What is a Centrifugal Pump?
Centrifugal pumps are the most common type of pump used in industry, agriculture, municipal (water and wastewater plants), power generation plants, petroleum and many other industries.
Centrifugal pumps are the primary pump type in the class of pumps called “kinetic” pumps and are distinctly different than “positive displacement” pumps. All centrifugal pumps include a shaft-driven impeller that rotates (usually at 1750 or 3500 RPM) inside a casing. Liquid flows into the suction port (inlet) of the casing and is thrown to the outside of the casing and then exits the discharge port. The velocity imparted to the liquid by the impeller is converted to pressure energy or “head”.
Centrifugal pumps are unique because they can provide high or very high flowrates (much higher than most positive displacement pumps) and because their flowrate varies considerably with changes in the Total Dynamic Head (TDH) of the particular piping system. This allows the flowrate to be “throttled” considerably with a simple valve placed into the discharge piping, without causing excessive pressure buildup in the piping or requiring a pressure relief valve. Therefore, centrifugal pumps can cover a very wide range of liquid pumping applications.
As described above, one key advantage of centrifugal pumps is the ability to “throttle” their flowrates over a wide range. Throttling centrifugal pumps with a discharge valve is not as energy-efficient as using a Variable Frequency Drive (VFD) to slow the pump/motor speed down, but it is much less expensive to install. Of course, throttling a centrifugal pump’s flowrate has certain limits.
They should not be throttled below the “minimum safe flowrate” indicated by the pump manufacturer for other than a minute or so; otherwise excessive recirculation can occur inside the pump casing which can cause excessive heat buildup of the liquid. In addition, too much “throttling” will cause excessive shaft deflection which will increase the wear on bearings and seals inside the pump.
Therefore, the ideal flowrate for a centrifugal pump is near its “Best Efficiency Point” (BEP). The BEP can be found on many pump Head-Flowrate Curves that have Efficiency curves shown on the same drawing. The BEP for a given model, speed and impeller diameter is the point where Efficiency is highest; this maximizes energy efficiency as well as seal and bearing life inside the pump.
Another important point is that running centrifugal pumps at 1750 RPM motor speeds instead of 3500 RPM motor speeds will reduce wear on seals and bearings by almost 4 times and the pump will also be less likely to cavitate when less favorable suction conditions (long suction pipes, high “lifts” from ponds or pits, low supply tank levels, or liquids with high vapor pressures such as hot water, gasoline, etc) are involved. However, centrifugal pumps running at 1750 RPM require much larger casings and impellers than those running at 3500 RPM and therefore, cost considerably more money.
Head – Flow Curves
Most centrifugal pump manufacturers publish “Head-Flow” Curves for each model, impeller diameter, and rated speed (RPM) for the centrifugal pumps they manufacture. A key point regarding these Head-Flow Curves is that all centrifugal pumps will always run along their Head-Flow Curve and the resulting flowrate will always be at the intersection of the pump’s Head-Flow Curve and the “System” Curve which is unique for each piping system, fluid and application.
System curves can be developed quite easily using Hydraulic Modeling Software and compared to various pump Head-Flow Curves in order to properly select centrifugal pumps that meet each user’s unique system and flowrate requirements.
Another important point is that centrifugal pumps will require their maximum horsepower, for a given impeller diameter and RPM, at maximum flowrate on their Head-Flow curve. As the Head (or Discharge Pressure) a centrifugal pump is working against is increased (i.e.-throttling valve being closed, tank filling up, strainer clogging, longer or smaller diameter piping, etc), the flowrate will decrease and horsepower will also decrease.
Centrifugal pumps are designed for liquids with relatively low viscosity that pour like water or like a very light oil. They can be used with slightly more viscous liquids such as 10 or 20 wt. oils at 68-70 deg F (ambient temperatures) but additional horsepower must be added because centrifugal pumps become less inefficient with even slight increases in viscosity and require more horsepower.
When viscosity of the liquids exceed those of 30 wt oils at ambient temps (approx. 440 centistokes or 2,000 SSU), centrifugal pumps become very inefficient and require much more horsepower. In those cases, most pump manufacturers start recommending positive displacement pumps (such as gear pumps, progressive cavity pumps) instead of centrifugal pumps in order to keep horsepower requirements and energy usage lower.
Centrifugal pumps also require increases in horsepower when pumping non-viscous liquids that are more dense than water such as fertilizer and many chemicals used in industry. Water has a density of 8.34 lbs/gallon. The specific gravity of any liquid is the density in lbs/gallon of that liquid divided by 8.34. The required increase in horsepower for a centrifugal pump used for a more dense liquid than water is directly proportional to the increase in specific gravity of the liquid.
For example, if a particular fertilizer has a specific gravity of 1.40 (i.e.-1.4 times the density of water or 11.68 lbs/gallon), then the increased horsepower for the pump would be 1.4 times the horsepower required when pumping water with the same pump. Therefore, in this example, if a 20HP motor was required for pumping water, then a 30HP motor would be required for pumping the fertilizer (actually, 28HP would be required which is 1.4 x 20 HP but the next largest motor commonly available is 30HP, since 25HP would not be sufficient).