Pump impellers - Types and Workings

Centrifugal pumps are the most preferred hydraulic pumps used in the domestic or industrial world. They are used to induce flow or raise the pressure of the liquid, and are used in water, sewage, petroleum and petrochemical pumping. The center to the working of a centrifugal pump is the pump impeller casting. Pump impellers are made of iron, brass, bronze, aluminium or plastic and transfer energy from the motor that drives the pump to the fluid being pumped, which accelerates the fluid outward from the center of rotation. This article focuses on the working of an impeller, its types and workings.

Pump Impeller Casting

 1.  How does a pump impeller work?

Pump impellers are made of brass, bronze, aluminium or plastic and are shaped like small cylinders with an opening. The pump impeller works by the alternating entry and discharge of water into the “eye” of the impeller by centrifugal force. The low pressure created inside the inlet causes more water inflow. This increases the velocity as water flows at a high speed on the shaft. The water is then collected by the volute and converted into pressure by directing the flow discharge into a piping system.

The design of an impeller helps in increasing the flow rate power consumption of the pump stabilisers after reaching the limit point. The inlets accept the incoming fluid, while the vanes push the fluid in a radial direction, and a threaded bore accepts a driveshaft. The projecting “eye” of an impeller helps in a better swirl of the flow with a high negative pressure at the suction.

2.  Pump impeller types

Pump impellers come in specific designs, which depend on the number of points water can enter the impeller and the amount of webbing between the impeller blades.

a.  Single-suction pump – Single-suction pump impellers are designed to have a single opening for the water to enter. The design is suited for higher axial thrust imbalance due to the inflow of water from only one side.

b.  Double-suction impeller – Double-section impellers have a two-sided design for the entry of water. This is a more balanced design, as it balances the axial thrust loads on the impeller and transmits water to the driveshaft to the piping system. If suction is not done correctly, there is the risk of uneven flow on both sides of the impeller.

c.  Open impellers – An open impeller has a single blade or vane attached to a central hub. They have a weaker structure and need higher NPSHR values for proper functioning. They are much more sensitive than closed impellers, and constant erosion may hamper their efficiency. Open impellers are used in small-diameter and inexpensive pumps or in pumps that handle suspension solids.

d.  Semi-open impellers – A semi-open impeller has a circular plate attached to one side of the impeller, with a back for mechanical strength. They require lower NPSHR values than open impellers and are efficient in medium-diameter pumps and liquid with small suspended solids. A small clearance must be provided between the vane and the housing, otherwise, efficiency will be reduced due to overheating and re-circulation of wastewater.

e.  Closed impellers – Closed impellers have circular plates attached to both sides of the impeller, with a back and front wall. They also have wear-rings, which separate the inflow pressure within the pump. Also known as shrouded impellers, they are used in centrifugal pumps that handle clear liquids. They are also efficient in large pumps and suspended solids service, with high wear-and-tear resistance. They are designed to reduce axial load thrust and maintain efficiency.

When choosing the best pump impeller casting, one needs to check for quality material. The casting should corrosion-resistant, durable and sturdy. One should also check the number of blades, impeller dimensions, turbulent power number and flow number for the best performance.