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Flow Rates: Flow rate will vary by type of fixture and water pressure at the fixture. Each fixture in a plumbing system is designed to operate at a specific flow rate, expressed in gallons per minute (gpm), liters per second (L/s), or liters per minute (L/min) of flow. Residual water pressure (pressure available at the outlet before a fixture) affects the flow rate of a fixture. A higher residual pressure results in a greater flow rate and thus more water consumption. Approximate flow rates and typical consumption by type of plumbing fixture and location are shown in Tbl. 5. Water Consumption Water use in many homes is lowest from about midnight to 5 AM, averaging less than one gallon per person per hour. Use climbs sharply in the morning around 6 AM, to about 3 gallons per person per hour. During the day, water use drops off moderately and rises again in the early evening hours. Weekly peak flows may occur in some homes on weekends, especially when all adults work during the week. In U.S. homes, average water use is approximately 45 gallons per person per day, but may range from 30 to 60 gallons or more. Peak flows at retail stores and other businesses typically occur during business hours. Peak flow occurs during meal times at restaurants. Rental properties, resorts, and commercial establishments in tourist areas typically have flow rates that vary by season. Approximate water requirements for selected buildings are shown in Tbl. 6. Water Demand: The instantaneous peak demand for water in a pipe serving a number of plumbing fixtures or serving an entire building is referred to as the design load. The design load is the maximum probable or peak instantaneous demand for domestic water by a group of fixtures. The design load is typically expressed in gpm, L/min or L/s. The design load of a pipe serving a group of plumbing fixtures or for the total fixtures installed on a project will depend not only on the number and type of fixtures installed but on the operation of the fixtures. Some fixtures may, at times, have continuous flow (e.g., faucets, hose bibbs, and showerheads), while other fixtures have an intermittent flow (e.g., water closets and urinals). Nevertheless, it would be highly unlikely that every sink, dishwasher, water closet, bathtub, shower, clothes washer, and garden hose in a building would be all operating at one time. So, simply totaling fixture flow rates for all fixtures in an entire building distribution system would give the total demand for water usage only if all fixtures were used at one time. In most in stances, totaling fixture requirements provides a very high estimate that results in overdesign of the piping. Method 1: Simple Empirical Design Method Pipe sizes for the water supply system of a single-family house and similar simple structures can be determined on the basis of experience and pertinent code requirements. Detailed analysis is not necessary in the design of simple systems. The fixture fed by the branch will influence branch pipe diameter. Pipe diameter is determined by the pipe size serving the fixture. In the empirical design method, piping is sized with rules of thumb based on observation and experience. For example, the mains that serve fixture branches can be sized as follows: • Up to three 1/2 inch branches can be served by a 3/4 inch main • Up to three 3/4 inch branches or up to six 1/2 inch branches can be served by a 1 inch main • Up to five 3/4 inch branches or up to ten 1/2 inch branches can be served by a 1 1/4 inch main Branch pipes can be sized from minimum branch requirements cited in the building code, such as those provided in Tbl. 7. The empirical approach is used in design of plumbing systems for residences and similar buildings with simple plumbing systems. Typically, a qualified plumber does design during rough-in of the piping. This approach can lead to system problems in complex piping arrangements. Fixture Units A method of estimating the design load for a group of plumbing fixtures is typically based on a quantity called the fixture unit. The fixture unit is an arbitrarily chosen measure that allows all types of plumbing fixtures to be expressed in common terms; that is, a fixture having twice the instantaneous flow rate of a second fixture would have a fixture unit value twice as large. The sole purpose of the fixture unit