Turbulent flow is detrimental to a centrifugal pump during handling of abrasive fluids. The drilling industry has standardized centrifugal pumps with concentric casings and wide impellers, a design that has proven to offer less turbulence and greatest pump life. The walls of a concentric style of casing (Figure 1) are an equal distance from the impeller throughout the impeller circumference, resulting in a smooth flow pattern. A volute style of casing (Figure 2) has a cutwater point that disturbs the fluid flow pattern, creating an eddy.
Wide impellers and larger casing cavities utilized by concentric pumps (Figure 3) reduce the effect of the fluid velocity when it exits the impeller OD. This wider area allows fluid to smoothly blend with recirculating fluid within the casing, thus reducing turbulent flow patterns that exist in volute pumps. These characteristics also reduce the sandblasting effect on the ID of the casing that is present with narrow impellers and close-proximity casing walls (Figure 4). Smooth flow patterns and wider recirculation area extend fluid end life and reduces operating costs of the centrifugal pump.
Friction Loss Tables
When selecting pipe size, both friction losses and line velocity must be considered. Friction losses should be kept to a minimum in order to reduce hp requirements. Discharge piping should be sized to achieve a flow rate of 5 to 12½ ft/sec. Suction piping should be sized to achieve a flow rate of 5 to 8½ft/sec. A minimum line velocity of 5 ft/sec is recommended because at lower velocities, solids within the liquid can settle in the piping. When settling occurs, it becomes difficult to open and close valves. Exceeding 12½ ft/sec on the discharge line will cause excessive wear of valves, elbows, and tees. Exceeding 8½ ft/sec on the suction line will cause excessive wear of the pump fluid end due to turbulent flow patterns that will occur as the fluid impacts the impeller.
Centrifugal Pumps Velocity Measured provide line velocities and friction losses that occurwhen fluid travels through new SCH 40 steel pipe. Scaled pipe and piping with different IDs will have different values, and corrections to shown values must be made. A design factor of 15–20% minimum should be added to values in these tables. The values in boldface designate recommended optimum flow rates based on discharge line velocities.
To determine line velocities and friction losses from Centrifugal Pumps Velocity Measured, first locate the proper pipe diameter. In the ‘‘Flow Rate (gpm)’’ column, locate the maximum anticipated flow rate. Line velocities will be located in the ‘‘V (ft/sec)’’ column. Friction loss values will be located in the column ‘‘Friction Loss in Feet of Head per 100 Ft of Pipe.’’ For example, for a 6-inch nominal pipe size at 1000 gpm, line velocities are 11.1 ft/sec and friction losses per 100 feet of pipe will be 6.17 feet of head.