revised capstone a

8/12/2019 Revised Capstone A

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48016- Capstone

Project A

V i b r a t i o n i n p i p e l i n e s O m e r T o k h

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48016- Capstone Project Part A- vibration in pipelinesOmer Tokh -11075761 1

ABSTRACTConstruction of pipelines dates back to earlier civilizations. Its progress reflects the steady evolution of cultures

around the world, the need of developing agricultures, the growth of cities, the industrial revolution and the use of steam power,the discovery and use of oil, the improvements in steel making and welding technology, the discovery and use of plastics, the

fast growth of the chemical and power industries, and the increasing need for reliable water, oil and gas pipelines[1].

The infrastructure of many nations has become a very complex inter-connection of supply chain distribution systems, oil and

natural gas production facilities. In addition, [2] each of these systems is intertwined and heavily dependent on each other. The

challenge is to model these interdependencies, identify vulnerabilities, and determine specific recovery strategies.

At the beginning of the 20thcentury, there has been an unprecedented growth and improvements in welding, in materials and inpumping technologies. At the same time, standardization of materials and designs became a financial and safety necessity, and

industries came to rely more on codes and standards [2], while national engineering societies and industry institutes became an

important source of innovation and improvements. Pipelines are generally the most economical way to transport large quantities

of oil, refined oil products or natural gas over land, under soil or sub- Sea. Pipelines are also most commonly used for the

transportation of water from reservoirs to end users.

Distribution of water in dry cities of the world is becoming an increasing challenge for the Governments [3]. For example, Las

Vegas is a city constructed in the desert, in the United States. The city ofLas Vegas requires pumping 300 billion litres ofwater

a year out of this landscape and transports it 300 miles, approximately 483 Km south to the thirsty metropolis of casinos and

golf courses. It is projected that its natural reservo ir will go dry in 20 years time according to Pat Mulroy [4] the manager of the

southern Nevada Water Authority (AWA) therefore the need for the water pipeline arises to keep the supply.

Similarly [5] , in the great dry land, Australians will be looking more to pipelines to deliver one of the countrys most valuableresources, water. Water pipeline projects are becoming an increasingly important part of Australian infrastructure development

in order to ensure water supply to the nations growing population [5]. With many areas of Australia facing severe drought,

water pipeline planning and construction has again thrived over the last year [6].

This paper discusses construction and manufacturing of pipelines and their usage. In addition to that, this paper addresses

several issues that cause the vibration such as pipeline leakage, speed, pressure and density of the fluid. Furthermore a thorough

research has been conducted on current issues with pipelines, maintenance problems and technical issues.

KeywordsPIPELINES, LEAKING, VIBRATION, SPEED, PRESSURE (INTERNAL- EXTERNAL), DENSITY, MATERIALS

1.0 INTRODUCTIONPipelines are now being constructed in various diameters, lengths and working pressure. To achieve a safe design and fit for the

purpose, it is imperative that engineers must take into consideration all aspects of the design such as what material is going toflow through the pipeline, is it for sub-sea, over the ground or underground transportation of the material etc. Engineers have foryears resorted to semi-empirical design formulae. Much work has recently been done in an effort to rationalize and standardize

the design of pipelines [7].

There are different types of pipelines used for different purposes for example Gathering pipelines are small groups of

interconnected pipeline , forming a complex networks with the purpose of bringing crude oil or natural gas from several nearby

wells to a treatment plant or processing facility. These pipelines have small diameters with only a few hundred meters long.Similar pipelines are also used in sub-sea to collect products from deep water.

The other type of pipeline is the Transportation pipelines which are mainly long and have larger diameters, these types are used

between cities, countries and even continents. These pipelines include several compressor stations in gas lines and pump

stations for crude and multi products pipelines.

However, the pipelines that are used to carry the product to the end consumer are called the Distribution pipelines. Thesepipelines have smaller diameters; feeder lines are deployed to distribute gas to homes and businesses downstream. A feeder line

is a peripheral route or branch in a network, which connects smaller or more remote nodes with a route or branch carrying

heavier traffic [8].

Below is a chart of materials adopted from George A. Antakis [1] book for Piping and Pipeline Engineering. This chart

indicates that not only pipelines can be manufactured from a single alloy, but from a mixture of two or three different alloys

depending on their usage and required strength.
http://www.guardian.co.uk/world/las-vegashttp://www.guardian.co.uk/environment/waterhttp://www.guardian.co.uk/environment/waterhttp://www.guardian.co.uk/world/las-vegas


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Figure 1 Pipe and Fitting Materials [1]

In addition to materials, there are also a number of other factors such as Internal Pressure, External Pressure, layout and

Supports, flexibility and fatigue that needs to be taken into consideration while designing a pipe.

2.0 LITERATURE REVIEWThis section of the report discusses various structural design and analysis of pipes. As it is mentioned earlier that pipes are made

of different alloys for different purposes therefore a thorough structural design analysis is required. Design must be based on

required lifetime performance and on limits of performance, sometimes referred to as failure. This report covers pipe

mechanics, design limitations, internal and external pressures, layout and supports, flexibility and fatigue. An understanding of

the principles of design is essential before applying the individual concepts.

3.0 INTERNAL PRESSURE (THIN WALL APPROXIMATION)To analyse the internal pressure in the pipe, we consider a straight section of pipe filled with a pressurized liquid or gas. The

internal pressure generates three principle stresses in the pipe wall: A hoop stress h, it is also referred to as circumferential or

tangential stress. A longitudinal stress 1 also referred to as axial stress and a Radial stress r. When the ratio of the pipediameter to its wall thickness, D/t is greater than 20, the pipe may be considered to be thin wall [cooper, BS 8010]. In this case,

the hoop stress nearly constant throughout the wall thickness and equal to h = PD /2t. Where P is the design pressure in

[Mpa], D is the outside pipe diameter in [mm] and tis the pipe wall thickness in [mm]. The longitudinal stress is also constant

through the wall and equal to half the hoop stress. 1= PD/4t. The radial stress varies through the wall, from P at the innersurface of the pipe to zero on the outer surface [1].


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Figure 2 Adopted from George A. Antaki, piping and pipeline engineering

Hoop (h), Longitudinal (L) and Radial (r) stress directions

3.1 PIPELINE DESIGN EQUATION

For oil and gas a pipeline, the thickness of the pipe wall is obtained by writing that the hoop stress, which is the largest stress in

the pipe. It must be limited to a certain allowable stress S. Using the thin wall approximation, this condition corresponds toPD / 2t < S, where S is the allowable stress in Mpa.

Pipelines that carry hazardous liquids, according to the George Antaki [1], the allowable stress is set at S=0.72 S y E. The value

0.72 is based on experiments and is the design factor value. E is the longitudinal weld joint factor. (The weld quality or joint

efficiency factor E is a factor introduced to account for the quality of the longitudinal or spiral seam in a pipe) and Sy is thespecified minimum yield strength in Mpa. However for the gas pipelines we consider two more factors which is the F = designfactor and T = temperature derating factor. The values for yield and factors for weld quality etc are based on experiments with

the various methods of pipe fabrication at the time.

It was first assumed that as the pipe deforms under pressure, the material maintains a constant volume. This corresponds to a

material Poisson ratio of 0.5, in which case, the Saint Venant, Tresca and Von Mises equivalent stress can be written in a

similar form [1].

Maximum strain energy (Saint Venant): = ( )

Maximum shear stress (Tresca): = tr=

Maximum energy (Von Mises): = (t r) =

All three stress expressions can be written in the form: =

3.2 PRESSURE RATINGBefore shipping the pipe to the consumer, all pipes are pressure tested. There are different methods of pressure testing to

determine the strength of the particular section of the pipe. The hoop stress distribution in pressurized fittings and components,

such as tees, reducers, elbows, nozzles, etc., cannot be expressed by a simple equation such as PD /(2t) for the hoop stress in astraight pipe. To eliminate the need for complex design calculations to size fittings, the pressure design for fittings and

components relies on a simple approach: first [9], fittings and components must meet standard dimensions specified in the

standards of the country where the pipe is going to be used, and secondly, fittings and components must be pressure rated by

means of proof tests.

For example to determine the strength of a tee pipe; the burst test is carried out. The specimen will be pressurized with water;

the pressure is then increased until assembly bursts. If the burst occurs in the tee, then the pressure at burst is B and the design

pressure or rated pressure is established as a fraction of B. If the burst occurs in the pipe extensions, then the tee is stronger than

the pipe and is assigned the pressure rating of the matching pipe [9].


