Preliminary Design Information Before starting the water system design, you must assemble a collection of information. This includes information from the owner, the local utility, and the architect. Things to consider in developing this design information include: • Utility information, such as contact information; requirements for the meter location (most utilities require the meter at the property line); requirements for backflow preventer locations (inside or outside, at the water meter); submittal requirements for permitting (some jurisdictions have special fixturecounting methodologies not included in the model plumbing codes); and special requirements of the local utilities (For example, the City of Houston will not allow direct pump suction from the city water mains, so you must have a break tank.) • Water supply flow test • Applicable plumbing codes, especially local amendments to national or state model codes • First pass at equipment needs and space requirements (While this is only a portion of the entire plumbing design, the elements included in the water design are water booster pumps and water heaters.) A word of caution: If you are working in a new jurisdiction, do not assume that the rules are the same as your home territory. While I started the bulk of my plumbing design portfolio in Florida, I designed a project in Ohio, where for some reason the architect had not provided the lavatories in the accessible water closet stalls. It was rather embarrassing when the architect told me that the Americans with Disabilities Act (ADA) does not require them. It turns out that Florida has its own ADA rules that are more stringent than the federal version. (I hate it when the authority having jurisdiction has to educate me on the local codes!)
O
ne of the fundamental design elements that every plumbing engineer faces is the design of domestic water systems. Water is used in just about every building of every type, and it is necessary for the plumbing engineer to be able to design and lay out a water system to be as efficient as possible in these days of lower fees and more demand from clients. This article outlines a methodology plumbing engineers can use to design an efficient domestic water system. It walks through the design process for a relatively large, multistory building, which will allow readers to look at all elements of the design of a domestic water system with the knowledge that smaller projects will not need to address every issue. It is up to the designer to audit the steps presented for each specific type of project. Please note that the scope of this article does not allow great depth on the design of water-heating systems or water booster pump systems. Designers should review ASPE design manuals, such as Domestic Water Heating Design Manual II or the Plumbing Engineering Design Handbooks, for further information on those topics. 30 Plumbing Systems & Design
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Is a Water Booster Pump Needed? To determine whether a water booster pump is required, the following information is used: • The minimum required water pressure typically is 20 pounds per square inch gauge (psig) if flush valves are used and 10 psig otherwise. For healthcare, laboratory, or industrial projects, you may have equipment that has higher minimum pressure requirements. • The elevation difference between the elevation of the water supply flow test and the highest fixture in the building must be determined. You typically do not have to worry about hose bibbs in mechanical penthouses. The elevation difference in feet multiplied by 0.454 gives the pressure drop in psi. • The longest run of piping in the building, including fittings, must be determined. This typically is figured at this stage in the design process by adding the height, width, and length of the building and multiplying by 1.5. • Assume a pressure drop of 1.5 feet per 100 feet (approximately 0.7 psi per 100 feet).
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If the sum of the minimum pressure, elevation, and friction exceeds the water supply pressure, then a water booster pump is required. As an example, let’s look at a three-story building, 200 feet long by 100 feet wide by 45 feet tall, with flush valve toilets and no major equipment. Assuming that the fixtures on the top floor are 3 feet above the floor slab, the calculated data is: • Flush valves = 20 psig • Elevation difference = 33 feet x 0.454 = 15 psig • Pressure drop = (200 feet + 100 feet + 45 feet) x 1.5 = 517.5 feet x (0.7 psi/100 feet) = 3.6 psig Thus, the minimum supply pressure is: 20 psig + 15 psig + 3.6 psig = 38.6 psig If the water supply flow test does not indicate a residual pressure of at least 38.6 psig, a water booster pump will be required. Please note that a safety factor has not been included in this calculation. The designer must pay special attention to the friction losses in pipe sizing to determine an appropriate factor. A safe practice is to add a 10 percent factor to the minimum. Some owners prefer water booster pumps to serve only the portions of the building that are higher than the pressure limits of the water service. This can save some operating costs for the pumps, but it also leads to problems with coordinating water heating. Do you provide two domestic hot water (DHW) systems or attempt to limit pressures to match the domestic cold water (DCW) system? If you are asked to do this, make sure you carefully examine the pressures in the systems.
