Fundamentals of SANITARY DRAINAGE SYSTEMS

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In the previous Section, building hot and cold water supply systems were described with pipes to provide sufficient running water to all of the fixtures throughout a building or project. Following the flow of the water through the plumbing system, the next step will be to design a system to dispose of the sanitary waste and wastewater.

Wastewater, sometimes referred to as sewage, is used water. It comes from almost all sections of the building, including bathrooms, kitchens, and laundry areas, and in commercial projects, equipment being serviced. Because organic waste in wastewater tends to decompose quickly, one of the primary objectives of the sanitary drainage system is to dispose of decaying wastes rapidly, before they cause objectionable odors or become hazardous to health. Wastewater treatment is covered in Section 15.

1 SANITARY DRAINAGE SYSTEM

Conventional Sanitary Drainage and Vent System

A sanitary drainage and vent system, sometimes referred to as the drain, waste, and vent (DWV) system, is a network of pipes that remove wastewater from a building. In this section the terminology and function of each of the parts are explained, but first a discussion of system operation is needed. (See Ill. 1.) In typical plumbing system operation, the sanitary drainage side of the system consists of traps at each fixture, and fixture branch, stack, and drain pipes that carry wastewater away from the plumbing fixtures and out of the building. Water transports wastes into the sanitary drainage piping and out of the building sewer line, leading to a community wastewater treatment plant or to a private sewage treatment system. Gravity is the driving force behind wastewater flow, so the sanitary drainage system is known as a gravity system.

The vent system side of the system introduces and circulates air in the system to maintain atmospheric pressure in the drain lines and ensure adequate gravity flow of wastewater.

Venting prevents a negative pressure (suction) in the system that could suck water from fixture traps and allow sewer gases to infiltrate the building. The vent system also exhausts sewer gases to the outdoors.

The chief components of a sanitary drainage and vent system are described as follows.

Ill. 1A Basic parts of residential sanitary drainage and vent plumbing systems.

Ill. 1B Basic parts of commercial sanitary drainage and vent plumbing systems

Ill. 2 An integral trap in a water closet.

Img 1 A P-trap.

Ill. 3 Drainage fittings and traps. Q-traps (sometimes called a 3/4 S-trap) and S-traps are generally not permitted.

Img 2 A P-trap under a bathroom lavatory.

Traps

A trap is a U-shaped pipe that catches and holds a small quantity of wastewater that's poured down a fixture drain. The trapped water prevents gases resulting from wastewater de composition from entering the building through the drain pipes and the fixture. (See Ill. 2.) Traps are made of copper, plastic, steel, wrought iron, or brass, with plastic most commonly used. The most acceptable type of trap is called a P-trap (see Ill. 3; see also Imgs 1 and 2). S-traps and U-traps can easily be siphoned, so they are prohibited by the building code. An integral trap is built in as part of the fixture. The integral trap in a vitreous china water closet is cast as part of the fixture.

In most instances, a trap is installed immediately down stream of the fixture, as close to the fixture as possible, usually within 2 ft (0.6 m) of it, unless the fixture is designed with an integral trap (e.g., a water closet). Because the trap may occasionally need to be cleaned, access is necessary. A removable plug in the trap may provide access or the trap may have screwed connections on each end for easy removal.

In locations where a fixture is infrequently used, water in the trap may evaporate and , with the water seal not working, gases may back up from the sewer and drainage pipes through the fixture and into the building. Floor drains, which are used to take away the water after washing floors or which may be used only in case of equipment malfunctions or repairs, present the most serious possibility of losing their water seal. When floor drains are connected to the drainage system, the possibility of a serious gas problem exists. The designer of the system can avoid such a situation by not tying the floor drain into the drainage system. Instead, the floor drains could be tied into a drywell, from which there will be no gases.

The water seal in a trap may be broken if there is a great deal of vacuum pressure in the pipes. A vent system is attached to the sanitary drainage system to reduce the vacuum and to equalize the pressure throughout the system.

Historically, a building trap was located at the end of the building drain (inside the building and just before it connected to the sewer line). It was theorized that this trap would act as a seal to keep gases from entering the building's sanitary drainage system from the sewer line. On the other hand, a building trap may impede the flow of wastes in the system. For this reason, codes disallow use of a building trap except in special installations.

Interceptors

Many substances (e.g., grease, fat, oil, hair, sand, clay, wax, or debris) are accidentally or intentionally placed into a building drain, potentially creating blockages that can cause backups and overflows, or contaminating wastewater, which makes treatment difficult and more costly. Interceptors are passive devices designed into a plumbing system that trap, separate, and retain these toxic or undesirable substances from wastewater before it's discharged into the sewer line.

Grease can solidify and coat the inner walls of pipes, creating a stoppage. Restaurants, cafeterias, and other commercial food establishments with cooking facilities must have a grease interceptor or grease trap that receives wastewater from sources such as sinks, dishwashers, floor drains, and washing area drains before draining to the municipal sewer system.

Manufacturing plants, vehicle service facilities, car washes, and other similar establishments must have an oil-sand interceptor to separate and remove floatable material (oils) and settleable materials (sands and metals) from wastewater before it's discharged to the municipal sewer system. Barber shops, beauty salons, pet grooming facilities, and any other establishments that discharge hair and /or other fibrous materials in wastewater must have a hair interceptor.

Typically, interceptors are sized for at least a 30-min peak wastewater flow detention time from all contributory sources. A grease interceptor for a restaurant can have a capacity of 1000 gal (3800 L) or more. Interceptors should be located as close to the discharging fixture as possible; they sometimes also serve as the trap, with some exceptions.

Grease traps must be located at least 10 ft (3 m) from hot water faucets. All hot water must cool to 120°F before entering the grease trap.

An interceptor must be readily accessible for periodic cleaning, inspection, and testing. Wastes captured in an interceptor must be disposed of following health standards. Precious metals (e.g., from polishing jewelry in manufacturing plants) can be recovered.

Img 3 Fixture branches for a kitchen sink and dishwasher tied into a waste stack (vertical pipe). Note the metal plates secured to the wood studs that are covering the pipes to protect them from screws when the wall surface is finished with drywall.