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Figure 3 Assembly for Burst Test of an ASME Tee

The pressure ratings of wrought steel butt-welding fittings will be the same as the straight seamless pipe of equivalent material.

3.3 PIPE ELBOW AND BENDSIf an elbow or bend is pressurized, the hoop stress will vary around the circumference, as shown in figure 4 below.

It is seen from experiments that the largest hoop stress is developed at the internal of the

bend where the wall of the pipe is thick on the other hand; small hoop stress arises at the

external of the pipe where the wall of the pipe is thinner. In a similar manner the

pressure test can be carried out on Branch Connections, Nozzles and End Fillets.

Figure 4 Distribution of Hoop Stress in a Pressurized Elbow

As we have seen so far, the minimum design wall thickness is proportional to the design pressure [1]. The design pressure is the

pressure that, taken with the concurrent temperature, results in the thickest pipe wall. It is the highest pressure at which the

piping system should operate at.

In the absence of a relief valve set pressure or rupture disc burst pressure, the design pressure is the highest pressure that thepipe should operate. While designing a pipe, it is vital to bear in mind the environment at which this pipe will be operating . A

system engineer familiar with the system function must develop the normal and abnormal operating scenarios. In addition to a

design pressure, the pipeline codes also define a maximum operating pressure (MOP) as the highest pressure at which a pipeline

is normally operated. The maximum allowable operating pressure (MAOP) is the maximum pressure at which a pipeline may be

operated in accordance with the code [1].

3.4 OVER-PRESSURE PROTECTIONDesigning a pressure protection for the pipeline system is different to that of a pressure vessel. In other words, if the pipe,

fittings and components are sufficiently thick to contain all credible pressure transients, the system does not need a safety orrelief device. A pressure relief valve gradually lifts and then recloses as the overpressure dissipates. It works the same way as a

liquid service where the safety valve pops open suddenly [9], remains open and recloses when the overpressure subsides. The

main idea behind a pressure relief valve is so that the existing pressure does not exceed the design pressure. The decision

whether the system requires a safety valve depends on the operation of the system and the environment. Experience has shown

that systems which are operated for many years have been the target of overpressure.

3.5 PIPE SPECIFICATIONA pipe specification is a document which indicates the right material for the project at hand. This document plays an important

role not only at the initial stages of the design but also at later stages if the pipes require replacement or repair. It contains vital

information such as what kind of pipe is installed and its function at the design stage, it is pressure ratings etc. Of the five

fundamental aspects of piping engineering (material, design, fabrication, examination and testing) a piping specification only

addresses materials and one aspect of design (pressure design) [2]. It is essential to understand this limitation: a plant piping

specification is not a code, but a series of additional requirement apply beyond material selection and pressure sizing.

4.0EXTERNAL PRESSURE4.1 BUCKLING PRESSUREThe behaviour of piping and tubing subject to external pressure has been well understood since the early 1900s [32]. Consider

a long, perfectly circular cylinder subject to uniform external pressure. By long cylinder, we mean a cylinder longer than a

critical length given by Lc= 1.11D where D is the diameter in [mm] and t is the wall thickness in [mm]. If the externalpressure is steadily increased, there will come a point where the cylinder will suddenly buckle. If the cylinder is long and thin,

this buckling will occur while the cylinder wall is still elastic. The external pressure at which elastic buckling occurs is called

the critical elastic pressure and is given by (Den Hartog). PCE

=

/ R

3(n

21) [32]


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Where:

PCE= Critical elastic external pressure at buckling, Mpa

V= Poisson ratio of material

E= Modulus of elasticity, Mpa

I= Cross section moment of inertia of cylinder wall per unit length (t3/12), mm

4

t= cylinder wall thickness, mm

R = radius of cylinder, mm

n= integer equal to 2,3, ...

A Pipe failing under external pressure is different from a pipe failing under internal pressure; there are different design methods

to overcome this failure. In internal pressure as discussed earlier, stresses exceed the pipes design pressure thus the pip e fails,

however during external pressure the pipe cannot support its shape and irreversibly takes on a new lower volume shape.

A pipe under external pressure (or vacuum internally) is subject to a potential stability problem and can undergo a form of

unstable collapse if the pressure is sufficiently high, through buckling of the pipe wall. These conditions apply when a pipe is

surrounded by water or concrete, or when vacuum is applied to a tube. Buried pipes receive support from the soil greatly

increasing their collapse pressure. However they can be subjected to higher external pressure from the overburden, and design

check for stability should be carried out [1]. Grouting is one such method which prevents the pipe from buckling, nonetheless

the process of applying grout needs careful consideration since grouting pressures must not exceed safe limits. If this cannot be

assured, internal support may be used to temporarily stiffen the pipe during the grouting process. For example, pressure

equalisation using internal water pressure or inflated air tubes may be applied [1].

4.2 VACUUM CONDITIONS

A vacuum condition inside a pipe is identical in effect to external pressure [1]. Designing a vacuum for large pipe diameters is

not common, but some large pipes have exhaust system with fan at the outlet end. According to George A. Antaki, [1] the more

usual examples are cases where surge produces negative pressure in a low-pressure water line on shutdown of flow. If the

negative surge exceeds the static head in the line the net pressure will go negative. This will commonly occur in a long flat line

with little positive head, e.g. Low-head irrigation lines and sewer pumping mains. Generally design and operation should avoid

such dynamic vacuum conditions arising, since possible column separation and rejoinder shock pressure can produce damage topipes and ancillary equipment.

The siphon effect is one such case where the pipeline passes over a hill and hydraulic gradient intersects the elevation of the

line. Gravity lines rarely operate in this mode because of the problems starting the siphon [35]. Pumping mains are designed

sometimes to operate thus, in order to save the cost of a break tank, and usually have problems. A negative pressure generatedin this way under steady state conditions must be assessed on the basis of the long term buckling performance of the pipe, which

is substantially less than that applicable to short term transients.

4.3 TEMPERATURE SELECTION

Selection of the temperature depends on the material of the pipe and where it will be used. The calculation must be the average

over a long period of time due to the uncertainty of the weather conditions. For pipes encased with concrete, the rise in

temperature is significant because the curing of cement compound is an exothermic reaction. Both factors and low thermal

conductivity of concrete result in a large rise in temperature in the interior of concrete mass [1]. Keeping the above variables in

mind a conventional figure is then recommended for concrete encasement applications. The above situation refers to the period

of concrete before it hardens and that is when it applies an external pressure on the pipe. After the concrete hardens, further

evolution and temperature rise will be irrelevant to the buckling of the pipe. It is however strongly suggested that in case of the

concrete encasement pipes, during the time that concrete is being hardened, the pipe must be filled with water. The internal

pressure will counter balance the external pressure to some extend and the water will help keep the temperature down and itfurther helps if the water keeps circulating [1].

5.0 LAYOUT AND SUPPORTS

5.1 SPACING OF PIPE SUPPORTS

The weight of piping and components must be supported to achieve five objectives (1) minimise stresses in the piping (2)

maintain the intended layout and slope (3) avoid excessive sag, (4) minimise reactions on equipment nozzles and (5) optimise

the type, size and location of pipe supports [8]. To achieve the above goals at the designing stage weight supports are placed atregular intervals. The distance between each weight differs for pipes made of different material. Tables are given in different

design books which specifies the support intervals based on the diameter of the pipe and the material which is intended to flow

inside it.

5.2 SUSTAINED STRESS

Under the effect of its own weight, including contents, insulation and in-line components, such as valves or strainers, the pipe

will tend to bend downwards, causing forces Fx, Fy and Fzand moments Mx, Myand Mzat each point along its length.


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These weight loads (The axial force Fx, shear Fyand Fz, bending Myand Mzand torsionMx) together with the operating pressure are sustained loads causing sustained stresses. In

practice, the objective of minimizing deadweight stress is accomplished by limiting the

sustained stress to a fraction of the allowable stress S at normal operating temperature. A

limit on weight bending stress of about 25% of the allowable stress would be a good

guideline for steel pipe [8].

Figure 5 Force and Moment Directions

5.3 SELECTION OF PIPE SUPPORTS

Pipe supports are assemblies which supports the weight of the pipes. It restricts pipes movement in lateral or axial direction.

Sway braces are used in fire sprinklers systems which prevent the pipe to move in lateral direction or sway.

There are numerous pipe supports used for different purposes such as variable spring hanger supports a hung pipe from above.

