There's more than one way to satisfy high head requirements in a pumping system. But which will work best for your process? Utilizing a multistage centrifugal pump, or setting up multiple centrifugal pumps in series are two design strategies available. We'll compare these in this post.
When considering whether to go with pumps in series, or a multistage centrifugal pump for a project, the most critical items to solve for are pump performance, system operation, and the cost. Let's compare how each method measures up in these areas.
Typically the multistage pump will win this battle. These pumps have smaller impeller diameters and tighter clearances for higher performance and efficiency. Additionally, expect less motor horsepower to be required. With only one motor, your energy usage should be less with this configuration.
Multi-stage pumps can save on floor space with their vertical configuration. Single stage can also be installed vertically, but multiple, single stage pumps will take up a lot of space in series, regardless of installing vertically or horizontally.
Assuming a constant flow, a VFD is needed to control the multistage pump and adjust any pressure build.
When piping multiple centrifugal pumps in series, with isolation/bypass options, the flow could potentially be diverted from one or more pumps, thus reducing the pressure generated in a “manual” way.
Multistage centrifugal pumps do not handle solids. If the process fluid contains debris or significant solids, a multistage pump will not be the right selection.
Depending on pump size and quality, single stage pumps will be easier to fix as they have fewer parts and stages. Additionally, with multiple pumps in the system, spares are technically available, or hopefully enough running to keep your process going. But keep in mind that more pumps mean more preventative maintenance as well.
It certainly depends on the pumps chosen and application type, but by evaluating the factors of equipment, piping, labor, energy usage, and maintenance, this category could go either way. In a clean fluid, simple application, generally, the multistage would be recommended.
Making the Choice
Multistage centrifugal pumps offer the benefit of better efficiency due to tighter impeller clearances and smaller impeller diameters. Higher pressure can be achieved with a smaller motor size and less energy. They also have a smaller footprint. Multistage centrifugal pumps can cost more than a little end suction, but running costs will be less. They also do not handle solids and would require a VFD for any sort of pressure variation. But perhaps the biggest watch out is, if the pump goes down, you run the risk of being dead in the water.
Single stage centrifugal pumps in series, with isolation/bypass valves, allow for system flexibility and manual pressure control. Piping can divert flow from a pump if less pressure build is desired, for example. This allows for spare pumps in the system, or at least a few running if failures occur. However, these pumps may not be as efficient, depending upon the impeller type. Motors may be larger than the multi-stage, not to mention the greater quantity of motors required.
The choice between generating more head with multiple pumps or multiple stages is one that is highly dependent upon each application. Be sure to involve an engineer well versed in pumping systems to make this decision.
Not sure which is right for your system? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.