Focus on Fundamentals (Part Five): Multiple Screw Pumps

Written by Dr. Lev Nelik and Jim Brennan, Pumping Technology, LLC

Pumps & Systems, August 2008

Within the rotary pumps family, twin and three screw pumps have earned reputations as particularly tight, robust pump types. Not to criticize their cousins, single-screw pumps (including progressive cavity, which we discussed in Part Four), but multiple screw pumps can handle high pressure, temperature, speed and power combined. While other pump types can handle these variables well individually, their combined force is a challenge.

Unlike gear and lobe pumps, screw pumps are axial flow rather than radial flow machines; flow moves along the axis of rotation rather than perpendicular to it. This axial flow allows multiple screw pumps to operate at relatively high direct drive speeds while still maintaining low fluid inlet velocities and low NPSH requirements. Figure 1 illustrates the flow path within these pumps.

Figure 1. Cross sections with flow paths (three screw top)

Each wrap of screw thread forms a cavity that moves axially from suction to discharge. The wrap, or cavity, acts as a pressure stage. Low pressure pumps have only one or two wraps (stages), while high pressure pumps may have 12 or more wraps. The staging effect allows each stage to handle a moderate pressure rise, resulting in low stress levels within the pump even at high pressure operation (Figure 2).

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<caption>Figure 2. Staging effect of multiple screw pumps

Multiple screw pump performance specifications are shown in the table below.

Pump Type	Flow Rate GPM	Pressure Range PSI	Viscosity Range cSt
Twin Screw	25 - 5,500	50 - 1,500	Gas - 1,000,000
Three Screw	1 - 3,500	50 - 4,500	0.5 - 1,000,000

Most three screw pumps are of the single suction design, but very large flow pumps are double suction. The vast majority of twin screw pumps are double suction designs, which effectively puts two single suction pumps in parallel in one casing. The double suction designs, both three-screw and two-screw, are inherently in hydraulic balance in the axial direction due to their symmetry.

In a radial direction, twin screw pumps are not hydraulically balanced and require radial bearings at each end of each shaft. In twin screw pumps, external timing gears and bearings keep the screws from contacting each other or their casing bores and do not rely on pumped fluid characteristics.

Both twin screw and three screw pumps share many applications including hydraulic services, machinery lubrication, compressor and expander gas sealing and some refinery (heavy fuel, asphalt, vacuum tower bottoms, bitumen) and chemical processing (synthetic fibers, explosives, polyol, isocyanate) applications.

Examples of critical applications include modified twin screw pumps used to handle multiphase flow, i.e., oil well head flows ranging from nearly 100 percent gas to 100 percent liquid including liquid slugs. Three screw pumps find use aboard combat ships for hydraulic services where extremely quiet operation is necessary to avoid acoustic detection. Both twin and three screw pumps are used in medium and heavy crude oil pipelines operating at efficiencies far above centrifugal pumps.

Multiple screw pumps operate at much higher efficiencies than centrifugal pumps on even moderately viscous liquids. Figure 3 illustrates the efficiency comparison on 400-gpm pumps operating at 1450-psi. Energy cost savings can be substantial. For example, three screw pumps operating in the 1,000-hp range on heavy crude can save more than $1 million annually in energy cost per three pump pipeline station, a significant amount even though they have lower power than big centrifugals.

Figure 3. Efficiency comparison between multiple screw pumps and centrifugal pumps

A twin screw pump can handle low viscosity fluids. The rotors are supported by the externally mounted bearings, which-in contrast to their three screw cousins-do not rely on the product for lubrication.

While centrifugal pumps are also evaluated on overall efficiency, screw pumps are typically evaluated on only one of the efficiency components, namely volumetric efficiency. Being positive displacement machines, screw pumps maintain constant or nearly constant flow at a wide range of pressures. However, some of the flow "slips" away, from discharge back to suction, which results in reduced flow at higher pressures, as compared with flow at low pressure. The ratio of flow at zero differential pressure to that at a given pressure is pump volumetric efficiency.

E_vol = Q_net / Q₀ = (Q₀ - Q_slip) / Q₀

Where:

Q₀ = Theoretical (ideal) flow; in the absence of slip, = Q₀ x RPM

Q_net = Net flow leaving a rotary pump

Q_slip = Internal recirculation

Q_slip is determined (usually by testing) for each design type and size by pump manufacturers. Pump catalogues report Q_slipas a function of pump speed, differential pressure and viscosity.

Volumetric efficiency only makes numerical significance when comparing similar designs (their comparative/competitive ability to maintain constant flow, with minimum slip), operating at the same speed. In rotary pumps, ideal flow is directly proportional to pump speed, but slip only depends on differential pressure, viscosity and internal clearances-not pump speed.

Three screw pumps handle low viscosity, non-lubricating fluids. Gas turbine fuel injection pumps may see all sorts of viscosities. Some handle 0.5-cSt naphtha and methanol-extremely non-lubricating fluids-at pressures at/above 1,500-psi.

Three screw pumps are not good at pumping gas, dry running and pumping water. Properly configured twin screw pumps can do all three. While three screw pumps can handle a substantial range of viscosities, twin screw pumps have almost no lower limit and, in fact, are used to pump high percentages of gas.

As always, a parting quiz to our readers: In three screw pumps, do all three screws pump, or does only the drive (center) screw? The first three people who answer correctly receive a free admission ticket to the PumpTec-2008 Conference in Atlanta, September 22-23.

Click here for "Focus on Fundamentals" Part 1, Part 2, Part 3 and Part 4.

Dr. Nelik (aka "Dr. Pump") is president of Pumping Machinery, LLC, an Atlanta-based firm specializing in pump consulting, training, equipment troubleshooting, and pump repairs. Dr. Nelik has 30 years experience in pumps and pumping equipment. He has published over 50 documents on pump operations, the engineering aspects of centrifugal and positive displacement pumps, and maintenance methods to improve reliability, increase energy savings, and optimize pump-to-system operations. With questions, comments, or to attend his Pump School, he can be contacted at www.PumpingMachinery.com.

Jim Brennan, principle of Pumping Technology LLC, has been involved with pumps and pumping applications since 1967. A 1993 graduate of Drexel University, BS Mechanical Industrial Engineering, Brennan is a 39 year veteran of Imo Pump, a business unit of the Colfax Corp., in marketing/sales, engineering management and project management. He has published over 50 pump articles worldwide, coauthored two books and been a 10 year speaker at Texas A&M's International Pump Users Symposium and 4 years at PumpTec Pump Conference. He can be reached at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

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