Centrifugal pump design and analysis

AIM:

Centrifugal Pump Design and Analysis

OBJECTIVES:

Design a Centrifugal Pump and do performance testing.

Obtain the relationship between Pressure ratio and mass flow rate.

INTRODUCTION:

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor.

The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from which it exits.

Common uses include water, sewage, agriculture, petroleum and petrochemical pumping. Centrifugal pumps are often chosen for their high flow rate capabilities, abrasive solution compatibility, mixing potential, as well as their relatively simple engineering.

A centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid. A portion of the energy goes into kinetic energy of the fluid. Fluid enters axially through eye of the casing, is caught up in the impeller blades, and is whirled tangentially and radially outward until it leaves through all circumferential parts of the impeller into the diffuser part of the casing. The fluid gains both velocity and pressure while passing through the impeller. The doughnut-shaped diffuser, or scroll, section of the casing decelerates the flow and further increases the 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. You should also consider the pressure and volume requirements of the specific operations for which you need the pump. The horsepower required is another important consideration when it comes to volume and discharge pressure.

BODY OF THE CONTENT:

• Creating the simple solid model of centrifugal pump as shown below:

• Now as the impeller will not be going to rotate as mesh cannot be deformed according to the motion of impeller but to analysis the rotational motion, we have to create a solid body around impeller which extrude smaller than the casing and large enough than the impeller as shown below.

• This solid region generated around the impeller will be the rotating element.
• Now inside the flow simulation creating the lids at the inlet and the outlet of the pump.
• Opening the wizard tab and taking the flow as inlet flow and adding rotation to it, water as the fluid medium and other parameters to the default setting.
• Now adding the rotational element to be the solid generated around the impeller and under component control un-check the rotating element/ solid body around impeller.
• Now adding the boundary conditions, at inlet the environmental pressure at outlet the outlet flow velocity to be 10 m/s.
• Under the calculation control options un-check all the parameters except the number of iterations and set it to be 500 so that the simulation will run for 500 iterations.
• Now running the simulation and the results obtained are shown below:

PRESSURE PLOT:

From the above cut-plot the pressure is increasing in the radial direction.

VELOCITY PLOT:

From the above cut-plot the velocity is also increasing in the radial direction and is max at the edge of the blades of impeller.

FLOW TRAJECTORY:

From the above flow tragectory animation it can be seen how the velocity is being changing .

From the above flow trajectory animation it can be seen that how the vaccum is being created at the centre of impeller and the pressure is increasing in radial direction and at the outlet the pressure generated is greater than the atmospheric pressure.

MASS FLOW RATE:

The -ve sign indicates the mass is flowing out of the system, the change in the mass flow rate generated above can be neglected and can be considered constant.

OUTLET PRESSURE:

The outlet pressure generated is about 250k pa and almost reamins constant.

INLET PRESSURE:

Now performing the parametric study by ranging the outlet velocity in between 10 to 30 m/s and taking 6 scenarios in between:

RESULTS:

The results obtained for 10m/s are the same as shown in the above case.

For outlet velocity of 14m/s.

Velocity cut-plot: -

Pressure cut-plot: -

Flow-trajectory: -

For outlet velocity of 18m/s.

Velocity cut-plot: -

Pressure cut-plot: -

Flow-trajectory: -

For outlet velocity of 22m/s.

Velocity cut-plot: -

Pressure cut-plot: -

Flow-trajectory: -

For outlet velocity of 26m/s.

Velocity cut-plot: -

Pressure cut-plot: -

Flow-trajectory: -

For outlet velocity of 30m/s.

Velocity cut-plot: -

Pressure cut-plot: -

Flow-trajectory: -

Change in the mass flow rate over 500 iterations:

it is almost constant in each and every flow simulation.

Comparing the overall results:

As from the above graph it can be seen that with the increase in the outlet velocity the mass flow rate is increasing linearly(-ve sign shows the flow is getting out of the system.)

Change in the Outlet Pressure over 500 iterations:

Comparing overall results:

From the above graph it can be seen that the pressure is decreasing drastically with the increase in outlet velocity and in the last scenario it can be seen that the pressure generated is -ve which is not appropriate as it is a mock up geometry.

Change in the Inlet Pressure over 500 iterations:

Comparing overall results:

It is same in all the cases which is equal to environmental pressure.

From the above generated data, we have to from a curve between pressure ratio and the mass flow rate

So, using the concept of data parsing in python copying the values into a txt file from the generated excel sheet as shown below,

The file is being saved with the name cp.txt now writing the python code to evaluate the graph.

"""
Importing the Modules
"""
import matplotlib.pyplot as plt 
import math 
import numpy as np 

# To extract the values
values =[]

#Atm pressure is constant in all the cases

atm_pr = 101325

#extracting the line

for line in open('cp.txt'):
	if 'SG' in line:

		values.append((line.split()[6:]))

#storing the values
outlet_pressure=values[0]

Mfr=values[1]

#for garph 
pr_ratio = []
mfr = []

for i in range(len(outlet_pressure)):
	pr_ratio.append((float(outlet_pressure[i])/atm_pr))
	mfr.append((float(Mfr[i])*-1))

#plotting

plt.plot(mfr,pr_ratio,'k-')
plt.xlabel('Mass flow Rate')
plt.ylabel('pressure Ratio')
plt.title('Pressure Ratio Vs Mass Flow Rate')
plt.savefig('Pressure Ratio Vs Mass Flow Rate')

From the above program the graph generated is shown below.

CONCLUSION:

With the increase in mass flow rate the pressure ratio is decreasing, which concludes that there is a limit up to which mass flow rate can be increased to get the desired pressure at the outlet.