The Power of Motion

Centrifugal pumps are a core part of the foundation of water and wastewater use and treatment. They come in a variety of types and their applications in the water industry are nearly limitless. There is always a pump (or two) to fit your needs, whether commercial, agricultural, public, or residential.

Centrifugal pumps are an ideal choice for clients in many scenarios due to their high rate of flow and relatively simple construction.

Types

There are many types of centrifugal pumps, and their application depends on the scenario at hand. It would take more time than we have to list all the pumps we may consider for a client’s water treatment needs, so here are five centrifugal pumps we see most often in the field:

Deep Well Vertical Turbine – These highly efficient pumps are commonly used in groundwater production wells where high flows, high reliability, and high efficiency are important. They also have the advantage of the motor being located above ground, allowing for easier maintenance.

Submersible Vertical Turbine – Submersible vertical turbine pumps have an impeller-and-bowl configuration similar to the deep well vertical turbine pumps. However, with the submersible pump, the motor is attached directly to the pump itself and is therefore below the water level. These pumps often have a lower capital cost, but can be more difficult to maintain since the pump must be removed to access the motor.

End Suction Horizontal– End coupled horizontal pumps are ubiquitous in the water industry, serving a wide range of applications. These pumps are efficient, economical, and readily available from a number of sources.

Horizontal Split Case – Horizontal split case pumps are ideal for clean water applications where high efficiency and ease of maintenance are a must. The impellers on these pumps are suspended between bearings and is easily accessible for maintenance.

Recessed Impeller – Recessed impeller pumps offer a clear passage for solids to enter and exit the volute without passing through the impeller vanes. This makes these pumps ideal for raw sewage or heavy solids applications. Due to the shape and location of the impeller, efficiencies can be lower with these types of pumps.

How They Work

A centrifugal pump works by converting rotational energy into energy in a moving fluid. The rotational energy is provided by a motor, or driver, which turns an impeller. Unlike a propeller (which turns rotational energy into linear motion), an impeller uses its rotation to impart centrifugal force to the water, causing it to move outwards. Both the pressure and the kinetic energy rise are captured by the volute which slows the velocity and increases the static pressure.

Impellers are the driving force for centrifugal pumps. Pump impellers are available in multiple configurations, depending on what our clients need.

  • Open impellers are often the choice for smaller applications because they are economical and easily maintained. They are able to function well in smaller sizes and are capable of handling suspended solids.
  • Closed impellers have greater strength, reliability, and longevity compared to open impeller pumps. They are often used on larger clearwater projects and have a wider range of operating speeds. Unfortunately, they are also more expensive, harder to clean and maintain, and they do not handle solids well.
  • Semi-open impellers combine the pros and cons of open and closed impellers; they can handle solid-liquid mixtures with added strength from a back wall. However, they are not as efficient as their open counterparts.
  • When mixing fluids or slurries in an agitated tank, clients can use either an axial or radial impeller construction. Both are designed to be submersed. Axial impellers are used on homogenous mixtures to increase their flow rate while radial impellers are used to force non-homogenous fluids together.

Shaft Seals

Years ago, most pump shafts were sealed using rings of soft packing, compressed by a packing gland. This type of shaft seal required a fair amount of leakage just to lubricate the packing and keep it cool. Then came the development of the “mechanical seal,” which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces (one stationary and one rotating). Even though these mechanical seal faces also require some (very small) leakage across the faces, to form a hydrodynamic film, this leakage normally evaporates and is not noticeable.

There are four major types of seals for centrifugal pumps which ensure longevity when applied in the correct context:

  • Stuffing seals uses packing within the case to create a seal so the impeller can function without leaking fluid and decreasing efficiency. These seals are commonly found on submersible pumps.
  • Mechanical cartridge seals usually have 3 seal points: an o-ring, a stationary face, and a rotating face. The o-ring rotates with the shaft of the pump while the stationary and rotating faces are pushed together to stop fluids from leaking. Due to the high friction present, lubrication is used to keep the part from wearing each other away.
  • Multiple mechanical seals are used to increase efficiency and longevity while preventing any leakage from the pumps. There are usually two seals present in this construction: a softer inner seal, which takes the brunt of the sealing duties, and a harder, wider outer seal that prevents any fluid not stopped by the first seal from escaping the pump. These two seals are always separated by a buffer, or fluid barrier, to prevent additional wear.
  • Flush water, or seal water, is an additional factor in pump sealing components whose function is cooling, lubrication, and flushing of the seal(s). This water can come from an outside source or the fluid which is being pumped can act as the sealant. In singular seals, the flush water remains inside the pump and any excess that escapes is used to care for the seal. In multiple seal constructions, the flush water is the fluid barrier between each seal, ensuring each seal remains at an optimal operating temperature with proper lubrication. If a pump is handling solids, flush water will also be used to clean the pump and seals of debris.

Blog author and MKN’s Chief Engineer, Jon Hanlon

 

Interested in learning more about our work with centrifugal pumps? Check out our projects page for an in-depth look at some of our recent work, or connect with us through our contact page, Facebook, or LinkedIn to discuss how we can help you.