Positive Displacement Screw Gear Pump
Positive Displacement Screw Gear Pump
What is a positive displacement screw gear pump?
A positive displacement screw gear pump is a type of screw pump consisting of two screws and timing gear. One screw is the driver, while the other is the driven screws. The screws intermesh to pressure liquid and move it within the pump system. An electric motor provides the required rotary motion to the driver screw coupled to its shaft. The driver screw rotates the timing gear, turning the driven screw so that male and female rotor assemblies do not create metal-to-metal contact. As such, the timing gear conveys the driver screw’s movement to the driven screw, which rotates around its axis and oppositely to the driver gear.
These pumps are self-priming with better suction capabilities and require less maintenance for the same speed range; hence they are one of the most reliable types of pumps for many operations. Positive displacement screw gear pump manufacturers design these pumps for excellent performance in many applications, such as marine and petrochemical industries, power generation, food processing, etc.
Figure: showing the configuration of a positive displacement screw gear pump
Positive displacement screw gear pumps have many attractive characteristics, such as the ability to operate at a wide range of speeds, a dry run for a short time, and tolerate pollution. It also has a low net pressure suction head requirement and can handle fluids with different viscosities and high-volume flow.
Components of a positive displacement screw gear pump
The pump casing is the part housing the screws, timing gear, bearing housing and other internal components of the pump. It also carries inlet and outlet ports. Pump casings are usually made from cast iron because it is cheaper. However, it can also be made of carbon steel, stainless steel, and aluminum alloys.
Figure: showing various parts of a positive displacement screw gear pump
The diving screw, or the male rotor, on one side, connects to the electric motor’s shaft and a timing gear on the other end. The primary function of the driving screw is to rotate the driven screw through a timing gear. Positive displacement screw gear pump manufacturers produce driver screws mainly from steel materials, including high-tensile steel, alloy steel, hardened steel, and surface-treated steel.
The driven screws or the female rotor fit well with the drive screw. It also has a constant clearance area which helps to trap liquid within the restricted space. It receives the rotary movement from the timing gear, rotating within its axis and in the opposite direction. The driver screw is also made from high-tensile steel, alloy steel and hardened steel because these materials are strong and can resist vibration.
The timing gear is the link between the drive screw and the driven screws. Its primary function is to rotate the drive screw and to ensure that the drive screw does not create metal-to-metal contact with the drive screw. The timing gear derives the rotary motion from the drive screw and transmits it to the driven screw.
The driving shaft connects the prime mover (i.e., electric motor) and the pump assembly. It is attached to the motor shaft and the drive screw using a set of flexible couplings. It facilitates the driver screw rotation when the motor runs.
The pump screws are supported by bearing on either end. The bearings help to keep the pump’s components aligned. They carry the screw mass and hydraulic loads imposed on the crew, driving shaft, and the loads due to the shaft coupling or belt drive. They help to keep the screw’s axial and lateral movement within acceptable limits. In most cases, the pump bearing is the pumping fluid that lubricates the bearings. They are part of the axial thrust balance for the pump since the discharge pressure on one side of the bearing, and the suction pressure on the other side cancels out while supplying liquid for cooling and lubrication.
A cooling jacket, also called a flow generator, ensures an adequate flow of water across the motor surface to cool it. It is used when the pump installation is in open water, where there is no well to guide the water over the surface of the motor.
Pressure relief valve
The function of the pressure relief valves is to safeguard the pump from overpressures which can damage the pump’s internal components. Screws pumps generate a lot of pressure during operation, and pressure relief valves help to return the extra pressure to the intake.
The seals prevent air from leaking into the pump when the pump is under vacuum conditions.
Inlet and outlet ports
The inlet and outlet ports of positive displacement screw gear pumps differ. The pump sucks in liquid through the inlet port and discharges it through the outlet port. Positive displacement screw gear pump manufacturers design the inlet and outlet port to maintain enough liquid medium when the pump eventually stops. Doing so helps to ensure that the suction tube does not dry out quickly.
Figure: Components of a positive displacement screw gear pump.
How does a positive displacement screw gear pump?
When the electric motors starts, the driving shaft transfers the torque to the driven screw via a flexible coupling. The driver screw rotates the timing gear, and the timing gear transmits that rotation to the driven screws. As the screws turn together oppositely, their profiles mesh, creating tight cavities, which help to trap and transport fluid from the suction to the discharge port. The trapped volume decreases towards the discharge side building up high pressure to overcome the downstream back pressure from the system. The pump cavities open and close smoothly, resulting in quiet pump operation and a discharge with low pulsations. The screws rotate with significant radial clearance to the bore eliminating abrasive wear and enhancing the pump’s suitability for transferring fluid with low lubricity.
Figure: Working of a positive displacement screw gear pump.
Types of positive displacement screw gear pump
Generally, positive displacement screw gear pumps’ design is single suction meaning that the liquid enters from one side and exits from the other. However, they can be classified as either horizontal or vertical positive displacement screw gear pumps
Horizontal positive displacement screw gear pumps
The driver screws and drive screws in a horizontal positive displacement screw gear pumps are oriented horizontally.
