Centrifugal pump design and analysis

Introduction:

A pump which is the heart of the hydraulic system, converts mechanical energy into hydraulic energy. The mechanical energy is delivered to the pump via a prime mover such as an electric motor. Due to mechanical action, the pump creates a partial vacuum at its inlet. This permits atmospheric pressure to force the fluid through the inlet line and into the pump. The pump then pushes the fluid into the hydraulic system. One thing is very important to understand that 'Pump do not pump pressure'. Instead they produce fluid flow. The resistance to this flow produced by the hydraulic system is what determines the pressure. There are two broad classifications of the pumps:

1. Dynamic (Non Positive Displacement) Pump:

2. Positive Displacement Pump

Dynamic (Non Positive Displacement) Pump:

This type is generally used for Low pressure. High volume flow applications. Because they are not capable of with-standing high pressure, they are of little use in fluid power field. Normally the maximum pressure capacity is 250-300 psi. This type of pump is primarily used for transporting fluid from one location to another. The two most common types of dynamic pumps are Centrifugal Pump and axial flow propeller pumps. Positive Displacement pumps: This type is universally used for fluid power systems. It ejects a fixed amount of fluid into the hydraulic system per revolution of the pump shaft rotation. Such a pump is capable of overcoming the pressure for the mechanical loads on the systems as well as the resistance to flow due to friction.

Centrifugal Pumps:

Centrifugal Pumps are the most popular and commonly used type of pump for the transfer of fluids. In simple words, it is a pump that uses a rotating impeller to move water or other fluids by using centrifugal force. These are the undisputed pump choice especially for delivering liquid from one location to another in numerous industries including agriculture, municipal (water and wastewater plants), industrial, power generation plants, petroleum, mining, chemical, pharmaceutical, and many others.
Centrifugal Pumps are useful since they can generally handle large quantities of fluids, provide very high flow rates (which may vary with the changes in the Total Dynamic Head (TDH) of the particular piping system) and have the ability to adjust their flow rates over a wide range.

Working of a Centrifugal Pump:

Centrifugal pumps are used to induce flow or raise a liquid from a low level to a high level. These pumps work on a very simple mechanism. A centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid. The two main parts that are responsible for the conversion of energy are the impeller and the casing. The impeller is the rotating part of the pump and the casing is the airtight passage which surrounds the impeller. In a centrifugal pump, fluid enters into the casing, falls on the impeller blades at the eye of the impeller, and is whirled tangentially and radially outward until it leaves the impeller into the diffuser part of the casing. While passing through the impeller, the fluid is gaining both velocity and pressure.

The impeller in a centrifugal pump creates velocity through rotation;

Casing converts the velocity into pressure.

The following chief factors affect the performance of a centrifugal pump and need to be considered while choosing a centrifugal pump:

• Working Fluid Viscosity - can be defined as resistance to shear when energy is applied. In general, a centrifugal pump is suitable for low viscosity fluids since the pumping action generates high liquid shear.

• Specific density and gravity of working fluid - The density of a fluid is its mass per unit of volume. A fluid's mass per unit volume and gravity of a fluid is the ratio of a fluid's density to the density of water. It directly affects the input power required to pump a particular liquid. If you are working with a fluid other than water, it is important to consider the specific density and gravity since the weight will have a direct effect on the amount of work performed by the pump.

• Operating temperature and pressure - Pumping conditions like temperature and pressures are an important consideration for any operation. For example - High-temperature pumping may require special gaskets, seals and mounting designs. Similarly, an adequately designed pressure retaining casing may be required for high-pressure conditions.

• Net Positive Suction Head (NPSH) and Cavitation - NPSH is a term that refers to the pressure of a fluid on the suction side of a pump to help determine if the pressure is high enough to avoid cavitation. Cavitation refers to the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an impeller and can cause serious damage to the impeller and lead to decreased flow/pressure rates among other things. One must ensure that the system's net positive suction head available (NPSHA) is greater than the pump's net positive suction head required (NPSHR), with an appropriate safety margin.

• Vapour pressure of the working fluid - The vapor pressure of a fluid is the pressure, at a given temperature, at which a fluid will change to a vapor. It must be determined in order to avoid cavitation as well as bearing damage caused by dry running when the fluid has evaporated.

Owing to the use in the diverse range of applications, pumps come with different capacities and in various sizes. The pressure and volume requirements of the specific operations for which we need the pump. The horsepower required is another important consideration when it comes to volume and discharge pressure.

