Eccentric Screw pump
Eccentric Screw pump
What is an eccentric screw pump?
An eccentric screw pump is a pump under rotary displacement pump. This pump has other names such as progressive cavity pump and cavity pump. This type of pump works by moving fluid by progressing it through the pump in sequence of fixed shape, small, discrete cavities while its rotor rotates. This makes the flow rate from the eccentric screw pump proportional to the rate of rotation of the rotor. The discrete cavities taper downwards to the ends and overlap. The cavities are such that as one terminates another one increases. Eccentric screw pumps are used in several applications such as metering fluid and transporting viscous and shear-sensitive materials. Eccentric screw pumps result in fluid flow with zero or little pulses.
Figure: Eccentric screw pump.
Components of eccentric screw pump
An eccentric screw pump has several parts that work together to enhance the flow of fluid being pumped. Among the main parts of this pump are the rotor, stator, joints, drive shaft, bearings, prime mover, coupling among others.
Figure: Components of an eccentric screw pump.
The primary design of an eccentric screw pump consists of a rotor and stator. The rotor is made up of a strong material such as high tensile steel. Helical screws are machined on the rotor. The rotor is positioned in a stator made of rubber.
This is where the rotor is fixed by linked arms allowing it to roll in the stator. The motion is enhanced by use of bearings.
Joint is used in eccentric screw pump to support forces created due to torque and axial force between the rotor and the prime mover.
Pump body and flanges
The part where fluid is pumped in is the pump body. In the pump body, the suction flange is fixed and the fluid enters into the pump via this suction flange.
Dry run sensor
This is a sensor used to protect stators from running dry by use of temperature sensor when stator temperature becomes higher than the programmed temperature. This helps to stop the prime mover thus saving the pump from damage.
This is a sensor used in eccentric screw pump so that the pump can switch on when a media is placed at the inlet or in the hopper.
Heating jackets are placed in the stator and also for hopper eccentric screw pumps in the hopper to help protect materials from solidifying.
This is a switch that helps to control the pump pressure at the inlet.
Pressure relief valve
It is a valve placed on the pump or on the pipes to prevent excess pressure build-up which can damage the piping system or the pump.
How does an eccentric screw pump work?
Eccentric screw pumps are powered by an electric motor or IC engine as the prime mover. Eccentric screw pumps have one rotor that rotates in a rubber stator. The rotor looks like an elongated corkscrew and it rotates in a rubber stator made of double internal helixes. The eccentric screw pump manufacturers make the stator helix two times that of the rotor. The pitch difference helps to form sealed cavities between stator and rotor. These cavities are made to flow axially along the stator when the rotor rotates causing the smooth axial flow of the fluid. The fluid is forced to flow through the pump until it exits through the discharge port.
Figure: Working of an eccentric screw pump.
Types of eccentric screw pump
There are various models of the eccentric screw pump. These models are designed based on the functions and orientation of the pump. Some of the types of eccentric screw pumps are as follows.
Hopper eccentric screw pump
This is an eccentric screw pump that has a hopper fitted on it. This type of eccentric screw pump helps to hold the material to be pumped. This eccentric screw pump is used for pumping viscous liquids, large materials that need to be cut into smaller pieces, solids that plasticize, and materials that contain high levels of dry matter.
Figure: Hopper eccentric screw pump.
Food grade eccentric screw pump
This is an eccentric screw pump that is mainly meant for pumping foodstuff. As the name suggests and considering this pump will work on foodstuffs, the eccentric screw pump manufacturers design it with a lot of consideration in enhancing its hygiene to the foods being pumped. This pump is designed in a way that makes it easy to clean. Some of the models of eccentric screw pumps also have trolleys mounted on them to enhance relocation between processes.
Figure: Food grade eccentric screw pump with a trolley.
Horizontal eccentric screw pump
A horizontal eccentric screw pump is a pump whose drive shaft is aligned horizontally. This type of pump is easy to install and do maintenance with its internal parts being easily accessible. However, this type of pump tends to take a lot of installation space. Also, this type of pump has limited capacity regarding its net positive suction head (NPSH). This type of eccentric screw pump works on the same principle of using rotors and stators to create fluid cavities just like other eccentric screw pumps.