concept is to make it possible to calculate the design load on a system composed of different types of fixtures, each having different flow rates. Fixture unit values are assigned to the different types of plumbing fixtures. The total number of fixture units is then used to establish the maximum probable water supply load and drainage load. (Drainage fixture unit load is discussed in Section 14.) Dr. Roy Hunter of the National Bureau of Standards (now the National Institute of Standards and Technology, NIST) developed this method over a half century ago and it still serves as the basis for estimating the design load of a plumbing system. The Hunter method assigns a water supply fixture unit to each fixture. The water supply fixture unit (WSFU) is a probability factor that represents each fixture connected to the water supply system and used to determine the total use of water within a given system. Tbl. 7 provides typical WSFU load requirements for common plumbing fixtures. The total WSFU quantity listed in the table relates to the WSFU load for that fixture. For example, a kitchen sink in a private residence (2 WSFU) has twice the flow rate in comparison to a lavatory in a private residence (1 WSFU). TBL. 7 SUPPLY, BRANCH SIZE, AND WATER SUPPLY FIXTURE UNIT LOAD REQUIREMENTS OF COMMON PLUMBING FIXTURES. TBL. 8 WATER SUPPLY FIXTURE UNIT (WSFU) LOAD AND RELATED DESIGN LOAD, IN GPM, L/MIN, AND L/S, BASED ON HUNTER'S WORK ON INSTANTANEOUS DOMESTIC WATER DEMAND. TBL. 9 WATER SUPPLY FIXTURE UNIT (WSFU) LOAD AND RECOMMENDED DESIGN LOAD, IN GPM, L/MIN, AND L/S, FOR SELECTED BUILDING TYPES. Most plumbing fixtures are connected to both cold water and hot water branches. Water is typically drawn from both the cold water and hot water supply lines. The fixture unit value for fixtures having both cold and hot water connections are taken as three-quarters (3/4) of the listed total value of the fixture. Thus, when calculating the peak demand flow rate for the hot or cold water distribution system, if both hot and cold branches supply a fixture (which, except for urinals, water closets, dish washers, and hose bibs, is nearly always the case), then the WSFU for the fixture is reduced by a factor of 0.75. These reduced WSFU values are provided in Tbl. 7 as cold water and hot water values. A standard one-bath home with kitchen sink, dishwasher, water heater, clothes washer, flush tank toilet, lavatory, tub/shower combo, and two hose bibbs would be counted as 18 WSFU. Most standard two-bath homes would be counted as about 24 WSFU. Most standard three-bath homes would be counted as 34 WSFU. Hunter developed a curve that establishes the flow rate for any given water supply fixture unit value. The design load, in gpm or L/min, is determined based on the number of WSFU using the Hunter method. See Tbl. 8. The design load is different, depending on whether the system consists of water closets and urinals in the system are predominantly flush tank or flush valve. Systems that consist of water closets and urinals with flush valves have a higher flow rate and thus a higher design load below about 1000 WSFU. Tbl.9 provides WSFU loads and the recommended de sign load, in gpm, L/min, and L/s, for selected building types. It’s based on a modification of Hunter's method. It’s modified from the original Hunter method because the original research assumed only two types of buildings: one that used tank-type water closets and one that used flush valve water closets. It did not account for variations in operation of the different types of buildings. A sports arena will have a higher demand on a water distribution system than an office building, a school, or a residence. Additionally, the original fixture unit design concept was based on water use over a half century ago, when fixtures discharged at a much higher flow rate. New research is suggesting changes in WSFU rates for many fixtures and different categories of buildings (i.e., one- and two family dwellings, multifamily dwellings, high-use assembly buildings, and other buildings or commercial buildings). The new WSFU method simply uses the adjusted WSFU values but still re lies upon the Hunter curve. Not all systems can be sized using the WSFU method. Like all sizing methods, there are restrictions regarding high and low limits of some water distribution systems. For example, the WSFU method will result in an undersized