The constant load hanger supports a hung pipe from above, the difference is that the support is placed below the pipe and

supports the pipe from underneath. Like the variable spring the constant load hanger supports the pipes deadweight while

allowing vertical movement due to expansion or contraction.

The other type of support is rigid frames; rigid frames are custom designed structures that hold the pipe rigid usually for heavier

pipes. Figure 7 illustrates the types of rigid frames.

Figure 6 Variable Spring Hanger

Figure 7 Example of custom made rigid frames

Rod hangers are smooth or threaded rods used to support the weight of hung pipes. They usually have turn buckles to permit

vertical adjustment. Rod hangers are standard catalogue components sized to act as tension members and can buckle in

compression. Pipe rolls are used where a pipe undergoes large longitudinal movement and little vertical or lateral movement for

example on long straight steam lines which will be permitted to expand axially with low friction of the rolls [8].

Rigid struts act in tension and compression along their axial. They can be sized to re-act cyclic and dynamic loads in which case

they are often referred to as restraints. Vibration Dampers are standard devices that absorb and dampen pipe vibration.


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It is preferable to eliminate the source of vibration rather than dampen its effects, but where this is

not feasible; a vibration damper can be used to reduce vibration amplitude. This should be done withcare since residual vibration may, in time, fail the damper itself or its attachments by fatigue.

Figure 8 Rigid Strut Tandem

Snubbers are shock absorbers that act somewhat like a seat belt: they extend or retract to accommodate a slow movement of the

pipe (due to thermal expansion or contraction) but locks under shock (seismic or water hammer load). They are either hydraulic

or mechanical devices, rated based on dynamic load and rage of static motion

Figure 9 Snubber Installed Horizontally

Saddles and piers are weight supports and if sufficiently deep, also act as lateral restraints. U-bolts or straps can be added toprovide upward and lateral restrain.

5.4 DESIGN OF STANDARD SUPPORT

Standard supports are designed in accordance with vendor catalogues, given the load applied by the pipe and the movement of

the pipe. In all cases the vendor catalogue will provide detailed dimensions and installation guidelines. Constant load hangers

are listed by travel and load carrying capacity. Variable spring supports are listed by spring deflection (travel range) and load

range. Rigid hangers are listed by maximum recommended load. Vibration dampers are listed by stiffness and spring travel.

Rigid struts are listed by load rating. U-bolts, clevis hangers, saddles, clamps, upper attachment brackets, turnbuckles, couplings

and pipe rolls are listed by maximum recommended load [9].

6.0 FLEXIBILITY AND FATIGUE

6.1 FLEXIBILITY

Changes in fluid or ambient temperature can have five effects on a piping system (1) a global or flexibility effect in the form of

movements and stresses as the pipe expands and contracts, (2) a local effect in the form of local temperature gradients in the

pipe wall as the temperature changes locally for example when injecting cold water in a hot line, (3) at sufficiently high

temperature, creep will take place accompanied by metallurgical changes, (4) changes in mechanical properties with a loss of

toughness at low temperature and a softening at high temperatures, and (5) changes in corrosion mechanisms or corrosion rate

[1].

In designing a stable pipeline system, different computer software are used to model the pipe. Depending on the material and

usage of the pipeline, effects such as temperature variation in pipe, supports where the pipe might buckle, restraints, guides and

anchors are added to get a good picture of the strength of the pipeline. Supports and anchors must be arranged to restrict all six

degrees of freedom rather than a few to just support the pipe. In this process it may be necessary to add expansion loops,

expansion joints or changes in direction to increase the system flexibility. This is an iterative process, until an optimumconfiguration is achieved.

6.2 FATIGUE

The allowable stress in the flexibility design equation is intended to avoid fatigue failure of the piping system as it undergoes

cold to hot cycles through its service life. To understand the fatigue limit, we review the process of fatigue and fatigue failure infive stages [35].

Stage 1, slip bands: when metal components are subject to alternating stresses, microscopic slip bands occur at the surface of

the metal. The slip bands occur along the planes of maximum shear stress, at 45 degrees from the applied tensile load.

Stage 2, Micro cracks: under continuing alternating stresses, microscopic cracks, in the order of microns in length, form along a

slip plane at grain boundaries. The duration of stages 1 and 2, the crack initiation phase, can be short if (a) the componentcontains initial cracks (for example the weld joints) and (b) the local stress is large (stress riser at the discontinuity such as a

nozzle or fillet weld).


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Stage 3, small cracks: if the applied stress range is sufficiently large, the micro-cracks will grow into small cracks, now visible

with dye penetrant. At this point the fatigue crack starts to extend through the grain (Trans granular) along the plane ofmaximum tensile stress (perpendicular to the applied tensile load) and, viewed under a microscope, exhibits the typical fatigue

surfaces (beach marks)

Stage 4: visible cracks: the crack professes and is now visible without the help of dye penetrant. If fatigue is due to cyclic

bending the crack will progress from the outer pipe diameter (maximum tensile stress) towards the inner diameter and

progressively around the circumference. The component will spring a leak when the crack has progressed through the wall and

if the leak can be detected in time, the system may be shut down before reaching the fracture stage. Stage 3 and 4 constitute thecrack propagation stages.

Stage 5, fracture: as the visible cracks progresses around the circumference, the remaining ligament of metal becomes too small

to resist the applied tensile load and will fracture, as indicated by stage 5. The fracture surface in phase 5 has a distinctly coarse

look when compared to the crack propagation striations of phase 4.

Figure 10 The five stages of Fatigue Crack Formation and Propagation

6.3 CORROSION FATIGUE

If the formation and propagation of a fatigue crack takes place in the presence of a corrosive fluid [37], then existing crack

which has been exposed to the corrosive fluid is corroded while the plane of metal just exposed during the last stress cycle is

bare. The corroded region, with its passive oxide film, acts as the cathodic pole, while the recently exposed bare steel is a nodic

and corrodes.

6.4 SHAKEDOWN

Fatigue cracks tend to originate at sharp structural discontinuities or at existing crack flaws, where the local peak stress is large.

This concentrated stress is often well above the material yield stress, creating a local plastic zone in the component. Fatigue can

also occur due to cold spring, creep damage and expansion joints [8]. A normal practice in industry is to drill a hole on the path

of a crack, this prevents the crack to stop growing and the stress due to the drill of the hole spreads the area of stress. Anexample of this would be a crack on the wing of an aircraft, since replacing the whole plate of the wing is not practical therefore

engineers then drill a hole on the path of the crack and then use a plate of the same material to patch the crack. It is common in

aviation industry, but requires constant monitoring to check if the crack has stop growing.

7.0 PIPELINES FOR NATURAL GAS TRANSPORT

The efficient and effective movement of natural gas from producing regions to consumption regions requires an extensive and

elaborate transportation system [10]. It is not uncommon to transport natural gas to great distances where it will either be storedor used. The network system designed to carry out this task is very complex and requires constant monitoring and evaluation.

Natural gas transported from wells may not be required immediately therefore there are storage facilities to store the gas and

supply when needed.

There are three major types of pipelines along the transportation route: the gathering system, the interstate pipeline system, and

the distribution system. The gathering system consists of low pressure, small diameter pipelines that transport raw natural gas

from the wellhead to the processing plant. Should natural gas from a particular well have high sulphur and carbon dioxide

contents (sour gas), a specialized sour gas gathering pipe must be installed. Sour gas is corrosive, thus its transportation from

the wellhead to the sweetening plant must be done carefully [11].

Pipelines can be characterized as interstate or intrastate. Interstate pipelines are similar to in the interstate highway system: they

carry natural gas across state boundaries and in some cases across the country. Intrastate pipelines, on the other hand, transport

natural gas within a particular state. This section will cover only the fundamentals of interstate natural gas pipelines; however

the technical and operational details discussed are essentially the same for intrastate pipelines [11].


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7.1 INTERSTATE NATURAL GAS PIPELINES

Natural gas that is transported through interstate pipelines travels at high pressure in the pipeline, at pressures anywhere from

200 to 1500 pounds per square inch (psi) or 1.4 to 10.4 (MPa). This reduces the volume of the natural gas being transported(by up to 600 times), as well as propelling natural gas through the pipeline [12].

7.2 PIPELINE COMPONENTS

Interstate pipelines consist of a number of components that ensure the efficiency and reliability of a system that delivers such an

important energy source year-round, twenty four hours a day, and includes a number of different components.

