Together
to success
since 1997
INTECH GmbH

# Pipeline Design and Selection. Optimum Pipeline Diameter

Pipelines for transporting various fluids are an integral part of units and plants, which implement working processes related to different fields of application. While selecting pipes and pipeline configurations, the cost of the pipes themselves and the cost of valves is of great importance. The final cost of medium transfer through the piping is largely defined by the size of pipes (diameter and length). Specially developed formulas, specific for certain types of operation, are used to calculate these values.

A pipe is a hollow cylinder made of metal, wood or other material used to transport fluid, gaseous and granular media. The transferred medium may include water, natural gas, steam, petroleum products, etc. Pipes are used everywhere, starting with various industries and ending household applications.

A variety of materials, such as steel, cast iron, copper, cement, plastic such as ABS plastic, polyvinyl chloride, chlorinated polyvinyl chloride, polybutylene, polyethylene, etc., can be used in pipe manufacturing.

Pipe diameter (outer, inner, etc.) and wall thickness, measured in millimeters or inches, are the main pipe dimensions. Such a value as a nominal diameter or nominal bore – a nominal value of the inner diameter of the pipe, also measured in millimeters (denoted by Ду) or inches (denoted by DN), is also used. Nominal diameter values ​​are standardized, being the main criterion in the pipe and connecting fitting selection.

The correspondence of the nominal bore in [mm] and [inches] is provided below.

Ду, mm DN, inches Ду, mm DN, inches
15 ½ 400 16
20 ¾ 450 18
25 1 500 20
40 600 24
50 2 650 26
80 3 700 28
100 4 750 30
150 6 800 32
200 8 900 36
250 10 1000 40
300 12
350 14

For a number of reasons, specified below, pipes with a circular (round) cross-section, are the preferred option as compared to other geometric cross-sections:

• Circle has a minimum perimeter-to-area ratio; as applicable to pipes, this means that, at an equal throughput, material consumption for round-shaped pipes will be minimal compared to pipes of another shape. This also implies minimum possible costs for insulation and protective coatings;
• Round cross-section is most advantageous option for moving fluid or gaseous media from a hydrodynamic point of view. Also, due to the minimum possible internal pipe area per unit of its length, the friction between the transferred fluid and the pipe is minimized.
• Round shape is most resistant to internal and external pressures;
• Round pipe production process is quite simple and easy to implement.

Pipes can vary greatly in diameter and configuration, dependent on the purpose and area of application. Since the main pipelines for transferring water products can reach almost half a meter in diameter at a rather simple configuration, and heating coils, also designed as a small-diameter pipe, have a complex shape with a lot of turns.

It’s impossible to imagine any sector of the industry without a piping network. Any piping network calculation includes selection of pipe materials, development of a bill of materials, which includes data on pipe thickness, size, route, etc. Feedstock, intermediate product and/or finished product pass various production stages, moving between different apparatuses and units, which are connected by pipelines and fittings. Correct calculation, selection and installation of the piping system is necessary for reliable implementation of the entire process and providing safe transfer of the working media as well as for sealing the system and preventing transferred substance leaks into the atmosphere.

There is no universal formula or rule to be used to select a pipeline for any possible application and working medium. Each pipeline application area involves a number of factors, which should be taken into account and are capable of making a significant impact on pipeline requirements. For example, while handling slurry, a large-sized pipeline will not only increase the cost of the unit, but also create operating difficulties.

Typically, pipes are selected following the optimization of material costs and operating costs. The larger the diameter of the pipeline, i.e. the higher the initial investments, the lower a pressure drop and, correspondingly, the lower the operating costs. Conversely, a small size of pipelines will allow the reduction in the initial cost of pipes and valves; however, the increased velocity will entail increased losses and result in spending additional energy to pump the medium through. Velocity rates, fixed for different applications, are based on optimal design conditions. These rates, with due consideration of the field of application, are used in pipeline size calculations.

## Pipeline Design

While designing pipelines, the following basic design parameters are taken as a basis:

• Required throughput capacity;
• Pipeline inlet and outlet points;
• Medium composition, including viscosity and specific gravity;
• Topographic conditions of pipeline routes;
• Maximum permissible operating pressure;
• Hydraulic calculation;
• Pipeline diameter, wall thickness, tensile yield stress of wall materials;
• Number and power consumption of pump stations as well as the distance between pump stations.