Img 4 An unfinished plumbing wall with cast iron drain and vent lines. Copper fixture branches extend from the drain pipes and will be connected to trap arms that will extend from the wall.

Img 5 A floor-mounted water closet flange.

Img 6 A flange and bracket connection to hold a wall-mounted water closet and connect it to a fixture branch.

Img 7 A drain stack passes through the wall above and picks up additional drain branches before transitioning to a short horizontal section and back to a vertical section to accommodate changes in wall alignment. The vent connecting to the horizontal section is under construction and not fully connected.

Ill. 4 A drain stack connected to an underfloor building drain.

Note the floor cleanout at the base of the stack. The building drain connects to sewer just outside the building foundation. The building sewer line connects to the large city sewer.

Img 8 Horizontal branches connecting to a horizontal line.

Img 9 Horizontal branches connecting to a horizontal line, which then connects to a stack. A floor cleanout is located at the base of the stack.

Img 10 Placement of drain lines (tall pipes), water closet flanges, and floor drain (far right) before a concrete floor is cast. Note that the pipes are covered with an expansion material to isolate them from the concrete slab. This will prevent damage to the pipes that could be caused by movement of the concrete floor slab.

Fixture Branches

Each plumbing fixture is connected horizontally to the sanitary drainage system by a drain line called a fixture branch. Beginning with the fixture farthest from the stack, the branch must slope 1/8 to 1/2 in per ft (10.4 to 41.6 mm per meter) for proper flow of wastes through the branch. Branch piping, which serves urinals, water closets, showers, or tubs, is usually run under the floor. When these fixtures are not on the branch, the piping may be run in the floor or in the wall behind the fixtures. Branch piping may be copper, approved plastic, galvanized steel, or cast iron.

Stacks

The fixture branches feed into a vertical pipe referred to as a stack. When the wastewater that the stack will carry includes human waste from water closets (or from fixtures that have similar functions), the stack is referred to as a soil stack. When the stack will carry all wastes except human waste, it's referred to as a waste stack. Soil and waste stacks may be copper, plastic, galvanized steel, or cast iron. These stacks service the fixture branches beginning at the top branch and go vertically downward to the building drain.

In larger buildings, the point where the stack ties into the building drain rests on a masonry pier or steel post so that the downward pressure of the wastes will not cause the piping system to sag. In addition, the stack must be supported at 10-ft (3-m) intervals to limit movement of the pipe. When a stack length is greater than 80 ft (24.4 m), horizontal offsets are used to reduce free-fall velocity and air turbulence. Connections to fixture branches and the building drain should be angled 45° or more to allow the smooth flow of wastewater.

Most designers try to layout plumbing fixtures to line up vertically floor after floor so that a minimum number of stacks will be required. Many times, a central core of a multistory building will be used as a plumbing core, and a pipe chase, a space that's left to put the pipes in, runs from the first floor to the roof of the building. (See Ill. 4.)

Building Drains

The soil or waste stacks feed into a main horizontal pipe referred to as the building drain. By definition, the building drain extends to a point 2 to 5 ft (0.6 to 1.5 m) outside the foundation wall of the building. The building drain slopes 1/16 to 1/2 in per foot (5.2 to 41.6 mm per meter) as it feeds the wastewater into the building sewer outside the building. Slopes of 1/8 to 1/4 in per foot (10.4 to 20.8 mm per meter) are common in most buildings.

Location of the building drain in the building depends primarily on the elevation of the community sewer line. Ideally, all of the plumbing wastes of the building will flow into the sewer (whether it's a community or a private sanitary system) by gravity. The drain is typically placed below the first floor or below the basement floor. If the height of the sewer requires the drain to be placed above the lowest fixtures, it will be necessary for the low fixtures to drain into a sump pit. When the level in the sump pit rises to a certain point, an automatic float or control will activate a pump that raises the wastewater out of the pit and into the building drain.

Building drains are usually made of approved plastics, copper (for above the floor), or extra-heavy cast iron (for below the floor) pipe.

Building Sewer

The building sewer is an extension of the building drain that carries wastewater from the building drain to a community sanitary sewer main or an individual on-site sewage treatment (OSST) system. In community sanitary wastewater systems, the building sewer may also be known as a house or building connection, or sanitary sewer lateral. The building sewer can have slopes that range from 1/16 to 1/2 in per foot (5.2 to 41.6 mm per meter). The extremely shallow slope of 1/16 per foot (5.2 mm per meter) is only common in large buildings serving hundreds of fixtures.

Sanitary Sewer Main

The sanitary sewer main is a pipe through which the waste water flows as it's conveyed from a building to the wastewater treatment plant. Typically, the minimum size of a community sanitary sewer main for a gravity-based system should be 8 in (200) mm in diameter.

Cleanouts

Provisions must be made to allow cleaning of the sanitary drainage system. Cleanouts are screw-type fittings with a cap that can unscrew to allow access to the inside of the sanitary drain pipes. A cleanout should not have a plumbing fixture in stalled in it or be used as a floor drain. Floor cleanouts (FCO) are found in horizontally positioned building drain or sewer lines that are installed in the floor or in the ground. Wall cleanouts (WCO) are placed in vertically positioned stacks. Al cleanouts in vertical stacks should be located no higher than 48 in (1.2 m) above the floor.

Cleanouts are generally required:

• At the base of soil and waste stacks

• At the upper end of building drains

• At each change of direction of the horizontal building drainage system greater than 60°; the total of the fit tings between cleanouts shall not exceed 120°

• At the junction between the building drain and building sewer (usually 2 to 4 ft away from the building foundation)

In addition:

• Cleanouts should be no more than 50 ft apart, including the developed length of the cleanout pipe, in horizontal drainage lines of 4 in or less size.

• Cleanouts should be no more than 100 ft apart, including the developed length of the cleanout pipe, in horizontal drainage lines of sizes over 4 to 10 in.

• Cleanouts should not be more than 150 ft apart, including the developed length of the cleanout pipe, in horizontal drainage lines exceeding sizes of 10 in.