7.3 TRANSMISSION PIPES

Transmission pipes are used in transporting natural gas from facilities near the well to it is destination; they are also sometimes

called mainline transmission pipes. The diameter of transmission pipes varies between (152 to 1220 mm) depending on their

function. Gathering /distribution pipelines are then connected to transmission pipes to provide the gas to consumers. Most majorinterstate pipelines are between 24 and 36 inches in diameter (~610 to ~915 mm). The actual pipeline itself, commonly called

'line pipe', consists of a strong carbon steel material. In contrast, some distribution pipe is made of highly advanced plastic,

because of the need for flexibility, versatility and the ease of replacement [11].

Natural gas pipelines are produced in steel mills; there are two different production techniques, one for small diameter pipes and

one for large diameter pipes. For large diameter pipes, from 20 to 42 inches in diameter (508 to 1067 mm), the pipes are

produced from sheets of metal which are folded into a tube shape, with the ends welded together to form a pipe section. Small

diameter pipe, on the other hand, can be produced seamlessly [11]. This involves heating a metal bar to very high temperatures,then punching a hole through the middle of the bar to produce a hollow tube. In either case, the pipe is tested before being

shipped from the steel mill, to ensure that it can meet the pressure and strength standards for transporting natural gas.

Gas pipelines can be both underground and over ground, therefore the pipes are coated to ensure that it does not corrode. The

coating protects the pipes from moisture which the main cause of corrosion and rusting. There are a number of different coating

techniques. In the past, pipelines were coated with specialized coal tar enamel. Today, pipes are often protected with what isknown as a fusion bond epoxy, which gives the pipe a noticeable light blue colour. In addition, cathodic protection is often

used; which is a technique of running an electric current through the pipe to ward off corrosion and rusting [12].

7.4 COMPRESSOR STATIONS

Natural gas is pressurized as it travels through states. To ensure that the natural gas flowing through any one pipeline remains

pressurized, compression of this natural gas is required periodically along the pipe. This is accomplished by compressor

stations, usually placed at 65 to 160 Km intervals along the pipeline. The natural gas enters the compressor station, where it is

compressed by a turbine, motor, or engine [12].

Turbine compressors gain their energy by using up a small proportion of the natural gas that they compress. The turbine itself

serves to operate a centrifugal compressor, which contains a type of fan that compresses and pumps the natural gas through thepipeline. Some compressor stations are operated by using an electric motor to turn the same type of centrifugal compressor.

This type of compression does not require the use of any of the natural gas from the pipe; however it does require a reliable

source of electricity nearby. Reciprocating natural gas engines are also used to power some compressor stations. These engines

resemble a very large automobile engine, and are powered by natural gas from the pipeline. The combustion of the natural gas

powers pistons on the outside of the engine, which serves to compress the natural gas [12].

In addition to compressing natural gas, compressor stations also usually contain some type of liquid separator, much like the

ones used to dehydrate natural gas during its processing. Usually, these separators consist of scrubbers and filters that capture

any liquids or other unwanted particles from the natural gas in the pipeline. Although natural gas in pipelines is considered 'dry'

gas, it is not uncommon for a certain amount of water and hydrocarbons to condense out of the gas stream while in transit. Theliquid separators at compressor stations ensure that the natural gas in the pipeline is as pure as possible, and usually filter the gas

prior to compression [13].

7.5 METERING STATIONS

In addition to compressing natural gas to reduce its volume and push it through the pipe, metering stations are placed

periodically along interstate natural gas pipelines. These stations allow pipeline companies to monitor the natural gas in theirpipes. Essentially, these metering stations measure the flow of gas along the pipeline, and allow pipeline companies to 'track'

natural gas as it flows along the pipeline. These metering stations employ specialized meters to measure the natural gas as it

flows through the pipeline, without impeding its movement.

7.6 VALVES

Interstate pipelines include a great number of valves along their entire length. These valves work like gateways; they are usually

open and allow natural gas to flow freely, or they can be used to stop gas flow along a certain section of pipe. There are manyreasons why a pipeline may need to restrict gas flow in certain areas. For example, if a section of pipe requires replacement or


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maintenance, valves on either end of that section of pipe can be closed to allow engineers and work crewssafe access. These

large valves can be placed every 9 to 32 Km along the pipeline, and are subject to regulation by safety codes.

Figure 11 A ground valve

7.7 PIPELINE CONSTRUCTION

As the demand for natural gas increases so is the need to construct reliable and safe pipelines. Pipelines are normally

manufactured in lengths of between 12 to 25 meters. Once the pipelines are shipped to the required area, they are then lowered

into trenches typically 1.5 to 1.8 meters deep. The depth of the trenches differs for example if the pipeline is to cross the road ornear domestic areas it is then dug even deeper. The pipelines are welded, bent, coated and inspected before it is lowered into thetrenches. Then the pipelines are hydrostatically tested [10]. This consists of running water, at pressures higher than will be

needed for natural gas transportation, through the entire length of the pipe. This serves as a test to ensure that the pipeline is

strong enough, and absent of any leaks of fissures, before natural gas is pumped through the pipeline.

Laying pipe across streams or rivers can be accomplished in one of two ways. Open cut crossing involves the digging of

trenches on the floor of the river to house the pipe. When this is done, the pipe itself is usually fitted with a concrete casing,

which both ensures that the pipe stays on the bottom of the river and adds an extra protective coating to prevent any natural gasleaks into the water [11]. Alternatively, a form of directional drilling may be employed, in which a 'tunnel' is drilled under the

river through which the pipe may be passed. The same techniques are used for road crossings - either an open trench is

excavated across the road and replaced once the pipe is installed, or a tunnel may be drilled underneath the road.

7.8 PIPELINE INSPECTION AND SAFETY

In order to ensure the efficient and safe operation of the extensive network of natural gas pipelines, pipeline companies

routinely inspect their pipelines for corrosion and defects. This is done through the use of sophisticated pieces of equipment

known as smart pigs. Smart pigs are intelligent robotic devices that are propelled down pipelines to evaluate the interior of the

pipe. Smart pigs can test pipe thickness, and roundness, check for signs of corrosion, detect minute leaks, and any other defect

along the interior of the pipeline that may either impede the flow of gas, or pose a potential safety risk to the operation of thepipeline. Sending a smart pig down a pipeline is fittingly known as 'pigging' the pipeline [13]. In addition to inspection with

smart pigs, there are a number of safety precautions and procedures in place to minimize the risk of accidents. In fact, the

transportation of natural gas is one of the safest ways of transporting energy, mostly due to the fact that the infrastructure is

fixed, and buried underground.

7.9 VIBRATION IN GAS PIPELINES

The motion of gas does not have significant impact on vibrations; however structural vibrations of significant magnitude can

occur in the pipelines (loading lines) under various operating conditions, including normal design operation. If these highamplitude vibrations are sustained, they often lead to problems such as misalignment, equipment malfunction, and structural

failures which are very costly in terms of down-time and component replacement. As such, structural vibrations of natural gascompressor installations are highly undesirable and have a significant influence on the operational reliability, maintenance and

safety of these installations. In the extreme, vibration induced failures of compressor components have been catastrophic in a

few cases [38].

7.10 LIMITATIONS OF PULSATION DAMPERS

Because of the oscillatory piston action of reciprocating compressors, pressure pulses are superimposed on the flow of gas from

the compressor into the loading lines [41]. Since the piston motion is not strictly a sinusoidal motion, these pressure pulses

contain a band of frequencies. Additional frequencies within the gas result from pressure wave reflections at valves, pipe bends

and other impedance discontinuities in the flow [41].

As the loading lines oscillates due to the gas flow generation, the pressure oscillation coincides with resonance frequencies inthe pipeline system thus resonant builds up and results in vibration. These further worsens when the resonant amplitude of the

pipe are even higher when driven by resonances within the gas. To control or minimise the amplitude of pulsation-induced pipe


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vibrations, pulsation dampers are inserted in the loading lines. These dampers act essentially like acoustic filters which remove

a specific narrow band of frequencies from the troublesome gas pressure pulses, thus decreasing the amplitudes of the pressurepulses and the attendant pipe vibrations [12]. The design of the pulsation dampers plays a vital role in minimising this effect. It

must match the pressure pulse characteristics, operating speeds, and gas pressures and temperatures. Vibration of this kind is

discussed further in the vibration section of this report.