## Reliability of Pipelines

Design reliability of pipelines is ensured by observing proper design standards. Personnel training is also a key factor in providing a long service life, tightness and reliability of pipelines. Continuous or periodic pipeline operation monitoring can be performed by monitoring, metering, control, regulation and automation systems, personal process monitoring instruments, safety devices.

Corrosion-resistant coating is applied onto outer surface of most pipes with the aim to prevent disruptive effect of corrosion made by the ambient environment. If corrosive media is pumped through, a protective lining can be applied onto inner surface of pipes. Before commissioning, all new pipes, intended for hazardous fluid transportation, are tested to identify defects and leaks.

## Basic Provisions for Pipeline Flow Calculation

The nature of medium flow in a pipeline and while flowing around obstacles may significantly vary from fluid to fluid. Medium viscosity, characterized by such a parameter as a viscosity coefficient, is one of critical indicators. Osborne Reynolds, an Irish engineer-physicist, conducted a series of experiments in 1880; based on the results of these experiments, he managed to derive a dimensionless value that characterizes the nature of a viscous fluid flow, called the Reynolds criterion and denoted by Re.

Re = (v·L·ρ)/μ

Where:
ρ — fluid density;
v — flow velocity;
L — characteristic flow element length;
μ – dynamic viscosity coefficient.

In other words, the Reynolds criterion characterizes the ratio of inertia forces to viscous frictional forces in a fluid flow. A change in the value of this criterion reflects the change in the ratio of these forces, which, in its turn, influences the fluid flow behavior. In this regard, three flow regimes are usually distinguished dependent on the Reynolds criterion value. At Re<2300, the so-called laminar flow is observed, when a fluid moves in thin layers, which are almost immiscible with each other; also, a gradual increase in the flow velocity is observed in the direction from the wall towards the center of the pipe. A further increase in the Reynolds number destabilizes this flow structure, and at 2300<Re<4000 a transient regime occurs, at which individual layers begin to mix with each other. At Re>4000, a stable behavior is already observed; this behavior is characterized by an erratic change in the flow velocity and direction at each individual point of the flow, resulting in a flow velocity equalization throughout the volume. This behavior is called a turbulent regime. The Reynolds number depends on a discharge head, developed by the pump, medium viscosity at the operating temperature, and dimensions and a shape of a cross-section of the pipe through which the flow passes.

The Reynolds criterion is a similarity criterion for a viscous fluid flow. I.e., it can be used to simulate a real process at a reduced scale, convenient for study. This is extremely important, because often it is extremely difficult, and sometimes it is completely impossible to study the nature of fluid flows in real units because of their large size.

## Pipeline Calculation. Pipeline Diameter Calculation

If a pipeline is not heat insulated, i.e. heat exchange between the transferred medium and ambient environment may occur, then the flow pattern in the pipeline may change even at a constant velocity (flow rate). This can take place at a rather high transferred medium temperature at the inlet and a turbulent flow. The temperature of the transported medium tends to fall along the pipe length due to heat losses to the environment, which may entail a change in the flow regime (to laminar or transient regime). The temperature, at which the flow regime is changed, is called the critical temperature. Since the fluid viscosity values directly depend on the temperature, such a parameter as the critical viscosity, corresponding to the flow regime change point at the critical Reynolds criterion value, is used in similar cases:

vкр = (v·D)/Reкр = (4·Q)/(π·D·Reкр)

Where:
νкр – critical kinematic viscosity;
Reкр – Reynolds criterion critical value;
D – pipe diameter;
v – flow velocity;
Q – flow rate.

Another important factor to consider is a friction that occurs between the pipe walls and the moving flow. In this case, the coefficient of friction largely depends on the pipe wall roughness. The relationship between the coefficient of friction, the Reynolds criterion and the roughness is established by the Moody diagram, which allows one of the parameters to be determined, knowing the other two parameters.

The Colebrook-White formula is also used to calculate the coefficient of friction of a turbulent flow. Based on this formula, it’s possible to plot charts to be used to establish the coefficient of friction.

(√λ)-1 = -2·log(2,51/(Re·√λ) + k/(3,71·d))

Where:
k – pipe roughness coefficient;
λ – coefficient of friction.

There are also other formulas for an approximate calculation of frictional losses at a pressure fluid flow in pipes. The Darcy-Weisbach equation is one of the most frequently used equations in this case. It is based on empirical data and is used primarily in system modeling. Frictional losses are a function of fluid velocity and pipe resistance to fluid motion, expressed through the roughness value of the pipeline walls.

