5.2 Sizing Water Distribution Piping: Pressure, Friction & Velocity

Allowed sizing methods (SPS 382.40 (7))

Water supply piping may be sized by:

1. The empirical method in SPS 382.40 (7) and Tables 382.40-4 to 382.40-15.
2. An approved alternate standard under SPS 381.20 (2). A code note names the IAPMO Peak Water Demand Calculator as an example.
3. An analysis by a Wisconsin master plumber, registered architect, professional engineer or permitted plumbing designer. That is an "engineered system," and under Table 382.20-1 only the department may review it.

The exam tests the empirical method.

Step 1: Pressure budget (SPS 382.40 (7) (a))

Calculations must account for:

1. The load in wsfu or gpm.
2. The minimum pressure available from the main or pressure tank.
3. Elevation loss from the main or tank to the building control valve, and from there to the controlling fixture. Use 0.433 psi per foot of rise.
4. Losses through water heaters, treatment devices, meters and backflow preventers (from manufacturer data).
5. The minimum flow pressure at the controlling fixture.
6. Friction losses through piping and fittings. Equivalent length may be taken as developed length x 1.5.

Minimum flow pressures (SPS 382.40 (7) (d) 1.)

Step 2: Friction per 100 feet and Step 3: the table

$$\text{Friction per 100 ft} = \frac{P_{\text{main}} - P_{\text{elevation}} - P_{\text{devices}} - P_{\text{flow}}}{\text{developed length} \times 1.5} \times 100$$

Tables 382.40-4 to 382.40-15 cover Type K, L and M copper, galvanized steel, PEX, CPVC, PEX-AL-PEX and PVC (PVC tables apply only to water services and private water mains). Each row is a friction loss in psi per 100 feet, and each pipe size lists maximum gpm, maximum wsfu (FM) for predominantly flushometer systems and maximum wsfu (FT) for predominantly flush-tank systems. "NP" marks loads whose velocity would exceed 8 ft/s. The table note says to round the calculated friction loss to the next higher number shown.

Excerpt from Table 382.40-5, Type L copper, at 10 psi per 100 feet:

Worked example

A house has a 55 psig minimum main pressure. The controlling fixture, a pressure-balance shower, is 20 feet above the main, and the developed length to it is 120 feet. The meter and softener lose a combined 10 psi, and the load is the 19.5 wsfu flush-tank house from section 5.1.

1. Elevation loss: 20 x 0.433 = 8.7 psi.
2. Flow pressure for a pressure-balance valve: 20 psig.
3. Available for friction: 55 - 8.7 - 10 - 20 = 16.3 psi.
4. Equivalent length: 120 x 1.5 = 180 ft.
5. Friction per 100 ft: 16.3 / 1.8 = 9.1, rounded to the next higher row, 10.
6. Type L at 10: 3/4 inch carries 13.0 wsfu FT, which is too small for 19.5; 1 inch carries 30.0 wsfu FT. The building supply is 1 inch.

Branches are sized the same way with their own loads. A branch serving one bathroom group (4.0 wsfu FT) fits in 3/4 inch Type L at this friction rate.

Pressure limits and booster systems (SPS 382.40 (7) (d) 2. to 4.)

• If the main or private supply exceeds 80 psig, install a pressure-reducing valve with a strainer (separate or built in). It goes upstream of all fixtures and downstream of the utility meter. One is not required on piping that feeds a booster pump directly.
• If pressure or volume is inadequate, install a hydropneumatic booster system or booster pump. Each pump needs an automatic low-pressure cutoff on the inlet set to stop the pump below 10 psig, plus gauges on the inlet and outlet.
• A pressure tank whose bottom is more than 20 feet above any outlet it serves needs a vacuum relief valve of at least 1/2 inch.

Velocity and minimum sizes

• Velocity may not exceed 8 ft/s, except in a multipurpose piping system (SPS 382.40 (7) (e)).
• Water services and private water mains: at least 3/4 inch (7) (b).
• 1/2-inch distribution piping serving 2 or more fixtures may carry no more than 2 wsfu. A 1/2-inch line serving a shower that is not individually pressure-balanced or thermostatic may serve no other fixture (7) (f).
• Fixture supplies (7) (g): 1/2 inch minimum; syphon-jet urinals 3/4 inch; flushometer water closets 1 inch; emergency eyewash and showers per manufacturer. A supply carrying 0.5 wsfu or less with a developed length of 25 feet or less may be 1/4 inch.
• Fixture supply connectors (7) (h): no more than 24 inches upstream of a fixture or faucet body, and no more than 10 feet upstream of a plumbing appliance. A connector downstream of a water cooler, treatment device or heater serving one outlet may be 10 feet, as may one upstream of a treatment device serving no more than 2 outlets.

Exam strategy

Write the pressure budget as a column of subtractions before touching a table. Most errors are skipped steps: forgetting the 1.5 multiplier, using 0.5 psi per foot instead of 0.433, or using 8 psig for a mixing valve that needs 20.