7.11 MODIFICATION AND ADDITION OF PIPING SUPPORTS

In some instances the pulsation-induced loading line vibrations are significant despite the presence of pulsation dampers

therefore to minimise the vibration pipeline supports are added to achieve the desired reduction in vibration. Supports for

pipelines are discussed thoroughly in section 5.0 of this report. Modelling the pipeline already in operation for supports can only

be of limited help because of the uncertain characteristics of the compressor and on the field evaluation to obtain an exact value

for the placement of the supports is not possible therefore it is then necessary to make the best estimate derivable on the basis of

whatever data can be obtained under the existing operational conditions - an approach that is less than desirable if the first

solution to be implemented must be adequate [9].

8.0 PIPELINES FOR OIL TRANSPORT

Pipelines are the most efficient method to transport crude oil and refined products as well. Pipelines are used to move crude

oil from the wellhead to gathering and processing facilities and from there to refineries and tanker loading facilities [14].

Product pipelines ship gasoline, jet fuel, and diesel fuel from the refinery to local distribution facilities. Crude oil is collected

from field gathering systems consisting of pipelines that move oil from the wellhead to storage tanks and treatment facilities

where the oil is measured and tested. From the gathering system the crude oil is sent to a pump station where the oil deliveredto the pipeline. The pipeline may have many collection and delivery points along route. Booster pumps are located along the

pipeline to maintain the pressure and keeps the oil flowing. The delivery points may be refineries, oil is processed into

products, or shipping terminals, then oil is loaded onto tankers.

A pipeline may handle several types of crude oil. The pipeline will schedule its operation to ensure that the right crude oil is

sent to the correct destination. Crude oil may also move over more than one pipeline system as it journeys from the oil field to

the refinery or shipping port. Storage is located along the pipeline to ensure smooth continuous pipeline operation.

After crude oil is converted into refined products such as gasoline, pipelines are used to transport the products to terminals formovement to gasoline stations. In addition to gasoline, product pipelines are used to ship diesel fuel, home heating fuel,

kerosene, and jet fuel. Because product pipelines are used to move many different products, the different types of products are

shipped in batches.

Batching is used to move two or more different liquids through the same pipeline. The liquid are transported in a series of

batches. The adjoining batches mix where they come into contact. This mixed stream may be sent to refinery for re-refining,sold as a lower valued product such as a mixture of premium unleaded gasoline with regular unleaded gasoline, or sold as

mixture. Many product pipelines have standard product specifications.

Considerations in regards to designing the pipeline for oil is similar to that of the gas mentioned earlier [15], however there

are a few points that need to be mentioned. The most important of all is the volume of the oil that needs to be transported.

There is often some uncertainty in volume estimates, and making the best projection of volume to be handled throughout the

life of the pipeline is the key to a profitable project. With projected volumes and the origin and destination of the pipeline

known, pipeline design typically follows these general steps:

8.1 PRESSURE DROP

Bernoullis theorem describes the flow of fluids in a pipe. The general equation for the flow of liquid in a pipe is Darcys

formula. To determine pressure drop, for instance, the equation is used in this form: [16]

p = Where

p = pressure drop over length,L in Mpa

P= density of fluid in kg/m3

f= friction factor, dimensionless

L= length of pipe, m

V= velocity of flow m/s

D= inside diameter of pipe in m

G= acceleration of gravity 9.8 m/s2


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In this equation, L can be either the entire length of the pipeline or another specified length. If the length of the entire pipeline

is uses, is total pressure drop in the line. Since the velocity is a function of flow volume [17], velocity can determined usingthe desired flow volume and an assumed pipe size. Then that assumed pipe size and calculated velocity are used in the equation

to determine pressure drop [18].

8.2 VALVE AND FITTINGS

In addition to the pressure loss due to friction of the flowing fluid with the walls of the pipeline, valve and fittings also

contribute to overall system pressure loss. The pressure loss due to a single valve in several thousand meters of straight piping

will be relatively insignificant. But in a pumping station, for example, where many valves exist and many changes in flow

direction occurs, pressure loss in valves and fittings is important. The type of crude must be considered in pipeline design

because of viscosity and other physical properties affect throughout and pumping calculations. For most crude, no specialequipment is required in the pipeline system for different types of crudes [19]. But there are some crude such as oil with very

high pour points or high with contents that require pipelines of special design. Pour point can indicate the amount of different

types of hydrocarbons in the crude.

9.0 PIPELINES FOR WATER TRANSPORT

9.1 FLOW IN PIPES

Fluid flow is classified as external and internal, depending on whether the fluid is forced to flow over a surface or in a channel.

Internal and external flows exhibit very different characteristics [20]. The internal flow,where the channel is completely filledwith the fluid and flow is driven primarily by a pressure difference. This is different to open-channel flowwhere the duct is

partially filled by the fluid and thus the flow is partially bounded by solid surfaces, as in an irrigation ditch, and flow is drivenby gravity alone [21].

The flow through most pipes is turbulent. Analysing the flow using classical analytical techniques is challenging. Available

techniques are based on experimental data and empirical formulae. The working equations are often derived from dimensional

analysis using dimensionless forms [22]. It is often desirable to determine the head loss, hLso that the energy equation can beused. Pipe systems come with valves, bends, pipe diameter changes, elbows which also contribute to the energy (head) loss. The

overall head loss is divided into two parts major loss hLmajor, and minor loss hLminor. The major loss comes from viscosity (instraight pipe) while the minor loss is due to energy loss in the components [22]. The major loss can actually be smaller than the

minor loss for a pipe system containing short pipes and many bends and valves.

Designing pipelines for water is no different to gas or oil pipelines mentioned above. Like the methods for oil and gas pipelines,

many factors need to be considered in designing pipes to carry water. For water pumping mains the flow velocity at the

optimum diameter varies from 0.7 m/s to 2 m/s, depending on flow and working pressure. It is about 1 m/s for low pressureheads and a flow of 100 L/s increasing to 2 m/s for a flow of 1000 L/s and pressure heads at about 400m of water, and may be

even higher for higher pressures. The capacity factor and power cost structures also influence the optimum flow velocity or

conversely the diameter for any particular flow. In planning a pipeline system it should be borne in mind that the scale of

operation of a pipeline has considerable effect on the unit costs. By doubling the diameter of the pipe, other factors such as head

remaining constant, the capacity increases six fold. On the other hand the cost approximately doubles so that the cost per unit

delivered decreases to 1/3 of the original. It is this scale effect which justifies multi- product lines. Whether it is in fact

economical to install a large diameter main at the outset depends on the following factors as well[23]:

Rate of growth in demand (it may be uneconomical to operate at low capacity factors during initial years). (Capacity

factor is the ratio of actual average discharge to design capacity).

Operating factor (the ratio of average throughput at any time to maximum throughput during the same period), which

will depend on the rate of draw-off and can be improved by installing storage at the consumer's end.

Reduced power costs due to low friction losses while the pipeline is not operating at full capacity.

Certainty of future demands.

Varying costs with time (both capital and operating).

Rates of interest and capital availability.

Physical difficulties in the construction of a second pipeline if required.

Pipelines are designed to last 10 to 30 years [24]. Longer planning stages are normally justified for small bores and low

pressures. For high pressure pipelines it is recommended to reduce the diameter and wall thickness. Systems analysis techniques

such as linear programming and dynamic programming are ideally suited for such studies. Booster pump stations may be

installed along lines instead of pumping to a high pressure head at the input end and maintaining a high pressure along the entire

line. By providing for intermediate booster pumps at the design stage instead of pumping to a high head at the input end, the

pressure heads and consequently the pipe wall thicknesses may be minimized. There may be a saving in overall cost, even

though additional pumping stations are required [25]. The booster stations may not be required for some time. The capacity of

the pipeline may often be increased by installing booster pumps at a later stage although it should be realised that this is not

always economical. The friction losses along a pipeline increase approximately with the square of the flow, consequently powerlosses increased considerably for higher flows.


