How do pumps work

The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing. There are two basic designs of pump casing: volute and diffuser. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure.

In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet. This design causes the fluid pressure to increase towards the outlet Figure 2.

The same basic principle applies to diffuser designs. In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller Figure 3. Diffuser designs can be tailored for specific applications and can therefore be more efficient. Volute cases are better suited to applications involving entrained solids or high viscosity fluids when it is advantageous to avoid the added constrictions of diffuser vanes.

The asymmetry of the volute design can result in greater wear on the impeller and drive shaft. There are two main families of pumps: centrifugal and positive displacement pumps. In comparison to the latter, centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids, down to 0.

However, there are a number of applications for which positive displacement pumps are preferred. A pump is any device meant to facilitate the motion of a fluid. Pumps displace fluids, causing it to move down or out of a pipe. Most pumps use some sort of compressional action to displace the fluid.

This compressional action sometimes necessitates a motor that acts to put pressure on the fluid in order to displace it. This motor can be powered by a variety of fuels, as long as it has the necessary power to displace the fluid. Most pumps are either positive displacement or rotodynamic. The impeller is always submerged in water, and when the pump is operational the impeller spins rapidly.

The centrifugal force applied to the water from this rotation forces the water outside of the casing, where it exits a discharge port. More liquid is introduced through a suction port, or inlet. 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. 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. 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 RPM motor speeds instead of 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 RPM require much larger casings and impellers than those running at RPM and therefore, cost considerably more money. 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. The performance curves reflect standard testing. Pump manufacturers typically calculate performance curves using a pressure gauge and a flow meter connected to the discharge port. For any anticipated total head, the discharge capacity can be determined. Pump performance curves can be found on each model page.

The performance curves are useful in selecting a particular water pump. When a question regarding the performance of a specific pump must be answered, refer to the pump specifications for the particular model. Determine how high the pump will sit above the water source static suction head. Determine how high the discharge end will be elevated above the pump static discharge head.

Determine what the discharge capacity gpm of the pump must be. Keep in mind, the actual discharge performance may be significantly less than predicted by using static head alone because of friction losses in the system. Pressure can be calculated for total head by multiplying total head by. Pressure available at the end of the hose at zero flow for a given total head less then the maximum total head can be calculated by multiplying the total head by.

Example: The maximum pressure for a WH20X is 71 psi. The maximum available pressure at a total head of feet is 71 - 52 x. The total static head is often only considered when selecting a pump. However, because of frictional losses, this method can often lead to large error, and in many cases, the pump performance will not meet expectations. The selection process becomes even more complicated when a nozzle or sprinklers are used. In order to accurately predict the performance of a centrifugal pump in a specific application, the total head losses must be considered.

These losses include, but are not limited to: total static head, losses due to pipe size, length, and material, and losses due to sprinklers or a nozzle. Accurately predicting the discharge and pressure for a given pump in a specific application requires tedious calculations and a lot of trial and error.

Another fact of nature, is that a liquid moving through a hose creates heat due to the friction of the two surfaces water against hose. Steel pipe will produce more friction than will smooth PVC or vinyl pipe.

Mother nature plays an important role by exerting only This limits the suction head of centrifugal pumps to However, this would only be obtained if we could achieve a perfect vacuum in the pump. In reality, the suction head of centrifugal pumps is limited to about 26 feet.