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What is a Thrust Block? It’s Design, Construction, Types, Working, Differences with Anchor Block

Written by Anup Kumar Dey in Civil,Pipeline,Piping Design Basics,Piping Stress Analysis,Piping Stress Basics

A thrust block is a concrete pipe restraint that prevents the mainline from moving by transferring pipe loads (mainly due to pressure thrust) to a wider load-bearing surface. Usually, thrust blocks are provided for buried pipelines at fittings requiring branching or direction change. The thrust forces generated at the directional changes or tee junctions due to internal pressure thrust are taken care of by these thrust blocks, which prevents the separation of pipe joints on these pipe fitting locations. Thrust blocks are also known as thrust restraints. Thrust blocks are also sometimes referred to as anchor blocks.

What is a Thrust Block?

A thrust block is a concrete structure made to handle the forces from fluid moving through pipes. These forces generated by pressure thrust are directed along the axis of the pipe. The thrust force, if not mitigated/absorbed, can cause significant movement or stress within the pipe system. This could damage the pipe system. The thrust block helps by transferring these forces over a larger area, like the surrounding soil or a concrete base, to keep the pipe stable and in position.

Thrust blocks in piping systems are strategically placed at locations where the direction or diameter of the pipes changes, such as bends, tees, dead ends, and reducers. These points experience significant stress, making thrust blocks crucial for managing this stress.

Why are Thrust Blocks Required? | Purpose of Thrust Blocks

A thrust block’s main purpose is to handle the thrust force and stabilize the pipes, preventing unwanted movement. This helps maintain the system’s integrity and reliability.

Fluids traveling through a piping system under internal pressure exert a thrust force at all bends, tee junctions, reducers, and stop ends. The magnitude of these forces is usually so high that they can easily weaken the joints and even cause leakage or failure of the piping/pipeline system. With an increase in the piping size, these forces increase further. Installation of a thrust anchor block partially absorbs that pressure thrust force and the remaining is transferred to the surrounding soil.

However, note that anchor thrust blocks are rarely used for steel pipes, as the thickness of welded pipes is normally sufficient to prevent joint separation. However, thrust blocks are quite common in ductile iron, GRP/FRP, PE/HPDE, and PVC piping systems.

How Thrust Blocks Work | Thrust Block Working

Thrust blocks or anchor blocks are key parts of unrestrained piping systems. They manage the forces created by moving fluids in the pipes. These forces are strongest where the pipe changes direction, like at bends, or where the pipe ends, such as at caps or valves. Here’s how they work:

Functionality of Thrust Anchor Blocks

Counteracting Forces:

When fluid moves through a pipe, especially under high pressure, it exerts a force in the flow direction. At a bend or change in direction, it generates a reaction force in the opposite direction to fluid flow. At the elbow center, both these forces add up and can cause the pipe to move or shift. A thrust block absorbs these forces and keeps the pipe in place, preventing joint failures or leaks.

Force Distribution:

Thrust blocks spread the forces over a larger area. Instead of the pipe handling all the stress, which could cause damage, the thrust block distributes the force across a wider area, often into the surrounding soil or concrete.

Thrust Block Design

As already mentioned, a thrust block is a large concrete block. It has to be sized properly so that the thrust block is capable of withstanding the pressure thrust force. Even though thrust blocks are specifically designed to absorb pressure thrust force, they should be designed to withstand thermal forces as well. Sometimes the thermal load can be more than the pressure thrust load. So, it is suggested to find out both thermal and pressure loads and consider the maximum force value for the calculation of the thrust block design. So, to size a thrust block, the first requirement is the thrust force.

Thrust Force Calculation for Anchor Thrust Block Design

The raw formula for the calculation of thrust force is Pressure multiplied by the internal cross-sectional area of the pipe. However, depending on various pipe configurations, this formula requires modification. The following image (Fig. 1) provides some typical thrust force calculation formulas for ductile iron pipes (Reference: Ductile Iron Pipe Research Association)

Thrust force formula for various piping configurations
Fig. 1: Thrust force formula for various piping configurations
  • Thrust Force on an Elbow or Bend: To Calculate the design thrust force or resultant force for bends the following formula can be used. Thrust force, F = 2 P A sin (ϕ/ 2) Where: P = design pressure, A = cross-sectional area of the pipe, and ϕ = angle of the bend.
  • Thrust force on Plugs or Caps: The Thrust force in a plug or cap is equal to the design pressure (P) times the cross-sectional area (A) of the pipe. (Thrust force, F = P A).
  • Thrust force for Tee connections: The thrust force generated in a Tee connection is calculated as F=P Ab. Where P=internal design pressure and Ab= cross-sectional area of the branch pipe.
  • Thrust force calculation of Pipe reducers: The design thrust force for piping reducers/expanders is equal to the design pressure (P) times the difference of the cross-sectional areas of the large (A1) and small end (A2) sizes of the reducer. Hence, thrust restraint force, F = P (A1 − A2)

The thermal load can be directly taken from any stress analysis software like Caesar II, Start-Prof, Rohr-II, Caepipe, or Autopipe.

Once thrust force (Let’s assume the calculated value of thrust force is F) is known, we have to calculate the area required to withstand that thrust force. The area can be calculated by knowing the soil properties where the thrust restraint will be installed. The required soil parameter is the bearing pressure (Let’s assume it to be Pb) of the soil. So, the minimum area required (A) can be easily calculated by dividing the thrust force by soil bearing pressure. Hence, the minimum required area, A=F/Pb. This minimum area should be multiplied by a factor of safety (usually 1.5) to get the actual area.

Once the minimum required area is known, the thrust block geometry can be designed after knowing the type of pipe fitting where the thrust block will be installed. So, the term “thrust block calculation” means the calculation of thrust force and the required area for a thrust block that is sufficient to absorb the thrust force.

Thrust Block Construction

Concrete is the most popular element for constructing thrust blocks. Concrete material is strong and dense enough to arrest the generated thrust forces. The design of these blocks—both in size and shape—depends on several key factors:

  • Pipe Diameter and Material: The dimensions and the material of the pipe influence the thrust block’s requirements.
  • Fluid Pressure and Velocity: Higher pressure and faster-moving fluids necessitate larger or more robust thrust blocks.
  • Pipe Bend Angles: The configuration of pipe bends affects how forces are distributed, impacting thrust block design.
  • Soil Conditions: The type of soil surrounding the pipe must support the thrust block adequately to prevent shifting or compression.

For thrust blocks to perform their function effectively, they must interact properly with the surrounding environment.

  • Soil: The soil must support the thrust block without excessive compression or shifting. Often, soil testing is conducted to determine the correct design and sizing.
  • Concrete and Reinforcement: To enhance their durability, concrete thrust blocks are typically reinforced with steel bars or mesh, which helps them withstand the forces without cracking or deteriorating.

Factors Affecting the Size of a Thrust Block

So, as specified above, there are four parameters required for sizing a thrust block. Those are:

  1. Maximum Internal pressure to calculate thrust force
  2. Pipe Size to calculate pipe cross-sectional area for calculating thrust force.
  3. Soil bearing load to find out the area required for the thrust block and
  4. Type of fitting (& Degree of angle in case of bends) to define the geometry of the thrust block

Fig. 2 below are some images of typical thrust blocks.

Typical thrust block images
Fig. 2: Typical thrust block images

Key Components of a Thrust Block and Their Roles

Thrust blocks are essential elements in piping systems designed to manage and distribute the forces generated by fluid flow. Understanding the thrust block component details that make up a thrust block and their functions is crucial for ensuring its effectiveness and durability. Now we will describe the primary components and their roles in the construction of thrust blocks.

