14 - Pumps

Abstract

This chapter focuses on the various pumps used in pipeline industry. Centrifugal pumps are best suited for large volume applications or for smaller volumes when the ratio of volume to pressure is high. The selection of the proper pump depends on the system throughput, viscosity, specific gravity, and head requirements for a particular application. The first step in selecting centrifugal pumps is to analyze the pipeline system and determine its characteristics such as the pressure required to move the desired flow rate. Initial and future conditions should be evaluated so that pumps with sufficient flexibility to handle both conditions with minor changes may be selected. Pipelines handling liquids in excess of about 500 SSU viscosity normally use rotary, positive displacement pumps both for optimum efficiency and lower initial cost. The rotary, positive displacement pumps are often used when products such as heavy crude oil, bunker fuels, low sulfur fuels, asphalt, Orimulsion ®, and similar products are transported. Although there are various types of rotary pumps, the most common on pipeline services are two- and three-screw pumps, gear pumps (internal and external), and vane pumps for low pressure services. Two- and three-screw pumps are used for higher pressure services, especially at medium to high flow rates. Gear and vane pumps tend to have the lower first cost and may or may not have the best lifecycle cost. They will normally operate at relatively low speeds, requiring speed reducers on all but the very smallest sizes. Multiple-screw pumps move liquid axially rather than in a radial direction, so fluid velocities are relatively low even when directly driven at four- and six-pole motor speeds.