Sizing PVC pipe for water supply is a critical engineering process that ensures every fixture in a building receives adequate water pressure and flow, even when multiple fixtures are in use simultaneously. Undersized pipes lead to frustrating low flow (a phenomenon often called "water starvation"), while grossly oversized pipes unnecessarily increase material costs and can lead to poor drainage of sediment. Proper sizing is based on calculating the total demand load of the system and ensuring the pipe diameter is large enough to handle peak flow while keeping friction losses within acceptable limits.

1. Determining Fixture Units (Demand Load)

The first step in sizing any PVC pipe for water supply system is calculating the total estimated water demand. This is done using a standardized measure called Fixture Units (F.U.). Every fixture—from a toilet to a shower—is assigned an F.U. value based on its expected flow rate and duration of use. For example, a bathroom sink might be assigned 1.5 F.U., and a toilet 3 F.U. The F.U. values for all fixtures served by a specific pipe section are summed up to determine the total demand load on that section. This provides a quantifiable value for the system's peak water requirement.

2. Converting Fixture Units to Gallons Per Minute (GPM)

Once the total F.U. for a pipe section is calculated, this number must be converted into a flow rate, typically measured in Gallons Per Minute (GPM). This conversion is achieved using specialized plumbing charts (often based on the Hunter's Curve). These charts account for the statistical probability that not all fixtures will be running at exactly the same moment. The resulting GPM is the required peak flow that the PVC pipe for water supply must be capable of delivering.

3. Calculating Allowable Pressure Loss (Head Loss)

Friction loss, or "head loss," occurs when water rubs against the inside walls of the PVC pipe for water supply and fittings. Every system has a limited amount of available pressure (measured in PSI). A proper design must ensure that the pressure loss from the municipal connection to the highest or furthest fixture remains within acceptable limits (usually about 15 to 20 PSI total loss). Engineers use formulas like Hazen-Williams to calculate the friction loss for different pipe diameters at the required GPM.

4. Selecting the Correct Pipe Diameter

The final step is selecting the smallest pipe diameter (e.g., 3/4-inch, 1-inch, etc.) that can deliver the required GPM while meeting two key criteria:

a) Velocity Limit: The water velocity must be kept below a maximum threshold (usually 5 to 8 feet per second) to prevent erosion, minimize noise, and avoid water hammer.

b) Friction Limit: The friction loss over the pipe run must not exceed the available pressure budget.

By iterating through different PVC pipe sizes and checking them against the GPM demand and allowable pressure loss, the most efficient and effective pipe size is determined for each segment of the building's water supply system.

Conclusion

Correctly sizing PVC pipe for water supply is a careful balancing act that requires calculating the total Fixture Unit load and optimizing pipe diameter to minimize friction and control water velocity. Supported by long-standing manufacturing experience, LESSO applies strict quality standards across LESSO's plastic piping solutions. Our industrial and commercial pipeline systems are developed with application-specific designs to ensure consistent durability and reliable performance in sectors such as mining, chemical processing, and other demanding environments.