∆H = λ · L/d · v²/(2·g)

Where:
λ – coefficient of friction;
L – pipe section length;
d – pipe diameter;
v – flow velocity;
g – gravity acceleration.

The pressure loss due to friction for water is calculated, using the Hazen-Williams formula.

∆H = 11,23 · L · 1/С1,85 · Q1,85/D4,87

Where:
L – pipe section length;
С – Haizen-Williams roughness coefficient;
Q – flow rate;
D – pipe diameter.

## Pressure

Pipeline operating pressure is the biggest gauge pressure that ensures a specified mode of operation of the pipeline. A decision on the pipeline size and the number of pump stations is usually taken with the consideration of the pipe operating pressure, pump capacity and flow rates. Maximum and minimum pipeline pressure as well as the working medium properties define the distance between and power consumption of the pump stations.

Nominal pressure (PN) is a nominal value corresponding to the maximum working medium pressure at 20°C, at which a continuous operation of the pipeline with the specified dimensions is possible.

As the temperature increases, the throughput capacity as well as the permissible gauge pressure of the pipe tend to decrease. The value (Pe, zul) indicates the maximum gauge pressure in the pipeline system at increased operating temperature.

The permissible excess pressure chart is provided below (permissible gauge pressure vs. operating temperature):

## Pipeline Pressure Drop Calculation

Pipeline pressure drop is calculated according to the formula:

∆p = λ · L/d · ρ/2 · v²

Where:
Δp – pressure drop across the pipe section;
L – pipe section length;
λ – coefficient of friction;
d – pipe diameter;
ρ – transferred medium density;
v – flow velocity.

## Transported Working Media

Pipes are most often used to transport water, but they can also be used to transport slurry, suspensions, steam, etc.

Pipelines are also used to handle:

• Refined products such as gasoline, aviation fuel, kerosene, diesel fuel, etc.;
• Petrochemical feedstock: benzene, styrene, propylene, etc.;
• Aromatic hydrocarbons: xylene, toluene, cumene, etc.;
• Liquefied petroleum fuel such as liquefied natural gas, liquefied petroleum gas, propane (gases with standard temperatures and pressures, but liquefied under pressure);
• Carbon dioxide, fluid ammonia (transported under pressure as liquids);
• Bitumen and viscous fuels, which are too viscous to be transported through pipelines;
• Hydrogen (at short distances).

## Transported Environment Quality

Physical properties and parameters of transported media determine, to a great extent, the design and operating parameters of pipelines. Specific gravity, compressibility, temperature, viscosity, pour point and vapor pressure are the main working medium parameters to be taken into account.

Specific fluid gravity is a fluid weight per unit of volume. Many gases are transported through pipelines under pressure, and when a certain pressure is reached, some gases can even be liquefied. Therefore, the degree of compression of the medium is a critical parameter for designing pipelines and determining their throughput capacity.

The temperature indirectly and directly affects the throughput of pipelines. This is expressed by fluid volume increase following the temperature increase, provided that the pressure remains constant. Lowering the temperature can also affect both the throughput and overall efficiency of the system. Typically, when the temperature of the fluid decreases, this is accompanied by an increase in its viscosity, which creates additional friction resistance along the inner wall of the pipe, requiring more energy to pump through the same amount of fluid. Very viscous media are sensitive to changes in operating temperatures. Viscosity is the resistance of the medium to the flow and is measured in centistokes (cSt). Viscosity predetermines not only the choice of pumps, but also the distance between pump stations.

As soon as the medium temperature drops below the pour point, the pipeline operation becomes impossible and some options should be taken to resume its operation:

• Heat the medium or apply heat insulation on pipes to maintain operating temperature of the medium above the pour point;
• Change chemical composition of the medium before entering the pipeline;
• Dilute transported medium with water.

## Types of Main Line Pipes

Main line pipes are made welded or seamless. Seamless steel pipes are produced without longitudinal welds by steel sections with heat treatment to achieve desired size and properties. Welded pipes are manufactured using a number of production processes. These two pipe types differ from each other in the number of longitudinal joints in the pipe and the type of welding equipment used. Steel welded pipes are the most commonly type used in the petrochemical industry.