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The three most important equations in fluid mechanics are the continuity equation [26], the momentum equation and the energy

equation. For steady, incompressible, one-dimensional flow the continuity equation is simply obtained by equating the flow rateat any section to the flow rate at another section along the stream tube. By 'steady flow' is meant that there is no variation in

velocity at any point with time. 'One-dimensional' flow implies that the flow is along a stream tube and there is no lateral flow

across the boundaries of stream tubes. It also implies that the flow is irrotional. The momentum equation stems from Newton's

basic law of motion and states that the change in momentum flux between two sections equals the sum of the forces onthe fluid

causing the change. For steady, one-dimensional flow this is:

AFx= PQAVxWhere F is the force, p is the fluid mass density, Q is the volumetric flow rate [26], V is velocity and subscript x refers to the ' x

' direction. The basic energy equation is derived by equating the work done on an element of fluid by gravitational and pressure

forces to the change in energy. Mechanical and heat energy transfer are excluded from the equation. In most systems there is

energy loss due to friction and turbulence and a term is included in the equation to account for this. The resulting equation for

steady flow of incompressible fluids is termed the Bernoulli equation and is conveniently written as:

p/y = pressure head (units of length)

= u n it weight of fluid

Z = elevation above an arbitrary datum

he= head loss due to friction or turbulence between sections 1 and 2The sum of the velocity head plus pressure head plus elevation is termed the total head. Strictly the velocity head should be

multiplied by a coefficient to account for the variation in velocity across the section of the conduit. The average value of the

coefficient for turbulent flow is 1.06 and for laminar flow it is 2.0. flow through a conduit is termed either uniform or non-

uniform depending on whether or not there is a variation in the cross-sectional velocity distribution along the conduit.

For the Bernoulli equation to apply the flow should be steady, i.e. there should be no change in velocity at any point with time.

The flow is assumed to be one-dimensional and irrotational. The fluid should be incompressible, although the equation may be

applied to gases with reservations. (Albertson et all., 1960).

9.2 PIPELINE SYSTEM ANALYSIS AND DESIGN

The flows through a system of interlinked pipes or networks are controlled by the difference between the pressure heads at the

input points and the residual pressure heads at the draw off points. A steady state flow pattern will be established in a network

such that the following two criteria are satisfied [27].

1. The net flow towards any junction or node is zero i.e., inflow must equal outflow, and

2. The net head loss around any closed loop is zero i.e., only one head can exist at any point at any time.

The line head losses are usually the only significant head losses and most methods of analysis are based on this assumption.

Head loss relationships for pipes are usually assumed to be of the form h=kLQN/Dmwhere h is the head loss, L is the pipe

length, Q the flow and D the internal diameter of the pipe. The calculations are simplified if the friction factor K can beassumed the same for all pipes in a network.

9.3 WATER HAMMER AND SURGE

Water hammer occurs whenever the fluid velocity in pipe systems suddenly changes direction [28]. For example at pump stop,

pump start up or valve opening and closure. It is important to design pump systems to prevent water hammer in order to avoid

potentially devastating consequences, such as damage to components and equipment and risks to personnel. Determining howto prevent water hammer requires a fundamental understanding of fluid properties, governing equations and the design and

operation of pipe systems, valves, pumps and pump stations. This section of the report focuses on water hammer, application


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equations, risk potential as well as methods of evaluating water hammer and/ or eliminating the consequences of these types of

transient events.

9.3.1 HYDRAULIC TRANSIENT ANALYSIS: PREVENTING WATER HAMMER

Water hammer is a type of hydraulic transient that refers to rapid changes of pressure in a pipe system that can have devastating

consequences, such as collapsing pipes and ruptured valves [29]. It is therefore important to understand the phenomena thatcontribute to transient formation and be able to accurately calculate and analyse changes as well as maximum and minimum

pressures occurring in a pipe system [30].

9.3.2 CAUSES AND EFFECTS OF WATER HAMMER

Rapid pressure changes are a result of rapid changes in flow, which generally occur in a pipe system after pump shut-off,although it may also occur at pump start or at valve opening or closing. Because of the compressibility of water and the

elasticity of pipes, pressure waves will then propagate in the pipe until they are attenuated at a velocity, which is dependent

upon pipe material and wall thickness [31]. The effects of the water hammer vary, ranging from slight changes in pressure and

velocity to sufficiently high pressure or vacuum through to failure of fittings, burst pipes and pump damage. Pump stop can

create hard-to-handle water hammer conditions; the most severe conditions result from a sudden power failure that causes all

pumps to stop simultaneously.

9.3.3 CALCULATING MAXIMUM PRESSURE INCREASE

Joukowskys formula, which originates from Newtons laws of motion, describes the pressure change that results from a rapidchange in velocity. By analysing the formula, it is clear that the larger the magnitude of the velocity change and the larger the

magnitude of the wave speed, the greater the change in pressure will be [26].

Joukowskys formula is expressed as:

H=

H = Change in pressure

a = Velocity of pressure wave

Q = Change in flow

g =Acceleration due to gravity

A = Pipe area

9.3.4 FACTORS THAT AFFECT THE CONSEQUENCES OF WATER HAMMER

To perform water hammer calculations several factors needs to be taken into consideration. To determine when this calculation

is necessary is mostly based on experience and experiments. Design books however suggest some points to as when this

calculation should be performed. The length of the pipe has a significant influence on the inertia of water inside the pipe, thus

the longer the pipe, the longer the reflection time [32]. This means that the time it takes for wave to reflect at the outlet and

return to the starting point. Conversely, the length of pipe has a greater effect because, the larger the mass of water that will

affect the moment of inertia of the water column. It is strongly advised that if the length of the pipe exceeds 300 m, the subpressures exist and water hammer calculations should be conducted.

Moment of inertia: A pumps moment of inertia plays a critical role in water hammer events. The higher the moment of

inertia, the longer the pump will continue to rotate after shut-off. A higher moment of inertia minimises pressure drops beforethe reflecting wave raises the pressure again.

Pipe material and dimensions:Joukowskys equation states that the magnitude of water hammer is directly proportional to the

velocity of the wave propagation. Wave propagation velocity depends on the elasticity of the pipe walls and the compressibilityof the liquid.

Filling around the pipeline:the type of filling and packing method used around the pipeline has a direct impact on the external

pressure on the pipelines. Due to the pressure changes created by water hammer, there will be oscillations of the pipe in the

ground; therefore the filling around the pipe will have a great effect on the wear of the pipe. Sharp stones, for example, will tear

the pipe exterior. For submerged pipes, consideration must also be given to the depth of the pipe because the pipe wall issubject to the difference in pressure between the pressure inside the pipe and the external pressure from the surrounding water.

If the pressure from the surrounding water is greater than the pressure inside the pipe, there is a risk of collapse or buckling

[32].


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Figure 12 examples of different kinds of filling around a pipe

9.3.5 CONSEQUENCES OF WATER HAMMERWater hammer can have devastating effects on the pump system. These include instant pipe failure, weakening of pipe

sections, fatigue and external wear.

9.3.5.1INSTANT PIPELINE FAILURE

Pipelines are mostly subject to low-pressure and over-pressure and may collapse due to either, but mostly because of over-pressure. Separations of columns or joints at particular locations of the pipe are due to vapour pressure of the pumped liquid

which causes vacuum conditions. Cavitation usually occurs at high points in the pipeline but may also occur in flat areas of the

pipe system. The collapse of the vapour pockets can cause dramatic high-pressure transients if the water columns re-join toorapidly. This, in turn, may cause the pipeline to rupture [33]. Vaporous cavitation may also result in pipe flexure, which can

damage pipe linings. The pipes ability to withstand subpressure depends on the material properties of the pipe, wall thickness,

how the pipes are laid, type of filling used as well as how the filling is packed. Only soft earth of good quality that does not

contain stones, boulders, root or vegetation should be used as filling to prevent the pipes from assuming a shape that is more

oval than round. Pipes with an oval shape do not tolerate pressure variations as well as pipes with a circular shape.

Manufacturers of pipes supply a data sheet which includes the minimum and maximum pressures that a pipe can withstand.

Pipeline design books also contain tables and graphs based on experiments which help decide if the pipe is suitable for the

situation. When determining the risk of collapse for submerged pipes [34], it is critical to take the surrounding water pressure

into account because the pipe wall will be exposed to the differences in pressure.

9.3.5.2 WEAKENED PIPELINE SECTIONPipe failure can also occur after a period of time due to a weakened pipeline section. The cause of the weakened section may be

corrosion, erosion due to flow or cavitation implosion. Regardless of cause, the weakened section is sensitive to water hammer,

which can lead to upsurge, down surge, cracking or rupture.

9.3.5.3 FATIGUE AND EXTERNAL WEAR

Pipe fatigue and external wear are also common occurrences. Axial pipe movement due to water hammer causes wear on the

pipe, especially in a pump system with frequent starts and stops. Most pipeline materials are more sensitive to fatigue due to sub

pressure rather than overpressure, and pipe fatigue is more pronounced when using plastic pipes. Dimensioning of sub pressure

depends largely on the pipe material and wall thickness and therefore this should be obtained from the pipe manufacturer.

