parameters. In addition to these theoretical concepts of efficiencies and suitability of pressure ranges, other important benefits such as reliability, availability, overall performance and operation should be respected. Among positive displacement pumps, gear pumps possess some vital advantages.
The gear pump principle features low-pressure pulsations due to the large number of tooth gaps conveying the fluid, which leads to excellent suction behavior and helps prevent cavitation.
Various pressure compensation measures and characteristics of gear pumps can offer desirable differential pressure and flow characteristics curve for many applications, and gear pumps can also offer high efficiencies for many targeted services.
The gear pump is simple and consists of a few components, leading to low manufacturing and operating costs.
Employing an appropriate combination of self-lubricating materials, a gear pump can be safely operated even when gas bubbles are trapped in the flow subsequent to cavitation phenomena.
Design & Operation
As the gears come out of mesh, they create expanding volume on the suction side of a gear pump. Liquid flows into the gear teeth cavity and is trapped by the gear teeth as they rotate. Liquid could also travel around the interior of the casing in the pockets between the teeth and the casing. This small flow does not pass between the gears. The meshing of gears forces liquid through the discharge port under pressure.
In gear pumps, running clearances between gear faces, gear tooth crests and the housing creates a relatively constant loss in any pumped volume at a fixed pressure. This means that volumetric efficiency at low speeds and low flows might be poor, so gear pumps should be run close to their maximum rated speeds.
Although the loss through the running clearances, or “slip,” increases with pressure, it is nearly constant with different speeds and flows, and it changes linearly as pressure changes. Change in slip with pressure change usually has little effect on performance when operated at higher speeds and outputs.
Many pumping applications of viscous liquids require adjusted flow independent of discharge pressure and also pressure-independent volumetric efficiency. Some gear pumps consist of a pressure-compensating sealing element that can reduce the face and tip clearances to decrease the internal leakage and increase the volumetric efficiency. The design of the sealing elements is usually based on theoretical predictions combined with practical experience. The seal’s geometry and designs should be optimized in several stages. Operational experience with gear pumps using properly designed pressure-compensating sealing elements has shown that when a critical differential pressure (say around 6-10 Barg) is exceeded, the desirable characteristics and an almost pressure-independent volumetric efficiency around 74 to 88 percent could be achieved.
Moreover, the pressure pulsations induced by the unsteady discharge of a gear pump should be measured to verify trouble-free operation of a gear pump. Pressure pulsations or ripples (suction or discharge) can arise from an interaction of the pumping dynamics with the dynamic behavior of the suction and discharge piping system. The presence of pressure pulsation would lead to a fluctuating pressure differential, and hence a fluctuating flow into the gear inter-tooth space. If the minimum pressure pulsation points coincide with the expansion phase as the side flow areas open up, it might result in some malfunctions or poor performance.
In a gear pump, the friction torque and consequent pump operation and required power can be affected by liquid temperature as well as operating pressure and pump speed. When the pressure differential is large, the friction torque decreases first and then increases with an increase in pump speed. For a large pressure differential, the friction torque could become higher with an increase in liquid temperature in a low pump speed region, but it could have the opposite tendency in a high pump speed region.
Transient Operation & Cavitation
When a gear pump operates with a relatively low suction pressure (for instance, when liquid is from a tank at a lower level), pressures in the suction piping and chamber get closer to vapor pressure, and cavitation can take place upstream from the gear meshing region.
Another common operational problem is cavitation in the case of transient operations. One frequent cause of cavitation is insufficient flow into the expanding inter-tooth volumes. In many theoretical or operational studies on these topics, the inter-tooth volumes that are formed at the roots of the driver and driven gears should be considered. Compressible flow into and out of these volumes plays important roles in cavitation and transient operation.
To study the effects of operation parameters such as suction pressure on pump operation, in a case study a gear pump has been operated at 1,200 rpm and 3,400 rpm speeds with around 20 Barg discharge pressure. The pump suction is from an atmospheric tank. An 0.8 bar pressure drop in the suction was observed when pump was operated at 3,400 rpm. In other words, at around 3,400 rpm, the gear pump should be operated with a mean suction absolute pressure of 0.2 bar absolute (Bara), which is relatively close to the pump limit, and cavitation should be expected. At 1,500 rpm, this same situation represented a smaller suction pressure drop of only about 0.5 bar; this resulted in a mean suction absolute pressure of approximately 0.5 Bara with some good margin against cavitation.
Manufacturing & Performance
Gear pumps can usually come in single or double (two sets of gears) pump configurations with different types of gear such as spur, helical, herringbone gears. Helical and herringbone gears typically offer a smoother flow compared to spur gears, although all gear types are relatively smooth. Straight spur gears are easiest to cut and are the most widely used. Helical and herringbone gears run more quietly but cost more. They are typically used in large capacity gear pumps.
Displacement volumes of a gear pump are directly affected by the gear tooth profile. Since the involute gear tooth profile is easily manufactured and the technology for the power transmission gear can be applied, this profile is usually adopted for a low cost gear pump. In an involute gear, the profiles of the teeth are involutes of a circle.
The pressure angle is the acute angle between the line of action and a normal to the line connecting the gear centers. Theoretically, gear manufacturers can produce any pressure angle. However, the most common gears have a 20 degree pressure angle, with 14.5 degree and 25 degree pressure angle gears as other common options. Increasing the pressure angle increases the width of the base of the gear tooth, leading to greater strength and load carrying capacity. Decreasing the pressure angle provides lower backlash, smoother operation and less sensitivity to manufacturing errors. Only used in limited situations are helical involute gears, where the spirals of the two involutes are of different “hand” and the “line of action” is the external tangents to the base circles.
Many gear pumps use helical gears. The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement. This gradual engagement makes helical gears operate more smoothly and quietly than spur gears. Because of the angle of the teeth on helical gears, a thrust load (axial load) is created on the gear when they mesh.
This load should be properly addressed, for example, by using thrust (axial) bearings. The use of helical gears is indicated when the application involves relatively high speeds, relatively high power pumps or where noise abatement is important.
As an indication, the speed might be considered to be high when the pitch line velocity exceeds 20 meters per second.
A herringbone gear is a specific type of double helical gear that is a side-to-side combination of two helical gears of opposite hands. From above, the helical grooves of this gear looks like the letter “V.” Unlike helical gears, herringbone gears do not produce an additional axial load. Like helical gears, herringbone gears have the advantage of operating smoothly because more than two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that the side-thrust of one half is balanced by that of the other half. This means that herringbone gears can be used without requiring a substantial thrust bearing.
Precision herringbone gears are more difficult to manufacture than equivalent spur or single helical gears and consequently are more expensive. A disadvantage of the herringbone gear is that it cannot be cut by simple gear hobbing machines, as the cutter would run into the other half of the gear. Therefore, advanced, expensive manufacturing machineries such as modern CNCs are needed.
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