Centrifugal pumps are the most common type of pump used in industry, agriculture, municipal (water and sewage plants), power plants, petroleum, and many other industries. They are the predominant type of pump in a class of pumps called "dynamic" pumps and are distinct from "positive displacement" pumps.
How do they work?
All centrifugal pumps include a shaft-driven impeller that rotates within a housing (usually at 1750 or 3500 RPM). The impeller is always submerged in water, and when the pump is running, the impeller spins rapidly. The centrifugal force acting on the water by this rotation pushes the water out of the housing and out of the drain from there. More liquid is introduced through the suction port or inlet. The velocity imparted to the liquid by the impeller is converted into pressure energy or "head."
Centrifugal pumps are unique because they can provide a high or very high flow (much higher than most positive displacement pumps) and their flow varies greatly with the total dynamic head (TDH) of a particular piping system The change. This allows the flow to be significantly "throttled" through a simple valve placed in the discharge line without causing excessive pressure build-up in the line or requiring a pressure relief valve. Therefore, centrifugal pumps can cover a very wide range of liquid pumping applications.
As mentioned above, a key advantage of centrifugal pumps is the ability to "throttle" their flow over a wide range. A throttling centrifugal pump with a bleed valve is not as energy efficient as using a variable frequency drive (VFD) to reduce pump/motor speed but is much less expensive to install. Of course, there are limits to throttling the flow of a centrifugal pump.
They should not be throttled below the "minimum safe flow" indicated by the pump manufacturer for a minute or so; otherwise, excessive recirculation may occur in the pump housing, causing the liquid to overheat. Additionally, too much "throttling" can cause excessive shaft deflection, which increases wear on bearings and seals in the pump.
Therefore, the ideal flow rate of a centrifugal pump is close to its "Best Efficiency Point" (BEP). BEP can be found on many pump head flow curves, their efficiency curves are shown in the same graph. The BEP for a given model, speed, and impeller diameter is the point of highest efficiency; this maximizes energy efficiency as well as seal and bearing life within the pump.
Another important point is that running the centrifugal pump at 1750 RPM motor speed instead of 3500 RPM motor speed will reduce wear on seals and bearings by a factor of nearly 4, and the pump is also less likely to suffer from adverse suction conditions (long suction lines, high "lift" from ponds or pits, low supply tank levels or liquids with high vapor pressure such as hot water, gasoline, etc.). However, a centrifugal pump running at 1750RPM requires a larger casing and impeller than a centrifugal pump running at 3500RPM and therefore costs significantly more.
Another important point is that for a given impeller diameter and RPM, centrifugal pumps will require their maximum horsepower, at their maximum flow on their Head-Flow curve. As the head (or discharge pressure) at which the centrifugal pump operates is increased (i.e. throttle valve is closed, the tank is full, the filter is clogged, the pipe diameter is longer or smaller, etc.), flow decreases and horsepower decreases.
Centrifugal pumps are designed for relatively low viscosity liquids that are poured like water or very light oil. They can be used with slightly more viscous liquids, such as 10 or 20wt. Oil at 68-70 degrees F (ambient), but additional horsepower must be added as centrifugal pumps become less efficient and even slightly increase in viscosity, and requires more horsepower.
When the viscosity of the liquid exceeds that of a 30wt oil at ambient temperature (about 440 centistokes or 2,000 SSU), centrifugal pumps become very inefficient and require more horsepower. In these cases, most pump manufacturers began recommending positive displacement pumps (eg gear pumps, progressive cavity pumps) over centrifugal pumps to keep horsepower requirements and energy consumption low.
Centrifugal pumps also require increased horsepower when pumping non-viscous liquids that are denser than water, such as fertilizers and many chemicals used in industry. The density of water is 8.34 pounds per gallon. The specific gravity of any liquid is the density (lbs/gallon) of that liquid divided by 8.34. The increase in horsepower required by a centrifugal pump for liquids denser than water is proportional to the increase in the specific gravity of the liquid.
For example, if a specific fertilizer has a specific gravity of 1.40 (that is, 1.4 times the density of water or 11.68 lbs/gal), then the increased horsepower of the pump will be 1.4 times the horsepower required to pump water with the same pump. So, in this example, if A 20HP motor is required to pump water, and a 30HP motor is required to pump fertilizer (actually, 28HP is required, which is 1.4 x 20HP, but the second largest motor is usually available is 30HP, as 25HP is not enough).
The above describes the working principle of centrifugal pumps in detail. If you want to know more or want to buy centrifugal pumps, please contact us.
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