Concrete

Concrete serves as the primary material for thrust blocks due to its impressive strength and durability. It is capable of absorbing and redistributing large forces, which helps prevent pipe displacement or damage under pressure.

  • Mix Composition: The concrete used in thrust blocks typically includes a carefully measured blend of cement, water, aggregates, and sometimes additives. These elements are combined to enhance strength, reduce curing time, and improve overall durability.
  • Curing Process: Achieving the desired strength and longevity of the concrete requires proper curing. This process involves maintaining the right moisture levels and temperature until the concrete reaches its optimal strength.

Reinforcement

To bolster the structural integrity of concrete thrust blocks, reinforcement is often used. This reinforcement generally comes in the form of steel rebar or wire mesh.

  • Steel Rebar: Reinforcing bars are embedded within the concrete to resist tensile and shear stresses. The specific layout and sizing of rebar are determined based on expected loads and environmental conditions.
  • Wire Mesh: For smaller or less critical thrust blocks, wire mesh may be employed. It helps evenly distribute stresses and mitigate the risk of cracking.

Formwork

Formwork refers to the molds or frames used to shape the concrete as it sets. The design and material of the formwork are crucial for ensuring that the thrust block achieves its intended shape and effectiveness.

  • Materials: Formwork can be constructed from wood, metal, or plastic, depending on the project’s requirements and whether it will be reused.
  • Design: The formwork must be designed to handle the anticipated forces and accommodate the pipe and soil interface. It should also be easily removable to avoid damaging the set concrete.

Interface Material

Interface materials are placed between the pipe and the concrete to protect the pipe from potential damage due to direct contact with the concrete. They help distribute loads and accommodate minor movements and vibrations.

  • Rubber or Neoprene Pads: These materials are commonly used to cushion the pipe and reduce stress concentration at contact points.
  • Sand or Gravel: In some designs, a layer of compacted sand or gravel is used around the pipe before pouring concrete. This layer provides a softer bedding that helps absorb some of the forces.

Protective Coatings

To extend the lifespan of thrust blocks, particularly in harsh environments, protective coatings can be applied to the concrete.

  • Waterproofing Agents: These coatings prevent water ingress, which can cause freeze-thaw damage and corrosion of reinforcements.
  • Chemical Resistant Coatings: In industrial settings where chemical exposure is a concern, coatings resistant to chemicals protect both the concrete and reinforcement from deterioration.

Expansion Joints

In scenarios where temperature variations are significant or thrust blocks are large, expansion joints can be incorporated. These joints allow for the expansion and contraction of both the concrete and the pipe, preventing damage and undue stress.

  • Materials: Expansion joints are usually made from compressible materials such as fiberboard or rubber.
  • Placement: They are strategically positioned within the thrust block and around the piping to facilitate movement while maintaining structural integrity.

Types of Thrust Blocks

Based on specific applications and environmental conditions, Thrust blocks are designed from different materials. Depending on the constituent material, the most common types of thrust blocks are discussed below:

1. Concrete Thrust Blocks

Concrete thrust blocks are the most prevalent type due to their durability, strength, and versatility. Concrete’s ability to absorb and distribute large forces makes it well-suited for a wide range of applications.

  • Pre-cast Concrete Blocks: Manufactured in a controlled environment, pre-cast concrete blocks ensure consistent quality and are typically used in standard applications where forces are predictable and uniform. Their pre-made nature allows for quick installation and reliable performance.
  • Poured-in-Place Concrete Blocks: For more complex or customized applications, poured-in-place concrete blocks are ideal. This method allows for a tailored fit, adapting to specific site conditions and pipe configurations. It ensures that the thrust block integrates seamlessly with the surrounding environment.

2. Steel Thrust Blocks

Steel thrust blocks are less common but offer advantages in certain situations. They are particularly useful in applications where a high strength-to-weight ratio is critical, such as in elevated or suspended piping systems.

  • Steel Plate Blocks: These blocks are constructed from thick steel plates welded together to form a robust anchor for pipes. They are often employed in industrial settings, where high-pressure pipes require substantial support.
  • Steel Rods or Cable Systems: In some scenarios, steel rods or cables are used to anchor pipes to stable structures like bedrock or building foundations. This approach provides a secure attachment in environments where traditional concrete might not be practical.

3. Composite Material Thrust Blocks

Composite material thrust blocks are gaining popularity due to their advanced properties, which cater to specific needs such as weight reduction or enhanced environmental resistance.

  • Fiberglass: Renowned for its resistance to corrosion and lightweight nature, fiberglass thrust blocks are used in harsh environments like chemical processing facilities or marine applications. Their durability in aggressive conditions makes them a valuable choice.
  • Polymer Concrete: Combining the strength of traditional concrete with the advantages of plastics, polymer concrete is an excellent choice for areas with acidic soils or high moisture levels. It offers a balance of strength, light-weight, and resistance to environmental factors.

What is a thrust block on a pipeline?

Thrust blocks in a pipeline refer to the concrete blocks provided in buried pipelines for preventing movement and absorbing thrust forces. For large buried pipelines, the pressure thrust force becomes too large and is usually exerted on elbows and tees. Thrust clocks are therefore added in those regions to avoid the failure of piping components.

How does thrust block work?

Thrust blocks are sized to have a larger bearing area, which transfers the thrust force from pipe fittings to the soil and thereby safeguarding from joint failures.

Where are thrust blocks needed?

Conventionally, thrust blocks are needed in underground pipeline directional changes. So, in all the fittings like elbows, wyes, Tees, and Pipe Caps, wherever the pipeline changes its direction.

What is the Difference Between an Anchor Block and a Thrust Block?

The terms “anchor block” and “thrust block” are used synonymously in most of the cases as the major function is almost similar. Both thrust block and anchor block in pipelines serve as anchors and fix the pipeline at location. However, the actual difference arises in the use of proper application. The main aim of thrust blocks is to absorb and distribute pressure thrusts whereas the main aim of anchor blocks is to fix the pipeline in position. Even though at the end both serves the same purpose. Other differences between a thrust block and an anchor block are

  • Thrust blocks are installed only in changes in pipeline directions but anchor blocks can be installed in line with pipe fittings, in between flanges, tie-in connection with dissimilar materials, etc.
  • Thrust blocks are usually cast with a small part of the pipeline fitting inside it. However, the anchor block usually cast all around (360 Degrees) to encapsulate the full fitting or pipe part.

What is Viscosity? | Factors Affecting Viscosity | Newton’s law of Viscosity

Written by Anup Kumar Dey in Mechanical,Piping Design Basics

“What is viscosity?” is an often-asked term in fluid mechanics. Viscosity is a fluid property and is very important for studying fluid flow behavior. All kinds of fluids, whether they are in a liquid state or gaseous state, possess viscosity. Viscosity is a fundamental property of fluids that plays a crucial role in various fields, including physics, engineering, chemistry, and even biology. It describes a fluid’s resistance to deformation and flow, which makes it essential for understanding the behavior of liquids and gases in both natural and industrial processes. In this article, we will explore more details about the term “viscosity,” its definition, significance, units, equations, and values.

Definition of Viscosity

The term “viscosity” has its root in the Latin term “viscum,” which refers to a viscous glue originating from mistletoe berries. In fluid mechanics, viscosity is defined as the measure of a fluid’s resistance to fluid flow under an applied force. It can be intuitively understood as the “thickness” or “stickiness” of a fluid. For a fluid in motion, viscosity describes internal friction. So, a fluid having a large viscosity provides more internal friction to resist flow, whereas a fluid with a lower viscosity provides little friction. In general, liquids have more viscosity than gases.

