CIVE401 Team Project November 19, 2014

Centrifugal Pumps: A Closer Look

Travis Bell Jocelyn Bryant Seth Bosch

Introduction:

Pumps are used to overcome losses in a pipe system. Centrifugal pumps

convert mechanical energy to increase the kinetic energy of the moving fluid. Interestingly, the centrifugal force was considered a fictitious force until the modern era where it was defined with in the rotating reference plane (Saylor Academy 1). Centrifugal pumps are devices that convert the work of a spinning motor into the flow and head increase of a fluid. Centrifugal pumps can also be applied in series or in parallel, in order to create additional flow or head. The centrifugal pump has become commonly used in many industries over the last 60 years. These pumps can move almost any material if properly designed. They are most commonly used however to transport: water, sewage, petroleum and petrochemicals. As time has passed these pumps have been optimized and the design has been altered to fit various applications. For example closed impellers are the most efficient design but if the material being worked with is highly viscous impellers can be altered to an open design in order to pump the material. This is simply one instance amongst many that make centrifugal pumps so useful. These versatile and efficient pumps have become an essential tool to modern industry and are applied at some level to every industrial process in the modern world.

Description: Centrifugal pumps have a very basic design that has been tweaked slightly over the years to increase its efficiency. Every centrifugal pump is made of these basic parts: impeller, casing, and shaft. The impeller is contained within the body of the pump. It has blades that contain water connected to a hub plate. When the impeller is rotated, the blades impart centrifugal force on the water particles. As the water is spun out of the center, the pressure and kinetic energy increase. At the edge, water is discharged. In order to maintain conservation of mass over the control volume of the impeller, water must enter through the eye. The eye has a pressure lower than atmospheric pressure, which helps suck more water into the impeller. Centrifugal pumps must be primed with water before they can be used. If a pump is not primed,

the rotation of air within the pump does not produce significant negative pressure and cannot draw water up though the pump. The impeller is placed in a casing, keeping the water contained and fed through the exit pipe. The casing on the impeller has greater cross‐sectional area around the exterior of the impeller as the casing approaches the outlet. This increase of cross‐ sectional area reduces exit velocity and increases the static pressure of the water. As more water is spun out of the impeller, the mass flow rate increases from inlet to outlet. The increase in cross‐sectional area also accounts for the increase in the mass flow rate.

Pump Anatomy (“Centrifugal Pump”)



Impellers are typically closed for water, but open impellers are used for fluids with more viscosity. The closed impeller is more efficient than the open impeller. The impeller is driven by a shafted connected to the motor. This shaft must be properly sealed to prevent leaks from occurring within the highly pressurized pump system. Since one end of the impeller must be open to allow the entrance of water, the shaft is mounted on the other side like a cantilever. Ball bearings need a bearing housing with a coolant fluid in some high flow rate pumps. Centifugal pumps are the most common type of pump for fluid flow. They account for approximately 80% of all pumps in a petroleum plant (Girdhar 10). Generally,

they are split into two categories, vertically and horizontally oriented pumps. They are also frequently arranged in sets to increase the overall mass flow rate. Additionally, the blades attached to the interior of the impeller drastically affect the efficiency of the pump. Backward curved blades are the most efficient because they create the most tangential exit of the water from the blades. The efficiency is hampered by the blades imparting rotational forces on the fluid. The straight blades impart an equal force out as rotational, while the backward curved blades reduce the rotational force on the fluid. This explains why the backwards curved blades are the most efficient. As a textbook explains “if the vanes of the wheel are straight and radial; but if they are curved, as is more commonly the case, the outward force is partly produced through the medium of centrifugal force, and partly applied by the vanes to the water as a radial component of the oblique pressure, which, in consequence of their obliquity to the radius, they apply to the water as it moves outwards along them” (Saylor Academy 2). The figure below shows the flow rate compared to the power needed for three different types of blades.

Power Consumption vs. Outflow For Different Blade Geometries (“Centrifugal Pump”)







History: The centrifugal pump is one of the most efficient and versatile pumps ever designed and thus it is the most common pump design in use today. These pumps basically act in the opposite way a water wheel operates. Instead of using moving fluid to create shaft power shaft power is utilized in creating movement in a fluid. Italian renaissance engineer Francesco di Giorgio Martini first considered this concept. Martini designed a mud‐lifting machine in 1475 based off this principle (Segrest, Michelle). However, the first

Denis Papin (Isaac Papin Et Moi….)

example of a true centrifugal pump appeared in 1687. French inventor Denis Papin created a single stage centrifugal pump with straight vanes in order to assist with local drainage work, he also used the concept to help with ventilation in a coalmine. About a century later in 1781, the centrifugal pump became much more common as the steam engine was adapted to produce rotary motion (Segrest, Michelle). This adaptation made the centrifugal pump much more useful. As a result, centrifugal pumps began to be installed in municipal water systems in large cities like London. The curved vane was later introduced to centrifugal pumps in 1851 making the design significantly more efficient. This discovery was made by British inventor John Appold (Saylor Academy). Appold discovered that a large portion of energy in straight vane pumps was wasted by creating purely rotational movement in the fluid as opposed to forcing fluid directly outward through the pumps pressurized

outlet. Over the next century, the design was altered again to produce the multistage centrifugal pump (multiple impellers). These multistage pumps for the most part act the same as multiple pumps in either series or parallel. The only difference is the impellers are all contained inside a single housing. Multistage pumps can create higher head or higher output, but for the most part single stage pumps are still preferred for the majority of applications. The centrifugal pump has become the most popular and versatile pump design in the last 60 years with the development of powerful high‐speed electric motors, which are used to drive the impeller. Today these pumps are used in almost all conditions and have numerous applications.