water distribution system for sports stadiums and arenas because of high peak demands. In these cases, the pipe size is selected and the pressure loss in the piping system is evaluated. Any sizing method needs a common sense approach for establishing the flow rate in the piping system. Method 2: WSFU Design Table Method In residential and small commercial buildings, WSFU design tables can be used to establish meter and distribution pipe size based on the total demand in WSFUs and the supply pressure (the available static pressure after static head loss). Tbl.10 represents of WSFU tables used to size building supply and branch lines, and meter and service lines. Meter and distribution pipe can be sized using the following methods: 1. Obtain minimum service water pressure for the location of construction. Usually this is available through the municipal water department. 2. Compute the total WSFUs, including proposed and projected future plumbing fixtures. 3. Calculate the maximum developed length of water piping: the actual length of pipe between the source of sup ply and the most remote fixture plus the developed length of fittings. Developed length can be approximated by multiplying the actual length to the most remote fixture by 1.2 to compensate for loss of meter and fittings. 4. Compute the static head (the pressure loss from elevation change) and subtract it from the service water pressure. Static head (P_static) is found by multiplying the vertical height (Z), in feet or meters: 5. Use Tbl.10 to determine the meter and distribution pipe sizing based on the total demand in WSFUs, maximum developed length of water piping, and the supply pressure (the available static pressure after static head loss). Tbl. **10 MAXIMUM WATER SUPPLY FIXTURE UNITS (WFSU) FOR A SUPPLY PRESSURE (THE AVAILABLE STATIC PRESSURE AFTER HEAD LOSS) AND SPECIFIC PIPE SIZE. Supply Pressure; Meter and Street Service Pipes (in); Building Supply and Branch Pipes (in) ; Maximum Developed Length (ft) Ex. **6: Using the WSFU design table method, determine the minimum meter and distribution pipe sizes. Assume the following: • Minimum service water pressure for the location is 65 psi. • Total WSFUs is 28. static head, in kPa, ? P_static __9.8Z static head, in psi, ? P_static __0.433Z • Actual length of pipe between the source of supply and the most remote fixture is 83 ft. • Elevation above the source of supply is 23 ft. For the maximum developed length of 100 ft and a sup ply pressure in the range of 45 to 60 psi, at 27 WSFU (33 WSFU in 100 ft column): • The meter and street service pipe size: 3/4 in. • The building supply and branch pipe size: 1 in Method 3: Velocity Design Method The velocity design method entails selecting the smallest pipe diameter without exceeding a pre-established maximum velocity for the design load in the pipe. It’s typically used accurately in a downfeed system and works well in preliminary design of a plumbing system provided the system layout is reasonably symmetrical. This method does require an investigation of pressure loss to ensure that residual pressure at the most remote fixture is adequate. The velocity design method involves computing the number of WSFUs served by a pipe, converting total WSFUs to a design load (in gpm or L/min), and then sizing the pipe based Supply pressure is found by: 65 psi _ 10 psi _ 55 psi __9.96 psi _ _10 psi __0.433Z __0.433(23 ft) P_static The pressure loss from static head, in psi, is found by: [...] approximated by: 83 ft _ 1.2 _ 99.6 ft _ 100 ft The maximum developed length of water piping is on a maximum velocity. Maximum velocities in plumbing systems typically range from 5 to 10 ft/s (1.5 to 3.1 m/s). Pipe sizes are calculated at strategic points in the system (e.g., wherever the WSFU served and, therefore, the design load change). The procedure is outlined below: 1. Sum the total number of WSFUs for hot water and cold water. (See Tbl.7.) 2. Determine maximum probable demand in gpm. (See Tbl.8 or Tbl.9.) 