The concept of viscosity will be more clear if we consider the following example. If we take water and honey in a pot and try to pour, we find that water is flowing more smoothly and quicker than honey. This is because honey is more viscous than water. So, honey provides more resistance to motion than water, and that is why water moves more freely than honey.

Viscosity of Water vs Honey
Fig. 1: Viscosity of Water vs Honey

Significance of Viscosity

The viscosity of a fluid is opposite to the fluidity that denotes how easily a fluid can flow. It is basically the internal friction between the molecules comprising the fluid. For fluid transportation and lubrication engineering, injection molding, spraying, and surface coating applications, viscosity plays a major role as it controls the flow of the liquid. Knowing the viscosity data is very important to predict fluid behavior. For example, if the tomato ketchup inside the tube does not have the correct viscosity, it may not flow from the tube or flow too much, all of certain.

Understanding viscosity is vital in a range of disciplines:

  • In engineering, it influences the design of pipelines and pumps.
  • In food science, it affects texture and mouthfeel.
  • In pharmaceuticals, it determines how drugs are delivered.

Symbol of Viscosity

Mathematically, viscosity can be defined as the ratio of viscous stress (shear stress) to the rate of change of deformation. The symbol of viscosity is µ (Greek letter mu). Hence,

Viscosity, µ=Shear Stress/strain rate=τ / (du/dy)
So, µ=τ / (du/dy)……(1)

The above symbol for viscosity µ is widely used. However, some physicists and chemists prefer to use η (the Greek letter eta) as the symbol of viscosity.

Units of Viscosity

From the above mathematical equation, we know that viscosity = stress/strain rate. The unit of stress in SI unit =N/m2. The unit of Strain rate = (m/s)/m.=1/s
Hence, the unit of viscosity = (N/m2)/(1/s)=N-s/m2, and the dimension of viscosity is (force X time/area).
The above viscosity discussed is also popular as dynamic viscosity or absolute viscosity.
Hence, the unit of viscosity or dynamic viscosity in the SI system is N-s/m2 or pascal-second.

Often, the unit of viscosity is denoted by Poise or Centipoise. In the CGS unit system, the unit of dynamic viscosity is “Poise” named after Jean Léonard Marie Poiseuille. The relation between Pascal-Second and poise is given below:

1 Pascal-Second= 10 Poise or 1Pa-s=10P

Types of Viscosity

There are two types of viscosity, relevant in different contexts.

Dynamic (Absolute) Viscosity

Dynamic viscosity, often denoted by the symbol μ (mu), is a measure of a fluid’s internal resistance to flow when an external force is applied. It is expressed in units of Pascal-seconds (Pa·s) or poise (P), where 1 P = 0.1 Pa·s.

Kinematic Viscosity

Kinematic viscosity, represented by the symbol ν (nu), measures the fluid’s resistance to flow under gravity. It is defined as the ratio of dynamic viscosity to fluid density and is usually expressed in square meters per second (m²/s) or centistokes (cSt).

What is Kinematic Viscosity?

Kinematic viscosity or momentum diffusivity is defined as the ratio of dynamic viscosity to fluid density. The symbol of kinematic viscosity is ν (Greek letter nu). So, mathematically the formula for kinematic viscosity is given by ν=µ/ρ.

Now we just learned that the unit of dynamic viscosity=N-s/m2. Unit of fluid density=Kg/m3. Hence, the unit of kinematic viscosity =(N-s/m2)/(Kg/m3)=(Kg-m/s2)*(s/m2)*(m3/Kg)=m2/s.
Accordingly, the dimension of kinematic viscosity is (length2/time). In fluid dynamics, working with kinematic viscosity is more convenient.

Measuring Viscosity

It is well known that measuring the viscosity of fluids is very important to understand the flow characteristics of those fluids. There are various types of instruments by which viscosity can be measured. Those viscosity-measuring devices are known as viscometers and rheometers. Common, widely used instruments for measuring viscosity are:

  • Capillary Viscometer
  • Falling Sphere Viscometer
  • Vibrating Viscometer
  • Rotational Viscometer
  • Microfluidic Rheometers
  • Zahn Cup
  • Fluorescence correlation spectroscopy
  • Acoustic rheometer

Factors Affecting Viscosity

There are various factors that affect the viscosity of a fluid. Those are:

  • Fluid Temperature: Usually the viscosity of liquids decreases with an increase in temperature. On the contrary, the viscosity of gases increases with an increase in temperature.
  • Flow Conditions: For laminar flow, the viscosity of liquid remains constant while for turbulent flow viscosity changes.
  • Pressure: With an increase in pressure, the viscosity of gases usually increases. Liquids being incompressible does not have much impact.
  • Multiphase flow: The viscosity of multiphase flow is affected by the volume of each phase.
  • Suspended Particles: Suspended materials increase viscosity.
  • Shear Rate: In non-Newtonian fluids, viscosity is dependent on shear rate. For example, shear-thinning fluids decrease in viscosity with increased shear rate, while shear-thickening fluids increase in viscosity under the same conditions.

Viscosity Measurement Methods

Measuring viscosity is essential for both theoretical and practical applications. Several methods exist, each with its advantages and disadvantages.

Capillary Viscometers

Capillary viscometers operate by allowing a fluid to flow through a narrow tube. The time taken for a certain volume of fluid to pass through the tube is measured, allowing for the calculation of viscosity. These are particularly effective for low-viscosity fluids.

Rotational Viscometers

Rotational viscometers measure the torque required to rotate a spindle immersed in the fluid. The viscosity is derived from the relationship between torque and rotational speed. This method is suitable for a wide range of viscosities and provides real-time data.

Falling Sphere Viscometers

This method involves dropping a sphere through the fluid and measuring the time it takes to fall a certain distance. The viscosity can be calculated based on the sphere’s terminal velocity, density of the sphere, and the fluid’s density.

Oscillatory Viscometers

Oscillatory viscometers use a vibrating element to measure the fluid’s resistance to oscillatory motion. This method is particularly useful for non-Newtonian fluids and provides information about both viscosity and elasticity.

Viscosity of Water

The viscosity of water at 200 C is 1 centipoise or 1 cP. As for liquids, the viscosity decreases with an increase in temperature, the same is true for water. The following table provides the dynamic viscosity of water with respect to various temperatures.

Temperature (°C)Viscosity (cP or mPa·s)
101.3059
201.0016
300.79722
500.54652
700.40355
900.31417
1000.2822
Table 1: Viscosity of Water with respect to Temperature

Kinematic Viscosity of water

The kinematic viscosity of water can easily be obtained by dividing the above dynamic viscosity values by the water density. Table 2 below provides the kinematic viscosity of water.

Temperature (°C)Kinematic Viscosity (m2/s X 10-6)
101.3059
201.004
300.801
500.553
700.413
900.326
1000.294
Table 2: Kinematic Viscosity of water with respect to temperature

Viscosity of Some Common Substances

The following table states the viscosity of some popular substances.

SubstanceTemperature (°C)Viscosity (mPa·s)
Benzene250.604
Air2518.5×10-3
Mercury251.526
Whole milk202.12
Dark beer202.53
Olive oil2656.2
Honey202,000–10,000
Ketchup255,000–20,000
Peanut butter –104–106
Pitch10–30 (variable)2.3×1011
Table 3: Viscosity of some common fluids

Newton’s law of Viscosity

The relationship between the shear stress and the shear rate of fluid under mechanical stress is established by Newton’s law of viscosity. For a given temperature and pressure, Newton’s viscosity law states that the shear stress between two adjacent layers in a fluid is proportional to the velocity gradients between those layers. In another way, it can be stated that the ratio of shear stress to shear rate in a fluid is a constant, and is defined as the coefficient of viscosity. Newtonian fluids obey Newton’s law of viscosity. Non-Newtonian fluids do not follow Newton’s law of viscosity and hence their viscosity varies and is dependent on the shear rate. Dynamic viscosity is the coefficient of viscosity as defined in Newton’s law of viscosity. Equation 1 mentioned above is basically a mathematical representation of Newton’s law of viscosity.

