Calculating Thrust-Block Size

Preventing pipe thrust from harming a penstock is critical to a successful hydro installation. Here is a sample thrust-block design calculation for a 45-degree bend in the penstock. This case assumes that a 4-inch pipe under 87 psi of pressure is buried in medium, clay-type soil.

Determine pipeline thrust. From the “Pipe Fitting Thrust” table, the factor is 12.4 lbs./psi for a 4-inch pipe and a 45° elbow.

Calculate the total thrust at the fitting. 12.4 lbs./psi x 87 psi working pressure = 1,078.8 lbs. of thrust

Determine soil load strength. From the “Soil Load Strength” table, the soil load strength is 2,000 lbs. per sq. ft.

Calculate the size of the thrust block. 1,078.8 lbs. ÷ 2,000 lbs. per sq. ft. = 0.54 sq. ft. of thrust-block surface area

The computation means that 0.54 square feet of blocking is necessary to hold the bend against the medium clay-type soil. Concrete is usually poured over the pipe fitting, filling the area against the undisturbed trench sides for thrust blocking.

For unburied pipe or pipe in soft soils with little resistance to movement, thrust blocking needs to rely on mass instead of the area resting against trench wall. Here is an example calculation of thrust block weight for a 6-inch pipe at 139 psi with 8 degrees of deflection:

Determine the thrust factor. From the “Side Thrust” table, 6-inch pipe has 61 lbs. of thrust per 100 psi per degree of deflection.

Calculate the amount of thrust. 0.61 lbs./psi x 8° x 139 psi = 678 lbs.

This means that there needs to be 678 pounds of mass (usually poured concrete) to keep this penstock from moving at the bend.