Each pipe piece is connected by welded sections together to form a pipeline. Also, dependent on the field of application, pipes made of fiberglass, various plastics, asbestos cement, etc., are used in the main pipelines.

To connect straight pipe sections as well as to provide a transition between pipe sections of different diameters, specifically made connecting elements (bends, branches, valves) are used.

Special joints are used for assembling individual pipeline parts and fittings.

The use of the joints:

Welded joint is a non-detachable connection, used for all pressures and temperatures;
Flanged joint is a non-detachable connection, used for high pressures and temperatures;
Threaded joint is a detachable connection, used for medium pressures and temperatures;
Coupling is a detachable connection, used for low pressures and temperatures.

The out-of-roundness and wall thickness difference of seamless pipes shall not exceed permissible diameter and wall thickness deviations.

## Thermal Pipeline Elongation

When a pipeline is under pressure, all its internal surface is exposed to an evenly distributed load, which causes longitudinal internal forces in the pipe and additional loads onto end supports. Temperature fluctuations also affect the pipeline, causing pipe size changes. Forces in a fixed pipeline at temperature fluctuations may exceed permissible values and result in excessive stresses, dangerous to the pipeline strength, both in the pipe material and in the flanged connections. Transferred medium temperature fluctuations also develop temperature stressed in the pipeline, which may be transmitted to valves, pump stations, etc. This may entail depressurization of pipeline joints, failure of valves or other components.

## Pipeline Dimension Calculation at Varied Temperatures

Linear pipeline dimensions at temperature variations are calculated by the formula:

∆L = a·L·∆t

a – coefficient of temperature elongation, mm/(m°C) (see the table below);
L – pipeline length (distance between fixed supports), m;
Δt – difference between maximum and minimum temperatures of the transferred medium, °С.

Table of linear expansion of pipes made of various materials:

 Material coefficient of temperature elongation, mm/(m°C) Cast iron 0.0104 Stainless steel 0.011 Steel, black and galvanized 0.0115 Copper 0.017 Brass 0.017 Aluminum 0.023 Metal plastic 0.026 Polyvinyl chloride (PVC) 0.08 Polybutylene (PB) 0.13 Polypropylene (PP-R 80, PN 10, PN 20) 0.15 Polypropylene (PP-R 80, PN 25, aluminum) 0.03 Polypropylene (PP-R 80, PN 20, fiberglass) 0.035 Cross-linked polyethylene (PEX) 0.024

The figures indicated above are the average values ​​for the listed materials; as for calculating pipelines made of other materials, the data from this table should not be taken as a basis. While calculating pipelines, it’s recommended to use the linear elongation factor specified by the pipe manufacturer in the accompanying technical specification or data sheet.

Temperature elongation of pipelines is eliminated by the use of both special compensating pipeline sections and expansion joints, which may include elastic or moving parts.

Compensation sections consist of elastic straight pipeline parts located perpendicular to each other and fastened by offshoots. In case of a temperature elongation, an increase in one part is compensated by the deformation of the bend of the other part in the plane, or by the deformation of the bend and torsion in space. If pipelines are capable to compensate for the temperature expansion themselves, this effect is called self-compensation.

Compensation also occurs due to elastic offshoots. Part of elongation is compensated by the elasticity of the offshoots, the other part is eliminated by elastic properties of the material of the area downstream of the offshoot. Expansion joints are provided in case if compensation sections cannot be used or when self-compensation of the pipeline is insufficient.

In terms of design and principle of operation, expansion joints are divided in four types: U-shaped expansion joint, expansion bellows, hinged expansion joint and sliding expansion joint. In practice, L-, Z- or U-shaped flat expansion joints are often used. The 3D expansion joints are usually designed as two flat mutually perpendicular sections with one common shoulder. Elastic expansion joints are made of pipes or elastic discs, or bellows.

## Pipeline Optimum Diameter Determination

Optimum diameter of the pipeline can be found based on cost-performance calculations. Pipeline dimensions, including the size and functional capabilities of various components, as well as pipeline operating conditions define the throughput of the system. Larger-diameter pipes are suitable for a more intensive mass flow of the medium, provided that the other components of the system are properly selected and designed for these conditions. Usually, the longer the length of the main pipe between pump stations, the bigger the required pressure differential in the pipeline. In addition, a change in physical properties of the medium transferred (viscosity, etc.) can notably affect the pipeline pressure.

Optimum size is the smallest of pipe sizes suitable for specific application, which is cost-effective throughout the service life of the system.