Practical Applications of Viscosity

The concept of Viscosity is used widely in science and technology. The following examples can easily substantiate the applications of viscosity:

  • The molecular weight of organic liquids is determined using the knowledge of viscosity.
  • In lubrication engineering viscosity data and its variation with temperature is an absolute necessity to decide suitable lubrication for specific equipment. For example, light machines use low viscous liquids whereas highly viscous oils are used in heavy machines.
  • For preparing various medicines like syrups viscosity data is required.
  • Cooking oils, fats, butter, etc are manufactured to provide a specific viscosity.
  • Gums, coolants, petrol as a cleaner, brake fluid, cosmetics, food products, etc all require viscosity data during production to work smoothly.
  • Blood circulation inside our body depends on the viscosity of blood.
  • In the cosmetic industry, the texture of lotions and creams is largely determined by their viscosity.

Other applications of viscosity can be described as follows:

Viscosity has vast implications across various industries, impacting everything from manufacturing to healthcare.

Industrial Applications

In manufacturing, understanding the viscosity of materials is crucial for processes like mixing, coating, and pumping. For example, in the paint industry, the viscosity of the paint affects application methods and drying times.

Biological Applications

In biology, the viscosity of blood is a vital parameter that influences circulation and oxygen transport. Abnormal viscosity can indicate medical conditions, making its measurement essential in clinical settings.

Environmental Science

In environmental science, viscosity plays a role in the movement of pollutants in water and soil. Understanding how viscosity affects dispersion and degradation can inform remediation efforts.

Food Science

In the food industry, viscosity impacts the texture and stability of products like sauces, dressings, and emulsions. Quality control measures often include viscosity testing to ensure product consistency.

Frequently Asked Questions-Viscosity

What is viscosity?

Viscosity is a measure of a fluid’s resistance to flow. It describes how thick or sticky a fluid is, influencing how easily it can be poured or spread.

What does high viscosity mean?

High viscosity usually means the liquid is thicker. The fluid with high viscosity offers greater flow resistance. Let’s describe the term high-viscosity with an example. Among, the two fluids, water, and honey, honey has a high viscosity as compared to water. So, the resistance force against the flow of water will be less as compared to honey.

What are the Types of Viscosity?

Engineering fluid mechanics provides two types of viscosity; Dynamic Viscosity and Kinematic Viscosity.

Is water viscous?

Viscosity is the property of any fluid. Water being a fluid, specifically liquid, is vicious.

How is viscosity important?

Fluid Viscosity is an important parameter for fluids. For food industries, viscosity provides the texture of food. In some situations, high viscosity is considered to be of superior quality as compared to thin liquids.

Which is the most viscous?

Pitch is the most viscous liquid. Glycerine and lubricating oil also have high viscosities.

How is viscosity measured?

Viscosity can be measured using several methods, including:

  • Capillary Viscometers: Measure the time it takes for fluid to flow through a narrow tube.
  • Rotational Viscometers: Measure the torque required to rotate a spindle in the fluid.
  • Falling Sphere Viscometers: Calculate viscosity based on the time a sphere takes to fall through the fluid.
  • Oscillatory Viscometers: Assess the fluid’s resistance to oscillatory motion.

What factors affect viscosity?

Viscosity is influenced by several factors:

  • Temperature: Generally, higher temperatures decrease the viscosity of liquids.
  • Pressure: In most liquids, higher pressure increases viscosity.
  • Fluid Composition: The chemical makeup and additives can alter viscosity.
  • Shear Rate: In non-Newtonian fluids, viscosity can change with the rate of shear.

How does viscosity relate to everyday life?

Viscosity impacts various daily activities, such as cooking (the thickness of sauces), automotive (engine oil performance), and cosmetics (the texture of creams and lotions).

Can viscosity change with time?

Yes, some fluids can experience changes in viscosity over time due to factors like temperature fluctuations, chemical reactions, or aging.

What is apparent viscosity?

Apparent viscosity is a term often used in non-Newtonian fluids that do not have a constant viscosity. This type of viscosity can vary depending on the shear rate, making it essential for characterizing complex fluids like polymers and biological fluids.

Does viscosity mean thickness?

Viscosity and thickness are related but not synonymous. Viscosity refers to a fluid’s resistance to flow, while thickness (or consistency) describes the physical property of how “thick” or “thin” a fluid appears. For example, honey is both thick and has a high viscosity, meaning it flows slowly. Water, on the other hand, is thin and has low viscosity, flowing easily. In summary, while viscosity can influence how thick a fluid feels, it specifically measures flow resistance rather than just physical thickness.

What is Pipeline Welding? | Methods of Pipeline Welding

Written by Anup Kumar Dey in Construction,Mechanical,Pipeline,Piping Design Basics,Piping Interface

Pipelines are used to transport gas, water, oils, and other liquids from one point to the other. And as all pipes are produced at a fixed length, there is a requirement of welding to join pipes and make kilometers of pipelines. So, pipeline welding is a very important activity in pipeline construction and is always a demanding profession. Pipe and Pipeline welders are required in construction industries, oil and gas fields, water industries, fabrication shops, nuclear energy industries, etc to lay new pipelines or repair an old pipe.

Pipeline welding is popular as girth welding which is performed along the circumference of the pipes to be connected. As compared to pipe welding in usual plants, pipeline welding poses various additional challenges. Pipeline welding should conform to the relevant ASME codes like B31.4, B31.8, ASME BPVC, etc.

Note that there is a specific difference between pipe welding and pipeline welding. Pipe welding normally refers to the welding of pipes inside chemical plants and oil refineries. Inside the plant a varying range of pipe sizes and materials require welding. On the contrary, pipeline welding concentrates on welding pipes of uniform size and material spread over hundreds of kilometers.

Widely used Pipeline Welding Processes

Usually, the following types of welding processes are widely used for pipeline welding:

  • Shielded Metal Arc Welding
  • Gas Metal Arc Welding
  • Flux-Cored Arc Welding
  • Submerged Arc Welding
  • Gas Tungsten Arc welding

Pipeline Welding by Shielded Metal Arc Welding (SMAW)

Shielded Metal Arc Welding of pipelines is also known as Stovepipe welding. In SMAW the pipelines are welded by melting the electrodes with the heat generated by an electric arc. The number of passes required usually varies with the pipe thickness, electrode size, welding position, and the current used for pipeline welding. The usual thumb rule for pipeline welding by SMAW is to consider one pass for each 1/8 inch(3.2 mm) of pipe thickness. Electrode diameters for SMAW pipeline welding normally vary from 1/8 inch to 3.16 inches (3.2 mm to 4.8 mm).

The main advantage of pipeline welding by SMAW is that the welding equipment is simple and portable. Also, no flux or shielding gases are required during welding. However, the productivity of this method is less due to lower travel speed.

Pipeline Welding by Gas Metal Arc welding (GMAW)

Pipeline welding by gas metal arc welding provides high productivity as compared to the SMAW method But this method required better control of welding variables for efficient quality work. High deposition efficiency (90 to 97%) with low fume generation are advantages of GMAW. This is generally performed using semi-automatic/ automatic equipment and the welding cost is increased.

Pipeline Welding by Flux-cored Arc Welding

Gas Shielded Flux-cored arc welding is performed using semi-automatic machines. Along with high productivity and excellent welding capabilities, pipeline welding by this method provides a broader operating range compared to GMAW. However, the wind usually causes disturbance for the shielding gas which in turn may cause porosity defects.