Locating Water Heaters For the purposes of this article, the discussion of water heaters will be limited to where to locate them. To avoid the potential for large differences in water pressure between the hot and cold water systems, it is preferable to locate the water heaters as close to the service entrance as possible. I have provided design services for a hospital that requires a check valve in every faucet that does not have an integral check stop. Due to portions of the building having a 10- to 20-psig difference in pressure between the DHW and DCW systems, faucets that do not have check valves can allow the hot water to cross over to the DCW system.
Piping Distribution Once the preliminary design information has been gathered and the architect has provided base sheets, you can start the layout and design of the systems. The design of the water systems should start at the service entrance and proceed to the individual fixtures in the building. Of great importance in the early part of the design is to determine how often and where vertical distribution will be located. For the most part, it is recommended that you use a layout using primarily horizontal distribution. This layout will consist of vertical risers from the water service entrance to the highest floor, horizontal mains to distrib-
ute water on each floor, and individual branch lines to each toilet room and/or fixture. Shutoff valves should be provided for each riser, floor, and toilet room. Normally, shutoff valves are not included for branch lines serving an individual fixture unless the owner has a requirement for such. Since every fixture must have a stop, eliminating the shutoff valves for single fixture branches can save money, since most repair or maintenance can be performed with the stop in lieu of a shutoff valve. In today’s design practice, the piping serving a floor generally is located in the ceiling cavity for that floor. This is the simplest layout in terms of minimizing installation costs and allowing efficient access to shutoff valves. I believe that making maintenance personnel find a valve on a different floor in an emergency situation creates potential trouble for everyone. For example, it has been common practice in Veterans Health Administration hospitals to locate the DCW and DHW piping in the ceiling of the floor below the fixture location. While this does allow slightly less piping to be used and allows a completely drainable system, every time the water needs to be shut off, maintenance has to find the valves on the floor below. Other design philosophies that have been utilized in the past include branch piping on every other floor, with each horizontal distribution system serving one floor up and one floor down. This can reduce installation costs, but two floors must be shut down if major maintenance or renovations are performed. A vertical distribution concept makes more sense in some projects, such as any type of project that has stacked toilet rooms as with many hotels and prisons. Using vertical branches for these projects keeps all of the plumbing in vertical stacks and makes the building more efficient. In any case, the primary criterion for selecting a location for risers, mains, and branches is to minimize the cost of the piping installation.
Pipe Sizing Once all of the distribution piping has been laid out, it is time to start sizing the piping. The following approach to sizing piping is based on Hunter’s curve and the fixture units included in the various plumbing codes. (Refer to Tables 1 through 4 for supporting information.) The fixture units in Table 1 are from the Florida Plumbing Code. The flow rates are based on the fixture unit to gallons per minute (gpm) tables also included in the Florida Plumbing Code. This information is similar, if not the same, to the information provided in the International Plumbing Code, Uniform Plumbing Code, and various state-level plumbing codes that are in place around the country. Notice that this spreadsheet has three sections for different types of information. The top section is the water supply flow information and the limits for the various calculations for pressures and velocities. The middle section calculates the total number of fixture units. The bottom section includes adjustments to the available pressure at the entrance.