Pipe throughput is calculated by the formula:

Q = (π·d²)/4 · v

Q – transferred fluid flow rate;
d – pipeline diameter;
v – flow velocity.

In practice, optimum velocities ​​of the transferred medium, taken from reference materials and compiled based on experimental data, are used to calculate optimum pipeline diameters:

Medium transferred Range of optimum pipeline velocities, m/s
Liquids Gravity flow:
Viscous liquids 0.1 – 0.5
Low-viscous liquids 0.5 – 1
Pump-transferred:
Suction side 0.8 – 2
Discharge side 1.5 – 3

Gas Natural draft 2 – 4
Low pressure 4 – 15
High pressure 15 – 25

Steam Superheated steam 30 – 50
Saturated steam under pressure:
>105 Pa 15 – 25
(1 – 0.5) · 105 Pa 20 – 40
(0.5 – 0.2) · 105 Pa 40 – 60
(0.2 – 0.05) · 105 Pa 60 – 75

Therefrom, the formula for calculating optimum pipe diameters comes:

dо = √((4·Q) / (π·vо))

Q – specified flow rate of the fluid transferred;
d – optimum pipeline diameter;
v – optimum flow velocity.

At high flow velocities, smaller diameter pipes are typically used, thus reducing the costs of pipeline purchase, maintenance and installation (denoted by K1). As velocity increases, the frictional head losses and local resistances increase, thus resulting in increased costs required for fluid transfer (denoted by K2).

For large diameter pipelines, the K1 costs will be higher and the operating costs (K2) will be lower. If we add the values ​​of K1 and K2, we will receive the total minimum costs (K) and the optimum pipeline diameter. The K1 and K2 costs for this case are specified for the same time interval.

## Pipeline Capital Expenditures Calculation (Formula)

K1 = (m·CM·KM)/n

m – pipeline weight, t;
CM – cost of 1 t, RUB/t;
KM – coefficient, increasing the cost of installation, e.g. 1.8;
n – service life, years.

The indicated operating costs are associated with power consumption:

K2 = 24·N·nдн·CЭ RUB/yr

N – power, kW;
nДН – number of working days per year;
СЭ – cost per kWh of energy, RUB/kWh.

## ormulas for Determining Pipeline Dimensions

An example of general formulas for determining pipe sizes without consideration of possible additional impact factors, such as erosion, suspended solids, etc.:

Item Equation Possible restrictions
Pressurized fluid and gas flow
Darcy-Weisbach pressure head loss due to friction d = 12·[(0,0311·f·L·Q2)/(hf)]0,2 Q – volume flow rate, gal/min;
d – inner pipe diameter;
L – pipeline length, ft;
f – coefficient of friction;
V – flow velocity.
Total fluid flow equation d = 0,64·√(Q/V) Q – volume flow rate, gal/min
Pump suction line size to limit friction head loss d = √(0,0744·Q) Q – volume flow rate, gal/min
Total gas flow equation d = 0,29·√((Q·T)/(P·V)) Q – volume flow rate, ft³/min
T – temperature, K
Р – pressure psi (abs);
V – velocity
Gravity flow
Manning equation for pipe diameter calculation at a maximum flow d = [1525 · (Q·n)/√S]0,375 Q – volume flow rate;
n – coefficient of roughness;
S – slope.
Froude number (inertia force to gravity force ratio) Fr = V / √[(d/12) · g] g – gravity acceleration;
v – flow velocity;
L – pipe length or diameter.
Steam and evaporation
Equation for determining pipe diameter for steam d = 1,75·√[(W·v_g·x) / V] W – mass flow rate;
Vg – specific volume of saturated steam;
x – steam quality;
V – velocity.

## Optimum Flow Velocity for Various Piping Systems

Optimum pipe size is selected based on minimum costs for medium transfer through the pipeline, and pipe costs. However, it’s also necessary to take into account velocity limits. Sometimes, the pipeline size should meet technological process requirements. Also, the pipeline size is often associated with a pressure differential. In preliminary design calculations, where pressure loss is not taken into account, the process pipeline size is determined based on the permissible velocity.

If the flow direction in the pipeline changes, local pressures on the surface, perpendicular to the flow direction, significantly grow. This increase is a function of fluid velocity, density and initial pressure. Since velocity is inversely proportional to the diameter, selecting the size and configuration of pipelines for high-speed fluids requires particular attention. E.g., the optimum pipe size for sulfuric acid transportation restricts the velocity of the medium to a value, at which wall erosion in pipe bends is not allowed, thus to prevent damage to the pipe structure.

