Sizing of public water supply systems upstream of the water service pipe is typically done by engineers, or designed in accordance with good engineering practice, using a detailed engineering design method. However, the water service pipe and building water distribution system are typically sized by the plumber. The BC Plumbing Code allows for alternative methods, which apply to both public and private water supplies, to be used in determining the size of each section of the water system.

BC Plumbing Code Sentence 2.6.3.1 (2) states “Potable water systems shall be designed, fabricated and installed in accordance with good engineering practice, such as that described in the ASHRAE Handbooks and ASPE Data Books.” Sizing of the water service pipe and the building water distribution system is covered below in Competency B-2 of the Level 3 content.

The municipal water supply system designer will typically use accepted engineering methods to size the municipal water supply systems. These systems contain transmission pipes and distribution piping (water mains). Several factors will go into the designer’s choice of pipe size and material, such as the peak-demand flow rate required, and the maximum intended velocity of the water in the system. For smaller communities – those with up to approximately 50,000 people – sizing of the water distribution system is often based on supply requirements for fire fighting. Pipe sizes are selected that will supply the quantity of water needed at a velocity that will not cause too much pressure loss. Municipal water supply system pipe water velocity is kept to a maximum of approximately five feet per second. To ensure this velocity is not exceeded, and to allow the network to accommodate a reasonable amount of future expansion, the water mains are commonly upsized one pipe size above calculated size.

Water pressure in transmission and distribution pipes is usually 80 to 120 psi (approximately 550 to 825 kPa), although water purveyors can choose to operate at higher pressures. As discussed in previous content, if higher system pressures are encountered as services are connected to building distribution pipes, pressure-reducing valves are installed.

Fluctuations will occur in municipal water supply systems based on elevation and during peak demand periods of the day. For example, water pressure at lower elevations in the municipality will be greater than at higher elevations in the municipality. Water pressure at peak demand periods (times of the day when the most water is being used) can be significantly lower than the pressure during low-demand periods. It is commonly accepted that the minimum pressure for water services should not be less than 35 psi during normal use, but may drop to 20 psi during periods of high demand.

As previously mentioned, municipal water supply systems are usually sized by engineers, using current fluid dynamics engineering practices. Typically, software is used to determine municipal transmission and distribution system pipe sizes. The designer would enter relevant data into software that would use several accepted formulas and equations to determine the pipe sizes. One of these formulas is the Hazen-Williams formula.

Hazen-Williams Formula

The Hazen–Williams formula is an empirical relationship which relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction. It is used in the design of water pipe systems such as fire sprinkler systems, water supply systems, and irrigation systems, but its use is limited to the flow of water in pipes larger than 2.0 inch and smaller than 6.0 feet in diameter, and for water flowing at ordinary temperatures of 40 to 75 degrees Fahrenheit (4 to 25 degrees Celsius) through pressurized pipes.

The Hazen-Williams formula is commonly used by municipal water supply system designers to calculate pipe friction loss. The calculation is done during the engineering stage to ensure the design pipe size does not exceed the allowable friction loss. The Hazen-Williams formula shown below can be used to calculate friction loss.

[latex]P=\dfrac{{4.52Q}^{1.85}}{{C}^{1.85}{d}^{4.87}}[/latex]

Where:

• P = friction loss, psi per linear foot
• Q = flow, gpm
• d = average pipe ID, inches
• C = constant 150

Note: The constant “C” is different for the types of pipe materials being used. The smoother the inside wall of a pipe is, the higher the C factor.

The Hazen-Williams formula is often the method used to create pipe pressure loss tables. These tables are used by engineers and plumbers to make pipe sizing decisions once other factors are known.

An example of a table showing pressure loss of water due to friction is shown below.

Pressure Loss due to Friction Loss in Copper Tube (psi per Linear Foot of Tube)

Now complete Self-Test 2 and check your answers.

Self-Test 2