3. Based on the maximum desired velocity (e.g., 8 ft/s or 2.4 m/s) and design load (Q), solve for the minimum required diameter (Di-min): In customary units, minimum required diameter (Di-min) of the pipe, in inches, is based on the maximum desired velocity (v) of a fluid flowing through a pipe, in ft/s, and the volumetric flow rate (Q), in gpm: In SI (metric) units, minimum required diameter (Di-min) of the pipe, in mm, is based on the maximum desired velocity (v) of a fluid flowing through a pipe, in m/s, and the volumetric flow rate (Q), in L/min: 4. Select a pipe size for the appropriate pipe material (from design tables such as Tables 9 through 15, 17 through 22 in Section 12) with an inside diameter equal to or greater than the minimum required diameter. Ex. **7 Using the velocity design method, determine the minimum required size of hot and cold water supply pipes serving two apartments, each containing a kitchen sink and a bathroom group as noted in the listing that follows. Use a maximum velocity of 6 ft/s, because of noise limitations. Assume a system with predominantly flush tanks and Type L copper tube ( Tbl. 9 in Section 12). From Tbl.7, the WSFU loads for hot and cold sup ply pipes are determined: 2 bath tubs (1 1/2 WSFU each for hot and cold) 2 water closets-flush tank (3 WSFU for cold only) 2 lavatories (3/4 WSFU each for hot and cold) 2 kitchen sinks (1 1/2 WSFU each for hot and cold) Total WSFU: Ex. **8 Using the velocity design method, determine the minimum required size of cold water supply pipe serving two apartments with design load of 10 gpm and a system with predominant flush tanks and Type L copper tube (same as previous exercise). Use a maximum velocity of 8 ft/s (rather than the 6 ft/s used in previous example). In comparing the cold water supply pipe sizes determined in the previous two examples, it’s evident that the velocity limitation influences pipe size. Selection of a pipe size is a balance between economy (a smaller pipe diameter is less expensive) and noise and erosion (a smaller pipe diameter is noisier and will wear faster). Ex. **9 Using the velocity design method, determine the minimum required size of hot and cold water supply pipes serving 16 apartments, each containing a kitchen sink and a bathroom group as noted in the listing that follows. Use a maximum velocity of 8 ft/s. Assume a system with predominant flush tanks and Type L copper tube. 16 bath tubs (1 1/2 WSFU each for hot and cold) 16 water closets-flush tank (3 WSFU for cold only) 16 lavatories (3/4 WSFU each for hot and cold) 16 kitchen sinks (1 1/2 WSFU each for hot and cold) Total WSFU: In Ex. **9, the designer should select a 2 in diameter copper tube for the cold water pipe based on the 8 ft/s velocity limitation. If this velocity limitation is arbitrary (e.g., not precisely required by design constraints or code requirements), the designer may chose to select a 1 1/2 in diameter tube. The 1 1/2 in diameter tube has an inside diameter slightly smaller than the minimum required diameter (Di-min). As shown in the next example, if the smaller pipe is selected, the water velocity will exceed 8 ft/s slightly. The smaller pipe may not present a concern unless design or code velocity limits are exceeded. Ex. **11 A large 10-story office building has a demand load of 60 WSFU for cold water and 25 WSFU for hot water at each floor. An additional supply of 18 gpm of cold water is required in the basement for makeup water for mechanical equipment and a landscaping sprinkler system. The system will be plumbed with Type L copper tube ( Tbl.9 in Section 12). a. Use the velocity design method to determine the minimum required size of hot and cold water supply pipes serving the building, based on a maximum velocity of 8 ft/s. For cold water supply pipe: _ 600 WSFU Total cold water demand load _ 60 WSFU _ 10 floors 100 gpm _ 18 gpm _ 118 gpm 600 WSFU _ 100 gpm ( Tbl.9) A 2 in diameter Type L copper tube with an inside diameter of 1.985 in (from Tbl.9 in Section 12) is acceptable. b. Use the velocity design method to determine the minimum required size of the main supply pipe serving the building, based on a maximum velocity of 8 ft/s. _ 250 WSFU _ 850 WSFU Total demand load for hot and cold _ 600 WSFU Tbl.11 indicates the maximum demand at common maximum supply velocities for common Type L copper tube (pipe) sizes. Tbl. **11 TYPE L COPPER TUBE (PIPE) DIMENSIONS AND MAXIMUM DEMAND IN GPM AT COMMON MAXIMUM SUPPLY VELOCITIES. Maximum Demand, in gpm, at Common Maximum Supply Velocities Method 4: Equal Friction Design Method A more accurate approach to sizing the pipe diameter in a complex network of pipes is the equal friction design method. In this design method, it’s necessary to determine the total pressure drop required between the water service and the