Building Water Supply System: Water Supply Design Concerns

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Water Velocity

Noise, erosion of inner pipe walls and valves, and economy of installation, operation, and maintenance dictate the minimum and maximum water velocity in a plumbing system; as a result, these have a bearing on pipe diameter. If pipe diameters are small, cost is low but noise, erosion (from high velocities), and pumping costs (from high-pressure losses) are high. In contrast, large diameter pipes reduce noise erosion and pumping costs, but result in high installation costs. An intermediate pipe diameter is desirable.

Typically, plumbing codes set velocity limits in water sup ply piping. Maximum water velocities in plumbing water supply piping are usually limited to a range of 5 to 10 ft/s (1.5 to 3 m/s).

Maximum velocities of up to 15 ft/s (4.5 m/s) are allowed for equipment feed lines in mechanical rooms (e.g., boiler feed lines) where noise is less of a concern. The maximum safe velocity for thermoplastic pipe is about 5 ft/s (1.5 m/s).

Cavitation

Cavitation is a physical phenomenon that occurs in a liquid when it experiences a drastic drop in pressure that causes the liquid to vaporize into small vapor bubbles. Vaporization is a problem because the liquid being vaporized expands greatly; for example, 1 ft^3 (or 1 m^3 ) of water at room temperature be comes 1600 ft^3 (or 1600 m^3 ) of vapor at the same temperature.

As the low pressure returns to normal pressure levels, these bubbles implode as the vapor changes phase back to a liquid and thus drastically decreases its volume. This implosion causes noise and high levels of erosion where the imploding bubbles contact the walls of a pipe, fitting, pump, or valve. The noise that develops sounds similar to gravel flowing through the system in the area where the cavitation is developing. Over time, the erosion results in excessive wear; this eventually manifests itself as pinhole leaking.

Valves can develop cavitation when they are partially closed and flow is restricted. The result is noise and possible damage from erosion. Cavitation can also develop in a pump, which is noisy and can adversely affect pump performance by causing violent and damaging vibration and a sharp drop in discharge pressure. It occurs if the pumped liquid on the suction side of the pump drops below its vapor pressure. Eliminating this potential for cavitation is necessary because a cavitating pump can be completely damaged in a few hours of operation.

Cross-Connections

A cross-connection is an unsatisfactory connection or arrangement of piping that can cause non-potable water to enter the potable water system. A cross-connection can cause used or contaminated water to mix with the water supply. It is an unsanitary and potentially hazardous condition. For example, a garden hose with one end immersed in a bucket of soapy water or in a swimming pool are possible backflow conditions. The American Water Works Association (AWWA) estimates over 100 000 cross-connections occur each day-half of these from garden hoses.

Most plumbing fixtures are designed to prevent a cross connection. A gap exists between the faucet and the rim of the bowl in lavatories, sinks, and tubs to create a separation and avert a cross-connection. An air gap is the vertical distance through open air between an opening in a fixture or faucet conveying potable water to the flood level rim of a tank or fixture.

As a general rule, the minimum air gaps for cross-connection protection for fixtures against one wall are as follows:

  • Lavatory 1 in (25 mm)
  • Sink 1 1/2 in (38 mm)
  • Laundry tray 1 1/2 in (38 mm)
  • Bathtub 2 in (51 mm)

Air gaps must be increased if the lavatory, sink, or tray is against two walls instead of one. Building codes will cite the minimum air gap based on fixture type; this is typically two times the inner diameter of the pipe (2D inside) serving the fixture.

IMG. 23 A double check assembly (DCA) is a backflow prevention device that consists of two check valves assembled in series usually with a ball valve or gate valve installed at each end for isolation and testing. The DCA shown is for the water supply to a fire sprinkler system.

Backflow

Backflow is a type of cross-connection that occurs when contaminated water or some other liquid or substance unintentionally flows backwards into distribution pipes containing potable water. Simply, it's water flowing in the opposite direction from normal flow. Backflow can allow contaminants to enter the potable drinking water system through cross-connections. A backflow can be a serious plumbing problem that causes illness and even death. There are over 10 000 reported backflow situations that develop in the United States each year.

Backpressure or back siphoning is backflow caused by a negative pressure (vacuum) in a potable water system. A down stream pressure that's greater than the supply pressure causes backpressure backflow. It can result from a reduction in the sup ply pressure, an increase in downstream pressure, or a combination of both, such as from firefighting efforts, a water main break, consumer high-side pressure (pumped), or when a fire hydrant is opened for testing. Any buildings near such a break or unusual fire hydrant use will experience lower water pressure.

A backflow can be avoided by the use of proper protection devices. Backflow prevention protects the potable water system from minor, moderate, and severe hazards. A backflow prevention device, often called a vacuum breaker, is a device or plumbing assembly that when properly installed in a plumbing system prevents backflow. Different types of backflow prevention assemblies are required depending on the extent of the hazard. A high hazard exists when there is danger that back flow could create a health threat. Examples of this classification include lawn irrigation systems with chemical injection, hospitals, and manufacturing plants where dyes or chemicals are mixed. Moderate hazards occur when there is no health threat, but backflow could cause discolored, smelly, or objectionable water. Retail stores and office buildings are examples of this classification.

The atmospheric vacuum breaker (AVB), the most common type, consists of a body, a check valve-like member (to prevent backflow), and an atmospheric opening. The AVB is not a testable device. A pressure vacuum breaker (PVB) is a type of backflow prevention device used to keep nonpotable (or contaminated) water from entering the water supply. A PVB is similar to an AVB, except that the PVB contains a spring loaded poppet. This makes it acceptable for applications that are high hazard or where valves are downstream. PVB devices are generally required on small (residential size) irrigation (sprinkler) systems to keep water contaminated with pesticides and fertilizers from reentering the building's plumbing system through the irrigation system. Generally, the PVB must be in stalled on the main line leading to the control valves. It must also be installed above ground and it must be 6 in (150 mm) higher than the highest sprinkler head or drip emitter controlled by any of the valves.