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Domestic Water 101 To summarize the process, the specific steps used for sizing piping are as follows: 1. Using the amount of piping in the longest run and the water supply flow information, the maximum allowable pressure drop can be determined. This value will be based on the difference between the water supply pressure and the minimum pressure at the fixtures. If a water booster pump is used, the available pressure is set at 55 psi, assuming a 75-psig discharge from the booster pump and a minimum of 20 psig at the fixture. (Refer to Table 1.) 2. For DHW systems, the maximum flow rate is set at 6 feet per second (fps) for most pipe materials, and the flow information for systems with flush tanks is used. (As an aside, it is recognized that according to the Copper Development Association, velocities for hot water piping should not exceed 5 fps. However, based on the DHW system not being a steady flow system and the actual flow rates based on Hunter’s curve being high with low-flow fixtures, I never have had a problem with erosion in a DHW system sized for 6-fps maximum velocity.) If the maximum allowable pressure drop does not allow for a velocity of 6 fps, the limit based on pressure drop is used. (Refer to Table 4.) 3. For DCW systems serving group toilets, the volume of water is based on only the maximum flow rate limit of 8 fps. For the most part, you are not worried about the pressure drop in the short runs of pipe that make up a toilet room. (Refer to Table 2.) 4. For the main DCW distribution system, the maximum flow rate is set at 8 fps, and the resultant flow is compared to the flow rate based on the maximum allowable pressure drop. Usually, the maximum pressure drop will be the limiting factor in the smaller pipe sizes, while the maximum flow rate will be the limiting factor in the larger pipe sizes. The breakpoint is determined by the actual allowable pressure drop. (Refer to Table 3.) 5. You should walk through the entire building and calculate the number of fixture units on each section of piping. Refer to Figure 1 for a sample of this method. In Figure 1, the blue numbers represent the DCW system fixture units, and the red numbers are the fixture units for the DHW system. 6. This fixture unit value then is compared to the appropriate Table (2, 3, or 4) to determine the size of that section of piping. It is advised to keep your markup drawing as part of your project notebook just in case you have to verify or defend your sizing in the future.
Things to Think About Water Service Sizing The water service should be sized using the total water column indicated in Table 1. This value accommodates the fact that while faucets and shower valves have a maximum 32 Plumbing Systems & Design
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flow rate, you don’t need to worry about whether they are using hot or cold water. Backflow Preventers Unless you have a break tank in your system, the local utility may insist on a reduced-pressure backflow preventer (RPBP) on the water service to the building. Even if you are on a campus area that provides a master RPBP to protect the utility, you still should have a building-level RPBP so that one building does not compromise the campus system. Further, if at all possible, the RPBP should be installed outside or at least in a mechanical room that is above grade. RPBP vent sections are designed to flow full under some types of failure conditions. Figure 2 indicates how much water can flow from this vent. As you can see, a 6-inch RPBP installed in a basement level mechanical room with a 50-gpm sump pump will not work. I speak from personal observation! Type of Building Most buildings can be sized safely based on the fixture units and Hunter’s curve flow rates in the plumbing codes. However, especially for buildings with large assembly occupancies, you will have to increase the pipe sizes to get everything to work. For example, when the Tennessee Titans opened their new stadium in Nashville, the DCW system was designed for all the flush valves in the facility to be actuated at the same time. If there was enough system pressure to close the diaphragms, the system would work. They even brought in the local Boy Scouts and other volunteers to test the system. The same can be true for theaters, conference centers, and large auditoriums. On the other end of the scale are building such as hospitals where most rooms have a bathroom. In that case, using Hunter’s curve will drastically oversize the piping system unless you figure everything as “private” use. Temperature Maintenance If a recirculation system is used, the layout of the hot water system must include circulation legs for any branch that exceeds 100 feet in length. In reality, especially for branches that serve only public lavatories, the minimum branch lengths really need to be evaluated. Further, I recommend using a minimum pipe size of ⅜ inch in lieu of ½ inch as is normally used. The reasoning for this change is apparent by looking at the velocity of water in the various pipe sizes. Table 5 provides some comparison information for ⅜-inch and ½-inch piping. For a lavatory with a maximum flow rate of 0.5 gpm, this results in a velocity of 44 feet per minute in ½-inch pipe and 76 feet per minute in ⅜-inch pipe. Thus, if you want water to get to the fixture in a reasonable time, the smaller pipe is the better choice. Further, using a maximum of 6 fps for hot water systems, a ⅜-inch pipe can carry 3.2 gpm while a ½-inch pipe can carry 5.6 gpm. This means that without diversity, a ⅜-inch pipe could serve seven lavatories while a ½-inch pipe could serve 11 lavatories.
