Sizing domestic water pipes

When sizing pipes in nonresidential buildings, engineers should pay close attention to local code, available water pressure, fixtures, and a host of other factors.

01/15/2018

Learning objectives

• Know the various pipe-sizing methods outlined in plumbing codes, such as International Plumbing Code.
• Understand the source of water supplied to a nonresidential building, and what factors influence design of the piping system based on this source.
• Learn how to calculate the size of pipes and related equipment.

Design professionals should consider several factors to dictate how water piping is sized for nonresidential buildings, including occupancy type, pressure availability from the source, specialized equipment requirements, and building layout. This article looks at sizing methods outlined in the International Plumbing Code (IPC), as well as explanations to determine when it is necessary to go above and beyond code-minimum requirements. The American Society of Plumbing Engineers (ASPE) and codes based on the Uniform Plumbing Code (UPC) use a similar method.

The goal of any water system is to reliably deliver water to all fixtures in a building at the pressure and flow rate required for proper operation. It is the responsibility of the design professional to gather all information on the types of fixtures and equipment being connected to the water system.

Components connected to the domestic water system, such as flush-valve water closets and urinals, sterilizers, commercial dishwashing equipment, and industrial equipment, often have minimum pressure requirements to operate. All of these components affect the level of pressure that is available to the end user; therefore, design always starts at the source.

Water-source considerations

Domestic water is typically provided by a municipal water system or well, and each source has its own operational criteria. The most common system (municipal) will be considered here.

Municipal water supplies vary across the country, and design professionals need to work with local utilities to verify requirements regarding meters and backflow prevention. Design professionals also must find out what minimum pressure is available by local utilities. For example, water pressure available at the street may be 80 psi at the time of testing, but some public utilities only guarantee 20 psi. Variations in pressure need to be considered in the design because the municipality's minimum pressure might not meet the minimum required to operate fixtures properly. In this case, a booster pump would be required.

Establishing a baseline can be done with a flow test at the street. Flow tests are either done by utility companies or outside contractors. Since flow tests often have to be performed for fire protection design, contacting the fire protection designer on record is a good place to start when verifying whether the test has already been performed.

The flow test will show the water-supply curve of the city main. This can be used to verify that the utility's water main can satisfy the building's demand. It is important to note that water demand for fire flow typically exceeds the demand for domestic water use. Because of this, domestic water consumption is assumed to happen near the static pressure. Using a flow test, if a building has a domestic water demand of 500 gpm, the residual pressure is approximately 89 psi.

Therefore, it is prudent to use the static pressure as the baseline for domestic water-service demand. It should be noted that 2012 IPC, Section 604.8, requires a maximum domestic water pressure of 80 psi. A pressure-reducing valve (PRV) should be added to this system downstream of the backflow preventer to ensure pressure does not exceed 80 psi.

Sizing the system

Before sizes can be determined, the amount of flow required must be established. It may seem obvious to start by taking all the fixtures in the building, adding up their flow rates, and using the total for system demand. However, the likelihood of all fixtures being in operation simultaneously is very small; some diversity factor has to be established for design.

To account for this diversity, most codes (IPC and UPC) have established a system wherein each fixture is assigned a value in fixture units. Fixture-unit values are shown in Table E103.3(2) of the 2012 IPC. The codes that do not use fixture-unit calculations apply a diversity factor to the gallons per minute (gpm) based on occupancy type and the number of fixtures.

For example, consider a 3-story office building placed 100 ft away from the city water main. The building has centrally located restrooms, drinking fountains, mop sinks, and four break rooms on each floor. The fixtures are listed in Table 1.

Of note in the table is that water closets are specified with flush valves. It is critical to account for the difference between tank toilets and flush-valve toilets. A tank water closet is assigned half the fixture units of a flush-valve type. Both fixtures use 1.6 gal/flush, the difference being the time it takes each type to pull 1.6 gal from the system. A flush valve typically operates in roughly 4 seconds (an average of 25 gpm), while a tank-type water closet might take 30 seconds to refill (an average of 3.2 gpm). Furthermore, flush-valve water closets typically have a minimum pressure of 35 psi to operate properly. With the total fixture units established, refer to Table E103.3(3) of the 2012 IPC for gallons per minute values. This value can be interpolated to be 151 gpm.

At this point, system demand is established at 151 gpm. With this information, it is possible to make a preliminary judgment on the building's main water line size. This figure might be adjusted later if the chosen size produces excessive pressure loss.

Now that the 100% cold-water flow rate is established, it is time to consider the pressure loss of the system. Items that affect system pressure are friction loss through pipe, valves, meters, fittings, and head loss due to elevation. Analysis will need to prove that the most remote fixture in the system will have adequate pressure to operate. For our example, this will be a flush-valve water closet on the 3rd floor that requires 35 psi to operate.

From the street to the building

When connecting to a public utility water main, a typical arrangement will be a tapping tee, isolation valve, and then a water meter. Utility companies have guidelines on meter type, sizing, and location requirements. They also will be able to provide flow data on the required meters. It is important to size the meter appropriately, because utility companies often have tap fees that can vary by the size of the meter. These fees can be accrued monthly, thus oversizing a meter could end up costing the building owner an enormous amount of money over the lifespan of the facility.

Downstream of the meter, the water main needs to pass through a backflow preventer. The backflow-preventer location will vary based on climate and the requirements of local authorities having jurisdiction (AHJ). It can be located inside the building or outside on the building site. When locating a backflow preventer inside the building, it is important to have the least amount of piping possible between the water line entering the building and the backflow. No taps can be made off the water line that are upstream of the backflow preventer. This is to prevent any contaminated water within a building from backflowing into the public water supply.

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