Subscribe to our 

  How Screw Pumps Can Achieve 80%-Plus Efficiency in Crude Oil Pipelines, Part 2 of 3Multiple screw pumps for crude oil pipeline service are produced in two basic configurations. The twin screw pump is illustrated in figure 1. It is a double suction design. Each shaft is independently supported with bearings external to the pumped liquid. The mesh of the screw set is synchronized through the outboard timing gears; the screws do not touch each other.

Figure 2 shows the construction of a three screw pipeline pump of the single suction design. It incorporates a replaceable liner and single, external support bearing. The smaller outside screws, called idler rotors, are driven by the center screw, called a power rotor. Thus there is metal to metal contact and these designs cannot be used with high gas content or for 100% water. Each type has its merits depending on pressure, flow and the condition of the crude oil.

Screw pumps are positive displacement machines. Every revolution of the pump shaft causes a specific volume of space to be opened to the system inlet pressure environment and then closed off from the inlet. The volume moves in an axial direction and is expelled from the pump by the next succeeding volume. Flow is very smooth and almost completely free of any measurable pressure or flow pulsation. At constant speed, screw pumps have a theoretical displacement dependent upon the size and geometry of the screws and the screw pitch or lead.

The pumps, obviously, have internal running clearances and will not be able to deliver 100% of their theoretical flow when pumping against a differential pressure. Slip flow occurs through the running clearances. The slip flow is a function of differential pressure and fluid viscosity. Increasing differential pressure and decreasing viscosity cause slip to increase. Slip flow is the volumetric inefficiency.

A pump having a theoretical flow of 432 GPM operating at 1000 PSID and 20 centistokes (100 SSU) might have a slip flow of 59 GPM. Thus the pump would deliver 432-59 or 373 GPM. The volumetric efficiency of the pump would be 373/432 or 86%. At higher viscosities, common on crude oils with an API gravity less than 200, the volumetric efficiency of a multiple screw pump can reach well into the 90 to 95% range.In order to limit the slip flow characteristics of multiple screw pumps, higher pressure designs use more “wraps” of screw thread than lower pressure designs. Each wrap acts as a barrier to slip flow, effectively causing the pump pressure rise to occur in stages, figure 3. The staging effect lowers the loading on rotating pump components as well as providing greater resistance to slip flow.

At low viscosities, slip flow is the major contributor to the inefficiency of a multiple screw pump. At increasing viscosity, the slip flow is reduced, sometimes to a negligible level. However, as viscosity increases, more power is required to rotate the pumping screws within their close clearance stationary boundaries. Viscosity is defined as a liquid’s resistance to shear. The pumping screws shear the liquid that is within the running clearances and this is the major contributor to inefficiency when operating at high viscosity.

Proper selection of pump size and speed can keep these viscous shear losses within reason and usually not require and pump speed reduction devices except under the most severe conditions. Figure 4 shows the performance curve of a typical crude oil pipeline screw pump at constant speed. The effects of viscosity and differential pressure are clearly evident.

Note that, like all machines, the pump efficiency is zero at zero differential pressure. This is the point where the machine would pump its theoretical flow but since there is no pressure rise, there is no power output yet it requires about 15 HP minimum to keep the pump rotating. Also note that there is no “best efficiency” point as with a centrifugal pump. Multiple screw pumps have a rapidly rising efficiency curve which then holds fairly high throughout its design pressure range.

Check back in a few days for additional content on how screw pumps are an efficient option for crude oil pipelines.

jimb@pumpxpert.com, and visit www.colfaxcorp.com

This entry was posted on Wednesday, December 10th, 2008 at 1:23 pm and is filed under Jim Brennan Blog. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.