Water Pressure

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

regulator.

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

pipe size.

Tables D-1

and

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.

1-17.

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.

Table 1-1

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?

Use

Table 1-1

and the following steps:

Step 1.

Multiply the number of each fixture by the GPM of that type fixture

(

from

Table 1-1)

.

Step 2.

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.

1-20.

Simultaneous-Use Factor.

The simultaneous-use factor is the

percentage of fixtures potentially in use at a given time

(Table 1-2)

.Itisan

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

Table 1-2

to determine

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

gallons.

1-22. Continuing the example in

paragraph 1-18,

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

(

refer to

Appendix D,

Tables D-1

and

D-2)

?

Step 1.

Read down the 60-foot column in

Tables D-1

or

D-2

,to11/2inch

diameter.

Step 2.

Read across (left) to the psi column and establish the given as 45 psi.

Step 3.

Read back to the 60-foot column.

TableD-1

shows 150 GPM (the quantity

that includes 110 GPM);

Table D-2

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.

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