Cleanout size is related to pipe size: 1 1/2 and 2 in diameter pipe have a 1 1/2 in cleanout; 2 1/2 and 3 in diameter pipe have a 2 1 in cleanout; and 4 in diameter and larger pipe have a 3 1/2 in cleanout. (See Imgs 11 and 12.)

Img 11 A close-up of a wall cleanout.

Img 12 A cleanout fitting allows easy access to the drain system.

Img 13 A vent connection and penetration through the roof decking.

Venting

Vents are pipes that introduce sufficient air into the drainage system to reduce air turbulence (from siphoning or back pressure) and to release sewer gases to the outside. (See Img 13.) The prime purpose of venting is to protect the trap seal. If traps did not exist in a drainage system, a venting could be eliminated.

Without a vent, as water drains from a fixture, the moving wastewater tends to siphon water from the trap of another fixture as it falls through the drain pipes. As a result, vents must serve the various fixtures, or groups of fixtures, as well as the rest of the drainage system. Vent piping may be copper, plastic, cast iron, or steel. Types of venting methods are as follows.

Individual Vents

The individual venting technique is defined as the installation of a vent pipe for every trap or trapped fixture. It is the easiest method of ensuring the preservation of a trap seal but the most costly because of the number of vent pipes required in the venting system. An individual vent must be located in close proximity to the trap to properly vent it. A more effective way of reducing the cost of venting has been in the combining of vents into a system. This would include common venting, circuit venting, wet venting, combination drain and vent, waste stack venting, and single stack systems. Venting methods are described below.

Common Vents

The common venting method serves two fixtures located on the same floor; it's essentially an individual vent that serves no more than two traps or trapped fixtures. This type of vent must be located close to the traps it vents to properly vent it. When the fixture connects at different levels, the drainage pipe between the two traps must be increased to compensate for the combined water and airflow.

Wet Vents

The wet venting method uses a single vent pipe to provide venting for all of the fixtures of one or two bathroom groups (e.g., a water closet, lavatory, shower, bathtub, and bidet) that are located on the same floor. The vent pipe for the lavatory typically serves as the vent for the other fixtures in the bathroom. Plumbing codes used to require the water closet to be the last fixture in line on a wet vent system. However, recent tests provided evidence that the order of the fixtures does not influence the overall performance of the wet vent system. The most recent standard permits the fixtures to be located in any order when connecting to the system.

Circuit Vents

A circuit venting system is a horizontal venting pipe serving up to eight fixtures. Each fixture must be connected to a single horizontal drain in this technique. The vent connection is made between the two upstream fixtures-that is, those fixtures connected to the horizontal drain pipe that are the farthest away from the vent stack. In this system, all of connections and the main piping must remain in the horizontal orientation. Vertical drops are generally not permitted.

Combination Drain and Vent

A combination drain and vent system allows the distance from trap to vent to be extended infinitely, provided the drain stays in the horizontal orientation and there is a vent somewhere within the horizontal branch. It is based on over-sizing the horizontal drain, so there is an increased likelihood of stoppage in the drain line. This is the most popular method of venting a floor drain or venting island fixtures. A combination drain and vent is a marginally effective venting method.

Relief Vent

A relief vent is a continuous pipe of lesser or equal diameter running parallel and alongside the soil and waste stack in a multistory plumbing system. It is used to equalize air pressure within the stack.

Vent stack configurations are shown in Ills 5 through 9. Codes limit the distance between the trap outlet and the vent to ensure proper venting. These distances depend on the venting technique and size of the drain and lines.

A vent stack extends vertically through the building and up through the roof to the exterior of the building. Vents from a fixture or group of fixtures ties in with the main vent stack, which extends to the exterior. It must extend beyond the roof at least 6 in (152 mm) and terminate to open air well beyond attic vents, windows, doors, or intake air vents. A vent stack is used in multistory buildings where a pipe is required to provide the flow of air throughout the drainage system. The vent stack can also begin at the soil or waste pipe, just below the lowest horizontal connection, and may go through the roof or connect back into the soil or waste pipe not less than 6 in (150 mm) above the top of the highest fixture. See Ill. 10.

Ill. 5 A vent-to-vent stack configuration.

Ill. 6 A vent-to-stack vent is typically used on upper floors. Fixtures are individually vented.

Ill. 7 A stack-to-stack vent is typically used in multistory buildings.

Ill. 8 A vent-to-soil-stack connection.

Ill. 9 Multiple stacks are required in most multistory buildings. These stacks connect to the building drain below the bottom floor.

Ill. 10 Pipe chases run from floor to floor to allow soil stacks and vents to pass vertically between floors. Chases are typically located alongside elevator hoist ways and common plumbing walls.

Img 14 An air admittance valve (AAV) is a pressure-activated, one-way mechanical venting port used to eliminate the need for venting through roof penetrations. (Used with permission of ABC)

Air Admittance Valves

An air admittance valve (AAV) is a pressure-activated, one-way mechanical venting port used to eliminate the need for expensive venting and roof penetrations (See Img ) Wastewater discharges cause the AAV to open, allowing air to circulate in the vent system. When there is no discharge, the valve remains closed, preventing the escape of sewer gas and maintaining the trap seal. Individual or branch-type air admittance valves may be used for venting individual, branch, and circuited fixtures.

AAVs are not permitted for venting combination drain and vent systems and wet vented systems.

AAVs are typically made from polyvinyl chloride (PVC) plastic materials with ethylene propylene diene monomer (EPDM) rubber valve diaphragms. Valves come in two sizes: one for fixture venting and a larger size for system venting. The valves fit standard diameter pipes, ranging from 1 1/4 to 4 in. Screening protects the valves from foreign objects and vermin. Using AAVs can significantly reduce the amount of venting materials needed for a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of fixtures, and reduce long-term maintenance problems where conventional vents penetrate the roof surface.

