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.
Why to provide thrust blocks?
Fluids traveling through a piping system under internal pressure exert a thrust force at all bends, tee junctions, and stop ends. The magnitude of these forces usually is so high that can easily weaken the joints and even can cause leakage or failure of the piping/pipeline system. With an increase in the piping size, these forces increase further. Installation of a thrust block partially absorbs that pressure thrust force and the remaining is transferred to the surrounding soil.
However, note that thrust blocks are rarely used for steel pipes as the thickness of welded pipes is normally sufficient to prevent joint separation. But the use of thrust blocks is quite common for Ductile iron, GRP/FRP, PE/HPDE, PVC piping systems.
Thrust block design
As already mentioned that a thrust block is a large concrete block. It has to be sized properly so that the thrust block is capable to withstand the pressure thrust force. Even though thrust blocks are specifically designed to absorb pressure thrust force, they should be designed to withstand the 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 load 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 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 to be modified. The following image (Fig. 1) provides some typical thrust force calculation formulas for ductile iron pipes (Reference: Ductile Iron Pipe Research Association)
- 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)
Thermal load can be directly taken from any stress analysis software like Caesar II, Start-Prof, Rohr-II 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.
Factors Affecting the Size of a Thrust Block
So, as specified above, there are four parameters required for sizing a thrust block. Those are:
- Maximum Internal pressure to calculate thrust force
- Pipe Size to calculate pipe cross-sectional area for calculating thrust force.
- Soil bearing load to find out the area required for the thrust block and
- Type of fitting (& Degree of angle in case of bends) to define the geometry of the thrust block
Fig. 2 below some images of typical thrust blocks.