These positive displacement pumps can be a good choice for high-pressure applications.
by Amin Almasi
August 22, 2017

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.