Figure: Horizontal eccentric screw pump
Vertical eccentric screw pump
These are eccentric screw pumps installed in the vertical direction. These pumps fit well where there is limited space for installation relative to the horizontal counterpart. Eccentric screw pump manufacturers make the pumps with a customizable length of immersion. This helps the end-user to install the pump without the challenges of insufficient length of immersion. These pumps have been used in applications such as pumping sewage to the main sewage treatment plant. These pumps are manufactured in short and long designs allowing for a range of applications. The short version of this eccentric screw pump has the mounting plate as the support when being installed while the long version has the guide and support to reduce oscillations and vibrations when used for deep immersion applications. Both of these versions have close-coupled configurations and stainless steel to protect them against corrosion of parts like stator housing.
Figure: Vertical eccentric screw pump.
Macerator eccentric screw pump
This pump is used to shred and cut solid particles in the fluid being pumped. This eccentric screw pump has a cutter made of hardened stainless steel. This cutter is responsible for cutting the solid particles. This helps the pump to effectively pump the fluid through small diameter pipes over a high lift or long distances without difficulties when the media has large solid components. This type of eccentric screw pump is manufactured in a way that helps it to achieve a high suction lift which makes it suitable for such difficult applications such as those where media to be pumped has solid components as well as very viscous fluids. After the solids are shredded into smaller pieces, they are transferred through the pipes without causing pump failure or blockage.
Figure: Macerator eccentric screw pump.
Multiphase eccentric screw pump
This is an eccentric screw pump designed to handle media in solid, liquid, and vapor phases. This is a very important pump in applications that have the media mixed with the three phases. The solids could lead to abrasion while vapor could lead to overheating and thus these could lower the life span of the pump. However, multiphase eccentric screw pump manufacturers make these pumps with features that guarantee enough strength to handle fluid with the three phases. This is often seen in crude oil applications whereby the crude oil is mixed with sand, gas, and water. Conventionally, it is required that the gas be removed from the crude oil. However, with the use of a multiphase eccentric screw pump, it is not a problem as the pump will work on all three phases of the media. Because of the ability of this pump to work on multiphase media, its design and cost tend to be higher relative to other eccentric screw pumps.
Figure: multiphase eccentric screw pump.
When to use an eccentric screw pump?
- When pumping viscous fluids. An eccentric screw pump is a good option for pumping very viscous fluids relative to other pumps such as centrifugal pumps. When centrifugal pumps are used to pump such very viscous fluids they become inefficient. As such, the fluid flow becomes very low and after a while, it may stop and also it results in more power consumption thus being uneconomical. On the other hand, when an eccentric screw pump is used to pump viscous fluids it becomes mechanically efficient and reduces the power required as well as rising flow. An eccentric screw pump, despite the level of viscosity, there will be a continuous flow. Also, in terms of variable viscosity, an eccentric screw pump would be very suitable as it can provide constant flow irrespective of the fluid viscosity.
- When in need of variable flow. Eccentric screw pump is capable of providing variable fluid flow as it is easy to regulate fluid flow as per the user’s needs. The flow can be regulated by varying the speed of the pump via the gearbox. The speed of the pump is proportional to the flow rate.
- When you need to separate fluid. An eccentric screw pump is a good option when fluids need to be separated. For example, assuming somebody wants a pump to transfer water and oil in a certain separating device. To make the separation process easier, then the water droplets need to be smaller than the oil droplets. If such fluids are pumped using other pumps like centrifugal pumps then they blend the oil and thus oil droplets become very small which interferes with the separation process. If the pump used is an eccentric screw pump it will be easier because this pump does not emulsify or blend the oil droplets.
- Suctioning conditions. Eccentric screw pumps need low torque to draw fluid into the pump because it has low pump speed. These pumps can also fill despite very low suction pressure. Centrifugal pumps cannot perform in low-pressure conditions and also they run at very high speed meaning their torque would be low.
- Used in metering and dosing. These pumps transfer fluids at low pulsation and thus they are suitable for metering and dosing.
- When precision flow is needed. An eccentric screw pump can provide precise flow for each revolution as each revolution results in a specific volume of fluid.