In the case of self-shielded flux-cored arc welding, external shielding gas is not required. However, this has lower deposition rates as compared to the gas-shielded arc welding process.

Welding of Pipelines by Submerged Arc Welding

In the semi-automatic Submerged arc welding process, the arc is not visible and it provides the highest deposition efficiencies as compared to all other pipeline welding methods. Such welding provides sound, defect-free welding surfaces but tracing is very difficult due to the invisible arc.

Pipeline Welding by Gas Tungsten Arc welding of Tungsten Inert Gas welding

TIG welding is used for welding critical joints requiring precision welds as this method produces high-quality pipeline welding utilizing tungsten electrodes. But, welding of pipelines by gas tungsten arc welding has lower deposition rates and higher equipment costs.

Pipeline Welding

Steps for Pipeline Welding

Usually, the following steps are performed for welding a pipeline:

  • Joint Preparation: Usually, guidelines provided in API 1104 are used for joint preparation.
  • Pipe End Cleaning: Undesirable moisture, paint, primer, rust, oil, varnish, and other coatings must be removed by cleaning the pipeline ends. Note that, proper cleanliness prevents defect generation leading to rejected welds and costly repair.
  • Follow the Welding Process specific to the pipes and pipeline materials (Electrode selection, Preheat requirement, etc). All these details are available in the Project welding specifications.
  • Welding the Root Pass
  • Welding the Hot Passes
  • Welding Fill and Cap Passes
  • Repairs in cases of unacceptable defects arise.

Pipeline Welding Process Selection

Pipeline welding is the backbone of pipeline construction. Hence, the pipeline welding process selection must be done, considering various factors as mentioned below:

  • Pipeline material
  • Pipe Diameter and Wall thickness
  • Welding location
  • Weldment properties
  • Welding Direction (Uphill or downhill)
  • Welding quality
  • Economic Consideration
  • HSE Consideration

Pipeline Welding Machine

Appropriate equipment for welding pipelines is a must for maintaining weld quality and production efficiency. Various parameters must be checked prior to selecting a proper pipeline welding machine such as

  • Output power
  • Duty cycle
  • Portability
  • Versatility
  • Safety

Pipeline Welding Jobs

Pipeline welding jobs are very demanding with long working hours. A daily review of the pipeline welders’ work is carried out. As the commissioning of the pipelines depends on the pipeline weld completion, pipeline welders always remain under high work pressure. At the same time, the job is unsafe, there are huge chances of accidents happening at any moment. However, all these come with good salaries for pipeline welders. Depending on the experience and skills of pipeline welders they make in the range of $2,000 to $11,000 per month in the USA.

To get into the pipeline welding jobs, a person must have

  • High school diploma
  • Welding school certificate or completed apprenticeship
  • Welding certificate (AWS/CSWIP)
  • Prior welding experience (pipeline welding preferred)
  • Knowledge of AWS, ANSI, ASME, and API standards
  • Experience and working knowledge of using all welding tools and equipment,
  • Excellent technical and communication skills
  • Ability to interpret schematics

In pipeline welding jobs, a pipeline welder typically is responsible for

  • Reading and understanding blueprints, and schematics
  • Determining required tools, materials, and welding methods
  • Assembling pipe components and systems
  • Preparing the materials
  • Installing and repairing these systems
  • Inspecting and maintaining supplies, materials, tools, and equipment
  • Ensuring produced welds are up to specifications
  • Following safety procedures and guidelines

Few more welding articles for you.

Welding Galvanized Steel
Overview of Pipeline Welding
Welding Positions: Pipe Welding Positions
Welding Defects: Defects in Welding: Types of Welding Defects
Welding Inspector: CSWIP and AWS-CWI
General requirements for Field Welding
Underwater Welding & Inspection Overview
Methods for Welding Stainless Steel

Video Courses in Welding

To learn more about welding the following video courses you can refer to:

Introduction to FRP Pipes | Their Properties, Specification, Codes, Joining, Supporting, and Applications

Written by Anup Kumar Dey in Engineering Materials,Piping Design Basics

The full form of FRP is fiberglass-reinforced plastic which is a composite material consisting of a polymer matrix reinforced with fibers. So, an FRP pipe is a pipe manufactured of FRP material by contact molding or filament winding method. Various types of resins like thermosetting polyester, epoxy, phenolic resin, etc are used to get specific FRP pipe properties in the final product. The most widely used reinforcement is the glass fiber “E-glass”. As a corrosion-resistant alternative to metallic piping, the FRP piping system has found worldwide application. By selecting FRP as the pipe material, the need for internal lining, external coating, and cathodic protection can easily be eliminated. FRP piping system is available in a wide range of sizes starting from 1 inch to 144 inches.

Applications of FRP Pipes

Due to its high durability, corrosion resistance, and moderate strength, the use of FRP pipes is increasing day by day. FRP piping systems are used in various industries like:

  • Potable Water and desalination industries
  • Chemical, Petrochemical, Oil & Gas industries.
  • Ducting and Vent piping
  • Irrigation and Sanitary services
  • Water distribution and transmission
  • Slurry piping
  • Power plants, etc.

Properties of FRP Piping

The main properties that make FRP Pipes an ideal selection for various industries worldwide are

  • Excellent corrosion resistance
  • Excellent strength-to-weight ratio. Note that, the strength-to-weight ratio of FRP pipes is higher than steel or other metallic pipes.
  • Lightweight which makes it easy for handling and transport.
  • Dimensional stability
  • Non-toxicity
  • Low coefficient of friction (>25% better than steel) that ensures excellent flow characteristics.
  • Good abrasion resistance
  • Suitable for both aboveground and buried piping
  • Resistance to biological attacks like bacteria
  • Non-conductive to electricity and
  • Low maintenance cost

Typical mechanical properties of the FRP piping system are provided in the table below:

Mechanical Properties of FRP PipeTypical Range
Tensile Strength14 to 550 Mpa
Tensile Modulus3.5 to 34.5 Gpa
Flexural Strength28-480 Mpa
Flexural Modulus6.9 to 34.5 Gpa
Poisson’s Ratio0.3
Thermal Co-efficient14 to 54 mm/mm/0C
Specific gravity1.2 to 2.3
Compressive Strength69-275 Mpa
Table 1: Range of Mechanical Properties for FRP Pipe

Joining of FRP Pipes

As the FRP pipe lengths are limited by transportation and handling, they are required to be joined. Also, various FRP Pipe fittings need to be joined as per the requirement. The joining system of the FRP pipe should be such that it does not leak for the intended service condition at the operating pressure. Depending on the specific joint configuration and design conditions, the FRP pipe joints may be restrained or unrestrained.

Unrestrained FRP Pipe Joints

Joints that can withstand the internal pressure but can not withstand the longitudinal tensile loads are known as Unrestrained FRP Pipe joints. Examples of such joints are Coupling joints, bell and spigot joints, mechanical coupling joints with elastomeric seals, flanged joints, butt joints with laminated overlay, etc.

Restrained FRP Pipe joints

Such pipe joints are capable of withstanding both internal pressure and longitudinal tensile loads. For these joints, supplemental restraining elements are added to restrict the longitudinal loads. Threaded joints, bell, and spigot joints with laminated overlay or adhesive bonds are examples of Restrained FRP pipe joints.

Note that FRP pipe joint tightness must be ensured following ASTM D4161.

