Plumbing System Design

1. Water Quality and Quantity

Water supplied to buildings must be of suitable quality for consumption and use, with sufficient quantity for operational needs.

2. Pipe Materials

Plumbing pipe materials are standardized for water distribution. Building plumbing typically uses Galvanized Steel Pipe, Black Steel Pipe, Copper Pipe, Cast Iron Pipe, or Plastic Pipe (PE, PVC).

3. Building Plumbing System

As a principle, pipes should be installed in straight lines to achieve the shortest distance (for cost efficiency).

Building water distribution systems can be divided into 2 types based on design:

1. Upfeed System

The upfeed system distributes water upward through the building height, suitable for buildings up to 10 floors and less than 10,000 square meters.

2. Downfeed System

The downfeed system distributes water downward from the top floor, suitable for tall buildings and large buildings.

Water distribution in high-rise buildings can be divided into multiple zones for easier maintenance and pressure management.

In downfeed systems with rooftop water tanks, plumbing fixtures and water-using equipment on upper floors may have insufficient water pressure and volume. Therefore, the upper floor water system is separated and a “pressure tank” is added to increase pressure and water volume for adequate usage.

Water hammer occurs when flowing water is suddenly blocked (by closing valves or faucets). When water cannot continue flowing, it creates impact and backflow, causing pipe vibration, noise, and accelerated pipe damage.

Water hammer pressure can be calculated using the formula:

P = 14.8V

Where:

P = Pressure increase from water hammer (unit: bar)

V = Water velocity (unit: meters/second)

We can see that internal pipe pressure increases 4-8 times normal operating pressure.

We can prevent water hammer by installing devices that “absorb pressure wave energy” (water hammer arrestor) in the pipe system. The simplest device is called an Air Chamber, which is a water pipe extended beyond the fixture connection.

Each type of plumbing fixture requires specific pressure and flow rates as recommended by manufacturers. Minimum pressure and flow rates for common fixtures can be found in the table below.

Due to varying pipe pressure, installing flow control devices for fixtures provides these benefits:

1. Can limit necessary flow rates for each type of fixture
2. Enables more precise pipe design
3. Saves energy in the water system

Multiply Water Factor by flow rate from Hunter’s Curve

We can calculate water flow rate using the basic equation:

Q = AV

Where:

Q = Flow rate (unit: gpm)

A = Pipe cross-sectional area

V = Water velocity in pipe

Due to pipe wall roughness causing turbulent flow, resulting in pipe friction and pressure drop, we can calculate pressure drop using the Hazen-Williams formula:

h_f = (4.727/D^4.87) L(Q/C)^1.85

Where:

h_f = Pressure drop (unit: ft_w)

D = Pipe diameter (unit: ft)

L = Pipe length (unit: ft)

Q = Flow rate (unit: ft³/s)

C = Surface Coefficient of pipe

C value depends on pipe wall roughness. In practice, using Q = AV and Hazen-Williams formula is not practical, so the formula is converted to graphs for direct reading of pipe size, water velocity, flow rate and pressure drop.

In design, water velocity in pipes should be between 1.2-2.4 meters/second or 4-8 feet/second, and should not exceed 3 meters/second to prevent noise and reduce pipe and valve erosion.The table below provides data to help design branch pipes

In water pipe systems, water flows through straight pipes, fittings, and various valves. In design, we use equivalent length, meaning the length of straight pipe of the same size as the fitting or valve that would give the same pressure drop as the device. The table below shows equivalent lengths of fittings and devices in meters.

A two-story building has the following fixtures. What size main water pipe is needed for adequate supply?

• 2 Water closets with flush tanks
• 3 Lavatories
• 1 Kitchen sink
• 1 15mm faucet

We can find fixture quantities from the FU table:

• 2 Water closets = 2 x 3 = 6 FU
• 3 Lavatories = 3 x 1 = 3 FU
• 1 Kitchen sink = 1 x 2 = 2 FU

Total = 11 FU

From Hunter’s Curve table, 11 FU equals 8.5 gpm

From water factor table, we get 1.0

If the faucet is used simultaneously with other fixtures, and faucet flow rate is 5 gpm

Water flow rate in pipe = 8.5 + 5 = 13.5 gpm or (13.5 x 3.785)/60 = 0.85 lps (liters/second)

From the pipe sizing graph, we find:

Should use 20 mm pipe with water velocity 2.8 meters/second and pressure drop about 80-100 meters, but since pressure drop is too high, we’ll change to 25 mm pipe (larger) with water velocity about 1.7 meters/second and pressure drop 25 meters