At what velocity is terminal velocity achieved in a drainage stack?

Terminal velocity in a drainage stack is a crucial concept because it relates to the efficiency of drainage systems. When fluid flows through a pipe, it accelerates due to gravity until the forces acting on it (including friction and air resistance) become equal, resulting in a constant velocity—this is known as terminal velocity.

The range of 10 - 15 feet per second (fps) is considered a practical velocity for achieving effective drainage without excessive turbulence that can lead to issues such as splashing and blockages. This velocity ensures the fluid can carry away solids effectively while minimizing potential noise and other related issues that can arise at higher velocities.

At velocities below this range, the flow may not be sufficient to remove waste effectively, leading to stagnant areas and debris accumulation. Conversely, velocities above this range might lead to air entrainment which disrupts the flow dynamics in the drainage system and can create problems in venting and pressure recovery.

This understanding is pivotal for plumbing design; selecting the right velocities helps ensure that drainage systems function optimally and are compliant with relevant plumbing codes and standards.