FRP Pipe Fittings

Various FRP pipe fittings are available for proper piping layout needs. Common FRP pipe fittings are

Codes and Standards for FRP Pipes

Frequently used Codes and Standards that govern the FRP piping details are

  • ISO 7370
  • ANSI/AWWA C950
  • AWWA M45
  • ISO 14692
  • AWWA C590
  • ISO 10467
  • BS 5480
  • ISO 10639
  • ASTM D2996, ASTM D2997, ASTM D5421, ASTM D4024, ASTM D5685

Specification of FRP Pipes

While ordering FRP pipes the following data need to be provided to the vendor:

  • Pipe Diameter
  • Design and Operating temperature of the service fluid.
  • Design, operating, Surge, vacuum, and test pressures.
  • Live loads in case of buried piping.
  • Maximum/minimum buried depth and trench Widths.
  • Details of Soil properties and trench preparation.

Supporting of FRP Piping System

FRP piping systems must be supported properly to avoid excessive sagging. Maximum acceptable sagging is the lower of 12.5 mm or 0.5% of span length. The manufacturer’s guidelines with respect to the supporting shall be followed. Usually, clamped supports with an elastomeric pad are used for support.

FRP Pipe Supporting
Fig. 1: FRP Pipe Supporting

Drawbacks of FRP Piping

The main drawbacks of FRP piping systems are

  • FRP pipe is not recommended for carrying fluid with temperatures more than 1000 C
  • Slight degradation from UV rays is found to occur which can be reduced by using pigments, dyes, UV stabilizers, fillers, etc in the resin system.

FRP vs GRP: Difference between FRP and GRP

FRP stands for fiber-reinforced plastic while GRP stands for Glass reinforced plastic. So, from the name, it is clear that there is a change in the reinforcing fiber. But, both FRP and GRP are normally used to indicate the same plastic piping products.

FRP vs Steel: Differences between FRP and Steel

As FRP Pipes have superior corrosion resistance capabilities and over the long term it is economic, Steel pipes are replaced by FRP pipes. So, in this section, it will be great to find the differences between FRP and Steel.

  • Steel pipes are isotropic while FRP pipes are anisotropic and the properties change with respect to direction.
  • FRP pipes are more flexible than Steel pipes due to the lower modulus of elasticity.
  • FRP piping systems are designed considering a higher factor of safety than steel piping. The usual factor of safety in the design of FRP pipes varies in the range of 5 to 10.

Other major differences between FRP and Steel pipes are provided in Table 2 below:

PropertyFRPCSRemarks
Density1850 kg/cu.m7800 kg/cu.mLoads on support are less in the case of FRP pipe as compared to Steel. Handling and transportation of FRP are easier than Steel pipes.
Co-efficient of thermal expansion27 X 10-6 mm/mm 0C11 X 10-6 mm/mm 0CExpansion is almost 2.5 times of Carbon Steel Pipe. So more thermal growth in the case of the FRP Piping system.
* This value may change from vendor to vendor
ModulusAxial= 12000 N/sq.mmElastic= 211365 N/sq.mmConsiderable difference in the strength of FRP & CS. Anchor loads are less in FRP Pipes as compared to steel pipes.
Shear= 11400 N/sq.mm
Tensile Strength80-135 MPa456 MPaMechanical Strength is higher for Steel material as compared to FRP.
Yield Strength70-135 MPa227 MPaThe yield strength of FRP is lesser than that of Steel.
Allowable Stress4,000 PSI to 20,000 PSI20,000 PSIThe strength of the GRE varies drastically and hence proper vendor data is a must.
Corrosion resistanceSuperiorInferiorThe corrosion resistance of carbon steel is much lower than that of FRP pipes.
JointsThreaded or gluedWeldedFRP joints are to be checked for higher axial loads and pressure
Table 2: FRP vs Steel

FRP vs HDPE: Differences between FRP and HDPE Pipes

The main differences between FRP and HDPE pipes are listed in Table 3 below:

FRP PipeHDPE Pipe
FRP is Orthotropic composite materialHDPE is isotropic material.
The cost of FRP pipe is very highThe cost of HDPE pipes is considerably lower than FRP Pipes.
Lower thermal expansion coefficientThe thermal expansion coefficients of HDPE pipes are extensively higher as compared to FRP pipes.
FRP pipes have a comparatively higher temperature range than HDPE PipesLowe temperature range
Fabrication time is comparatively longerQuicker fabrication.
The strength and Elastic modulus for FRP pipes are higher than HDPELower strength and elastic modulus.
Easy installation at the siteCostly complex equipment is required for installation.
Highly skilled professionals are required for site work of FRP piping systemsHDPE pipe works can be done by semi-skilled operators.
Much lighter in weight due to lower all thickness even though the density of FRP is normally higher than HDPEHeavier due to higher wall thickness.
A fire-retardant version of FRP pipes can be made.HDPE pipes are highly flammable
Table 3: FRP vs HDPE

The initial cost of FRP pipes is normally higher than the metallic piping systems. But when comparing the total cost over the complete service life FRP Pipes come as a winner due to their long service life.

Stress Analysis of FRP Piping System

In piping stress analysis guides or flexibility specifications, FRP lines are considered critical irrespective of their sizes. So, a proper stress analysis must be performed to investigate the stresses, loads, displacements, supports, etc to decide if the FRP piping system will work smoothly throughout its design life. I have developed an online course explaining step-by-step procedures for FRP piping stress analysis. You can check it out here.

How is Steel Made? Steel Production Process

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics,Piping Interface

Steel is widely used in every known industry starting from construction to manufacturing. The worldwide requirement for steel is growing every year. Steel is strong, inexpensive and so versatile that the modern world can not be thought of without Steel. Studies show that after cement, Steel is the second most mass-produced commodity in the world. Steel is environmentally friendly, recyclable, sustainable, and produced in various shapes and forms. In this article, we will learn about the process of steelmaking i.e “How is steel made”?

What is Steel?

Steel is a ferrous alloy material composed of iron and carbon (0.002 to 2.14%) as the primary elements. Many other elements like manganese, silicon, chromium, nickel, phosphorus, oxygen, molybdenum, boron, cobalt, etc can be present or added purposefully to get some required properties. They are used in infrastructure, ships, weapons, railways, buildings, tools, cars, machines, etc. Steel is available in two crystalline forms; body-centered cubic and face-centered cubic. Depending on the alloying constituents the density of steel ranges between 7,750 and 8,050 kg/m3. Steel is ductile, hard, durable, possesses good strength, and can withstand shocks.

How is Steel Made? Steel Manufacturing Process

To produce steel first the iron ore is heated and melted in a furnace. The impurities are then removed from the molten iron. Carbon and other required alloying elements are added to make steel. Modern-day steelmaking is highly improved and specialized. There are two processes that are widely popular for modern-day steelmaking. They are:

  • Blast Furnace
  • Electric Arc Furnace.

Blast Furnace Steel Production Process

A blast furnace is a huge steel roughly cylindrical shell lined with heat-resistant brick. Steel production in a blast furnace is a continuous process. Hence, once steel manufacturing started, the production continues for years until the refractory lining is damaged or demand dropped. There are two processes by which steel is produced in a blast furnace

  • The Bessemer Process for Steel production and
  • The Open Hearth Process of Steelmaking

Steel manufacturing Principle of the Bessemer Process

The main raw materials for Steel production in a blast furnace are Iron ore, limestone, coke, and some scrap steel. These materials are charged into the blast furnace from the top and converted into molten pig iron. Next, a current of air is blown through the molten pig iron containing carbon, silicon, and manganese. These elements and some iron are oxidized. The carbon forms gas and escapes, But the oxidized silicon, manganese, and iron form a mixed ferrous-manganese silicate slag. This process is known as the Acid Bessemer Process.

The above oxidation process, especially the oxidation of silicon, produces a large amount of heat. This heat maintains the temperature and fluidity of the molten pig iron. If the content of silicon in the pig iron is very high, the heat evolved may be excessive, and cold steel scraps are added to the converter to lower the temperature.