Img 15 A positive air pressure attenuator (PAPA) is a product developed to protect buildings of 10 or more stories against the unwanted positive pressures (i.e., back pressure/positive transients) generated in the DWV system. PAPAs are installed at the base of the soil and waste stack and at various floor intervals, depending on the height of the building. (

Positive Air Pressure Attenuator

A positive air pressure attenuator (PAPA) is a product developed to protect buildings of 10 or more stories against the unwanted positive pressures (i.e., back pressure/positive transients) generated in the DWV system. PAPAs are installed at the base of the soil and waste stack and at various floor intervals, depending on the height of the building (See Img 15.) The unsteady nature of the water flows cause pressure fluctuations (known as pressure transients), which can compromise water trap seals and provide a path for sewer gases to enter the habitable space. A PAPA/AAV system counters the tendency for the loss of trap water seals resulting from positive pressure pulses in a soil and waste stack. The PAPA/AAV system may be used in sanitary plumbing systems as an alternative to relief venting, eliminating the need for a continuous parallel relief vent pipe. It is a viable option to the Sovent system.

Sovent Drain and Vent System

Multi-story buildings traditionally rely on a complex drain and vent system with two stacks that run vertically from floor to floor and vents and branches to every fixture. In high-rise buildings, if it works successfully, a drain/vent scheme with a single stack and branches without vents is an effective substitute for the traditional two-pipe, drain and vent system.

The Sovent system is a system that combines the drain stack, branches, and vents into one pipe system by using patented Sovent fittings. Fritz Sommer of Switzerland, whose work was mainly driven by a need for resource-conserving construction techniques, developed and patented the Sovent fit tings in the 1950s.

The system consists of four components: vertical stack piping, horizontal branches to the fixtures, aerator fitting, and de-aerator fittings. These components work together to collect wastes from the plumbing fixtures and transport them down a stack to the building drain.

The system works on the principle that wastewater flowing down a vertical pipe tends to cling to the interior wall surface and continue downward in a swirling motion. As the wastewater travels down the walls of the pipe, the pipe center remains open and serves as an airway. The airway provides venting so there is a balance of pressures within the drainage system. It eliminates the need for a separate venting system.

However, if the fall rate of wastewater is uncontrolled, the falling water will increase speed and meet air resistance, which will flatten out the falling waste until it blocks the stack. This downward moving blockage can throw off the pressure balance in the system and suck water out of fixture traps. Specially designed fittings are placed in the vertical stack at each floor to eliminate speed buildup and blockage, thereby maintaining the airway and allowing for good drainage.

Horizontal branches and branch runouts connect to the plumbing fixture and transport the wastes to a specially designed stack. Generally, vents to individual fixtures are not required if fixture placement is near the stack; for example, a 4-in soil/ waste line may be run horizontally out to 27 ft from the Sovent stack without the use of traditional venting methods.

The Sovent stack is a vertical pipe that conveys wastes from the upper levels of a building to the base of the stack. The stack begins just above the bottom-most de-aerator fitting (to be described later) and continues to just above the highest fixture connection. The main difference between the specially designed stack and the traditional waste and vent stack is the Sovent stack will remain one size throughout its entire length.

It is not permitted to change diameter because it functions for both drainage and venting purposes. Sovent stack size is based on the total number of drainage fixture units that connect to that stack. The stack will penetrate the roof to the atmosphere much like traditional vent systems.

The Sovent aerator fitting is made of two separate chambers. The first chamber, called an offset chamber, allows falling waste from the upper floors to enter the chamber and pass around the horizontal branch inlets. This offset reduces the falling waste's velocity, eliminating blockage before it's allowed to form. The second chamber, named the mixing chamber, is fully separated from vertical stack flow with an internal separation baffle. As horizontal branch flows enter the aerator fitting, it must transition to a vertical flow, smoothly uniting with any vertical stack flow that may exist. Aerator fit tings can have several branch inlets. The mixing chamber pro vides the branches with sufficient air circulation to balance any pressure fluctuations that may occur. A second internal baffle in the mixing chamber is located perpendicular to the separation baffle to prevent crossflow from opposing branch inlets on that floor.

A Sovent de-aerator fitting must be located at the base of each Sovent stack and at any horizontal stack offset. This fit ting is designed to effectively deal with pressure fluctuations that occur when vertical falling wastes suddenly turn horizontal.

Sewage Ejection

For the most part, sanitary drainage systems rely on the force of gravity to create flow to discharge wastewater. In some building installations, however, a fixture or group of fixtures must to be installed below the level of the nearest available sewer line. In these cases, wastewater must be lifted to the level of the main drain or sewer by a pumping system called a sewage ejector. Typically, a sewage ejector can pump solids from 2 to 4 in (50 to 100 mm) in size or grinds solid wastes before passing them through the ejector. Img 16 shows an installed swage ejector.

A sewage ejector system consists of the sump basin, a motor-pump assembly, and a system of automatic electrical controls. Wastewater from the sanitary pipes flow by gravity into the sump basin, a pit that collects wastewater. As the waste water level rises, it triggers a float switch that activates the pump. The pump then lifts the wastewater through a check valve and discharge line into a typical building drain line, where it gravity flows into the building sewer. It operates much like a sump pump.

The check valve in the discharge line prevents backflow. Without it, the pump will cycle continuously. A vent pipe connects to the sump basin to relieve the suction created by the pump. A high water alarm is generally added to the system, to warn of pump failure or backup to prevent flooding. Basins are typically fabricated of fiberglass, cast iron, or high-density polyethylene thermoplastic; they are typically set in a hole in a concrete floor slab.

A single ejector pump is installed in a small system, such as a single-family residence or small commercial building.

Larger commercial and industrial installations require two pumps to ensure continued operation if one pump fails. The additional pump also provides extra capacity in times of extra heavy loads.

The size and capacity of a sewage ejector system is determined by the application. The manufacturers' literature specifies the capacity of the pump and the maximum size of wastewater solids that can be handled by a particular pump. At tempting to eject solid matter that exceeds this rated size or materials that expand in water have the potential to clog the system.

Typically, residential ejector systems must have the capacity of ejecting solids up to 2” (50 mm) in size. Depending on pump impeller design, a 4-in pump will normally handle spherical solids from 2 to 3 in and typically range in motor size from 1/3 to 2 horsepower. Additionally, these systems are generally rated to a maximum temperature of 180°F (82°F).

