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What Is a Thrust Block?

Andy Hill
Andy Hill

Most commonly found in drainage applications, a thrust block is a concrete pipe restraint. The block is utilized to prevent pipe joints separating under the force of carried fluids traversing a bend or tee junction. Thrust block dimensions need to be calculated based on the type of fluid being carried, the pipe diameter, and existing ground conditions. The pressure that the fluid will be under within the pipe also needs to be taken into consideration.

Fluids traveling through a pipe exert a pressure, or thrust force, at bends and at tee junctions or stop-ends. These forces can be sufficient to weaken the joints between the adjacent pipes. This effect can be exaggerated when the surrounding ground is weak or friable or where a high groundwater table exists. By installing a large block of concrete at the bend or junction, the thrust energy of the fluid is partially absorbed and then redirected along the pipe.

Man with a drill
Man with a drill

When considering the size of the required thrust block, the angle of the bend is a key factor. As an example, in the case of ductile iron pipes, an 11.25-degree bend will require a smaller thrust block than a 45-degree bend. This is due to the forces exerted on the bend being reduced as the internal angle increases.

Using concrete to form a thrust block is commonly only found where socket-spigot pipe jointing techniques have been used. If the installed pipe has flanged joints — where the flanges of the pipes are bolted together — the bolts themselves will act as the thrust restraint. Similarly, welded steel pipe generally requires no additional thrust restraint depending on the thickness of the pipe material and the specified weld type.

An alternative form of restraint when using socket-spigot pipes exists in the form of an anchor joint, or anchor gasket. Socket-spigot pipe joints traditionally use rubber gaskets between the pipes to prevent dirt ingress and leakage. An anchor gasket is similar in appearance to a regular pipe gasket but features small steel teeth, which lock the pipe into position once jointed with an adjacent pipe. These steel teeth will then act as the thrust restraint under pressure.

The term thrust block is also used in the shipbuilding industry. In this application, the thrust block — alternatively known as a thrust box or thrust bearing — is a restraint system in propeller mechanisms. This bearing resists the thrust of the propeller shaft and transfers the energy into the hull of the ship. By utilizing this system, shipbuilders can optimize the force produced by the propeller to move the ship, minimizing lost energy through propeller shaft movement.

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