9.3.5.3 SLAMMING VALVES

Slamming valves are often misunderstood to be caused by water hammer, but this is generally not the case. Instead slammingvalves are typically the cause of very high water column occurring at pump stop. When the pump is stopped, the water

accelerates and reverses direction. A fast water column retardation is often generated in systems where we do not have

problems with water hammer. Typically slamming valves can be seen in a system with a short pipe length and a relatively high

static head while water hammer typically appears in systems with long pipe length and small static head. A high head and a

short pipe length will cause a high water column deceleration. Calculations to predict the possibility of a slamming valve can be

done manually; however, to be more precise, the use of water hammer calculation software is recommended [35].


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Figure 13 Different effects of a weakened section of the pipe.

10.0 SELECTING APPROPRIATE PROTECTIONWhen selecting the appropriate method of protection for a pipe system, it is important to consider various factors, such as thenumber of pumps in operation, conditions during normal stop and power failure as well as the risk of buckling, fatigue and

clogging. It is critical that the protection method used is based on thorough understanding of the effect that the method will have

on the system and that the protection method is dimensioned accordingly on a case-by-case basis. Protection equipment can be

divided in two groups: active protection and passive protection [35].

10.1 ACTIVE PROTECTION

Devices used to actively protect the pump station against the effects of water hammer are dependent upon power supply.Therefore these methods only protect the pipeline during normal pump stops [36]. Examples of active protection include

variable frequency drives, soft starters and slow-closing valves.

10.2 PASSIVE PROTECTIONPassive protection equipment operates without the need for additional power supply and can therefore be used to protect the

pipe system in the event of a power failure. Air chambers, surge towers and air inlet/release valves are methods used to providepassive protection.

11.0 AIR IN PIPELINES

It is recognised that air is present in many water pipelines [28]. The air may be absorbed at free surfaces, or entrained in

turbulent flow at the entrance to the line. The air may thus be in solution or in free form in bubbles or pockets. An air pocket

implies a relatively large volume of air, likely to accumulate on top of the pipe cross section. The pockets may travel along the

line to peaks. There they will either remain in equilibrium, be entrained by the flowing water or be released through air valves.

Air in solution is not likely to present many engineering problems. It is only when the pressure reduces sufficiently to permitdissolved air to form bubbles that problems arise. The water bulks and head losses increase. The bubbles may merge and rise to

the top of the pipe to form large pockets. Flow conditions then become similar to those in partly full drain pipes, except that in a

pipeline it is likely that system, including the free air, will be pressurised.

EXTERNAL LOADS:Low pressure pipes, especially sewers, gravity mains or even large diameter pumping mains should bedesigned for external loads as well as internal loads. The vertical soil load acting in combination with vacuum pressure inside

the pipe could cause the pipe to collapse unless the pipe is adequately supported or stiffened. Internal and external load and

pressures are discussed in section 3.0 and 4.0.

SOIL LOADS: The load transmitted to a pipe from the external surroundings depends on a number of factors.

RIGIDITY OF PIPE: The more rigid relative to the trench side fill the more loads it will take. The side fill tends to settle, this

causing a large part of the backfill to rest on the pipe. This occurs with flexible pipe too to some extent, as a pipe is supported

laterally by the fill and will not yield as much as a free standing pipe.


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TYPE OF TRENCH FILL: the load transmitted to the pipe varies with the width and depth of trench since friction on the

sides of the trench affects the resultant load [35]. Embankment fills may also transmit different loads to a pipe, depending on therelative settlement of side fill and top fill. Figure 12 illustrates various installation methods.

There are several issues to consider such as trench condition, embankment condition etc.

Figure 14 illustrates various possible installation conditions of pipes

12.0 CONCRETE PIPES

12.1 THE EFFECT OF BEDDING

Non- pressure sewer or drain pipes are designed to withstand external loads, not internal pressures. Various standards specify

the design load per unit run of pipes for different classes and the pipes are reinforced accordingly. The main stresses due to

vertical external loads are compressive stress at the haunches and the bending stresses at the crown, the bottom and the

haunches. The main stresses are caused by live loads, vertical and horizontal soil loads, self-weight and weight of water

(internal water pressure and transient pressures are neglected for non-pressure pipes.) concrete pipes and the other rigid non

pressure pipes are normally designed to withstand a vertical line load while supported on a flat rigid bed [37]. The load per unitlength required to fracture the pipe loaded this is called the laboratory strength. Although the laboratory strength could be

calculated theoretically, a number of practical factors influence the theoretical load and experimental determination of the load

is more reliable. (The tensile strength of concrete is very uncertain and the effect of lateral constraint of the supports may be

appreciable)

12.2 PRESTRESSED CONCRETE PIPES

Prestressed concrete is becoming a popular medium for large-bore pressure pipes. Pre-stressed concrete competes economically

with steel for long pipelines over approximately 800 mm diameter. It has the advantage that the pre-stressing steel can be

stressed to higher stresses than the plain-walled pipes [37]. The wall thickness of plain walled steel pipes must be reasonablythick to prevent buckling, collapse and distortion even if the thickness is not required to resist internal pressures. Consequently

the use of high-tensile steels is restricted when manufacturing plain-walled steel pipes, but this is not the case with pre-stressed

concrete pipes which are more rigid than steel pipes.


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13.0 BURIED FLEXIBLE STEEL PIPE

When designing steel pipe, the designer must consider issues beyond the thickness of the steel cylinder. These considerations

include the type of coating and linings to be applied and the type of joint configuration consistent with the application. Certain

coatings and linings are appropriate for some installation conditions and inappropriate for others [9]. The same holds true for

the various joint configurations.

As with any design, the designer should always be aware of the nature of the input data and the impact the results of acalculation have on the economics of a project. Rules of thumb do not have to be considered as absolute values. The designer

should use discretion when evaluating requirements for project design. Performance limits are not synonymous with failures.

Both performance limits and failure mechanisms must be recognised. Pipes are an efficient and economical means of

transporting anything that can flow-fluid, slurry, gas, wire conduits, pedestrians, traffic, and so on. Pipes even provide storage.

The first step for transportation of fluids in pipe is to determine [9]:

1. What is to be transported?

2. What is the rate (quantity) of flow?3. What are the pressure and pressure variations?

Structural design of welded steel pipe is based on the principles of pipe performance and the conditions for performance limit.

A performance limit may be a leak or excessive deformation-either ring deformation or longitudinal deformation. Deformation

is based on pipe mechanics, soil mechanics, and pipe soil interactions. Soil is part of the conduit structural system- not simply aload on the pipe.

Typically, design of pipe proceeds as follows:

1. Internal pressure-steel cylinder (wall) thickness

2. Handling and installation-ring stiffness

3. External pressure-ring compression

4. Ring deflection (deformation)

5. Longitudinal stress analysis

6. Joints, linings and coatings7. Miscellaneous, special design cases

Internal pressure design- already discussed refer to section 3.0

Minimum wall thickness for the steel cylinder of the pipe is often governed by what can be safety handled and installed in thefield. Designers commonly use D/ts ratios up to 240. With special design, manufacturing, shipping, and installation

considerations, increased D/tsratios of 288 and higher have been successfully installed. Irrigation and hydroelectric systems are

two examples that use these higher D/ts values.

Ring stiffness

Stiffness is resistance to deflection. Pipe stiffness is defined as the ratio of the concentrated load F to a cylinder over the

resulting deflection D or F/D. Ring stiffness is defined as EI/r3per unit length of pipe, the dimensions of the ring stiffness are

force per unit area, commonly Mpa. Because L=t3/12 per mm length of pipe EI / r

3= 2E / 3(D/t)

3.

Ring compression

If the pipe ring is held in the circular shape when external pressure is applied, stress in the pipe wall is ring compression stress,

s= P (D0) /2ts [55]. Performance limit for common pipe diameters and thicknesses is wall crushing or wall buckling at yieldstress y. External pressure is caused by the soil embedment and the pipe does not fail at yield stress, but any additional

pressure must be supported by the embedment. Nevertheless, for design, yield stress is set as the performance limit. Other

factors that need to be considered while designing the flexible steel buried pipes are the ring deflection, yield stress.

13.1 SOIL MECHANICSAN INTRODUCTION

Structural behaviour of buried pipe is not elastic, especially at performance limits. Design and analysis is based on pipe-soil

interaction using correct properties of both pipe and soil. At performance limit (beyond elastic limit), steel pipes are ductile.

Soil varies from particulate (granular) to viscous (mud). Basic principles of soil mechanics are required for rational design and

analysis of buried pipe. Soil mechanics is not discussed in this report.