Sizing commercial or industrial installations involves application of complex formulas. In each case, the design must consider total dynamic head, the highest vertical point, and the size of the basin provided. The farther the distance the waste must be lifted, the more powerful the pump must be to do the job. Regardless of peak flow requirement for a given application, the pump must always be able to provide a mini mum velocity of 2 ft per second through the line. Typically, in residential and small commercial applications, the water sup ply fixture unit (WSFU) load can be used to estimate usage demands of plumbing fixtures served. Larger capacity systems are required in motels, apartment complexes, and large office buildings because of higher peak demands. The installation must conform to the local building code.

Img 16 A sewage ejector.

2 DRAIN AND VENT PIPE DESIGN

Drainage Fixture Units

The draining rate for plumbing fixtures is based upon the drainage fixture unit (DFU). Refer to Table 1. Similar to the water supply fixture unit introduced in Section 13, the DFU is an arbitrarily chosen measure that allows all of types of plumbing fixtures to be expressed in common terms; that's , a fixture having twice the instantaneous drainage flow rate of a second fixture would have a fixture unit value twice as large. The WSFU and DFU may differ slightly for a single fixture, be cause the rates of filling and draining are different.

Design Approach

The approach used to size drain and vent lines relies on tabular information found in code. Table 2 indicates the maximum load in DFU and maximum pipe length for a given pipe diameter. The minimum pipe diameter is based on the total connected DFU. In the case of vent lines, maximum developed length for a given pipe is also a criterion. Developed length is the "center line" length of the lines, excluding traps and trap arms. It is important to ensure that a larger pipe diameter does not flow into a pipe having a smaller diameter.

tbl 1 DRAINAGE FIXTURE UNITS (DFU) AND MINIMUM TRAP SIZE FOR SELECTED PLUMBING FIXTURES.

Type of Fixture or Group of Fixtures Automatic dishwasher Bathtub group (water closet, lavatory, and bathtub or shower stall)

Bathtub Bidet Clothes washer Combination sink/tray with waste disposal (grinder)

Combination sink/tray with one 1 1/2 in (38 mm) trap Combination sink/tray with separate 1 1/2 in (38 mm) trap Dental unit or cuspidor Dental lavatory Drinking fountain Dishwasher (domestic) Dishwasher (commercial)

Floor drain with 2 in (50 mm) waste Kitchen sink (domestic) with 1 1/2 in (38 mm) trap Kitchen sink (domestic) with waste disposal (grinder)

Kitchen sink (domestic) with waste disposal and dishwasher, 1 1/2 in (38 mm) trap Kitchen sink (domestic) with dishwasher, 1 1/2 in (38 mm) trap Lavatory with 1 1/4 in (32 mm) waste Laundry tray (1 or 2 compartments)

Shower stall (domestic) Showers per head (group)

Sink (surgeon's) Sink (flushing rim with valve)

Sink (service-trap standard)

Sink (pot, scullery) Sink per faucet (circular or multiple wash)

Urinal (siphon jet blowout)

Urinal--Urinal (waterless) Water closet (private) Water closet (general use)

Waste disposal (commercial)

Waste disposal (domestic)

Fixture not listed-trap size of 1 1/4 in (32 mm) or less Fixture not listed-trap size of 1 1/2 in (38 mm) or less Fixture not listed-trap size of 2 in (50 mm) or less Fixture not listed-trap size of 21/2 in (65 mm) or less Fixture not listed-trap size of 3 in (75 mm) or less Fixture not listed-trap size of 4 in (100 mm) or less

Traps and trap arms are sized based on a specific type of fixture. Refer to Table 1 for minimum trap sizes.

Some fixtures such as urinals and water closets have integral traps built into the fixture so trap size does not need to be specified.

tbl 2 MAXIMUM DRAINAGE FIXTURE UNITS (DFU) THAT MAY BE CONNECTED TO HORIZONTAL FIXTURE BRANCH AND STACK BASED ON PIPE SIZE.

tbl 3 MAXIMUM DRAINAGE FIXTURE UNITS (DFU) THAT MAY BE CONNECTED TO A BUILDING DRAIN OR BUILDING SEWER BASED ON PIPE SIZE WHEN THE BUILDING DRAIN AND SEWER SERVES ONE BUILDING.

Example 1

The following number and type of plumbing fixtures serve two apartment units: two bathtubs, two water closets, two lavatories, and two kitchen sinks. Assume the horizontal fixture branch serving these fixtures flows into the waste stack. Assume the vent stack extends through the roof and is 22 ft long. Determine the minimum pipe diameter required for the horizontal fixture branch, waste stack, and vent stack.

From Table 1, the DFU values for the plumbing fixtures are extracted. DFU are then totaled:

2 Bathtubs (2 DFU each) 4 2 Water closets-flush tank (4 DFU each) 8 2 Lavatories (1 DFU each) 2 2 Kitchen sinks (2 DFU each) 4 Total DFU: 18 For the horizontal fixture branch, from Table 2, a 3-in diameter pipe is selected. A 3-in diameter pipe used as a horizontal fixture branch can serve up to 20 DFU.

For the waste stack, from Table 2, a 21/2 in diameter pipe can be selected but the 3 in diameter horizontal fixture branch would then flow into a smaller pipe. A 3 in diameter waste stack is a prudent choice.

For the vent stack, from Table 4a, a 1 1/2 in diameter pipe is selected, based on a 3 in diameter soil and waste stack and a capacity of up to 30 DFU.

Example 2

The following number and type of plumbing fixtures serve six apartment units with two apartments on each floor: six bathtubs, six water closets, six lavatories, and six kitchen sinks. Assume horizontal fixture branches serving these fixtures flow into the waste stack at three locations (three intervals), two apartments per interval. Assume the building drain is sloped at 1/4 in per ft and the vent stack extends through the roof and is 42 ft long. Deter mine the minimum pipe diameter required for the horizontal fixture branches, waste stack, building drain, and main vent stack.

6 Bath tubs (2 DFU each)

12 6 Water closets-flush tank (4 DFU each) 24 6 Lavatories (1 DFU each) 6 6 Kitchen sinks (2 DFU each) 12 Total DFU: 54

For the horizontal fixture branch, from Table 2, a 3 in diameter pipe is selected. A 3 in diameter pipe used as a horizontal fixture branch can serve up to 20 DFU. Two apartment units have two bathtubs, two water closets, two lavatories, and two kitchen sinks-a total of 18 DFU (see Example 2).

