Determining the right size centrifugal pump for the job, whether a sewer bypass, industrial site or construction area, is no simple task. The job is riddled with enough friction loss calculations, net positive suction head (NPSH) formulas and total dynamic head figures to make anyone’s head spin. Fortunately, pump distributors and manufacturers are well-versed in sizing pumps and can be the best resource during the selection process. Even so, it’s a good idea to be familiar with what goes into the decision.
First and foremost, the easiest part of selecting a centrifugal pump is determining the type of material the pump will need to move. If the pump is handling water laden with sticks, debris or waste, choose a wastewater pump. If it’s moving thick or sandy water, then a slurry pump is in order. Choosing one that is made to handle the type of material being pumped ensures two things: it can effectively move all material, and it can do so without damage to the pump components.
Suction Lift & Friction Losses
The next step is calculating the amount of suction that is required to move the fluid from the source up to the pump centerline, or eye of the impeller. This number is also called net positive suction head required, or NPSHr. But before determining the NPSHr to effectively move fluids, the amount of net positive suction head available, or NPSHa, must be calculated. This is basically the amount of pressure the system inherently has based on the elevation and system setup. Fluid moves from areas of high pressure to low pressure, and NPSHa indicates the amount of pressure the pump system has to work with to move the fluid. NPSHa includes atmospheric pressure as well as any friction losses that occur because of distance, elbows in the system or valves—basically everything that will create resistance.
The maximum suction lift a pump system can have due to gravity is 32 feet, which is at sea level. As the elevation rises, this number drops 1.2 feet per 1,000 feet. For example, Miami is at sea level so it has a NPSHa of 32 feet, while Denver, at 5,280 feet, has a NPSHa of 25.66 feet. This means the pump system has 25.66 feet of suction lift, the naturally occurring pressure to move fluid from the source to the impeller. When adding this number to the friction losses that occur within the system, the suction head available drops.
For the pump to operate correctly, the system must have more natural pressure working for it than required pressure, so the amount of suction lift required to move the fluid must always be less than the amount of suction lift available.
The ideal speed, or gallons per minute (gpm), required to effectively pump fluids is the next consideration in the pump-sizing process. Many system components should be considered when determining the ideal gpm for the job. These include pipe diameter, fluid velocity and materials being pumped, for example. It’s best to work closely with a pump dealer or manufacturer who can help determine the required gpm.
Distance & Elevation
How far the fluid needs to travel from the suction to the discharge point, as well as how high it will travel, must also be considered. This can be determined by adding suction lift available to the total elevation the fluid will travel.
For example, if the system has 10 feet of suction lift and the fluid needs to travel 50 feet up, the total elevation the system must take into account is 60 feet. The resulting number is the total dynamic head, which is the total elevation minus any friction losses, such as distance.
A Clearer Picture
When sizing a centrifugal pump, what you see is what you get. The calculations are the same across the board. But when choosing a pump to minimize downtime and enhance efficiency, the waters become a bit more muddied.
Keep in mind pumps are just like any other equipment in terms of design. They can vary from manufacturer to manufacturer.
Be sure to look for features that enhance uptime, efficiency and performance. For example, some units feature cleanout doors that swing open for easy access to the impeller to remove stuck materials, such as rags and diapers. This also allows technicians to perform impeller replacements without taking the entire pump apart. Standard setups require operators to remove several bolts with a special tool before accessing the impeller.
Often this involves using a crane to move the pump offsite for maintenance. Easy open doors can save contractors considerably in terms of downtime and hassle. Mechanical shaft seals that are placed in an oil bath, eliminating risk of dry running, are another feature than can minimize downtime.
Also, open-face impellers and large separators effectively separate air from the liquid, resulting in automatic and efficient priming in wet- or dry-prime applications. Units with this type of impeller are ideal for pumping fluids that also contain suspended solids.
Much goes into selecting the right pump for the job, but knowing how the process works as well as what features to look for can make the process a bit less daunting and keep productivity flowing.