Applications of Centrifugal Pumps:

The fact that centrifugal pumps are the most popular choice for fluid movement makes them a strong contender for many applications and as mentioned previously, they are used across numerous industries. Supplying water, boosting pressure, pumping water for domestic requirements, assisting fire protection systems, hot water circulation, sewage drainage and regulating boiler water are among the most common applications. Outlined below are some of the major sectors that make use of these pumps:

1. Oil and Energy- pumping crude oil, slurry, mud; used by refineries, power generation plants

2. Industrial & Fire Protection Industry - Heating and ventilation, boiler feed applications, air conditioning, pressure boosting, fire protection sprinkler systems.

3. Waste Management. Agriculture & Manufacturing - Wastewater processing plants, municipal industry, drainage, gas processing, irrigation, and flood protection

4. Pharmaceutical, Chemical & Food Industries - paints, hydrocarbons. petrochemical. cellulose, sugar refining, food and beverage production

5. Various industries (Manufacturing. Industrial. Chemicals. Pharmaceutical. Food Production. Aerospace etc.) - for the purposes of cryogenics and ref rigerants.

Types of centrifugal pumps

Centrifugal pumps can be classified into several types depending on factors such as design, construction, application, service, compliance with a national or industry standard, etc. Therefore, one specific pump can belong to different groups and at times pump is known by its description itself. Some of these groups have been highlighted below:

Depending on the number of impellers in the pump, pumps can be classified as per the following:

1. Single Stage— A one impeller pump, single stage pump has a simple design and easy maintenance. Ideal for large flow rates and low-pressure installations. They are commonly used in pumping services of high flow and low to moderate TDH (Total Dynamic Head).
1. Two-Stage — This type of pump has two impellers operating side by side which are used for medium head applications.

2. Multi Stage— pump has three or more impellers in series; for high head service.

Impeller:

Impellers are rotating devices designed to alter the flow and/or pressure of liquids, gases, and vapors. Impellers consist of various vanes —often blade-shaped — arranged around a short central shaft. When the shaft and vanes rotate, they suck in fluids or gases and impel them out the other side.
Impellers are typically found in — and are integral components of — pumps, agitation tanks, washing machines, and other devices requiring fluids or gases to move in a specific direction. The image below shows the position and function of an impeller in an impeller pump. In this example, fluid enters through the orifice on the right and is moved through the pump using a combination of suction and pressure until the fluid is moved through the output orifice on the left. Note that this particular impeller is designed to rotate clockwise unlike the one in the image above; this rotation coupled with the design of the vanes causes the pump impeller to move fluid in a roughly clockwise direction.

Types of Impellers in Centrifugal Pumps:

Impeller design is the most significant factor for determining performance of a centrifugal pump. A properly designed impeller optimizes flow while minimizing turbulence and maximizing efficiency.

The impeller of a centrifugal pump can be of three basic types:

• Open impeller. Open impellers have the vanes free on both sides. Open impellers are structurally weak. They are typically used in small-diameter, inexpensive pumps and pumps handling suspended solids.

• Semi-open impeller. The vanes are free on one side and enclosed on the other. The shroud adds mechanical strength. They also offer higher efficiencies than open impellers. They can be used in medium-diameter pumps and with liquids containing small amounts of suspended solids. Because of minimization of recirculation and other losses, it is very important that a small clearance exists between the impeller vanes and the casing.

• Closed impeller. The vanes are located between the two discs, all in a single casting. They are used in large pumps with high efficiencies and low required Net Positive Suction Head. The centrifugal pumps with closed impeller are the most widely used pumps handling clear liquids. They rely on close-clearance wear rings on the impeller and on the pump casing. The closed impeller is a more complicated and expensive design not only because of the impeller, but the additional wear rings are needed.

The impeller blades can be:

• Backward-curved blade design (prefered design due to negative slope of performance curve)

• Radial blade design

• Forward-curved blade design (due to positive slope conditions this design can cause pump surge)

Impellers can be either:

• Single-suction. A single-suction impeller allows liquid to enter the center of the blades from only one direction.