Equations: Below are listed various basic equations used to design and optimize systems using centrifugal pumps. (“Centrifugal Pump”) V1 = Fluid Velocity @ Inlet U1 = Impeller Velocity @ Inlet V2 = Fluid Velocity @ Outlet U2 = Impeller Velocity @ Outlet

Flow and impeller Velocities at Inlet and Outlet (“Centrifugal Pump”)



The work required to increase the fluid velocity between the pump inlet and outlet can be expressed by:











It is known that tangential velocity at the inlet is nonexistent so it can be said 0. Thus:











Rise in energy head through the pump can be found using:









Again it can be assumed that Vθ1=0:









If impeller blade angle at the outlet is unknown it can be found using:



Specific Speed of the pump, which relates the pump head and discharge for a given rotational speed of the pump, can be found using: . .



Finally Power can be found in centrifugal pumps using the below equations if H (ft) and Q (cfs):

or



Advantages of Centrifugal Pumps: There are many advantages of centrifugal pumps, making them extremely popular in the modern world, most being a result of the simplicity at which they operate. The simplicity allows for centrifugal pumps to generally be small in size and easy to maintain, which permits the facility to be economical with space and inexpensive upkeep. Essentially, they are a low‐risk piece of equipment, as there is no danger of damage to the pump if the discharge valve is closed while starting, which is not the case with many other pump types. An additional attribute that makes the use of centrifugal pumps an attractive option is that they are able to deal with large volumes and medium to low head and medium to low viscous fluids.

Disadvantages of Centrifugal Pumps: Although the concepts at work for a centrifugal pump are extremely rudimentary, there are several problems that coincide with the use of centrifugal pumps. Most of these issues however, can be prevented with thorough maintenance and upkeep, along with proper usage. For example, centrifugal pumps must be primed prior to being initiated. If the pump is not filled with a liquid before being started, gases and vapors can enter the system and impair the pumps ability to operate properly. Similarly, a problem that arises with centrifugal pumps is the occurrence of cavitation. Cavitation in a pump will occur first, at the inlet – where the suction happens. This can be caused when the net positive suction head, or the head value required to prevent cavitation from occurring, is too low. Cavitation can lead to reduced efficiency along with damage to the pump. Wear and corrosion of the impeller can ensue if suspended solids are present in the fluid. The properties of said fluid can also cause damage to the pump if they are not ideal; for example, fluids with a high viscosity.

Lastly, variables of the pump must be closely monitored in order to quickly detect defects. If the flow is too low, the pump can overheat, reducing efficiency. It is also possible for water to leak around the impeller.

Net Positive Suction Head (NPSH): As previously mentioned, cavitation can occur if the net positive suction head or NPSH is too low. The NPSH is a measure of the head value taken as the difference of the suction head at the impeller and the liquid vapor head. If this quantity is maintained at a large value, cavitation should not occur. However, if this number is not maintained, cavitation becomes a large risk. Suction Head:



Where:



hs = suction head close to the impeller



ps = static pressure in the fluid close to the impeller



γ = specific weight of the fluid



Vs = velocity of fluid



g = gravity constant

Liquid Vapor Head:



Where:



hv = vapor head



pv = vapor pressure

Net Positive Suction Head (NPSH):

or



Conclusion: Centrifugal pumps are a low cost, simple design that is reliable and applicable to a broad range of scenarios. These pumps are most often used in buildings to pump the general water supply and the fire suppression water. Centrifugal pumps are also used in petroleum refineries to transport the oil in its different refined stages. The centrifugal pumps’ low risk of damage, even if not operated properly, makes them ideal for these applications. Invented in the 17th Century and refined over the years, centrifugal pumps are governed by a fairly simple set of equations that allow for fairly rudimentary design. Major refinements included the introduction of backward swept blades, which reduce the tangential velocity applied to the water and therefore increase the efficiency of the pump. These pumps can be used for a variety of liquids, including more viscous fluids that other pumps can generally not handle. Closed casings are best for Newtonian fluids, while open casings are best for the more viscous fluids. Multistage centrifugal pumps are a further design option that can be designed to create either remarkably high discharge or high head based on the application. As modern electric motors improve further the centrifugal pump only becomes more effective in a wider spectrum of applications. The simplicity and practicality of centrifugal pumps have proven effective ever since their conception and they will likely continue to be used for years to come.



Works Cited: "Centrifugal Pump." LearnEngineering.org. Learn Engineering, n.d. Web. 05 Oct.2014. Evans, Joe. “A Brief Introduction to Centrifugal Pumps.” Pacific Liquid and Air Systems, n.d. Web. 17 Oct 2014. Girdhar, Paresh, and Octo Moniz. Practical Centrifugal Pumps: Design, Operation and Maintenance. Oxford: Newnes, 2005. Print. "Isaac Papin Et Moi…." Revue De Web Expo Denis Papin. N.p., n.d. Web. 30 Sept. 2014. “NPSH – Net Positive Suction Head.” The Engineering Toolbox, n.d. Web. 17 Oct 2014. Saylor Academy. Centrifugal Pumps. N.p.: Xlibris, 2010. Web. 1 Oct. 2014. Segrest, Michelle. "The History of Pumps: Through the Years." Pumps & Systems. Pumps and Systems, 22 Dec. 2011. Web. 01 Oct. 2014. “Utilitiesman Volume 01 – Manual for electric, plumbing, water and other utilities.” n.p., n.d. Web. 17 Oct 2014.