Basic differences between Open and Closed Impellers in Centrifugal Pumps

What is a pump impeller?

A pump impeller is a component used in centrifugal pumps to transfer mechanical energy from the prime mover (electric motor or diesel engine) to the fluid being pumped. The impeller is made of metallic material such as steel, and it comes with vanes.

How does an impeller transfer fluid through a pump?

When a pump starts (turned on), the prime mover (motor or engine) shaft starts rotating at high speed from 1000 to 3000 rpm. Since the impeller is attached to the motor/engine shaft, it also turns at the same rate. Due to the impeller rotation, a vacuum is created inside the pump. A vacuum is a low-pressure level relative to the atmospheric pressure. Due to this low pressure inside the pump, the fluid in the storage tank enters the pump via the suction pipe, aiming to create a pressure balance. As the fluid enters the pump, it meets the impeller and vanes, rotating at high speed. The rotation energy on the impeller is imparted onto the liquid, increasing its kinetic energy. As the fluid proceeds to the pump casing, the kinetic energy is reduced while pressure energy increases. The increased pressure energy helps to move the fluid to the required destination.

Figure: Impeller working to transfer fluid inside a pump.

Types of centrifugal pump impellers

There are two main types of impellers made by centrifugal pump manufacturers: the open and closed impeller.

Open impeller

An open impeller is the one whose vanes are open on both sides. This impeller does not have a protective shroud. As such, the impeller is used in inexpensive and smaller pumps that will not be operated at high strains since they are weak. These impellers have the advantage of pumping fluid with some solids.

Figure: Open impeller.

Closed impeller

A closed impeller is enclosed at the front and back. This design enhances strength, making the impeller suitable for heavy-duty applications. Closed impellers have low net positive suction heads and more efficient flow. However, these impellers are complex in design and expensive since they employ close-clearance rings to maintain efficiency and lessen axial loads. Centrifugal pump manufacturers use closed impellers in large pumps to transfer clean fluids and prevent clogging.

Figure: Closed impeller.

Difference between open and closed impeller

  • A pump that uses a closed impeller must be disassembled when the user needs to check the status of the wear rings. On the other hand, an open impeller does not need the user to disassemble the pump since the rings are mounted outside the impeller. This makes the wear rings easily accessible and visible. As such, open impellers are cheaper to repair and saves on time.
  • Closed impellers have high efficiency at the early stages of use. However, the efficiency reduces with time since wear ring clearance increases. The clearance grows as the impeller vanes wear, leading to a gap between the impeller’s edge and the wear ring, thus reducing efficiency. On the other hand, an open impeller can keep its efficiency with regular clearance adjustments. This adjustment is performed by moving the impeller forward or backward in its casing. By doing so, the impeller remains close to the wear ring, enhancing high efficiency over an extended duration.
  • Closed impellers are common in most industries as they can handle explosive and volatile fluids. The design of closed impellers ensures a tight seal, preventing leakage of hazardous liquids. Also, a closed impeller produces a higher head and pressure than an open impeller.
  • The vanes in an open impeller are attached to a central hub without any shroud or housing enclosing them. As such, there is enough space between vanes, reducing the possibility of clogging when working on fluids with solids. On the other hand, a closed impeller has its vanes fully closed by a shroud or housing, increasing the chances of clogging if the pump handles stringy materials or solids.

Factors to consider when selecting pump impeller

  • Flow rate. A flow rate is the fluid the pump impeller will deliver in a given period. It is given in terms of gallons per minute. A pump impeller that can provide 100 GPM is suitable for high-flow rate applications relative to 50 GPM.
  • Head. The head is the maximum height at which the fluid will be lifted. It is given in meters or feet.
  • Specific gravity. Specific gravity is the ratio of the density of a fluid to the density of water. A liquid with a specific gravity of more than 1 is heavier than water; therefore, it would need a powerful pump.
  • Solid material. If the fluid has solid particles, it is advisable to use open impeller to avoid pump clogging.
  • Fluid viscosity. A viscous fluid needs an open impeller to transfer.