HOW TO DETERMINE SIZE OF PIPES FOR WATER DISTRIBUTION IN BUILDINGS

Pipes for water distribution in a building are available in various sizes. Determination of suitable pipe sizes for various purposes in a building based on various factors such as cost, pressure etc. are discussed.

Following points regarding pipe sizes in buildings are discussed:

o    What are the factors that influence the selection of water distribution pipe size in buildings?

o    Procedures used for the determination pipe sizes for water distribution in buildings.

Fig.1: Different Pipe Sizes to Distribute Water in Buildings

Factors Influencing Selection of Water Distribution Pipe Size in Buildings

o    Cost effectiveness

o    Pressure at the source of water supply

o    Required pressure at each outlet fixture (examples of outlet fixture are bathtubs, channel drains, bidets, drinking fountains, kitchen sinks, bathroom sinks, showers and others)

o    The loss of pressure of outlets placed above water source. Loss of pressure is commonly caused by friction of water. The friction is occurred due to flow of water in pipes, water flow meter, and backflow preventer.

o    Restriction on water flow velocity to avoid noises and pipe erosion.

o    Extra capacity of possible expansion in the future. Generally, the additional capacity if around 10 percent.

o    Economic considerations govern the selection process to a great extent. But other factors explained above may enforce the use specific pipe size other than the most economical ones.

Procedures for Determination of Pipe Sizes for Water Distribution in Buildings

o    Draw all suggested horizontal mains, risers and branch lines with necessary information includes types and number of fixtures and flow that is needed.

o    Calculate fixtures demand weight employing Table 1 and Table 2.

o    Specify water demand in gallon per millimeter using Figure 2 and Figure 3 and total number of fixture units.

o    Estimate equivalent length of the pipe for each stack in the system. This step should start from the street main.

o    Determine average minimum pressure in the street main. This can be obtained from Water Company or by test.

o    Specify minimum pressure required for the highest fixture

o    Calculate the loss of pressure in the pipes using the computed equivalent length of the pipe.

o    Finally, select the size of the pipe using Figure 4.

Note:

o    Unit sizes provided in the table 1 and table 2 is in mm, but the size of pipes in the chart is in inch. 1 inch= 25.4mm, 1 ft. = 304.8mm, 1 gallon/m=3.785L/m.

o    Water velocity should not be greater than 2.438m/s at valve seat to prevent noise and erosion. So, Pipe area should be at least equal to water flow rate divided by eight.

o    Sizes for fixture supply pipes provided in table 1 and table 2 are minimums.

Table-1: Fixture Units, Trap and Connection Sizes for Plumbing Fixtures for Domestic Waters
Types of fixture	Domestic water
	Fixture-unit value as load factors		Minimum size of connections, mm
	Private	Public	Hot water	Cold water
Bathtub (with or without overhead shower	2	4	12.7	12.7
Bidet
Combination sink and tray	3	–	12.7	12.7
Combination unit and tray with food disposal unit	4
Dental unit		1		9.525
Dental lavatory	1	2	12.7	12.7
Dish water, domestic	2
Drinking fountain	1	2		9.525
Floor drains	1
Kitchen sink	2	4	12.7	12.7
Kitchen sink, domestic, with food waste grinder	3
Lavatory	1		19.05	9.525
Lavatory		2	12.7	12.7
Lavatory, barber, beauty parlor		2
Lavatory, surgeon’s	2
Laundry tray (1 or 2 compartments)	2	4	12.7	12.7
Shower, per head	2	4	12.7	12.7
Sinks Surgeon’s	3		12.7	12.7
Sinks Flushing rim (with valve)		2	19.05	19.05
Sinks Service (trap standard)	3		12.7	12.7
Sinks Service (P trap)	2	4	12.7	12.7
Sinks Pot, scullery,		4
Urinal, pedestal, siphon jet, blowout		10		25.4
Urinal, wall lip		5		12.7
Urinal stall		5	50.8	19.05
Urinal with flush tank		3
Wash sink (circular or multiple) each set of faucets		2	12.7	12.7
Water closet, tank-operated	3	5		19.05
Water closet, valve-operated	6	10		25.4

 

Table-2: Fixture Units, Trap and Connection Sizes for Plumbing Fixtures for Drainage
Types of fixture	Drainage
	Fixture-unit value as load factors	Minimum size of trap, mm
Bathtub (with or without overhead shower	2	38.1
Bidet
Combination sink and tray	2
Combination unit and tray with food disposal unit	3
Dental unit	1	31.75
Dental lavatory	2
Dish water, domestic	2	38.1
Drinking fountain	1	31.75
Floor drains	2	50.8
Kitchen sink	2 or 3	38.1
Kitchen sink, domestic, with food waste grinder	2
Lavatory	1	31.75
Lavatory	2	38.1
Lavatory, barber, beauty parlor	2
Lavatory, surgeon’s	2
Laundry tray (1 or 2 compartments)	2
Shower, per head	2	50.8
Sinks Surgeon’s	3	38.1
Sinks Flushing rim (with valve)	6	76.2
Sinks Service (trap standard)	3
Sinks Service (P trap)	3	50.8
Sinks Pot, scullery,	3	38.1
Urinal, pedestal, siphon jet, blowout	6	76.2
Urinal, wall lip	2	38.1
Urinal stall	2	50.8
Urinal with flush tank	2	38.1
Wash sink (circular or multiple) each set of faucets	3
Water closet, tank-operated	4	76.2
Water closet, valve-operated	6

 

Fig.2: Domestic Water Demand Curve Specify Rate of Flow Using the Number of Fixture Units Served

Fig.3: Domestic Water Curve, Enlarged Portion for Low Water Demand

 

Fig.4: Determination of flow in copper tubing and other pipes that will be smoothed after service of 15 to 20 years

 References:-

ASPE. Plumbing engineering design handbook: Engineer’s guide to system design and construction. Chicago: American Society of plumbing engineers , v. I, 2004.

FREDERICK S. MERRITT, JONATHAN T. RICKETTS. Building design and construction hand book. 6th edition. ed. New York: McGRAW-HILL, 2001.