## Fluid Gravity Flow

Pipeline size calculations for a gravity flow are rather complicated. At this flow pattern in the pipeline, the flow can be single-phase (full pipe) and two-phase (partial filling). A two-phase flow is formed in case if fluid and gas are simultaneously present in the pipe.

Dependent on a fluid-to-gas ratio and associated velocities, a two-phase flow regime may vary from a bubble flow to a dispersed flow.

A driving force for gravity flowing fluids is provided by the difference in the heights of the start and end points, and the location of the start point above the end point is a mandatory condition. In other words, the height difference defines the difference in the potential energy of the fluid at these points. This parameter is also taken into account while selecting an appropriate pipeline. In addition, the driving force value depends on the pressure ​​at the start and end points. An increase in the pressure differential entails an increase in the flow velocity, which, in its turn, allows a smaller diameter pipeline to be selected, and vice versa.

If the end point is connected to a pressurized system, e.g., a distillation column, the equivalent pressure should be subtracted from the available height difference to estimate the actual effective pressure differential. Also, if the pipeline start point is under vacuum, the impact of vacuum on the total pressure differential should also be taken into account while selecting a pipeline. The differential pressure, which takes into account all the above mentioned factors and which is not based only on the height difference of the start and end points, shall be used for a final pipe selection.

## Hot Fluid Flow

Generally, process plants face different problems while handling hot or boiling media. The main reason is the evaporation of part of a hot fluid flow, i.e., a phase transformation of fluid into steam inside the pipeline or equipment. A typical example is a centrifugal pump cavitation, accompanied by a spot fluid boiling with a subsequent formation of vapor bubbles (vapor cavitation) or the release of dissolved gases into bubbles (gas cavitation).

Larger-diameter pipelines seem preferred due to reduced flow velocities, as compared to smaller-diameter pipelines, at a constant flow rate, which allows the achievement of a higher NPSH value in the pump suction line. Also, points of a sudden change in the flow direction or a reduced pipeline size may the cause of cavitation at a loss of pressure. The resulting vapor-gas mixture creates obstacles to the flow and can cause damage to the pipeline. That’s why the cavitation effect is extremely undesirable in the pipeline operations.

## Equipment/ Instrument Bypass Piping

Equipment and instruments, especially those which can create significant pressure differentials, i.e. heat exchangers, control valves, etc., are equipped with bypass piping (to prevent process interruption even during maintenance). This piping usually has 2 shut-off valves, installed in the unit line, and a flow control valve, provided in parallel to this unit.

During normal operation, a fluid flow, passing through major assemblies of the unit, experiences an additional pressure drop. In accordance with this, its discharge pressure, developed by the connected equipment, e.g., a centrifugal pump, is calculated. The pump is selected based on the total pressure differential across the unit. When the flow passes through this bypass line, the additional pressure differential does not exist, whilst the running pump discharges the flow with a former force, in accordance to its operating data. To avoid differences in the condition of the flow, passing through the unit and through the bypass line, it is recommended to use a smaller bypass line with a trim valve to create a pressure, equivalent to the pressure in the main unit.

## Sampling Line

Usually, a small amount of fluid is collected for analysis in order to determine its composition. Sampling may be implemented at any stage of the process with the aim to determine the composition of feedstock, intermediate products, finished products or transported substances such as waste water, coolant, etc. The size of the pipeline section for sampling typically depends on the type of fluid to be tested, working medium and sampling point location.

For example, for pressurized gases, small pipelines with valves for collecting a required number of samples are sufficient enough. An increase in the sampling line diameter will reduce the working medium fraction collected for analysis, but these sampling lines become more difficult to control. At the same time, small sampling lines are not very suitable for analyzing various suspensions in which solids can clog the flow path. Thus, the size of sampling lines for suspension analysis largely depends on the size of solids and medium properties. Similar conclusions are also applicable to viscous liquids.

While selecting the size of sampling lines, the following is usually taken into account:

• Properties of the fluid to be sampled;
• Working medium loss during sample collection;
• Safety requirements while sampling;
• Ease of operation;
• Sampling point location.

