Installing And Maintaining Gear Pumps

While not as common as centrifugal pumps in the CPI, gear pumps play important roles in handling many of today’s more difficult-to-pump fluids. Because they operate at lower speeds — generally, 900 rpm or less — their seals and bearings tend to last longer than those of centrifugal models. In addition, unlike centrifugal pumps, gear pumps’ flows are independent of their systems’ pressure curves, and they can handle a wider range of viscosities.

Although high-flow, low-head applications remain the domain of centrifugal pumps, the use of gear pumps is increasing in the chemical process industries (CPI). While some application boundaries between gears and centrifugals are blurring, there are some crucial differences between the way the two are operated and maintained — for example, where pressure relief is concerned. This article provides a general summary of gear pump characteristics and applications, highlighting critical aspects of installation, operation and maintenance.

Generally, gear pumps are used in both metering and general fluid transfer applications. Metering units tend to be smaller and built to closer tolerance levels than general transfer pumps.

In the past, most applications ran at fixed speeds, but today, variable speed drives are gaining popularity. Standard industrial pumps are available with flow rates to 2,000 gal/min and differential pressure to 1,500 psi.

How gear pumps work

Gear pumps are rotary, positive displacement pumps. In centrifugal pumps, liquid displacement and pumping depend largely on the velocity imparted to the process fluid. In gear pumps, two or more gears, in either internal or external designs (Figures 2 and 3) mesh together to provide the pumping action. Typically, one of the gears must drive the others. Gear pump flow is a function of gear geometry, fluid viscosity, differential pressure and rotation speed.

Spaces between gear teeth, and between the gears, casing and endplates are carefully controlled to minimize “slip.” Analogous to leakage flow through a centrifugal pump’s wear clearances, slip is fluid that seeps back from the pump’s high pressure to its low pressure side. Manufacturers’ data include information on how to predict the slip, and therefore the delivered flow, for various combinations of speed and viscosity.

Gears can be of the spur, helical, or herringbone design. Spur gears are normally used when differential pressures are high, to minimize the loading on the endplates from any thrust reaction. Helical gears are used to reduce noise levels and reduce hydraulic trapping between the meshing of the gear teeth. Herringbone gears provide the noise reduction of helical gears and the low end thrust of spur gears in one gear. However, they cost more than the others, and, because of their shape, fluid can more easily be trapped within gear teeth than in other models.

It is difficult to compare different gear pumps, since there are no industry standards defining performance, connections, mountings, or speeds, as there are for end-suction centrifugal pumps (ANSI B73.1). However, Hydraulic Institute (Cleveland, Ohio) standards [1,2] provide basic definitions and requirements for testing, general design and other parameters.

While not as common as centrifugal pumps in the CPI, gear pumps play important roles in handling many of today’s more difficult-to-pump fluids. Because they operate at lower speeds — generally, 900 rpm or less — their seals and bearings tend to last longer than those of centrifugal models. In addition, unlike centrifugal pumps, gear pumps’ flows are independent of their systems’ pressure curves, and they can handle a wider range of viscosities.

Although high-flow, low-head applications remain the domain of centrifugal pumps, the use of gear pumps is increasing in the chemical process industries (CPI). While some application boundaries between gears and centrifugals are blurring, there are some crucial differences between the way the two are operated and maintained — for example, where pressure relief is concerned. This article provides a general summary of gear pump characteristics and applications, highlighting critical aspects of installation, operation and maintenance.

Generally, gear pumps are used in both metering and general fluid transfer applications. Metering units tend to be smaller and built to closer tolerance levels than general transfer pumps.

In the past, most applications ran at fixed speeds, but today, variable speed drives are gaining popularity. Standard industrial pumps are available with...