Thrust Block

Thrust block, abbreviated as TB, is a structural component used in plumbing and pipeline systems to resist the axial forces or thrust generated by the fluid pressure within the pipes.  These forces can occur when the fluid inside the pipe changes direction, such as when there are bends, tees, or other fittings in the pipeline.  Thrust blocks are commonly used in underground water and sewer systems, as well as in other fluid transport systems.  The primary purpose of a thrust block is to prevent the pipes from moving or separating due to the internal pressure forces.

 

Thrust Block Advantages and Disadvantages

AdvantagesDisadvantages
  • Thrust blocks effectively resist the thrust forces generated at pipe bends or fittings, preventing movement or displacement of the pipeline.
  • Thrust blocks provide stability to the pipeline, ensuring that it remains securely in place despite changes in flow direction or pressure.
  • By resisting thrust forces, thrust blocks help prevent the separation of joints in the pipeline, reducing the risk of leaks or structural failures.
  • Thrust blocks are a relatively cost effective solution for managing thrust forces compared to more complex alternatives.  They are often simpler to design and install.
  • Thrust blocks, typically made of durable materials like concrete, contribute to the long term stability and reliability of the pipeline system.
  • Thrust blocks require adequate space around pipe fittings to be installed.  In confined spaces or areas with restricted access, this can be a limitation.
  • The construction of thrust blocks adds time to the overall installation process.  This can be a disadvantage in situations where rapid construction is crucial.
  • The design of thrust blocks needs to account for various factors such as soil conditions, pipe material, and pressure levels.  Achieving the right design may require engineering expertise.
  • While thrust blocks are durable, if maintenance is required, accessing and repairing them may be challenging, especially if they are located underground.
  • The use of concrete or other materials in thrust blocks may have environmental implications.  Additionally, the weight and composition of the materials may impact transportation and handling.

 

Here's How a Thrust Block Works
 
Counteracting Forces  -  When fluid flows through a pipe and encounters a change in direction, it exerts a force in the direction opposite to the change.  This force is known as thrust.  For example, if a pipe makes a 90-degree turn, there will be a thrust force trying to push the pipe straight ahead.
Thrust Block Construction  -  A thrust block is typically a mass of concrete or other strong and stable material that is placed around the pipe at points where thrust forces are expected to occur.  It is designed to distribute and absorb these forces.
Resistance to Movement  -  The thrust block's weight and structural strength resist the axial forces by preventing the pipe from moving or shifting.  It essentially anchors the pipe in place and ensures that it remains stable under pressure.
Preventing Damage  -  Without a thrust block, the thrust forces could potentially cause damage to the pipe joints, fittings, or even lead to pipe separation, which could result in leaks or a pipeline failure.

The size and design of a thrust block depend on various factors, including the pipe diameter, material, the angle of the bend, and the fluid pressure.  Engineers and plumbers calculate the necessary dimensions and reinforcement of thrust blocks to ensure they can effectively counteract the thrust forces and provide stability to the pipeline system.

 

Step 1  -  Resultant thrust

Resultant thrust Formulas

Resultant thrust is the overall thrust force acting on a pipeline at a change in direction or slope.  It is the combined effect of the horizontal and vertical components of the thrust force.

When a pipeline changes direction or slope, there is a tendency for the pipe to exert a force on the surrounding soil.

The resultant thrust (R) is the vector sum of these horizontal and vertical components and is calculated using the Pythagorean theorem.

\( R \;=\; \sqrt{ T_h^2 + T_v^2 }   \)     (Resultant Thrust)

\( T_a \;=\; p \; A_c \)     (Axial Thrust)

\( T_h \;=\; T_a \; tan \left(\theta\right) \)     (Horizontal Thrust)

\( T_v \;=\; T_a \; tan \left(m\right) \)     (Vertical Thrust)
Symbol English Metric
\( R \) = Resultant Thrust \( lbf \) \( N \)
\( T_h \) = Horizontal Thrust (Axial Thrust Caused by Changes in Slope) \( lbf \) \( N \)
\( T_v \) = Vertical Thrust  (Axial Thrust Caused by Changes in Slope) \( lbf \) \( N \)
\( T_a \) = Axial Thrust (Force Acting Along the Pipe due to Changes in Direction or Slope) \( lbf \) \( N \)
\( p \) = Pipeline Internal Pressure \(lbf \;/\; in^2\) \( Pa \) 
\( A_c \) = Pipeline Area Cross-section \( in^3 \) \( mm^3 \)
\( \theta \) = Bend Angle \( deg \) \( rad \)
\( m \) = Pipeline Slope \( ft \) \( m \)

 

Step 2  -  thrust Block Volume

thrust block Volume Formulas

Thrust block volume is the amount of concrete or material needed to construct a thrust block, which is a stabilizing structure used in the design of pipelines at changes in direction or slope. The purpose of a thrust block is to resist the forces generated by changes in the direction or slope of a pipeline and prevent movement or displacement.

It's important for the thrust block to extend below the frost line and provide sufficient resistance against the horizontal and vertical components of the thrust forces.

\( V_{tb} \;=\; M_b \;FS \;/\; S \; h \; FS_s \)     (Thrust Block Volume)

\( M_a \;=\; D \;/\; 2   \)     (Moment Arm)

\( M_b \;=\; R \; M_a \)     (Bending Moment)
Symbol English Metric
\( V_{tb} \) = Thrust Block Volume \( in^3 \) \( mm^3 \)
\( M_b \) = Bending Moment \(lbf \;/\; sec\) \(kg-m \;/\;s\)
\( FS \) = Factor of Safety \(dimensionless\) \(dimensionless\)
\( S \) = Concrete Allowable Stress \(lbf \;/\; in^2\) \(Pa\)
\( h \) = Depth of the Thrust Block (How Deep the Block Extends into the Ground)  \( ft \)  \( m \)
\( FS_s \) = Factor of Safety of Soil Bearing Capacity \(dimensionless\) \(dimensionless\)
\( M_a \) = Moment Arm \(lbf \;/\; sec\) \(kg-m \;/\; s\)  
\( D \) = OD of Pipe \( in \) \( mm \)
\( R \) = Resultant Thrust (see step 1) \( lbf \) \( N \)

 

Thrust Block Drawings

Isometric Drawing of a thrust block

Isometric Drawing of a thrust block

Isometric Drawing of a thrust block

 

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