## Coolant Circulation

For pipelines, handling circulating coolants, high velocities are the preferred option. This is mainly explained by the fact that coolant in a cooling tower is exposed to sunlight, which creates the conditions for an algal-containing layer formation. Part of this algal-containing volume enters the circulating coolant. At low flow velocities, algae begins to grow in pipelines and, after a while, creates difficulties for cooling fluid to circulate or pass into a heat exchanger. In this case, high circulation velocities are recommended to avoid the formation of algal congestions in pipelines. Usually, intensively circulating coolants are used in the chemical industry. To feed various heat exchangers, pipelines with large diameters and long lengths are required.

## Tank Overflow

Tanks are equipped with overflow pipes for the following reasons:

• Avoid fluid loss (excess fluid enters another tank, rather than flows out of the original tank);
• Prevent undesired fluid leaks out of tanks;
• Maintain fluid level in tanks.

In all above mentioned cases, overflow pipes are designed for a maximum permissible fluid flow, entering the tank, regardless of the fluid flow rate at the outlet. Other pipe selection principles are similar to those, applicable to the gravity-flow fluid line selection, i.e., in accordance with the available vertical height between the start and end points of the overflow pipeline.

The highest point of an overflow pipe, which is also its start point, is located at the point of its connection to a tank (tank overflow branch) and is typically situated virtually atop, while the lowest end point can be located near a drain chute, near the ground level. However, the overflow line may end at higher elevations. In this case, the available differential head will be lower.

## Slurry Flow

In the mining industry, ore is usually mined in hard-to-reach areas, which generally have no railway or road connections. In these cases, a hydraulic transportation of media with solid particles is considered as the most acceptable option, in particular, if mining & processing plants are located far away. Slurry pipelines are used in various sectors of the industry to transport crushed solids together with fluid. These pipelines proved to be the most cost-effective option in comparison with other methods of solid media transportation in large amounts. In addition, a rather high safety, provided by avoiding several types of transportation, and environmental friendliness, may be considered as an advantage of this method of transportation.

Suspensions and mixtures of suspended solids in liquids are stored being intermittently stirred to maintain uniformity. Otherwise, a separation process takes place, in which suspended particles, dependent on their physical properties, float to the fluid surface or settle to the bottom. Stirring is provided by special equipment, such as tanks with agitators. In pipelines, stirring is achieved by maintaining turbulent flow conditions of the medium.

Reducing the flow velocity while transporting particles, suspended in a fluid, is not desirable, since a phase separation process may start in the flow. This may result in the pipeline plugging and a changed concentration of the solid transported in the stream. The turbulent flow regime facilitates an intensive mixing in the flow volume.

On the other side, an excessive pipeline size reduction also often leads to the pipeline plugging. Therefore, selecting an appropriate pipeline size is an important and crucial step, requiring preliminary analysis and calculations. Each case should be treated individually, because different slurries behave differently at different fluid velocities.

## Pipeline Repair

In the course of the pipeline operation, various leaks may occur which should be immediately eliminated to maintain the operability of the system. Several repair methods, e.g., the replacement of the whole or small leaking pipeline sections, or the application of patches onto existing pipes, can be used to repair main pipelines. However, before choosing one or another repair method, the cause of the leak origination shall be thoroughly studied. In some cases, a rerouting, rather than a simply repair, may be required to prevent the reoccurrence of the pipe damage.

The first stage of repair is to identify the location of a pipe section that requires intervention. Further, dependent on the type of the pipeline, a list of equipment and activities, required to eliminate the leakage, is defined, and, in case if the pipe section to be repaired is located on the territory of another owner, necessary documents and permits are collected. Since most pipes are located underground, it may be necessary to remove a part of the pipe. Then, a general condition of the pipeline coating is checked; after that, part of the coating is removed to allow repair operations directly with the pipe. After the repair, various testing activities can be carried out: ultrasonic testing, color flaw detection, magnetic-powder flaw detection, etc.

Although some repair activities require a complete shutdown of the pipeline, often it is sufficient only to temporarily interrupt the operation to isolate the section, to be repaired, or to arrange a bypass line. However, in most cases, a complete shutdown of the pipeline is required for repair. Pipeline sections can be isolated by plugs or shut-off valves. Further, required equipment is mounted and repair is carried out. Repair operations are performed on the damaged section, which should be completely drained and depressurized before commencement of the work. Once the repair is complete, plugs are opened and the pipeline integrity is restored.