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Since full stroke length is considered optimum for metering pump performance, changing speed as opposed to stroke length to alter flow represents a significant improvement. The newest metering pumps use only VFD motors and controllers, always at full stroke length.

Multiple-diaphragm Design Reduces Pulsations

Typical of most metering pumps is the single-diaphragm configuration, which is responsible for non-linear flow in metering systems. Pulsations can be minimized with dampeners and by multiplexing together several pumps, sequencing the diaphragm strokes. These options, however, add significant costs, size and maintenance to the system.

Some of the newest metering pumps have at least three (and as many as five) diaphragms per liquid end, each with a corresponding set of valves and pistons. Multiple-diaphragm pumps reduce acceleration loses and pipe strain, providing virtually "pulse-free" liner flow. This can eliminate the need for expensive pulsation dampeners and broaden the range of application opportunities for the pumps.

Traditional metering pumps also need to increase the sizes of the plunger, diaphragm and liquid end to handle greater flow and pressure demands. With the newest metering pumps, the wet end can remain constant because it can employ different gearboxes with different ratios to cover a wide range of flows and pressures. Changes in process requirements can be addressed simply by changing the gearbox, thus reducing pump acquisition, maintenance and downtime costs. In addition, spare part kits remain the same, reducing inventories.

The New Definition

When is a metering pump not just a "controlled volume, positive displacement pump" for metering? When it uses VFD motors instead of manual and electronic stroke adjusters to increase response time and accuracy. When its multiple diaphragms per liquid end virtually eliminate pulsations. When it employs interchangeable gearboxes to broaden the performance envelope.

These changes may provide solutions to user problems, but they also give the newest metering pump technology cause to redefine what makes a metering pump.

Donelle Capriotti is the director of business development for Wanner Engineering, Inc., 1204 Chestnut Avenue, Minneapolis, MN 55403, 412-494-3848, Fax: 412-494-7484, www.Hydra-Cell.com/metering.

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