Buried pipe design analysis, following three basic procedures. The first step in buried pipe design is to determine the wall

thickness required because of internal pressure. The step assumes that pipe diameter, flow, pressure, and routing have beendetermined. The second step is to check for minimum steel wall thickness for handling. The final step the pipe-soil embedment

system is analysed for external loading during construction, which involves earth loads, live loads and water table conditions

and can also include negative pressure in the pipeline.


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14.0 ABOVE GROUND SYSTEM DESIGN- SUPPORTS, ANCHORS AND GUIDES

14.1 Piping support design

Above ground piping systems may be designed as restrained or unrestrained. Selection of the design method is dependent on

variables such as operating temperature, flow rates, pressures and piping layout. System designs combing the two methods often

lead to the most structurally efficient and economical piping layout [21].

14.2 Unrestrained system design

The unrestrained system is often referred to as a simple supported design. It makes use of the inherent flexibility of fiber glass

pipe to safely absorb deflections and bending stresses. Simple pipe hangers or steel beams are used to provide vertical support

to the pipe. These simple supports allow the piping system to expand the contract freely resulting in small axial stresses in thepiping system. Long straight runs often employ changes-in-direction to safely absorb movement due to thermal expansion and

contractions, flow rate changes, and internal pressure.

14.3 Restrained system design

The restrained system is often referred to as an anchored and guided design. The low modulus of elasticity for fiberglass

piping translates to significantly smaller thermal forces when compared to steel. Anchors are employed to restrain axial

movement and provide vertical support in horizontal pipelines. Anchors used to restrain thermal expansion create compressive

forces in the pipeline [21]. These must be controlled by the use of pipe guides to prevent the pipe from buckling. In cases where

axial loads created by anchoring a pipe run are excessively high, the use of expansion loops or expansion joints must be

employed. When using anchors, the effect of system contraction should be considered. Supports for pipes are discussed insection 5.0.

14.4 System design

The properly designed piping system provides safe and efficient long-term performance under varying thermal environments.The system design dictates how a piping system will react to changes in operating temperatures. The unrestrained piping system

undergoes expansion and contraction in proportion to changes in the pipe wall mean temperature. Fiberglass piping system that

operate at or near the installation temperature are normally unrestrained designs, where the most important design consideration

is the basic support span spacing. Since few piping systems operate under these conditions, some provisions must be made for

thermal expansion and contraction. The simplest unrestrained piping system use directional changes to provide flexibility to

compensate for thermal movements. When directional changes are available or provide insufficient flexibility, the use of

expansion loops or expansion joints should be designed into the system to prevent overstressing the piping system. Thesesystems are considered unrestrained even though partial anchoring and guiding of the pipe is required for proper expansion joint,

expansion loop performance and system stability.

The fully restrained anchored piping system eliminates axial thermal movement. Pipe and fittings generally benefit from

reduced bending stresses at directional changes. Restrained systems develop internal loads required to maintain equilibrium atthe anchors due to temperature changes. When the pipe is in compression, these internal loads require guided supports to keep

the pipe straight preventing Euler buckling. Thus, the commonly referred to name of restrained systems is anchored and

guided. Anchored and guided systems have anchors at the ends of straight runs that protect fittings from thermal movement

and stresses [21].

Anchors at directional changes (elbow and tees) transmit loads to the support substructure. Special attention should be given to

these loads by the piping engineer to ensure an adequate substructure design. When multiple anchors are used to break up longstraight runs, the loads between them and the substructure are generally small. The axial restraining loads are simply balanced

between the two opposing sides of the pipeline at the anchor.

15.0 VIBRATION

Every machine vibrates as it operates [38]. No matter how rigidly a machine is mounted, the machine and all attached structures

will experience some undesirable motion caused by various forces. These forces are usually related to the movement of variousparts within the machine [39]. If this vibration-related movement becomes too great, damage to the machine will result.

Vibration can be caused by a variety of conditions including bent shafts, unbalance in rotating parts, worn or bent gears,

damaged bearings, misaligned couplings or bearings, electromagnetic forces, etc. [40].

Pipeline vibrations are caused by many reasons. It depends on the flow, the equipment to which it is connected, supports,

selection of piping with respect to flow and pressure and types of valves and pipe fitting in the route. Piping vibration can be an

annoying problem which can consume unnecessary maintenance activity and can affect pumping system performance and

endurance [40]. The system includes the pipe, all piping supports, hangers, snubber, pipe to pipe interfaces, and machinery ordevices attached to the pipe. All these items can influence the pipe vibration patterns. Sudden closure and opening of valves


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creates "water hammer" effects in the piping and causes tremendous vibration. In-sufficient Net Positive Suction Head NPSH in

the pump also causes vibration. The Net Positive Suction Head Available must be greater than the Net Positive Suction HeadRequired (NPSH Available > NPSH Required) [41]. A pipe will not vibrate if it is prevented from moving. However, this does

not necessarily help the piping system design from the standpoint of its ability to absorb differential thermal expansion.

Therefore, when addressing a vibration problem, the flexibility design of the piping system must also be considered. Restraints

that are added to reduce vibration must not increase the pipe thermal expansion stresses or end-point reaction loads to an

acceptable levels [42]. It may sometimes be necessary to use hydraulic snubber to stop vibration rather than fixed restraints.

Such snubbers permit pipe thermal movement while still dampening vibration.

In reciprocating machineries, the flow through the pipeline is pulsating. If the pulsating is not dampened it will cause vibration

in the pipeline. Dynamic imbalance in centrifugal fan can cause heavy vibration and transmits it to piping which can be cured

by balancing the fan with shaft on balancing machine or "in-situ" balancing. Generally, the pipe supports should be a nodal

point with little or no motion. Excessive motion at these locations indicates that the support is faulty or improperly installed [43].

16.0TYPES OF VIBRATIONS

16.1Free vibration: occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely.

Examples of this type of vibration are pulling a child back on a swing and then letting go or hitting a tuning fork and letting it

ring. The mechanical system will then vibrate at one or more of its "natural frequency"and damp down to zero.

16.2 Forced vibration: is when an alternating force or motion is applied to a mechanical system. Examples of this type of

vibration include a shaking washing machine due to an imbalance, transportation vibration (caused by truck engine, springs,road, etc.), or the vibration of a building during an earthquake. In forced vibration the frequency of the vibration is the

frequency of the force or motion applied, with order of magnitude being dependent on the actual mechanical system.

17.0 WHAT TYPES OF VIBRATION ARE CAUSED IN PIPELINES

For the purposes of piping design and monitoring, vibration is typically divided into two types: steady-state and dynamic

transient vibrations. Each type has its own potential causes and effects that necessitate individualized treatment for prediction,

analysis, control, and monitoring [44].

17.1 Steady-State Vibration

Steadystate vibration in pipelines can be defined as a repetitive vibration that occurs for a relatively long period of time. This

type of vibration is caused by different forces acting on the pipeline for example force due to rotating or reciprocating

equipment such as a pump or by the fluid pressure pulses. Cavitation or flashing are also a major cause of vibration that resultsfrom pressure reducing valves, control valves and flash tanks. Flow-induced vibrations such as vortex shedding can causesteady-state vibrations in piping [45], and wind loadings can cause significant vibrations for exposed piping similar to that

typically found at outdoor boilers. Steady-state vibrations exist in a range from periodic to random. Material fatigue is also a

major cause of steady-state vibration. This type of vibration occurs in the piping itself, most likely at areas with stress risers

such as branch connections, elbows, threaded connections, or valves. However, this failure can also occur in various piping

system components and supports. Fatigue damage to wall penetrations can occur because of vibration in the attached piping,

snubbers, and supports; premature failures of machine bearings are another potential consequence [46].

17.2 Dynamic-Transient Vibration

Dynamic-transient vibration on the other hand is a more intense type of vibration. It is generated by much larger forces than the

steady state vibration and lasts for a relatively short period of time. In piping, the primary cause of dynamic transients is a high-

or low-pressure pulse traveling through the fluid [47]. Such a pulse can result in large forces acting in the axial direction of the

piping, the magnitude of which is normally proportional to the length of pipe legthat is, the longer the pipe leg, the larger thedynamic transient force the piping will experience ( pipe leg is defined as the run of straight pipe between bends) [47]. The

common examples of this type vibration are rapid pump starts and trips, and also the quick closing or opening of valves such as

turbine-stop valves and various types of control valves. Dynamic transients al

revised capstone a

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