For the waste stack, from Table 2, a 4 in diameter waste stack is selected. A 3 in diameter pipe used as a waste stack can serve up to 240 DFU.

For the building drain, from Table 3, a 4 in diameter pipe is required. A 4 in diameter pipe used as a building drain can serve up to 216 DFU at a slope of 1/4 in/ft.

For the vent stack, from Table 4a, a 21/2 in diameter pipe is selected, based on a 4 in diameter soil and waste stack, a capacity of up to 100 DFU, and a developed length of 42 ft.

tbl 4A REQUIRED VENT DIAMETER AND MAXIMUM LENGTH OF VENT BASED ON SOIL AND WASTE STACK SIZE AND CONNECTED DRAINAGE FIXTURE UNITS (DFU).

tbl 4B REQUIRED VENT DIAMETER AND MAXIMUM LENGTH OF VENT BASED ON SOIL AND WASTE STACK SIZE AND CONNECTED DRAINAGE FIXTURE UNITS (DFU), FOR SI (METRIC) UNITS.

Img 17 A view of the interior of a plumbing (wall) chase. DWV lines are cast iron (black).

Img 18 A drain stack interval in a high-rise condominium building.

Ill. 11 In large installations with multiple buildings, pipe funnels house pipes. They allow easy access.

3 SYSTEM INSTALLATION

On a small project, the drainage piping typically varies in size from 1 1/2 to 4 in. It can be much larger in large hotels, apartments, and office buildings. This larger size of pipe often re quires special provisions in wall width or furred-out areas.

Poured concrete slabs will require that the plumbing lay out be carefully considered. The pipes need to be placed in the ground before the slab is poured, so accurate placement is crucial. Typically, both the water supply and drainage pipes are laid out next to each other, as they go to the same areas of the building. String is usually stretched over the slab area to mark where the pipes should be located. Many times, they are planned so they will come up in a wall. However, the tub, shower, and water closet piping will need to be placed in the exact location where the fixture is to go. All piping must be carefully located and the system checked for leaks before the concrete is poured because any relocation or repairs of pipes would be costly.

On larger projects with concrete walls and ceilings, it's usually necessary to provide sleeves (holes) in the concrete for the pipes to pass through to get from space to space. It will also be necessary to provide inserts and hangers to support the pipes.

The open spaces provided in truss-type construction make it easier to run piping through to the desired location. The only points of difficulty would be where it needs to pass by ductwork or some other large pipe that's going in the opposite direction. This will require coordination with the contractor in stalling any heating, air conditioning, or ventilating ductwork.

In wood frame construction, the holes are sometimes drilled to allow the passage of the pipes. These should be at the middle of any load-bearing wood members so that a minimum of structural damage is done. There are times when the width of a wall needs to be increased to allow for pipes running horizontally to pass by drainage pipes (or other pipes) running vertically.

Pipe chases run from floor to floor to allow stacks and vents to pass vertically between floors. A view of the interior of a plumbing (wall) chase is shown in Img 17. Chases are typically located alongside elevator hoist ways and common plumbing walls. A drain stack is shown in Img 18. Pipe tunnels (Ill. 11) may be used on large projects to provide concealed space for the passage of mechanicals at ground level and from building to building. Hangers from the top or side of the tunnel are used to support the pipes. Access may be from either end of the tunnel or access floors may be provided.

4 SANITARY DRAINAGE AND VENTSYSTEM DESIGN EXAMPLE

Ill. 12 Soil and vent stacks run vertically from floor to floor and are located near plumbing fixtures.

Ill. 14 Fixture branches connect to a soil stack, which then connects to the building drain. Vents are not shown for clarity.

Ill. 13 Trap arms from the kitchen sink (ks), bathroom lavatory (lav), and bathtub (tub) connect to a fixture branch, which then connects to a soil stack. An integral trap is found in a water closet (wc), so a trap does not need to be constructed in the piping.

Ill. 15 A vent stack parallels the soil stack and extends to through the roof. Individual vents are not shown for clarity.

Ill. 16 Trap sizes are noted on the drawing. Individual vents are not shown for clarity.

Ill. 17 A fixture branch for the design example.

The following example conveys a typical design approach for the four-story apartment building used in the water supply de sign example in Section 13. A review of the drawings for the apartments (in Section A) shows that each floor contains two bathroom groups and two kitchen sinks. Assume the project will be connected to a community sewer. In this project, it's assumed that there will be two plumbing stacks, one for each tier of apartments (See Ill. 12).

1. The first step is to sketch an isometric drawing of the drainage piping. The stack is located on the plan as shown in Ill. 12. The vertical stack is then sketched as shown in Ill. 13, and beginning at the top floor, the fixture branch and the connection at each fixture. Next, the other three floors are sketched as shown in Ill. The vent stack is added from a point just below the bottom fixture branch to a point above the top fixture and the building drain (See Ill. 15).

2. Next, the minimum trap size for each fixture is selected from Table 1 and the trap size noted on the schematic pipe layout, as shown in Ill. 16. Assume the kitchen sink has a small P.O. (plumbing outlet) plug.

3. The fixture branch is the first drainage pipe to be sized.

The first portion of the branch to be sized is from the fixture farthest from the stack to the next fixture, as shown in Ill. 17. Begin by determining the fixture units that this short piece of pipe will serve from Ill. 17.

In this problem, the branch serves a bathtub with a fixture unit value of 2, based on Table 1.

4. Next, select the branch size to serve the bathtub from Table 2. In this case, the horizontal fixture branch size is being selected, so go to that column (Table 2). Go down the column until the fixture-unit number is the same as or more than the amount being served-in this case, 3 DFU, which is greater than the 2 DFU value of the bathtub. Now, move horizontally to the left and select the minimum pipe size required. In this case, a 1 1/2 in or 38 (40) mm. Now, note the information accumulated in tabular form as shown:

DFU Pipe Size

No. 1 tub 2 1 1/2 in (38 or 40 mm)

For all fixtures, there is a maximum horizontal distance between the fixture trap and a vent. The distance depends on the pipe size being used. In this case, the plumbing code states that this distance is 5 ft (1.5 m).

