What is Fluid Flow?
Fluid flow is the study of how fluids behave when in motion. It is a fundamental element of engineering, and is studied in the fields of hydraulics and fluid mechanics. Understanding the principles of fluid flow is essential for effectively sizing and designing piping and equipment.
What is a Fluid?
A fluid is any substance that is capable of flowing. This includes liquids, gases, or a combination of both. Fluids have a finite mass, occupy a space, and are tangible. They are made up of different molecules or particles and are considered to be in a state of flow when the particles of the substance have a relative change in position over time. Fluids have some specific properties, such as having a mass of their own, not possessing shape, and taking on the shape of the vessel or pipe in which it is flowing.
Forces Acting on Fluids
The forces that act upon a fluid are surface force and body force (gravitational force). Mechanics is the study of force and how it affects the fluid. By understanding the effects of these forces, engineers can effectively design systems that take into account the flow of the fluid.
What is Fluid Flow in Pipes?
Fluid flow in pipes is the movement of a fluid within a closed conduit under the influence of a certain pressure. It is used to transport chemicals, petroleum products, gas products, sewage flows, household water supply, etc. in different piping and pipeline systems. The flow of all real fluids is termed viscous flow since they possess a viscosity. This can be characterized by the shear stresses or the frictional forces between the fluid layers and fluid to a solid surface, as well as the physical parameters of the pipe and the external forces affecting the piping system.
How Does Fluid Flow in Pipes?
Fluid flow in pipes is caused by energy. This energy can come in the form of flow energy (pressure head), kinetic energy, and potential energy. Tilting the pipe so that the flow becomes downhill can create gravitational energy that transforms itself into kinetic energy. Another way to create fluid flow is to make a pressure difference through the use of different pumps. These pumps use impellers, which are connected to the motor, to accelerate the fluids into the discharge line. This will ultimately determine the pressure and flow rate for the system.
Pipe Flow Calculations
Flow Rate Through Pipes
Flow rate, or discharge, is the volume of fluid that passes through a pipe in a given period of time. It is calculated by multiplying the area of the pipe with the velocity at which the fluid flows through it. The unit of measurement for this is usually cubic meters per second (m3/s).
Types of Fluid Flow: Laminar and Turbulent Flow
Laminar Flow:
Laminar flow is a type of flow in which the fluid moves slowly in layers without much mixing. This type of flow usually occurs when the velocity is low or the fluid is very viscous. The maximum flow will be at the center of the pipes and the minimum flow at the pipe walls.
Turbulent Flow:
Turbulent flow is a type of flow in which the fluid becomes irregularly fluctuating with time due to its high velocity. This type of flow usually occurs when the velocity of the flow exceeds some threshold value for a given fluid in a pipe. The velocity at the center of the pipe is approximately equal to the average flow velocity.
Impact of Fluid Properties on Flow through Pipes
Flow Assurance Software:
Flow assurance activities can be found using surge analysis software such as AFT Impulse, AFT Fathom, and Bentley Water Hammer. AFT Fathom is commonly used to determine flow rates, pressure, and velocity in piping systems.
Effect of Density on Flow through Pipes
The density (mass/unit volume) of a fluid has a great impact on the piping system through which it flows. When the fluid density is high, the mass is also higher, making the system more compact and dense. This can result in a reduction in inflow and a more significant drop in head or pressure, while also leading to an increase in power draw. According to Bernoulli’s principle, as the pressure increases, the velocity of the fluid decreases since the pressure and velocity are inversely proportional.
Effect of Viscosity on Flow through Pipes
The viscosity of the fluid can also affect the performance of the pumps, as they need to be adjusted to account for the additional shear resistance. When the viscosity increases, the head or pressure can drop significantly and the power draw may go up.
Impact of Temperature on Fluid Properties
Temperature is another factor that can affect the properties of the fluid, including its viscosity and density. When the temperature changes, the viscosity and density of the fluid will also change, impacting the flow in the piping system.
Effect of Length, Diameter, and Roughness on Flow
The length and inner diameter of the pipe can have an effect on the flow, particularly in the case of turbulent flow. The internal roughness of the pipes, as well as the number and type of bends, valves, and other pipe fittings, can also affect the flow of the fluid.
Impact of Specific Gravity and Surface Tension on Flow
The specific gravity (relative density) of the fluid, which is the ratio of the density of the fluid to the density of a standard fluid, can also have an effect on the flow. When the specific gravity changes, the outlet pressure will change proportionally, while the pressure drop will remain unaffected. The surface tension of the fluid can also affect the hold-up of the fluid.
Losses in Flow through Pipes
When fluid flows through pipes, energy losses will occur. These losses can be categorized into two sections: Major Losses and Minor Losses.
Major Losses
Major losses are the result of pipe wall friction and are proportional to the volume of the fluid. Factors such as the density of the fluid, the nature of the surface, the nature of the fluid, and the solid walls in contact all play a role in determining the amount of energy lost. Major losses can be calculated using Darcy Weisbach equation and Chezys formula.
Minor Losses
Minor losses are caused by eddy formations in the fluid due to sudden increases or decreases in velocity. This can be caused by pipe bends, pipe fittings, and even obstacles. These minor losses can have an effect on the total energy lost.
Chezys formula equation and Darcy Weisbach equation
Chezys formula and Darcy Weisbach equation are two mathematical equations used to calculate major losses in fluid flow through pipes. Chezys formula is used for turbulent flow, while the Darcy Weisbach equation is used for laminar flow. Both equations take into account factors such as the velocity of the fluid, the pressure, the density of the fluid, and the pipe diameter.
Flow of Gases Through a Piping System
Gases flow through the piping system in various orientations and passages such as chokes for controlling their flow. As per the Joule-Thomson effect, when an ideal gas passes by at constant pressure, its temperature remains constant, but there is a pressure drop at some points as the inner energy is transformed into kinetic energy, which causes the temperature to fall. The velocity at which the gas flows through the pipe is calculated by the formula volumetric flow actual/area of the pipe. The factors to consider while sizing the pipe are design flow rate, design temperature, and minimum operating pressure.
Pipes in Series
When pipes of different diameters are connected from one end to another to form a pipeline, they are said to be in series. The total loss of energy (or head) is the sum of losses in each pipe and the local losses at the connection. The volume flow rate is constant and the head loss is the sum of all parts. The same discharge passes through the pipe, and the sum of the total head loss in the pipe is equal to the difference in liquid surface levels. Q=A1V1=A2V2=A3V3
Pipes in Parallel
When the main pipe is divided into two or more branches and then joined together downstream to form a single pipe, they are said to be parallel. In this case, the rate of fluid flow in the main pipe is the sum of the rate of flow in the branch pipes. The pressure loss across all the branches for the pipe parallel is the same. Q = Q1 + Q2.