fixture and equate this to a pressure drop per 100 ft over the equivalent length of pipe. Pressure available at the water service minus pressure head and desired pressure at a fixture determines the desired pressure drop. This method is more complex and more accurate, but usually requires several iterations before pipe diameters are selected. Sizing the piping using the equal friction design method requires several iterations. Often, it’s a matter of trial and error, even for experienced engineers; 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 the charts in Figs. **6 and **7. The chart used will depend on the type of pipe roughness (material). To make a pipe selection for a specific condition using the equal friction design method: 1. Find the volumetric flow rate along the side of the chart. 2. Move horizontally across the chart to the pipe diameters and, for specific nominal diameters, note associated pressure drops and velocities. 3. Select a pipe diameter having the desired pressure drop (including fittings) without exceeding the velocity limitation requirements. Ex. **12 a. Select a nominal pipe diameter of copper pipe (Type L) with a volumetric flow rate of 100 gpm. The desired pressure drop from friction loss is 16 psi in 48 ft. Use a 50% in crease in pressure drop from fitting losses. From Fig.6, these nominal pipe diameters of copper pipe have the associated pressure drops and velocities. Pressure drop and velocity for each pipe size were found at the intersection of the line representing nominal pipe size and the line representing a flow rate of 100 gpm. 4 in pipe 0.27 psi per 100 ft 2.6 ft/s 3 1/2 in pipe 0.53 psi per 100 ft 3.5 ft/s 3 in pipe 1.05 psi per 100 ft 4.7 ft/s^2 1/2 in pipe 2.70 psi per 100 ft 6.7 ft/s^2 in pipe 7.80 psi per 100 ft 10.4 ft/s 1 1/2 in pipe 29.0 psi per 100 ft 18.1 ft/s Pressure drop for the 48 ft long pipe for each pipe size is computed as follows: A pipe with a 2 in nominal diameter should be selected because it has a pressure drop from friction loss that is closest to 6 psi. b. Select a nominal pipe diameter of copper pipe (Type L) with a volumetric flow rate of 100 gpm. Limit velocity of water flow to a maximum of 8 ft/s. Based on the information shown above, a pipe with a 2 1/2 in nominal diameter should be selected because its velocity does not exceed 8 ft/s. c. The total water pressure available at one end of a 48 ft long pipe is 50 psi. The pipe will have a volumetric flow rate of 100 gpm and a vertical elevation increase of 24 ft. Limit velocity of water flow to a maximum of 8 ft/s. Use a 50% increase in pressure drop from fitting losses. Select a nominal pipe diameter of copper pipe (Type L) based on total pressure drop (including static and friction losses) so that the minimum pressure available at the fixture is 12 psi. Total pressure available: 50 psi Pressure loss from elevation change: Desired residual pressure: 12 psi Pressure available for friction is found by: A friction pressure loss in the pipe of 27.6 psi is the best selection. From the solution in a. above, a 2 in diameter pipe provides a pressure drop including fittings of 5.62 psi (much too small) and a 1 1/2 in diameter pipe of 20.9 psi (slightly too large). The 1 1/2 in diameter pipe would likely be selected initially but the water velocity, at 18.1 ft/s for this pipe size, exceeds the 8 ft/s significantly. With the velocity constraint, a 2 1/2 in diameter pipe is the only alternative, at a friction pressure drop of 1.94 psi. The remaining pressure of 37.66 psi (27.6 psi - 10.4 psi _ 1.94 psi _ 37.66 psi) is higher than the minimum pressure required at the fixture of 12 psi. It could be deemed accept able or a valve could be installed in the system to reduce the pressure further. d. Using the velocity design method, select a nominal pipe diameter of copper pipe (Type L) based on the design criteria in c. The velocity limitation dictated pipe diameter in the previous example. Had the pipe been 480 ft long (instead of 48 ft long) with the same pressure drop from fitting losses, friction pressure loss would have been **4 psi (ten times greater). The friction pressure loss in the longer pipe is much closer than the optimal pressure loss of 27.6 psi. Frequently, velocity requirements dictate pipe diameter in short lengths of pipe such as in the previous example. Prev: Water
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