5. DFU. The section of pipe that serves the lavatory and kitchen sink must be sized. From Table 1, the number of fixture units for the kitchen sink being served is 2 DFU and for the lavatory, 1 DFU, for a total of 3 DFU. Refer ring to Table 2, the minimum pipe size is 1 1/2 in or 38 mm. The maximum horizontal distance from fixture trap to vent is limited to 5 ft (1.5 m). The accumulated in formation is listed as follows.

DFU Pipe Size

No. 2 sink/lav. to branch 3 1 1/2 in (38 or 40 mm)

6. The section of branch marked No 3 serves the tub, kitchen sink, and lavatory. From Table 1:

Bathtub 2 DFU Kitchen sink 2 DFU Lavatory 1 DFU Total 5 DFU From Table 2 the pipe size is 2 in or 50 mm. Add this information to the tabular form.

DFU Pipe Size No. 3 sink, lav., and tub 5 2 in (50 mm)

7. Now, the last section (No. 4) is calculated and tabulated.

At this point, the branch services all of the fixtures, which are a fixture group and a kitchen sink. From Table 1, the fixture units are:

1 bathroom group 6 DFU 1 kitchen sink 2 DFU Total 8 DFU

From Table 2, the pipe size is 2 1/2 in or 65 mm, but since the pipe can't be smaller than the largest fixture trap, a 3-in or 75-mm pipe is required (water closet, 3-in or 75-mm trap).

Ill. 18 A horizontal fixture branch for the design example.

Ill. 19 Assigned drainage fixture units for the design example. Individual vents are not shown for clarity.

Ill. 20 Pipe sizes for the design example. Individual vents are not shown for clarity.

DFU Pipe Size

No. 4 bathroom and sink 8 3 in (75 mm)

8. With all branch sizes tabulated (Table 5), transfer the sizes to the isometric sketch, as shown in Ill. 18.

Because each floor is exactly the same, all of the branch sizes are the same, and all floors are noted on the isometric drawing. If any of the branches were different, the same procedure would be followed to determine the pipe sizes in these different branches.

9. The stack size is selected next. Because the stack handles human waste, it's a soil stack. The stack must be selected and sized for the total fixture units that it must handle. As noted on the sketch in Ill. 19, each floor has a total of 8 DFU, for an overall total of 32 DFU.

Based on 32 DFU for the entire stack, the minimum stack size from Table 2 would be 21/2 in, except that the stack must be at least as large as the fixture branch: in this de sign, it would be 3 in. Note the stack size on the sketch as shown in Ill. 20 and tabulated in Table 6.

In metric (SI) units, based on 32 DFU, for the entire stack, the minimum size from the chart would be 65 mm, except that the stack must be at least as large as the fixture branch- in this design, 75 mm. Note the stack size on the sketch as shown in Ill. 20, and tabulated in Table 6.

10. The vent stack is sized next, using the table in Table 4.

It is necessary to know the maximum size of the soil stack (left column) and the fixture units connected to the vent stack; the maximum length of vent stack is noted in feet. The soil stack has already been sized, and the DFU have been totaled as 32 for this stack. The developed length of the vent stack is the length of pipe required from the lowest point where it connects with the soil stack to the point where it terminates outside the building, as illustrated in Ill. 21.

The next step is to determine the size and location of the main (vent) stack. The code requires that every building "shall have at least one main stack." This main stack must be sized to handle the total fixture units on the system.

In this exercise, the developed length is about 52 ft. To size the stack use Table 4, find the soil stack size (4 in) and then move horizontally to the right to the fixture-unit column. There are three listings for a 4-in stack, all with different fixture-unit values. In this case, there are 64 DFU, which means the listing of 100 DFU must be used. Now move to the right; the next number represents the developed length of vent in the design; in this case, 52 ft. In the table, it's more than 35, so the 100 DFU column is used.

From the 100 DFU, move vertically up to read a vent size of 2 1/2 in.

The plumbing code states that the main vent and vent stack shall be sized in accordance with the table "and be not less than 3” in diameter." In addition, it states that the size of the vent stack must not be reduced all the way through the roof. So in this design, the minimum vent stack size of 3 in is selected.

In metric (SI) units, the developed length is about 15.9 m.

To size the stack, find the soil stack size (100 mm) and move horizontally to the right to check the fixture unit column. At this point, note that there are three listings for a 100 mm soil stack, all with different fixture-unit values.

In this case there are 64 DFU, which means the listing of 100 DFU must be used. Now move to the right; the next number represents the developed length of vent in the design-in this case, 15.9 m. In the table it's more than 11 m, so the 30 m column is used. From the 30 m, move vertically up to read a vent size of 65 mm.

The plumbing code states that the main vent and vent stack shall be sized in accordance with the table "and be not less than 75 mm in diameter." In addition, it states that the size of the vent stack must not be reduced all the way through the roof. So in this design, the minimum vent stack size of 75 mm is selected.

Code requirements also stipulate that the vent stack must terminate no less than 6 in (150 mm) above the roof (Ill. 22), and if the roof is to be used for other than weather protection (for example, as a terrace or balcony), the vent stack must run at least 5 ft (1.5 m) above the roof. Most codes require at least a 3-in (75-mm) vent through the roof.

Ill. 21 The developed length of a vent is computed to its terminating point at the roof.

Ill. 22 Vents must terminate away from walls, windows, and doors.

Ill. 23 Sketch of building drain for the design example.

Ill. 24 Tabulation and sketches for building drain size.

11. The next step in the design is to size the building drain. The building drain must be sized for the total amount of DFU connected to the point at which the main vent is connected.

In this exercise, there are two stacks serving the building, which means that the two stacks must be connected to the building drain. A sketch similar to Ill. 23 helps to prevent any confusion or the possibility that a stack may be forgotten. On the sketch, and in tabular form, list the accumulated fixture units.

Next, the slope of the drain must be determined and noted on the sketch; in this case, a 1/8-in slope per ft (10.4 mm per m) is selected. Building drain sizes are found in Table 3. In this design, the main vent is considered to be the end vent, as noted in Ill. 24, so the entire building drain must be sized for 64 DFU.

