A thrust block is used in drainage applications to prevent pipe joints from separating under the force of fluid passing through a bend or tee. The size of the block is determined by the type of fluid, pipe diameter, and ground conditions. The angle of the bend is also a key factor. Concrete is commonly used for spigot pipe joining techniques, while flanged joints and welded steel pipes generally do not require an additional thrust constraint. An alternative form of retention is an anchor joint or gasket. The term thrust block is also used in the shipbuilding industry to resist thrust from the propeller shaft and transfer energy into the ship’s hull.
Most commonly found in drainage applications, a thrust block is a constraint for concrete pipes. The block is used to prevent pipe joints from separating under the force of the fluid being carried passing through a bend or tee. The size of the thrust block should be calculated based on the type of fluid being carried, the diameter of the pipe and existing ground conditions. The pressure at which the fluid will be inside the pipe must also be taken into consideration.
Fluids traveling through a pipe exert pressure, or pushing force, at bends and tees or stops. These forces may be sufficient to weaken joints between adjacent pipes. This effect can be exaggerated when the surrounding soil is weak or friable or where a high water table exists. By installing a large concrete block at the bend or joint, the fluid’s buoyancy energy is partially absorbed and then redirected down the pipe.
When considering the size of thrust block required, the angle of the bend is a key factor. For example, for ductile iron pipe, an 11.25 degree bend will require a smaller thrust block than a 45 degree bend. This is due to the fact that the forces exerted on the bend decrease as the inside angle increases.
The use of concrete to form a thrust block is commonly found only where spigot pipe joining techniques have been used. If the installed pipe has flanged joints, where the pipe flanges are bolted together, the bolts themselves will act as a thrust constraint. Likewise, welded steel pipe generally does not require an additional thrust constraint depending on the thickness of the pipe material and the specified weld type.
There is an alternative form of retention when using socket pipes in the form of an anchor joint or an anchor gasket. Socket pipe joints traditionally use rubber seals between pipes to prevent dirt ingress and leaks. An anchor seal is similar in appearance to a regular pipe seal, but has small steel teeth, which lock the pipe in place once it is joined to an adjacent pipe. These steel teeth will then act as the thrust bond under pressure.
The term thrust block is also used in the shipbuilding industry. In this application, the thrust block, also known as the thrust case or thrust bearing, is a restraint in the propeller mechanisms. This bearing resists thrust from the propeller shaft and transfers the energy into the ship’s hull. Using this system, shipbuilders can optimize the force produced by the propeller to move the ship, while minimizing the energy lost through propeller shaft movement.
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