Once the oxidation is completed, the produced mild steel contains a considerable amount of oxygen and maybe hydrogen which is rectified by spiegeleisen or ferromanganese additions. This is usually done when the metal is poured from the converter into the casting ladle.

All the reactions taking place in the converter are reflected in the flame which issues from the mouth of the converter. During the early part of the blow, when the manganese and silicon are being oxidized, the flame is short and of low luminosity, gradually increasing in length and luminosity as the blow proceeds and the carbon is eliminated. But the flame drops when decarburization is completed, and blowing is discontinued. Any prolongation of blowing after the drop of the flame results in the oxidation of iron and this is indicated by the number of brown fumes in the flame.

In the Basic Bessemer Process, the blowing is prolonged after the flame drops, and this period is known as the after-blow. The phosphorus is removed during the after-blow. The pig iron used in the basic process contains a low silicon and manganese content but is high in phosphorus. It is the oxidation of phosphorus that produces the necessary heat.

The removal of phosphorus necessitates a basic lined converter and a basic slag which is formed from lime added to the converter just before the molten pig iron is poured into it.

The procedure adopted in either the acid or basic process is similar. The molten pig iron is poured into the converter, which is in a horizontal position turned down so that the metal is away from the tuyeres. These tuyeres are fixed in the bottom of the converter and they supply the current of air. The blast is then put on, and the converter is rotated into a vertical position. Immediately the flame drops, the vessel is turned down again and the blown metal is poured into a casting ladle. At this stage, the finishing additions are usually made.

Hence, the process, whether acid or basic, consists of the following three stages:

  1. A hot metal supply.
  2. A suitable vessel for blowing the air through the molten pig iron.
  3. A casting ladle and ingot mold with an auxiliary plant, for teeming the steel into ingot form.

Steel Making Principle by the Open Hearth Process

In the open-hearth process of steel production the slag is the refining medium and serves two purposes:

  • It is the oxidizing medium to remove the impurities.
  • It works as a receptacle to retain the oxidation products in a stable form.

The raw materials for the acid process are of a siliceous nature, but low in sulfur and phosphorus. The maintenance of an acid slag prevents the elimination of these elements. The pig iron and scrap are charged, and when melted the slag is rendered fluid and oxidized by iron-ore additions. The carbon, silicon, and manganese present in the charge are oxidized by these ore additions. The carbon escapes as carbon monoxide gas, while the silica and manganese oxide passes into the slag, which is virtually a ferrous-manganous silicate.

In the basic process, a wider range of raw materials may be employed. The addition of Limestone forms a basic slag that affects the removal of sulfur and phosphorus. As far as possible, Siliceous materials should be avoided because an excessive lime is required to neutralize the silica formed, and if the slag becomes too siliceous the removal of sulfur and phosphorus is hindered and the basic lining of the furnace is attacked.

In many cases, cold or solid pig iron is charged into the furnace and the process is referred to as “the cold pig and scrap” process. At composite works, where blast-furnace metal is available, molten pig iron is employed. This is referred to as the “hot metal” process. Whether the process is acid or basic, and whether hot or cold pig iron is used, the same type of furnace is employed.

Steel Production by Electric Arc Furnace

In the production of steel, the direct arc resistance electric furnace is employed. In this type of electric arc furnace, the heat is generated by the arcs formed between the electrodes and the charge. The resistance is provided by the charge to the passage of the current. There are two methods of operating the electric arc furnace:

  • The basic process.
  • The acid process.

The basic process is carried out in a basic lined furnace. The production of high-grade tool steel, alloy, and special steels in ingot form is ideal in the basic process. The basic process is also used for the production of steel castings. On the contrary, the acid process operating with a silica-lined hearth is used chiefly for the production of steel castings.

Basic Process: In the basic electric arc steelmaking process a wide range of steel scrap is employed. The normal steel manufacturing procedure is to melt the scrap, which is recarburized with coke, anthracite, or broken electrodes, to give sufficient carbon at the “melt out” to assure a good carbon boil. As a rule, no pig iron is employed.

Acid Process: Acid-lined electric arc furnaces for steel making have a very limited application and are chiefly employed in steel foundries. Acid furnaces require selected scrap and raw materials low in sulfur and phosphorus. In many foundries, acid furnaces are used for the remelting of the scrap arising in the foundry, whilst basic-lined furnaces are used for converting the miscellaneous brought scrap into low sulfur and phosphorus steel.

The following flowchart by steel.org provides a nice animation of the steel manufacturing processes:

Steel Production flowchart
Steel Production flowchart by steel.org

What is HDPE Pipe? | Types and Design of HDPE Pipes

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Pipeline,Piping Design Basics,Piping Interface

HDPE pipe is a flexible plastic pipe made of thermoplastic high-density polyethylene widely used for low-temperature fluid and gas transfer. In recent times, HDPE pipes got their extensive uses for carrying potable water, hazardous wastes, various gases, slurry, firewater, stormwater, etc. The strong molecular bond of HDPE pipe materials helps it to use for high-pressure pipelines. Polyethylene pipes have a long and distinguished service history for gas, oil, mining, water, and other industries. Due to its low weight and high corrosion resistance, the HDPE pipe industry is growing tremendously. In the year 1953, Karl Ziegler and Erhard Holzkamp discovered high-density polyethylene (HDPE). HDPE pipes can work satisfactorily in a wide temperature range of -2200 F to +1800 F. However, the use of HDPE Pipes is not suggested when the fluid temperature exceeds 1220 F (500 C).

HDPE pipes are made by the polymerization of ethylene, a by-product of oil. Various additives (stabilizers, fillers, plasticizers, softeners, lubricants, colorants, flame retardants, blowing agents, crosslinking agents, ultraviolet degradable additives, etc.) are added to produce the final HDPE pipe and components. HDPE pipe lengths are made by heating the HDPE resin. It is then extruded through a die, which determines the diameter of the pipeline. The Pipe wall thickness is determined by a combination of the die size, the speed of the screw, and the speed of the haul-off tractor. Usually, 3-5% carbon black is added to HDPE to make it UV resistant, which turns HDPE pipes black in color. Other color variants are available but usually not used frequently. Colored or striped HDPE pipe is usually 90-95% black material, where a colored stripe is provided on 5% of the outside surface.

Properties of HDPE Pipes

The physical and mechanical properties of a typical HDPE Pipe are listed below:

  • Governing Codes: ISO 1183, ISO 4427, AS/ NZS 4130, BS EN 12201, EN 12056, AWWA C901, AWWA C906, etc
  • Available size range: DN 16 to 1600 (However, other customized sizes can be manufactured)
  • The density of HDPE Pipe: 930-970 kg/m3
  • Typical Mechanical Properties of HDPE Pipe are given in Table 1 below. However, these values vary from one manufacturer to another. So, must be consulted with the manufacturer.
Mechanical PropertiesHDPE Properties
Yield Strength18-26 Mpa
Elongation10%
Elastic Modulus900 Mpa
Hardness61 Rockwell (Shore)
Poisson’s Ratio0.45
Linear Coefficient of thermal expansion0.17 mm/m-0C
Brittleness temperature<-700C
Table 1: Mechanical Properties of HDPE Pipe

Types of HDPE Pipes

Depending on maximum pressure withstanding capability, HDPE Pipes are classified into several pressure grades known as PN grades. They are

  • PN 2.5 – max pressure 2.5 bar
  • PN 4 – max pressure 4 bar
  • PN 6 – max pressure 6 bar
  • PN 10 – max pressure 10 bar
  • PN 16 – max pressure 16 bar
  • PN 20 – max pressure 20 bar
  • PN 25 – max pressure 25 bar

Again, based on the types of materials used HDPE pipes are grouped into the following types:

  • PE 63 – medium pressure piping systems
  • PE 80 – gas pipe for natural gas distribution network with pressure rate up to 4 bars or drinking water pipe with pressure rate up to 16 bar – sewers, outfall pipes, industrial pipes
  • PE 100 – high demands piping applications

The numbers following PE indicate the minimum required strength in Bar at 200 C for 50 years life span corresponding to ISO-4427. The rated working pressure of HDPE pipes is determined at 200 C. For higher temperatures, a derating factor (Table 2 Below) is used to determine the maximum pressure to which the HDPE Pipe can be subjected.

