UNIT 2

FLUID MECHANICS

Fluid mechanics is the discipline concerned with the behavior (movement / conduct) of liquids and gases at rest or in motion.

Lead-in

Free writing: Write down everything that comes to your mind about FLUID MECHANICS. Write only words. If you don’t know the English term, write down the Russian one. You have ONLY 1 minute to do it!

1. Pronounce the following words. Pay special attention to the letters in bold.

2. Pay attention to the stress in the following words. □ shows the position of stress.

device □ restrain □ laminar□ tur□bulent veloc □ ity pro□file

momen □tum mol□ecule cavita□tion repair□ relief□

replace□ment excess□ive expel□ collaps□ing reverse□

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3. Read the following minitexts, do the exercises.

Typical Pipe System Components

The transport of a fluid (liquid or gas) in a closed conduit (commonly called a pipe, if it is of round cross section) or duct (if it is not round) is extremely important.

Some of the basic components of a typical pipe system are depicted in Fig. 1. They include the pipes themselves, various fittings used to connect the individual pipes to form the desired system, the flow rate control devices (valves), and pumps (turbines) that add energy to or remove energy from the fluid.

In general, fluids have a well-known tendency to move or flow. It is very difficult to restrain fluid from moving. The slightest shear stress will cause fluid to move. Similarly, appropriate imbalance of normal stresses (pressure) will cause fluid motion.

General Characteristics of Pipe Flow

Although not all conduits used to transport fluid from one location to another are round in cross section. These include typical water pipes, hydraulic hoses and other conduits that are designed to withstand a considerable pressure difference. There are two situations where one pipe is completely filled with the fluid being transported (pipe flow) and one through which rainwater flows without completely filling the pipe (open-channel flow). The difference between open-channel flow and pipe flow is in the fundamental mechanism that drives the flow. For open-channel flow, gravity alone is the driving force. For pipe flow, gravity may be important, but the main driving force is likely to be a pressure gradient along the pipe.

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Laminar or Turbulent Flow

The flow of a fluid in a pipe may be laminar or turbulent. The distinction between laminar and turbulent pipe flow was first pointed out by Osborne Reynolds (1883). The actual transition or turbulent flow may take place at various Reynolds numbers, depending on how much the flow is disturbed by vibrations of the pipe, roughness of the entrance region, etc.

Fig. 3. Fluid Flow in a Pipe

The region of flow near where the fluid enters the pipe is the entrance region. A boundary layer is produced along the pipe wall so that the initial velocity profile changes with distance along the pipe, until the fluid reaches the end of the entrance length. The flow in long, straight constant diameter sections of a pipe becomes fully developed, i.e. the velocity profile is the same at any cross section of the pipe. The details of velocity profile are different for laminar and turbulent flows. The nature of the pipe flow is strongly dependent on whether the flow is laminar or turbulent. This is a direct consequence of the differences in the nature of shear stress in laminar and turbulent flows. The shear stress in laminar flow is a direct result of momentum transfer among the randomly moving molecules. The shear stress in turbulent flow is largely a result of momentum transfer among the randomly moving molecules, finite-sized bundles of fluid particles.

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Fig. 4. Entrance region, developing flow, and fully developed flow in a pipe system

Valves are mechanical devices that are installed in pipelines to control flow or pressure. Valves are an important part of piping systems and if not properly selected and operated, they can cause operation problems. The primary valve types, classified by their function, are:

•control valves – used to control flow, pressure, liquid level, cavitation and pressure transients;

•isolation (block) valves – placed on each side of control valves and pumps, allowing them to be removed for repair or replacement;

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•check valves – used to prevent reverse flow;

•relief valves – admit air to the pipe while the pipe is being drained to prevent excessive vacuum pressures and reduce the possibility of collapsing thin-walled pipes;

•air valves – designed to expel large amounts of air at low pressure during filling and release small amounts of pressurized air during operation.

(Kennedy, John L., Oil and Gas Pipeline Fundamentals, USA. 1999)

4. Find terms which are used with the following words.

Example: flow control valve

•flow

•valve

•fluid

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5. Compose collocations from the following words.

Example: fully developed

laminar turbulent

6. Match the definitions with the following terms.

Example: 11. laminar flow – G. a smooth flow of fluid in which no turbulence or cross flow of fluid particles occurs between adjacent stream line

7.Replace the underlined words with a suitable variant from the text.

1.Fluid mechanics is the discipline concerned with behavior (the movement or conduct) of liquids and gases in quiescent state or when they are moving.

2.Some of the basic components of a typical pipe system include the pipes themselves, the various parts which connect the individual pipes to form the desired system, the flowrate control devices and the pumps.

3.For the situation when the pipe is not completely filled, gravity alone is the driving force for fluid. For other situations where the pipe is completely filled with the fluid being transported, gravity may be important, but the main driving force is likely to be a pressure gradient along the pipe.

4.The region of flow near where the fluid comes into the pipe is termed the inlet area.

5.The primary valve types are valves used to regulate flow, pressure, liquid level, cavitation and pressure transients, valves placed on each side of valves and valves which are designed to prevent reverse flow.

8.Answer the following questions.

1.What is the difference between the following terms:

•Pipe

•Conduit

•Duct

2.What are the pipe system components and give their definitions?

3.What is the difference between pipe flow and open-channel flow? Give examples.

4.What factors can affect the Reynolds number during the transition fro laminar to turbulent flow?

5.Why is velocity profile important in understanding different flows?

6.What is a valve? Give examples, indicating their functions.

7.What are the most important criteria for control valves?

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9. Discussion. Complete the following diagrams and give all necessary explanations.

pipe system

flow

10.  You will hear a part of a lecture series on Fluid Mechanics. For questions 1-10, complete the sentences with a word or phrase.

In elementary mechanics we are used to describing a position of material

As Lagrangian measurements are very complicated, it is more convenient to

The fact that velocity at point of laboratory coordinates is not always referred to the same piece of matter leads to (9) between Lagrangian and Eularian representations.

(10) is an advantage of laboratory coordinates.

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11.Prepare the following presentations.

1.Laminar and turbulent flows

2.Valve and their functions

3.General Characteristics of Pipe Flow

4.Pipe system components

5.Fluid mechanics

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UNIT 3

JOINING PIPES

Piping refers to the overall network of pipes, fittings, flanges, valves and other components that comprise a conduit system to convey liquids. Whether a piping system is used to convey fluids from one point to another or to process and condition the fluid, piping connections serve an important role in the operation of the system.

Pipes can conduit fluid under gravity, vacuum, low-pressure, or high pressure in a process, pneumatic or hydraulic system. Depending on the application, pipe connections can take a variety of forms.

Lead-in

Many different kinds of joint are used to connect pipes. Three types of fittings for small D pipes and three for large D pipes are shown in the below diagrams. Fill in the missing words. Describe pipe joint types (WHAT DO YOU KNOW?)

Small diameter pipes

A.Each pipe ___ _______ into the connector; the connector _____ ________ to the pipes.

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B. The pipes _____ _______ into the coupling; then the nuts ___on.

C. Each pipe ______ ________ into the threaded coupling.

Large diameter pipes

are clamped are bolted(x3)

are welded (x2)

D. The flanges ____ _______ onto the pipe.

Then they ____ _______ together.

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E. The ends of the two pipes ____ ____ together.

F. The flanges _____ _______. Then ______ ________.

G. The two flanges _____ ____ together. Then the clamps ______

_________ together.

1.What are the joining processes?

2.What are the joining devices and materials?

(Basic Technical English, Oxford University Press. 2002)

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