The correct answer is D. All of the above.
The pressure exerted by the sewage when running full from inside is called internal pressure. This pressure can cause tensile stress in the pipe material. Pressure sewers are designed to be safe in tension, so that they can withstand the pressure of the sewage without breaking.
Here is a more detailed explanation of each option:
- Option A: The pressure exerted by the sewage when running full from inside is called internal pressure. This pressure is caused by the weight of the sewage and the friction of the sewage against the walls of the pipe. The internal pressure can be calculated using the following equation:
$P = \rho g h$
where:
- $P$ is the internal pressure,
- $\rho$ is the density of the sewage,
- $g$ is the acceleration due to gravity, and
$h$ is the depth of the sewage in the pipe.
Option B: The internal pressure if any, causes tensile stress in the pipe material. The tensile stress is caused by the difference in pressure between the inside and outside of the pipe. The tensile stress can be calculated using the following equation:
$\sigma = \frac{P}{t}$
where:
- $\sigma$ is the tensile stress,
- $P$ is the internal pressure, and
$t$ is the thickness of the pipe wall.
Option C: Pressure sewers are designed to be safe in tension. This means that the pipe material is strong enough to withstand the tensile stress caused by the internal pressure. The pipe material is typically made of steel or reinforced concrete.
I hope this explanation is helpful. Please let me know if you have any other questions.