Centrifugal pump design and analysis

Aim: To create a 3D model of a centrifugal pump and perform a baseline flow simulation. Create a parametric study for the same and show the performance diagram. Obtain graphs of mass flow rate, pressure ratio, and show the relationship between them.

Objective:

1. Create a 3D model of centrifugal pump
2. Initialize the simulation after providing lids
3. Assign boundary conditions and goals to the design.
4. Run the simulation for 500 iterations with basic mesh and check the advanced mesh refinement box as well.
5. Obtain the results and compare them.

Introduction:

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.

Centrifugal pumps are generally designed and suitable for liquids with a relatively low viscosity that pours like water or light oil. More viscous liquids such as 10 or 20 wt. oils at 68-70 deg F will require additional horsepower for centrifugal pumps to work. For viscous liquids of more than 30 wt. oils, positive displacement pumps are preferred over centrifugal pumps to help lower energy costs.

The Main parts of Centrifugal Pump are:

1. Impeller

It is a wheel or rotor which is provided with a series of backward curved blades or vanes. It is mounded on the shaft which is coupled to an external source of energy which imparts the liquid energy to the impeller thereby making it rotate.

Impellers are divided into 3 types,

1. Open Impeller
2. Semi-enclosed Impeller
3. Enclosed Impeller

2. Casing

It is a pipe which is connected at the upper end to the inlet of the pump to the centre of the impeller which is commonly known as the eye. The double end reaction pump consists of two suction pipe connected to the eye from both sides. The lower end dips into liquid in to lift. The lower end is fitted into foot valve and strainer.

Commonly three types of casing are used in a centrifugal pump,

1. Volute Casing
2. Vortex Casing
3. Casing with Guide Blades

3. Delivery Pipe

It is a pipe which is connected at its lower end to the outlet of the pump and it delivers the liquid to the required height. Near the outlet of the pump on the delivery pipe, a valve is provided which controls the flow from the pump into the delivery pipe.

4. Suction Pipe with Foot Valve and Strainer

The suction pipe is connected with the inlet of the impeller and the other end is dipped into the sump of water. At the water end, it consists of foot value and strainer. The foot valve is a one-way valve that opens in the upward direction. The strainer is used to filter the unwanted particle present in the water to prevent the centrifugal pump from blockage.

Working of Centrifugal Pump

The first step in the operation of a centrifugal pump is priming. Priming is the operation in which suction pipe casing of the pump and the position of fluid with the liquid which is to be pumped so that all the air from the position of pump is driven out and no air is left. The necessity of priming of a centrifugal pump is due to the fact that the pressure generated at the centrifugal pump impeller is directly proportional to the density of fluid that is in contact with it.

After the pump is primed the delivery valve is still kept closed and the electric motor is started to rotate the impeller. The delivery valve is kept closed in order to reduce valve is opened the liquid is made to flow in an outward radial direction thereby vanes of the impeller at the outer circumference with high velocity at outer circumference due to centrifugal action vacuum is created.

This cause liquid from the sump to rush through a suction pipe to the eye of impeller thereby replacing long discharge from the center circumference of the impeller is utilized in lifting liquid to required height through delivery pipe.

Types of Centrifugal Pumps

Centrifugal Pumps are classified into many types based on many categories, they are

Based on the number of impellers in the pump,

1. Single-stage pump
2. Two-stage pump
3. Multi-stage pump

Based on the orientation of case-split,

1. Axial split Pump
2. Radial split Pump

Based on the type of impeller design,

1. Single suction Pump
2. Double suction Pump

Based on the basis of compliance with industry standards,

1. ANSI pump – (American National Standards Institute)
2. API pump – (American Petroleum Institute)
3. DIN pump – DIN 24256 specifications
4. ISO pump – ISO 2858, 5199 specifications
5. Nuclear pump – ASME (American Society of Mechanical Engineers) specifications

Based on the type of volute

1. Single volute Pump
2. Double volute Pump

Based on where the bearing support is,

1. Overhung
2. Between-bearing

Based on shaft orientation

1. Horizontal Pump
2. Vertical Pump

Application of Centrifugal Pumps

1. Oil & 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, petro-chemical, cellulose, sugar refining, food and beverage production
5. Various industries (Manufacturing, Industrial, Chemicals, Pharmaceutical, Food Production, Aerospace etc.) – for the purposes of cryogenics and refrigerants.

Geometry and Modelling

Simulation Parameters:

Type - Static Simulation

Analysis type - fluid flow

Fluid used - Water

Type of Flow - Laminar and Turbulent

Rotation Type - Local Region Averaging

Inlet boundary conditions - Pressure 1 bar

Outlet boundary conditions - Velocity of 10 m/s

Procedure:

1. Design a 3D model of a centrifugal pump with specific dimensions
2. Start the simulation after providing lids on both openings to make the geometry watertight.
3. Select the above-mentioned parameters for simulation
4. Insert the boundary conditions for inlet as Environmental Pressure of 1 bar and outlet velocity of 20 m/s.
5. Surface goals are set at inlet and outlet faces.
6. Surface goals at outlet face are selected for mass flow rate and total pressure.
7. Mesh is created for the model at level 3 and advance channel refinement is also selected.
8. Run the simulation for 500 iterations and obtain cut plots for Velocity, pressure and flow trajectory is also obtained.

Baseline Simulation Results

Pressure Plot

Velocity Plot

Flow trajectory

Parametric Study

Set the input variables as the outlet velocity and select range by number and vary it with 5 divisions between 20 and 60, i.e., 20, 30, 40, 50, 60 m/s.

The outlet variables are set to surface goals at the outlet, cut plots for velocity and pressure and mass flow rate.

Results:

• At 20 m/s

Velocity Plot

Pressure plot

Flow trajectory

• At 30 m/s

Velocity Plot

Pressure plot

Flow Trajectory

• At 40 m/s

Velocity Plot

Pressure plot

Flow Trajectory

• At 50 m/s

Velocity Plot

Pressure plot

Flow Trajectory

• At 60 m/s

Velocity Plot

Pressure plot

Flow Trajectory

Mass Flow Rate graph

Total Pressure graph

Parametric Study

Pressure Ratio Graph

Pressure Ratio vs Mass flow rate

Conclusion:

From the mass flow rate graph, we conclude that as the velocity increases, the mass flow rate decreases.

The pressure ratio is calculated by calculating outlet pressure/inlet pressure.

The pressure ratio vs velocity graph concludes that as the velocity increases, the pressure ratio decreases.

The relationship between mass flow rate and pressure ratio says that it is inversely proportional. As the mass flow rate increases, pressure ratio decreases and vice versa.