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Domestic Water 101 Table 1 Tabulation of fixture units (FU) for a sample project Available Pressure (psi) Maximum Water Velocities (fps) Static 73 Cold water 8 Residual 67 Hot water 6 Flow rate 1,574 Required pressure: 25 psi
Highest elevation: 37 feet
Description
Number 5 35 11 44 4 39
EWC Lavatory, public Sink Shower, public Service receptor Water closet, valve, public Floor totals (FU) Flow rate (gpm) Supply pressure @ flow
Hot Water FU Total 0 0 1.5 52.5 1.5 16.5 3 132 2.25 9 0 0 210 67
Cold Water FU Total 0.25 1.25 1.5 52.5 1.5 16.5 3 132 2.25 9 10 390 601.25 155 72.90
Total Water FU Total 0.25 1.25 2 70 2 22 4 176 3 12 10 390 671.25 165
Table 4 DHW sizing Pipe Size (in.)
Pipe ID (in.)
Maximum Velocity (fps)
Flow at Maximum Velocity (gpm)
/8 ½ ¾ 1 1¼ 1½ 2 2½ 3 4
0.43 0.527 0.785 1.025 1.265 1.505 1.985 2.465 2.945 3.905
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
2.72 4.08 9.05 15.43 23.50 33.27 57.87 89.24 127.38 223.96
3
17 0.13
1 4 12 21 36 60 160 321 517 1,116
Table 5 Piping comparison 3
/8-inch Pipe
Inside diameter (inches)
0.402
Volume of 1 foot of pipe (gallons)
0.0066
Length of pipe for 0.5 gallon (feet)
Corrections Elevation Site piping (ft) 150 Inside diameter (in.) 6 C value 150 Water meter Backflow preventer Fixture minimum Available pressure Longest pipe run – Pipe + Fittings (equivalent length)
Maximum FU, Flush Tank
½-inch Pipe 0.529 0.0114
75.8
43.8
Capacity of pipe at 6 fps (gpm)
3.2
5.6
Number of public lavatories at 0.5 gpm, undiversified
6
11
6 11 20 18.77 100 ft
Table 2 Bathroom group DCW sizing Pipe Size (in.)
Pipe ID (in.)
/8 ½ ¾ 1 1¼ 1½ 2 2½ 3 4
0.43 0.545 0.785 1.025 1.265 1.505 1.985 2.465 2.945 3.905
3
Maximum Flow at Maximum Velocity (fps) Velocity (gpm) 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00
3.62 5.82 12.07 20.57 31.34 44.36 77.16 118.99 169.84 298.62
Maximum FU, Flush Valve
Maximum FU, Flush Tank
0 0 0 0 14 35 136 351 694 1,738
3 6 16 30 56 103 259 470 743 1,738
Table 3 DCW main sizing Pipe Size Pipe ID Allowable Allowable Velocity Maximum Flow at Maximum Maximum FU, (in.) (in.) Loss (psi) Flow (gpm) (fps) Velocity (fps) Velocity (gpm) Flush Valve /8 ½ ¾ 1 1¼ 1½ 2 2½ 3 4 3
0.43 0.545 0.785 1.025 1.265 1.505 1.985 2.465 2.945 3.905
0.188 0.188 0.188 0.188 0.188 0.188 0.188 0.188 0.188 0.188
2.91 5.44 14.20 28.66 49.87 78.78 163.25 288.69 461.12 969.08
6.44 7.48 9.42 11.14 12.73 14.21 16.93 19.41 21.72 25.96
8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00
3.62 5.82 12.07 20.57 31.34 44.36 77.16 118.99 169.84 298.62
0 0 0 0 14 35 136 351 694 1,738
Maximum FU, Flush Tank 1 6 16 30 56 103 259 470 743 1,738
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Domestic Water 101 There is so much more that can be talked about in the design of domestic water systems. We haven’t touched the principles behind proper backflow prevention based on the hazard classification or the impact of laboratory or medical equipment on water systems. Maybe in a future article. I often have been told that plumbing design is an art, not a science. Raymond F. Parham, PE, is a plumbing/fire protection engineer with Moses & Associates. He has more than 25 years of experience in mechanical design and building construction and has been a registered Professional Mechanical Engineer since 1986 and a registered Fire Protection Engineer since 1999. For more information or to comment on this article, e-mail
[email protected].
Figure 2 Maximum water flow from reduced-pressure backflow preventer vents Source: Watts Regulator Co.
Figure 1 Sample layout with fixture units
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