1-15. Pressure in the main usually ranges from 45 to 60 pounds per square
inch (psi). If the pressure is over 60 psi, a pressure-reducing valve must be
placed in the water service line at its entry to the building. The size of the
water service pipeline, the rate of use, the length of the line, and the outlet
height in the system control the pressure available at the outlet. If the water
pressure is less than 15 psi, use a tank and a pump or other means to provide
pressure. If the water pressure is over 80 psi, use an approved pressure
Calculations for Sizing Pipe
1-16. The minimum practical size for a water service line is 3/4 inch. This
size should be used even when calculations indicate a smaller one.
Calculations for factoring loss of pressure in complex systems are beyond the
range of this manual. For simple systems, use approximate figures to find the
D-2, Appendix D
, give capacities and psi for
galvanized-steel/iron pipe, copper tubing, and plastic pipe. Use these tables
combined with the maximum fixture demand and simultaneous use factor to
determine pipe sizes.
Maximum Fixture Demand.
The maximum fixture demand in
gallons per minute (GPM) is the total amount of water needed to supply all
the fixtures at the same time. Estimate the maximum fixture demand by
counting the number and types of fixtures in the plumbing system.
gives the maximum fixture demand for different fixtures.
- Water Closet 45GPM
- Lavatory 7.5GPM
- Shower 15GPM
- Urinal 39.5GPM
- Laundry Tub 15GPM
- Floor Drain 7.5GPM
1-18. For example, what is the maximum fixture demand for a plumbing
system which consists of the following 14 fixtures: 2 water closets, 4
lavatories, 2 showers, 3 urinals, 1 slop sink, 1 laundry tub, and 1 floor drain?
and the following steps:
Multiply the number of each fixture by the GPM of that type fixture
Total these figures.
1-19. The result is a maximum fixture demand of 313.5 GPM.
NOTE: Use the fixture demand (313.5 GPM) with the simultaneous-
use factor to select the pipe size.
The simultaneous-use factor is the
percentage of fixtures potentially in use at a given time
estimate of the total demand on a water supply system, expressed as water
supply fixture units. Simultaneous-use factors decrease as the number of
fixtures in a building increases. Use the formulas in
simultaneous use factor.
1-21. If a table for the simultaneous-use factor is not available, estimate the
probable demand by computing 30 percent of the maximum fixture demand in
1-22. Continuing the example in
the 14 fixtures would have
a simultaneous use of 42.72 percent (round up to 43 percent). Since the fixture
demand was 313.5 GPM, the water service line must have a capacity of 43
percent of 313.5 (110 GPM). What size of pipe would be needed for a 60-foot
long pipeline with a pressure at the main of 45 psi
Read down the 60-foot column in
Read across (left) to the psi column and establish the given as 45 psi.
Read back to the 60-foot column.
shows 150 GPM (the quantity
that includes 110 GPM);
shows 155 GPM (round up to 160 GPM).
1-23. Either 1 1/2-inch galvanized, copper, or plastic piping would be large
enough for the water service line.
NOTE: Remember, the minimum practical size for a water service
line is 3/4 inch. This size should be used even when calculations
indicate a smaller size. Visit here
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