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How Screw Pumps Can Achieve 80%-Plus Efficiency in Crude Oil Pipelines, Part 3 of 3
How Screw Pumps Can Achieve 80%-Plus Efficiency in Crude Oil Pipelines, Part 3 of 3
Precision timing gears can operate in the 98% range. The mechanical seal drag has a component of loss for the body rotating within the fluid as well as a component due to the shearing of the liquid film between the rotating face and stationary seat. All these losses are normally very small and might contribute only a few percentage points of inefficiency. This is true unless pumps are operated at very low hydraulic power levels.
As is true with most rotating equipment, larger machines are more efficient. In the case of screw pumps, the reason is that the theoretical flow rate is a function of the cube of the screw size while slip flow, everything else being constant, is a function of the square of the screw size. Figure 5 illustrates this effect for an 8 wrap, 1000 PSID, 1200 RPM screw pump handling 100 centistoke (500 SSU) crude oil.
The determination of pump efficiency is a straightforward calculation as follows:
E0 = Power out / Power in = (QD X PD X 100 / k) / W
E0 = EV X EM X 100
EV = QD / QT X 100
UNITS
E0 = overall pump efficiency % % %
EV = pump volumetric efficiency % % %
EM = pump mechanical efficiency % % %
QD = pump delivered flow rate GPM B/D M3/H
PD = pump differential pressure PSI PSI BAR
k = conversion constant 1714 58764 36.03
W = pump input power HP HP KW
QT = pump theoretical flow rate GPM B/D M3/H
Since pumps are frequently sized to operate over a range of pressure and viscosity, for cost calculations, use the power required at the pressure and viscosity that will be representative of normal operation. Screw pumps will require their maximum input power at maximum viscosity. The minimum flow will be delivered at the minimum viscosity. Do not use the minimum delivered flow and the maximum required power to calculate overall pump efficiency. This method understates the efficiency as simultaneous operation at these conditions is not possible.
Figure 6 shows a three screw crude oil emulsion shipping pump on a California offshore
platform.
There are three pumps on each of two platforms. Each pump delivers 800 GPM (27,500 B/D) at design discharge pressures to 1190 PSIG. The 800 HP, 1200 RPM electric motor drivers were sized to handle a maximum pumping viscosity of 350 centistokes (1610 SSU). The overall pump operating efficiency at this point is 82%, considerably better than available from centrifugal equipment.
(87,500 B/D) of Orimulsion, an emulsion of 30% water and 70% bitumen, that is exported as a power plant fuel. Design differential pressure is 531 PSID and the pumping viscosity range is 215 to 970 centistokes (1000 to 4500 SSU). The pumps are driven by 1250 HP, 1200 RPM electric motors and operate in the 78 to 82% efficiency range.
Figure 8 shows the range of multiple screw pump sizes available for transport of crude oil. 
In order to maintain high efficiencies over longer time periods when pumping crudes with sand, carbonates, sediment, etc., screw pump manufacturers use a number of techniques to enhance the life and prolong the running clearances within these pumps. Hardened or hard coated screws, hard chrome plated liner bores, hard/soft and hard/hard combinations of running surfaces and erosion resistant inlays and overlays all can be used to contribute to longer useful pumping life between overhauls.
SCREW PUMP CENTRIFUGAL PUMPFlow 583 GPM (20,000 B/D- 132 M3/H)
Pressure 1000 PSI (70 BAR)
Crude Viscosity 1000 SSU (200 cst)
Efficiency 82% 45%Power Req’d. 415 HP (309 KW) 756 HP (563 KW)
The power difference is 563 – 309 or 254 KW. With 8760 hours in a year, the annual
energy difference is 254 X 8760 or 2,225,040 KW-Hr. At an energy cost of $0.104 / KW-Hr (2008 U.S. average), the annual direct energy cost difference is $240,000. Add the cost of carrying money, currently about 10%, and the difference is over $268,000. per year for one pump, a significant amount of money.
jimb@pumpxpert.com, and visit www.colfaxcorp.com