Fluid Temperature (0C)Temperature Derating factor
0-201
>20-250.9
>25-300.8
>30-350.7
>35-400.6
>40-450.5
>45-500.4
Table 2: Temperature Derating Factor for HDPE Pipes

Length of HDPE Pipes

As HDPE Pipe manufacturing is a continuous process, pipes of any length can be manufactured. However, due to transportation requirements and ease of handling at the site, the HDPE pipe length is limited and normally produced in standard lengths of 6, 10, 12, 15, 24, and 30 meters. HDPE Pipe lengths of 50 and 100 meters can be produced on request.

Joining HDPE Pipes

HDPE Pipes and fittings are joined to each other by Welding or by mechanical fittings. There are two types of welding methods used to join HDPE pipes. They are

  • Electrofusion Welding: Electrofusion is a simple and quick method to permanently join individual pipes and HDPE fittings or prefabricated sections in tight places. This method is suitable for on-site joints.
  • Butt Welding: Butt-welding homogenously joins HDPE pipes and fittings into prefabricated sections using heat and pressure without using additional connection fittings.

Various types of mechanical joints can be used for joining HDPE pipes to HDPE or dissimilar pipes. They are

  • Plug-in joint
  • Snap joint
  • Screw-thread joint
  • Flanged joint
  • Contraction sleeve
  • Sleeve coupling
  • Metal coupling
  • Rubber collar joint
HDPE Pipe Joining
Fig. 1: HDPE Pipe Joining Methods

Fittings for HDPE Pipes

A complete range of HDPE Pipe fittings is available for any purpose. Typical HDPE pipe fittings include:

  • Bends
  • Elbows
  • Reducers
  • Branches
  • End caps
  • Electrofusion couplers
  • Mechanical connection fittings
  • Traps
  • Sanitary fittings
  • Spare parts
HDPE Pipe Fittings
Fig. 2: Typical HDPE Pipe Fittings

Design Criteria for Aboveground HDPE Pipe

Factors that can influence the behavior and thus, the design of above-ground HDPE piping systems are

  • Temperature: For pressure systems, the use of HDPE pipes is usually limited to -400C to 500 C. For non-pressure services, higher temperatures can be used. Consult with the manufacturer for getting information about their product. Note that, the coefficient of linear thermal expansion for HDPE pipes is approximately 10 times higher than that of metal pipes. So, the potential of thermal expansion due to temperature fluctuations is very high and must be considered in the design. A typical Thumb rule is HDPE pipes will expand or contract by 0.2mm per meter per °C increase or decrease in temperature.
  • Internal Pressure
  • Chemical exposure
  • Potential mechanical impact or loading: Extra protection must be given if the HDPE pipe is installed in a region of high traffic or mechanical abuse.
  • Ultraviolet radiation: To safeguard against ultraviolet radiation effects, black HDPE pipes containing a minimum of 2% carbon black are selected.

Pressure Consideration for HDPE Pipes

A number of variables determine the hydrostatic pressure capacity of HDPE pipes. Those are

  • Standard Dimension Ratio or SDR is defined as the ratio between the outside HDPE Pipe diameter and the wall thickness.
  • The hydrostatic design stress of the HDPE material being used (PE63, PE 80, PE100) for HDPE Pipe Manufacturing.
  • The maximum and minimum operating temperatures.
  • The duration and variability of the stress applied by the internal hydrostatic pressure during hydrostatic testing.
  • The chemical resistance of the HDPE pipe for the service chemical (the standard pressure rating is based on a pipe carrying water).

Usually, HDPE pipe can withstand short-term hydrostatic water pressures that are considerably higher than the pressure rating, or class. But the duty of the HDPE pipe should always be determined following the pipe’s long-term strength at 20°C to ensure a design life of at least 50 years.

Barlow’s Formula is also applicable for HDPE piping that is related to the internal pressure, pipe outer diameter, wall thickness, and circumferential hoop stress and is given by

p = 2 x t x σ /d or alternatively t = p x d/(2.σ + p)

Here:

  • p = internal pressure (MPa)
  • t = minimum wall thickness (mm)
  • d = mean outside diameter (mm)
  • σ = circumferential hoop stress (MPa)

The design hoop stresses for HDPE pipe materials are considered as follows as given in ISO 4427:

  • PE 63: 5 MPa
  • PE 80: 6.3 MPa
  • PE100: 8 MPa

Advantages of HDPE Pipes

HDPE pipes provide various advantages with respect to other piping materials like

  • Cost-effective. Hence, easily affordable
  • High flow rate due to the smooth inner surface
  • Wide temperature range
  • Non-leaching
  • Easy transportation due to low-weight
  • UV-resistant
  • Amazing Durability (More than 50 years)
  • Resistant to most chemical solvents
  • Stiff material
  • Environment friendly
  • Lower maintenance cost
  • High Quality
  • Reduced installation cost
  • Due to its flexible shape, simple to use on slopes.
  • Simple joining methods.
  • Not affected by underground movements (Seismic Resistant)
  • Bad conductor of electricity.
  • low-friction
  • Ability to dampen water-shock
  • Sun-resistant
  • Non-stick surface.
  • Leak-free due to its seamless nature.
  • Prefabricated HDPE pipe sections can be easily installed

Disadvantages of HDPE Pipes

The main disadvantage of HDPE pipes are

  • Poor weathering resistance
  • Highly Flammable
  • Sensitive to stress cracking
  • Difficult to bond
  • Poor High-Temperature capability
  • High thermal expansion

HDPE Pipe Frequently Asked Questions

What is the full meaning of HDPE pipe?

The full form of HDPE is High-Density Polyethylene. This hydrocarbon polymer is produced from petroleum by a catalytic process. Famous for its tensile strength in a similar product category, High-Density Poly Ethylene is a kind of thermoplastic.

Is HDPE the same as PVC?

Both HDPE and PVC are considered plastics. However, they are not the same. HDPE is a thermoplastic made from petroleum and PVC is a durable vinyl polymer. While comparing HDPE and PVC, HDPE due to its flexibility, high strength-to-density ratio, no corrosion, and chemical stability is a strong choice for low-pressure and low-temperature applications.

What is HDPE piping used for?

HDPE or High-density polyethylene pipe is widely used to carry potable water, slurries, wastewater, chemicals, hazardous wastes, and compressed gases. They have been used in various industries like gas, oil, mining, water, and other industries.

How long will HDPE pipe last?

Properly designed HDPE piping systems provide a long-lasting performance as compared to other piping systems. As per the estimates of the Plastic Pipe Institute, they are supposed to last 50 to 100 years.

Is HDPE pipe safe for drinking water?

Yes, HDPE is totally safe for carrying drinking water. This is one of the main reasons that in the water industry and desalination plants, high-density polyethylene pipes are widely used. The application of HDPE pipes for potable water applications is certified by NSF. Various disinfectants like chlorine and chloramine are approved for use in HDPE pipes.

Which is stronger PVC or HDPE?

PVC is stronger and stiffer as compared to HDPE.