What is Screw Pump?
A screw pump, also known as a water screw, is a positive displacement (PD) pump that uses one or more screws to move fluid solids or liquids along the screw axis. In its simplest form, a single screw rotates in a cylindrical cavity, moving the material along the screw’s spindle.
This ancient design is still used in many low-tech applications, such as irrigation systems and farm machinery used to transport grain and other solids.
The evolution of the screw pump has led to a variety of multi-axis technologies in which carefully crafted screws rotate in opposite directions or remain stationary in a cavity. The cavity can be profiled, creating cavities in which the conveyed material is “locked-in”.
In offshore and marine installations, a three-spindle screw pump is widely used to pump viscous fluid under high pressure. Three screws propel the pumped liquid in a closed chamber. As the screws rotate in opposite directions, the pumped liquid moves along the spindles of the screw.
Three-spindle screw pumps are used to transport viscous liquids with lubricating properties. They are suitable for a variety of applications such as fuel injection, oil burner, supercharging, hydraulic, fuel, lubrication, circulation, delivery, and so on.
Positive displacement pumps have several advantages over centrifugal pumps. The pumped fluid moves axially without turbulence, preventing foaming that would otherwise occur with viscous liquids. You can also pump higher viscosity liquids without losing flow rate. Changes in the pressure difference also have hardly any effect on PD pumps compared to centrifugal pumps.
How Do Screw Pumps Work?
Screw pumps operate using two counter-rotating screw rotors which are engineered so that they rotate “towards each other”. This traps the gas in the space between the “screws” of their rotors. As the screws rotate, this trapped volume decreases which not only compresses the gas but move it towards the exhaust.
In the first case, mechanical bearings support the rotors at both ends. The cantilever design solution supports the rotors at the high-pressure end and the rotors can be cooled internally. A motor drives the two rotors via a gear.
Gear and the bearings are lubricated but separated from the pumping mechanism (‘vacuum generator’) via shaft seals or labyrinth seals, thus the compression is oil-free. The rotors have no mechanical contact between each other and the pump housing, resulting in zero mechanical wear.
To keep the installed electrical power low, modern pumps are driven by an electronic frequency converter and rotate slower at pressure ranges near atmospheric pressure. Some versions use so-called blow-off valves instead to keep the rotating speed constant also at high pressures. Cooling is normally done by water.
The Performance of Screw Pumps
Screw pumps can reach ultimate pressures of ~ 1e-3 mbar. The operating pressure range is 103 to10-2 mbar. Various sizes with pumping speeds ~ 60 – 1200 m³/h are available. The pumping speed in the vacuum range < 10 – 50 mbar can be enhanced by placing a roots blower on top. These combinations, often in one single pump housing, are available up to 9000 m³/h.
Types of Screw Pumps
There different types of screw pumps are available in the market in different designs. They are briefly discussed below.
1. One-Screw Pump
These pumps are named PC pumps otherwise progressive cavity pumps and worm Pumps. Generally, these types of pumps are not considered within the family of the screw pump. Because the rotor in these pumps is not like a usual screw but somewhat a twisted round shaft.
A progressive cavity pump includes one shaft with somewhat twisted within the form of a screw, & is enclosed within a pumping hall that is generally rubber lined.
2. Two Screw Pump
These pumps are also named the double screw pump, and it is the most general type used high power applications like weighty oil tube transfer. The screws in the pumps are driven from the motor, & timing gears are generally included to turn the next screw.
3. Three Screw Pump
These pumps are also named triple screw pumps, and these are usually used in small applications like lubrication systems. The screws in the pump are determined from the motor to rotate the remaining two screws which are around it without using timing gears.
4. Four Screw Pump
These pumps are basically two screw pumps, however with two screws for each rotor in opposite directions. These pumps absorb the liquid within the suction port, then divide evenly & are routed to both pumps’ ends.
The two liquids flow with the help of the pump towards the center & connect jointly again before leaving the release port. Similar to the double screw pump, this pump includes a timing mechanism to make the second rotor run. These pumps are frequently used within multi-phase applications & oil transport pipelines.
5. Five-Screw Pump
These pumps are mostly similar to a triple screw pump; however, with five screws, not three screws. Similar to the 3-screw pump, this pump includes one diving rotor that drives all the remaining screws. This type of pump is frequently used in many applications like tube oil otherwise hydraulic.
Applications Of Screw Pumps
Three-spindle screw pumps are used for the transport of viscous fluids with lubricating properties. They are suited for a variety of applications such as fuel injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed, and so on.
The applications of screw pumps include the following.
- These pumps have many uses in a variety of applications in industries like oil, manufacturing, gas, and mining.
- Most of the pump applications are high viscosity fluids like asphalt, otherwise oil, multi-stage liquid, which means there is fluid and steam mixed jointly within the liquid stream.
- Some of the examples of these pumps used in systems include Multi-phase Pumping, Lubrication, Hydraulic Systems, and Weighty Oil Pipeline.
Advantages of Screw Pumps
Screw pumps contain multiple screws that engage with one another as rotated to form a sealed cavity within the pump casing. As the screw drives turn, the fluid shifts steadily and constantly through the pump. It creates a volumetrically consistent flow rate that is unaffected by pumping pressure or viscosity that might slow down a centrifugal pump.
Some screw pump advantages include:
- Versatility with different flow rates, pressures, liquid types and viscosities
- Constant flow rates
- High volumetric efficiency
- Controllable output
- Low internal velocities
- High tolerance for entrained gases or air
- Smooth and quiet operation
- Very low pulsation
- Self-priming characteristics
- Reduced mechanical vibration
Disadvantages of Screw Pumps
The disadvantages are as follows:
- They are bulky and heavy.
- They are sensitive to viscosity changes of the fluid.
- They have low volumetric and mechanical efficiencies.
- Manufacturing cost of precision screw is high.
FAQs
What is the purpose of a screw pump?
Screw pumps are more helpful in pumping heavy oil. They even have the ability to pump higher flow rates and higher viscosity fluids. These pumps are capable of pumping both liquids and gases. For this reason, these pumps are used in many pumping industries.
What are the pros and cons of screw pumps?
They are capable of constant flow rates, have a controllable output and operate quietly and smoothly with low pulsation. They also have high volumetric efficiency and are capable of self-priming. One of the main disadvantages of screw pumps is that they can be expensive.
What is the difference between screw pump and centrifugal pump?
Screw pump parts are inherently self-priming whereas Centrifugal pumps would require a priming system, foot valve or self-priming casing design. For clarity, a centrifugal pump with a helicoidal screw impeller should not be confused with a screw pump.
What is the difference between a gear pump and a screw pump?
The screw pump design has better mechanical efficiency as it uses less power for the same capacity relative to a gear pump, thus saving energy costs. The motor in gear pumps use higher power for the same capacity as a screw pump, resulting in increased energy costs and a larger motor required.
Can screw pump run dry?
The dry running of an eccentric screw pump can be costly. Repairs and plant downtimes are the result. The Jessberger dry run protection prevents this timely and reliable during operation or startup of the pump. The pump only starts when the medium is actually transferred.
Does a screw pump need priming?
A screw pump is self-priming, meaning this process doesn’t need to take place. This is because of the vacuum that is created within the pump. If a screw pump does become dry and continues to be used, it will cause damage. Eventually, dry running a screw pump will leave it irreparable.