12. The plumbing drainage system has now been designed, with the exception of checking the location of the vent in relation to the fixtures. As the fixture branch sizes were being selected, the maximum distance from the trap to a vent was also tabulated (Table 5).

In reviewing the fixture branch and trap sizes in Table 5, each of the tub, lavatory, and sink traps can be no more than 5 ft (1.5 m) from a vent, and the water closet no more than 10 ft (3 m). A review of the floor plan finds that the total width of the bathroom is 7 ft-6 in (2.25 m). This means that the vent could be located in the wall near the lavatory and be within the limits of all listed horizontal distances.

If fixtures are so spread out that one or more fixtures are beyond the maximum horizontal distance, there are two options:

a. Increase the size of the horizontal fixture branch, which automatically increases the allowable horizontal distance. For example, change length 1 from 1 1/2 in (38 mm) to 2 in (50 mm), and it changes the horizontal distance from 5 ft to 8 ft (1.5 m to 2.4 m). Many times this is the most economical solution.

b. Add more vents. Instead of trying to service a group of fixtures with one vent, perhaps a vent could be added to service any fixtures that are be yond the allowable distance. A variety of venting solutions for various groups of fixtures is shown in Figs. 25 through 28.

In this example, a single vent off the top of the lavatory- kitchen sink fixtures (referred to as a wet vent and discussed in the following paragraph) will be used.

The size of the wet vent is selected from Tables 4 and Ill.s 25 through 28. But a little explanation is needed.

First, a review of the sketch in Ill. 25 shows that the portion of the vent referred to as a wet vent is actually a part of the drainage system for the lavatory and the kitchen sink. Because it acts as both a vent and a drainage pipe, it's referred to as a wet vent.

For single bathroom groups, selection of the wet vent is based on the number of fixture units that the wet vent serves. A careful review of the code requirements for wet venting a multistory bathroom group (Ill. 28) indicates that a wet vent and its extension must be 2 in diameter and can serve the kitchen sink, lavatory, and bathtub. In Ill. 28 it states, "each water closet below the top floor is individually back vented" unless wet vented as illustrated in Ill. 26. When the minimum size of the vent is larger than the waste sizes previously selected, the larger size must be used. Note the size of the venting required on the schematic sketch, and the plumbing drainage design is completed for this project.

Ill. 25 Venting options.

Ill. 26 Vent for a single-family dwelling for the design example.

Ill. 27 Wet venting at the top floor for the design example.

Ill. 28 Wet venting below the top floor for the design example.

QUIZ:

1. What is a trap, where is it located, and how does it work?

2. Why are vents required on the waste system? Where are they located in reference to the fixture?

3. What is a wet vent, and how does it differ from other types of vents?

4. What is the difference between a stack vent and a vent stack? Using a sketch, show the location of a stack vent and a vent stack in a multistory design.

5. What is the difference between a soil stack and a waste stack?

6. Sketch and locate the house (building) drain and the sewer.

7. What provisions must be made to provide drainage for fixtures located below the level of the building drain and the sewer?

8. Why does the designer make sketches of the drainage piping?

Design Exercises

9. The following number and type of plumbing fixtures serve an apartment: one bathtub, one water closet, one lavatory, one kitchen sink, and one dishwasher. Assume the horizontal fixture branch serving these fixtures flows into a waste stack. Assume the vent stack extends through the roof and is 14 ft long. Determine the minimum pipe diameter required for:

a. The horizontal fixture branch

b. The waste stack at base of stack

c. The vent stack at top of stack

10. The following number and type of plumbing fixtures serve a condominium: two bathtubs, two water closets, two lavatories, one kitchen sink, and one dishwasher.

Assume the horizontal fixture branch serving these fixtures flows into a waste stack. Assume the vent stack extends through the roof and is 18 ft long. Determine the minimum pipe diameter required for:

a. The horizontal fixture branch

b. The waste stack at base of stack

c. The vent stack at top of stack

11. The following number and type of plumbing fixtures serve a residence: two bathtubs, three water closets, three lavatories, one kitchen sink, and one dishwasher.

The bathroom fixtures are served by one set of waste and vent stacks. The kitchen fixtures are served by a second set of waste and vent stacks. Assume the vent stacks extend through the roof and are 20 ft long.

Determine the minimum pipe diameter required for the kitchen fixtures:

a. The horizontal fixture branch

b. The waste stack at base of stack

c. The vent stack at top of stack Determine the minimum pipe diameter required for the bathroom fixtures:

d. The horizontal fixture branch

e. The waste stack at base of stack

f. The vent stack at top of stack

12. A high-rise condominium building has condominium units stacked symmetrically from floor to floor. The following number and type of plumbing fixtures serve each condominium: one bathtub, two water closets, two lavatories, one kitchen sink, and one dishwasher.

Assume that the horizontal fixture branches serving these fixtures flow into a single waste stack and single vent stack. Each stack serves seven condominium units. Assume the vent stack extends through the roof and is 78 ft long. Determine the minimum pipe diameter required for:

a. The horizontal fixture branch

b. The waste stack at base of stack

c. The vent stack at top of stack

13. A high-rise condominium building has condominium units stacked symmetrically from floor to floor. The following number and type of plumbing fixtures serve each condominium: one bathtub, two water closets, two lavatories, one kitchen sink, and one dishwasher. Assume that the horizontal fixture branches serving these fixtures flow into a single waste stack and single vent stack. Each stack serves ten condominium units. Assume the vent stack extends through the roof and is 112 ft long. Deter mine the minimum pipe diameter required for:

a. The horizontal fixture branch

b. The waste stack at base of stack

c. The vent stack at top of stack

14. Design the plumbing drainage for the apartment building in Section B. Use acrylonitrile-butadiene-styrene copolymer (ABS) pipe and tubing for this design problem.

15. Design the plumbing drainage for the residence in Section C. Use ABS pipe for this design problem.

16. Design the plumbing drainage for the residence in Section D. Use ABS pipe for this design problem.

















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Updated: Tuesday, February 21, 2012 11:38