- To handle abrasive fluids. Eccentric screw pumps have a low internal velocity relative to centrifugal pumps. When the fluid has some solid particles in other pumps made of metallic materials it can easily cause wear or damage to the parts because of low tolerance and high rpm.
Applications of eccentric screw pump
- Eccentric screw pumps are used in sludge and sewage pumping.
- They are used to pump viscous chemicals in manufacturing plants.
- They are used in paper and pulp industries.
- They are used to pump biomass.
- They are used in gas and oil production.
- They are used in dosing and metering applications.
- They are sued to pump shear-sensitive products.
- They are used in food and beverage processing.
Advantages of eccentric screw pump
- They have a low pulsating flow.
- They are reversible.
- They can pump very viscous fluids.
- They can shred and pump solid particles.
- An eccentric screw pump can work on abrasive fluids.
- It can produce high pressures.
- Its speed can be varied to suit the application.
Disadvantages of eccentric screw pump
- They move slowly reducing the flow rate.
- They can pump for a limited distance.
- The shafts can deflect under high loads.
Troubleshooting eccentric screw pump
The eccentric screw pump is not starting
- Stator and rotor adhesion. Lubricate stator and rotor using inert oil or glycerin.
- Outlet excess pressure. Use a bigger motor or pump. Also, try to change pipe configuration to reduce outlet pressure.
- Blockage. Remove solids in the inspection port. If it is not possible disassemble the pump so that you can easily remove all solids and clean the pump easily.
- Frozen fluid. This could be due to the very low temperature in the pump. Install trace heating.
- The stator is not compatible with the fluid. Check stator and fluid chemical compatibility.
- The pump speed is not correct. Check pump speed to make sure it is within the recommended range. Also, ascertain rpm, voltage, power absorbed, and frequency is as recommended by the manufacturer.
- Very high viscosity. Check fluid viscosity matches with the pump specifications.
Eccentric screw pump not producing suction
- Blockage. Remove solids in the inspection port or try to disassemble the pump for effective cleaning.
- Air ingress. This could be air permeating through inlet pipes or seals. Tighten joint bolts and replace worn-out seals.
- High pump speed. Ensure pump speed is within the recommended range.
- Wrong direction of rotation. Ensure the rotation direction is as given on the pump nameplate.
- Worn out stator; replace.
- Worn out rotor; replace.
Eccentric screw pump having incorrect flow rate
- Faulty or broken transmission. Check the transmission and repair where necessary.
Eccentric screw pump having very high vibrations
- Driveshaft. Check the movement of the drive shaft. It could be due to an incorrectly fitted bush.
- Shaft misalignment. Ensure all shafts are properly aligned.
- Large solids. Reduce the solid size or install a cutter to reduce the solids before being pumped.
- Seal system. The seal system used is not suitable for that pump and needs to be changed.
- Mechanical seal. Verify if seals are worn out and replace them where necessary.
- Blockage. Remove solids from the inspection port. If possible disassemble the pump for effective cleaning of the pump.
Eccentric screw pump is also known as progressive cavity pump. This pump works by getting power from a prime mover which is either an IC engine or electric motor. The prime mover via drive shaft and coupling helps to transmit power to the pump. The pump has a metallic rotor and stator made of rubber. The rotor is machined with helical screws that help the pump to create fluid cavities as the rotor rotates. The rotation of the rotor keeps moving the fluid from the inlet section to the outlet section. The eccentric screw pump moves fluids at low pulsations and has a constant flow rate. Every rotor rotation produces a specific volume of fluid which makes it easier to determine the flow rate from an eccentric screw pump unlike other pumps like a centrifugal pump.
This pump is used in many industries such as oil and gas production, food and beverages, paper and pulp, sludge and sewage transfer, biomass transfer, dosing, and metering applications among others. The reason why this pump is used in many applications is due to its unique features such as being reversible, ability to pump fluids of whatever viscosity level, ability to release constant flow rate, vary its sped, ability to work on fluids with solids, ability to work on fluids that need separating devices among other. However, eccentric screw pumps use shafts for power deflections which tend to deflect under heavy loads. Such deflections can damage pump bearings and even other components of the pump. However, the eccentric screw pump advantages by far outweigh the disadvantages.