Bigger is NOT Always Better - Pipe Clogging and Duct Choking

Clogging and choking have always been a consistent problem with piping and duct systems whether it be hydraulic or pneumatic. The common layman's solution to this is to use a bigger pipe or duct. This intuitive solution, however, is not always effective. In contrary, using larger diameter might actually be the source of pipe clogging and duct choking problems.

The Math

There are two major factors that influence the pipe/duct size selection process - flowrate or discharge and pressure. Flowrate and pressure are included in two independently different formulas which may be hard for some to correlate.

Q = AV ... (Equation 1)

Where:
Q = Flow Rate
A = Cross-Sectional Area
V = Velocity

P = F / A  ... (Equation 2)

Where:
P = Pressure
F = Force
A = Area

By analyzing these two equations, the science behind the relationship between clogging and pipe size can be understood. As you can see, Cross-Sectional Area is common to both Equations 1 & 2. By substitution, we can say that:

A = Q/ V
A = F / P

But,

A = A

Then,
Q / V = F / P

or

P = V F/Q  ... (Equation 3)

In practice, how is clogging dealt with? Pressure is applied to force the obstruction out. To prevent clogging, sufficient pressure should be specified at the design stage. The conclusions about Pressure that can be drawn out of the equations above are as follows:

1. Pressure is inversely proportional to area (from Equation 2). This means that as the cross-sectional area increases, the pressure decreases.

2. Pressure is directly proportional to design velocity (from Equation 3). This means that as the velocity increases, the pressure increases.

3. Velocity is inversely proportional to cross-sectional area (from Equation 1). This means that as the cross-sectional area increases, velocity increases.

One can see right away that by considering pressure’s major role in declogging, using bigger pipes or duct is not the solution because using larger diameter increases the cross-sectional area and increasing cross-sectional area decreases pressure and velocity. At the design stage, the designer must specify a sufficient design velocity and pressure that will not allow clogging. It can be done by identifying the mass, density and mass flow of the possible clogging materials and calculating the necessary force and pressure to keep such materials moving across the system. The viscosity of the fluid and the roughness coefficient of the pipe or duct must always be considered to arrive at an efficient design.

Self-cleaning action in waste pipes

Waste pipes that use gravity to allow water to move are the most prone to clogging because these merely rely on the slope of the pipe. The ideal slope to allow self-cleaning action is 2% (2 units vertical and 100 units horizontal). If the slope is too steep, the water would flow faster than the solid waste so it will be left out and accumulate at the bottom of the pipe and would eventually clog the pipe. If the slope is too small, the velocity will not be enough to carry the solid waste which will result in sedimentation, accumulation, and clogging. For gravity drainage systems, the design slope which dictates the design velocity is critical.