The design of impellers in Centrifugal Pumps can differ based on factors such as the nature of the fluid, the pressure requirements, and whether the unit is self-priming or designed to handle entrained gas. Therefore, choosing the impeller type is crucial to balance the desired outcome for the process and maintenance intervals. The objective should be to achieve optimal process efficiency while ensuring the longevity of the equipment.
A Deep Dive Into The Types And Applications Of Centrifugal Impellers
Impellers come in 10 different types, each with strengths and weaknesses tailored to specific applications. For example, the arrangement of radial vanes in an impeller impacts its performance, with impellers having radial vanes positioned closer together and a more enclosed design with tighter tolerances offering higher efficiency but lower solid handling capacity due to the limited free passage.
Below is a list of the ten centrifugal impeller types, outlining their respective benefits and the types of fluids for which they best suit.
Various Types Of Impellers And Their Flow Systems
- Closed or Enclosed (Multistage and First Stage of Side Channel and Centrifugal Pumps)
- Double Suction (Double Suction and Split Casing Designs)
This particular design is primarily utilized in Split casing pumps, and its main advantage is its ability to draw liquid through both sides of the impeller blade simultaneously. It is most commonly used for clean drinks that do not contain solid particles and applications that require high flows and relatively low heads.
The double spiral casing feeds both sides of the impeller blade simultaneously, which results in the highest flow possible within a single case.
Flexible Impeller Designs
This design employs curved rubber vanes that remain in contact with the pump casing, making the unit self-priming. As a result, it can handle clean, viscous, and solid-laden liquids, and the solid particles fit between the blades.
As a result, it can handle suspended solids like fruit chunks up to 25mm in diameter. However, the unit is designed to have low shear, and its rotational speed is limited to less than 1400rpm because the impeller touches the casing.
The Advantages Of Open Design Centrifugal Pumps
This design features visible vanes that allow the unit to handle large solids and make cleaning easy. However, its sizeable free passage area makes it less efficient than other designs. Additionally, the lack of side walls makes it challenging for this design to generate high pressures. Despite this limitation, the design is still able to accommodate large solids.
Use Of Semi-Open Design In Trash And Centrifugal Pumps
The Semi-Open design, commonly used in Centrifugal and Trash pumps, features wide channels that allow handling large solid particles without clogging. It is designed to accommodate large capacities and low pressure for sewage or aggressive process applications within the industry.
Sinosuidal Design Of Centrifugal Pumps
The Sinosuidal design of pumps has a unique appearance similar to that of a sinusoidal sound wave, setting it apart from other methods. It is specifically designed to scoop pieces and gently transfer them toward the outlet while preventing fluid from recirculating within the pump head using a scraper at the outlet. Instead, the scraper moves in and out, following the wave shape of the impeller.
Unlike other low-shear pump designs, the impeller in Sinosuidal pumps does not have a tip. This unique feature allows for transferring soft or damage-prone products without causing any breakage, such as fruit pieces that do not pass through the impeller.
Importance Of Vortex Pump In Industrial Process
In conventional pumps, the liquid moves through the impeller and exits through the casing outlet. However, the Vortex version generates a swirling motion inside the casing. This feature allows fibrous materials or solid particles to pass through without touching the impeller. This design is known as non-clogging because only a few particles come in contact with the rotating part of the pump.
Vortex pumps are ideal for low-head applications and can be installed within the pump casing to allow for more significant suspended solid passage without affecting the pumped solids.
Mixed Flow Industrial Pumps
Like the semi-axial designs used in vertical turbine and immersion models, mixed-flow pumps transport the fluid at a diagonal angle. This creates solid vertical currents that can prevent settling at the bottom of a tank if the pump is immersed.
The Function Of Cutter Pumps In Industrial Process
Cutter pumps can come in different designs depending on whether they are a cutter or a macerator. Macerator and cutter pumps are two standard pumps dealing with solids and debris. Macerator pumps utilize a tungsten carbide impeller combined with a diffuser plate and grinder to effectively break down solids into smaller particles.
Some macerator models include an agitator below the impeller further to aid the suction and transfer points breakdown process. In contrast, cutter pumps utilize a multi-vane channel impeller that incorporates a cutting mechanism against a stationary cutting ring and rotating knife to cut and transport solids through the pump.
This design enables the pump to deliver higher pressures than a solid handling pump, utilizing smaller pipework. This makes it ideal for situations where high pressures are necessary and pipework size needs to be kept as small as possible.
For example, it can be helpful in long discharge lines or sewage pumping applications, especially when an additional bathroom is added to a property within a basement.
Side Channel Pumps With Closed Impellers
Side channel pumps with closed impellers have unique multiple radial designs that allow them to handle entrained gas. The pump fits tightly between a suction and discharge casing, which helps maintain its efficiency. While it can’t handle solids, it can separate gas or air, allowing both to be pumped without vapor locking. This feature makes it self-priming, ensuring that the pump can continue to operate effectively.
10 Common Issues That May Affect The Functioning Of Pumps In Industrial Process
- Clogging occurs when fluid gathers within the casing, decreasing efficiency and flow rate.
- A chemical attack happens when liquids or chemicals incompatible with the pump material cause corrosion and parts to fail.
- Cavitation occurs when insufficient suction pressure leads to bubbles forming within the suction that rapidly erode impeller surfaces and can cause failure.
- Imbalance occurs when impellers are not adequately balanced during manufacturing, resulting in unequal radial forces and accelerated component wear due to heavy vibration. This can cause premature failure of the mechanical seal and bearings.
- Water hammer is when fluid is suddenly stopped, causing momentum to reverse and creating rapid back-and-forth movement that can crack impellers and casings due to the forces involved.
- Impellers may loosen due to vibrations, causing them to come into contact with wear rings or the pump casing at high speeds, damaging the unit.
- Variable Speed Drives can cause resonance in pump parts at certain RPMs, leading to vibration.
- Fouling can reduce efficiency by obstructing the free flow of liquids. For example, if pumps are left standing for long periods, growths can occur within the pump and suction pipework, leading to cavitation. In addition, closed impellers can become clogged with solid particles, while liquids that contain dissolved solids can cause calcification and limescale build-up, which reduces efficiency.
- Turbulent flow can cause separation of the pumped fluid, leading to cavitation and reduced efficiency.
- Overpressure from high inlet pressures can push impellers against wear rings or stage casings, drastically reducing the pump’s service life.
Conclusion
The various types of centrifugal impeller designs are essential components of pumps and significantly impact the pump’s performance and efficiency. The selection of the appropriate impeller design depends on the specific application, as each method has advantages and disadvantages.
Open, semi-open, closed, and recessed impeller designs offer unique benefits regarding fluid handling, energy consumption, and maintenance requirements. Proper selection, installation, and maintenance of impellers are critical for achieving optimal pump performance, reducing energy consumption, and maximizing the lifespan of the equipment.
A thorough understanding of impeller designs and their applications is necessary for ensuring the reliable and efficient operation of centrifugal pumps in various industrial and commercial settings.