• Double-suction. A double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously. This reduces forces exerted on the shaft.
The output pressure slightly changes according to the design used. The blades may be open or closed. Also the diffuser may be fitted with fixed vanes to help guide the flow toward the exit. The energy transferred to the liquid corresponds to the velocity at the edge of the impeller. The faster the impeller revolves or the bigger the impeller is, the higher will the velocity head be. In general, centrifugal pumps can be classified based on the manner in which fluid flows through the pump. It is not classification based on the impeller alone. but it is based on the design of pump casing and the impeller. The three types of flow through a centrifugal pump are:
• radial flow

• mixed flow (part radial, part axial)

• axial flow (propeller type)

Applications

While all impellers consist of similar design elements as described above, they may perform slightly different operations depending upon their intended application. Impellers in Pumps: Impellers are integral components in centrifugal pumps and vacuum pumps, among other pumping devices. Pump impellers rely on Bernoulli's principle — which states that an increase in fluid velocity is accompanied by a decrease in pressure or potential energy (and vice versa) — to operate.

When fluid or gas media enters an impeller pump, it becomes trapped between the impeller vanes and the pump wall and increases in velocity as it moves from the impeller eye (center) toward the outside diameter of the impeller. Once the media reaches a certain point nearest the outside diameter, it abruptly decreases in velocity and experiences an equal increase in pressure (according to Bernoulli's principle). The media becomes even more pressurized as it is discharged from the impeller and out of the pump orifice. Based on these principles of operation, it can be discerned that a pump impeller's rotational speed (affects velocity) and vane height (affects pressure and the possible formation of a vacuum) largely determine the pump's output pressure and flow.

Casing: A volute is a curved funnel that increases in area as it approaches the discharge port. The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, maintaining the velocity of the fluid through to the diffuser. As liquid exits the impeller it has high kinetic energy and the volute directs this flow through to the discharge. As the fluid travels along the volute it is joined by more and more fluid exiting the impeller but, as the cross sectional area of the volute increases, the velocity is maintained if the pump is running close to the design point.

If the pump has a low flow rate then the velocity will decrease across the volute leading to a pressure rise causing a cross thrust across the impeller that we see as vibration. If the pump flow is higher than design the velocity will increase across the volute and the pressure will decrease according to the first law of thermodynamics.
There are two types of casing in centrifugal pumps:

1. Volute Casing:

2. Diffuser

Volutes are designed to capture the velocity of liquid as it enters the outermost diameter of an impeller and convert the velocity of the liquid into pressure. The portion of the volute that extends closest to the impeller is called the cutwater.

Starting from the cutwater and proceeding in a counter-clockwise fashion, the distance between the volute and the impeller increases gradually. This has the effect of causing pressure to build within the volute as the distance increases. Once the point of greatest separation is reached — directly next to the cutwater moving in clockwise direction — the pressure is at its greatest, and water is forced out the casing when it encounters the cutwater.

What a cutwater is to a volute, vanes are to a diffuser. While volutes only have one (or sometimes two) points where the edge of the casing approaches the edge of the impeller in order to begin building pressure, diffusers often have many vanes. In the case of the assembly drawing shown the diffuser contains 10 vanes as compared the volute casing which only has one.
Also, while an impeller is placed in the center of a volute, an impeller generally sits directly adjacent to a diffuser and pushes water into the diffuser vanes.

The basic function of a diffuser is similar to that of a volute. Diffuser vanes are positioned such that they begin close to the outer edge of the impeller and then gradually extend away from the impeller periphery.

CFD Analysis: This project simulates a typical centrifugal water pump using the steady-state, multiple reference frame (MRF) method. The pressure-velocity coupling is performed through the SIMPLE algorithm. The MRF zone is assigned a rotational velocity of 104.66 rad/s (1500 rpm). The project is concerned with the influence of pressure and velocity changes with the change in outlet velocity and draw a relationship between Pressure ratio and mass flow rate of the centrifugal water pump. The performance characteristic curves, as well as the local and global flow variables, are numerically predicted using SolidWorks Flow Simulation.

Impeller Design: It is a semi-close impeller containing 6 Backward Curved Vanes. The impeller Diameter is 132mm.

Observation:

1. Firstly, we observe that with the increase in velocity, the total pressure at the output decreases gradually. So, this satisfies the working of an actual centrifuagal pump where we get maximum pressure at the outlet.

2. The pressure ratio which is calculated from the ratio of Outlet Pressure to Inlet pressure, which also seems to be decreasing.

3. The mass flow rate of the Pump is increasing with the increase in Outlet Velocity.

4. The graph plotted between Pressure Ratio and Mass Flow Rate, shows that mass flow rate is inversely proportional to Pressure Ratio.