Figure: showing a horizontal positive displacement screw gear pump
Vertical positive displacement screw gear pumps
Vertical types of positive displacement screw gear pumps have vertically oriented screws and driving shafts.
Figure: showing a vertical positive displacement screw gear pump
Applications of a positive displacement screw gear pump
Positive displacement screw gear pumps find uses in many applications such as food process, food drying, dairy, beverage, and cosmetics, pharmaceutical, chemical, oil, manufacturing, gas, and mining applications. Typical uses include
- Positive displacement screw gear pumps are used for pumping highly viscous fluids and low viscous fluid under high pressure.
- They are also used in high-power applications like weight oil lube transfer.
- They also apply to the transfer of sensitive structural products.
- They are also used in applications requiring pulsation-free flow.
- positive displacement screw gear pumps also apply in a large-scale coating such as architectural glass coaters.
- They are used in ships to move fuel oil from the double-bottom tank up to settling tanks.
- They are used as roughing pumps for high-vacuum pumps.
- They are also used in vacuum furnaces for blazing and sistering.
- They are also used in the hydraulic system to generate and overcome pressure.
- Also used for lubrication and multiphase pumping.
- They are also used in machine tools, windmills and presses.
- They may be used for shear-sensitive fluid, non-Newtonian fluid and emulsion because they experience low shear rates during operation.
Advantages of positive displacement screw gear pump
- Positive displacement screw gear pumps are highly reliable for a variety of operation.
- They have high application versatility as they can be used in diverse applications.
- They have better suction capabilities.
- They are self-priming and can operate with low suction pressure.
- They provide quite an operation and pulsation-free discharge.
- The pump requires minimal maintenance; the design is highly resistant to abrasive wear, and the timing gear prevents any contact between the screws, which causes them to wear out.
- They are perfect for viscous fluid and multiphase liquids, and gas mixtures.
- They pump fluid with lubricating properties; hence they don’t require lubrication service.
- They are gas tolerant and can handle fluid with entrained gases without vapor locking.
- They also can operate at the maximum flow speed.
Disadvantages of positive displacement screw gear pump
- Initial acquisition and installation costs may be very high.
- They require the installation of a gas ballast to transfer light gas.
- If used to pump light gases without a gas ballast, they result in low pumping speed and lower final pressure.
- It isn’t easy to scale the pumping rate below 50 cubic meters per hour.
Troubleshooting a positive displacement screw gear pump
The pump is unable to self-prime
- The motor direction is incorrect. Inspect the motor rotation direction and reverse if it is necessary. Ensure motor rotation align with the arrow provided on the nameplate by the positive displacement screw gear pump manufacturer.
The pump has increased noise and vibration
- There is wear and tear in the pump internals. Inspect the screws, bearings, and casing for wear and replace them if necessary.
- Some parts of the pump are contacting. Check if some parts of the pump are contacting correct.
- There is a system blockage. Inspect the system for blockage, and ensure the valves are working on both the suction and discharge sides.
- The relief valve is opening and closing rapidly. Increase the relief valve set pressure according to the pump manufacturer’s guide.
The pump gives out low-pressure discharge
- The pump doesn’t meet the pump manufacturer’s NPSH. Increase the suction pressure and level of the suction tank or reduce the pump speed.
- Screws are worn-out. Inspect and replace the screws if it is necessary.
- There is too much air ingress into the pump. Check pipework connections for wear and air ingress.
The pump takes in too much air
- Pipe work connections are severely worn out. Inspect, repair or replace the pipe connection.
- Defective sealing in the suction port. Inspect and repair pump sealing.
- The pump speed is low. Increase the pump speed to the level recommended by positive displacement screw gear pump manufacturer.
A positive displacement screw gear pump comprises two screws and timing gear. The timing gear connects the driver and the driven screws so that they do not create metal-to-metal contact, preventing any damage to the pump. The driver and driven screws intermesh to pressure liquid and move it within the pump system. During operation, an electric motor provides the mechanical power to turn the driver screw which rotates the timing gear. And the timing gear transmits the rotary motion to the screw gear, causing it to rotate in the opposite direction to the drive screw. As the screws rotate, their profiles intermesh, creating tight cavities, which help to trap and transport fluid from the suction to the discharge port. Positive displacement screw gear pump manufacturers design the pumps to perform excellently in many applications, such as marine and petrochemical industries, power generation, and food processing. The design of this pump is solely single suction, but the pumps are available in horizontal and vertical configurations.
Positive displacement screw gear pumps are equipped with high suction capabilities and are highly reliable and versatile in applications. They are self-priming and can handle liquid with entrained gas without vapor locking. In addition, they operate quietly and can provide pulsation-free discharge. However, they cannot run dry for too long because dry running can damage the pump, and they are expensive to acquire.