Building Water Supply System: WATER SUPPLY SYSTEM DESIGN EXAMPLE

Demand Load Example

1. The first step in determining the demand load is to list the plumbing fixtures required on the project. The following information is gathered:

4-story apartment building 3 in (75 mm) service main Street main pressure: 50 psi (345 kPa) Fixtures per floor:

2 flush valve water closets (toilets) 2 tubs with showerheads 2 lavatories (bathroom sinks) 2 kitchen sinks 4 hose bibbs on the first floor (3/8 in [10 mm] supply, general use)

It should be noted at this point that the apartment building being sized for a water system has a repetitive floor plan for each floor. This allows the entire apartment to be serviced from a single water pipe (riser) going up the building (Fgrs 19 through 22).

FGR.19 Location of water risers in apartment building design example are required on both sides of the hallway.

FGR.20 Water riser location in apartment building design example.

FGR.21 Elevation of water riser in apartment building design example.

FGR.22 Water zoning in apartment building design example is comprised of upfeed and downfeed zones.

Upfeed System Design Example

Water supply pipes must be of sufficient size to provide adequate pressure to all fixtures in the system at a reasonable cost.

Selection of economical sizes for the piping and the meter is based on the total demand for water (see earlier Demand Load Example); this process must take into account the available water pressure (street main pressure), the pressure loss from static head (pressure loss to raise the water up the pipe), the pressure loss from friction in piping and fittings (?P_friction), and the pressure required to operate the fixture requiring the most pressure on the top floor.

The main pressure in the street must be adequate to supply:

required fixture flow pressure _ static head (? P_static)

_ friction in pipe and meter (? P_friction) The accumulated information to this point (from above) is as follows:

1. The first step in sizing the main supply is to determine the pressure losses. The pressure loss from static head is computed by multiplying 0.434 psi per ft (9.8 kPa per m) times the vertical distance of the riser from the service main into the building to the top branch water line (fgr.21):

2. The residual pressure required at the most remote fixture on the top floor (e.g., the fixture requiring the most pressure) must be determined. The design should also leave some excess pressure so that more than one fixture can be operated properly at the same time:

Water closet: 15 psi (from tbl.1)

In metric (SI) units:

Water closet: 100 kPa (from tbl.1)

3. The pressure loss from friction in the main and riser pipes, the fittings, and the meter must be found next. Add the static head and the pressure required for the most re mote fixture, then subtract the total from the available street main pressure to find the pressure left, which is the maximum amount that can be lost to friction:

Street main pressure: 50.00 psi Static head _ fixture

Static head loss, ? P_static _ 9.8 kPa _ 12.2 m _ 120 kPa In metric (SI) units:

Static head loss, ? P_static _ 0.434 psi _ 40 ft _ 17.36 psi

Residual pressure available for the water closet is acceptable.

4. Sizing the piping using the equal friction design method (see Method 4: Equal Friction Design Method) is a matter of trial and error. The process involves first selecting a pipe size for the building main, which runs from the water system to the riser(s), and then determining the friction loss for the pipe used from friction loss charts (e.g., Fgrs 6 and 7). The chart used will depend on the type of pipe used.

From the friction loss chart (fgr.6 for Type L copper), the first pipe size servicing 59 gpm is 4 in. Because this is much larger than the street main, keep moving to the right to the 2 1/2 in pipe. Where the horizontal line from 59 gpm touches the line representing 2 1/2 in, draw a vertical line to the bottom (or top), and the friction loss reads

Prev: DETERMINING WATER HEATER SIZE
Next: PUMPS

top of page | Home