A double check assembly (DCA) or double check valve is a backflow prevention device assembly that consists of two check valves assembled in series usually with a ball valve or gate valve installed at each end for isolation and testing. See Img. 23.

Test cocks (small valves) are in place to attach test equipment for evaluating whether the double check assembly is functional. The DCA is suitable for prevention of back pressure and back siphonage, but is not suitable for high hazard applications. A DCA is commonly used on fire sprinkler and boiler systems.

Water Hammer

A large pressure develops when fluid moving through a pipe is suddenly stopped. In a plumbing supply system, the sudden closing of a valve will cause fast-flowing water to stop quickly, resulting in a large increase in pressure that's known as water hammer. For example, in a pipe with water flowing at 10 ft/s (3 m/s), the maximum theoretical pressure that will develop if a valve is instantly closed is 635 psi (4.35 MPa). Although the elasticity of the pipe material reduces this theoretical pressure to some extent, a large pressure and force still develops against the valve and inner walls of the pipe.

As a minimum, water hammer produces a force that makes pipes rattle with banging or thumping sounds as they expand and contract from exposure to an increase in water pres sure. This sound is frequently heard as a loud thump when the automatic shut-off valve on an older appliance such as a dish washer or clothes washer rapidly shuts off the supply of water.

In extreme cases when the water flow rate is high and the valve is closed very rapidly, water hammer can cause a valve to rupture and the pipe walls to burst.

To avert water hammer damage to a buildings plumbing system, air chambers or water hammer arrestors are used in the supply branches serving each fixture. These devices use trapped air to cushion the hydraulic shock.

Air Chambers

Air chambers are 15 in to 5 ft long pipes or pipe-like devices.

They are installed vertically above the fixture water connection and are concealed in the wall. Air is trapped within the air chamber. The trapped air is compressible, which cushions the pressure surge as the valve is closed and absorbs the hydraulic shock.

Water Hammer Arrestors

Water hammer arrestors are patented devices that absorb hydraulic shock. (See Ill. 9 and Img. 24). Such de vices, when installed, must be accessible for maintenance. One type should be placed at the end of the branch line between the last two fixtures served. Additional arrestors should be placed at the midpoint of a run longer than 20 ft.

ILL. 9 A cross-section view of a water hammer arrestor, a patented device that absorbs hydraulic shock.

ILL. 10 Types of expansion bends and loops. Thermal expansion in the pipes must be accommodated to minimize damage from thermal movement. Expansion bend Riser Expansion space Expansion loop Horizontal offset Horizontal offset Spring piece Compensating loop

IMG. 24 Water hammer arrestors (two) above shut-off valves at a clothes washing machine connection.

TBL. 4 DIMENSIONAL CHANGE FROM THERMAL EXPANSION OF SELECTED PIPE MATERIALS.

Pipe Material Cast iron Stainless steel (12% chromium)

Wrought iron/steel Monel (nickel/copper alloy)

Copper Brass Chlorinated polyvinyl chloride (CPVC) Acrylonitrile-butadiene-styrene (ABS) Polyvinyl chloride (PVC) Polybutylene (PB) Cross-linked polyethylene (PEX)

Thermal Expansion

No matter what type of piping material is used in the water system, some expansion in the pipe will occur. This expansion must be considered in the design of the system. The amount of expansion will depend on the type of piping material and the range of temperatures that the pipe will be subjected. Expansion of copper tubing is slightly greater than one inch in a 100 ft length in a 100°F temperature change. Plastic pipe will expand even more; up to 10 inches in a 100 ft length in a 100°F temperature change. See Tbl. 4.

Piping for commercial hot water may have to withstand a temperature range from about 68°F (20°C), the average indoor temperature, to over 180°F (82°C), the temperature of the sanitary rinse water. In a residence, the upper limit for hot water pipes is usually 125°F (52°C). Cold water piping will be subjected to a much smaller temperature range, usually with a low of 35°F (2°C) and a high of about 80°F (27°C). These ranges will vary, sometimes considerably, on projects and must be checked.

Thermal expansion in the pipes of a plumbing system must be accommodated to minimize damage from thermal movement. Expansion from temperature increases can push a pipe through a wall or cause it to burst. There are two methods in common use for providing for expansion in pipelines: expansion bends and expansion joints.

Expansion bends and expansion loops make use of pipe fabricated with U-shaped or circular bends. (see Ill. 10).

The increase in the length of pipe from thermal expansion is accommodated by flexing or springing of the bends or loops. A connection of four elbows and three short sections of straight pipe, connected in the form of a U-shape between the long lengths of pipe, can also be used to accommodate thermal expansion.

Expansion joints in common use include the slip expansion joint and the corrugated expansion joint. The slip expansion joint consists of a slip pipe and a flange, which is bolted to an adjoining pipe. The slip pipe fits into the main body of the joint, which is fastened to the end of the other adjoining pipe.

When piping expands with temperature change, the slip pipe slides into the joint body. To prevent leakage between the slip pipe and the joint body, packing is used around the outside of the slip pipe and the slip pipe moves within the packing.

A corrugated expansion joint consists of a flexible corrugated section. The corrugated, accordion-like section is able to absorb a certain amount of end movement of the pipe.

Viscosity

As water flows through a pipe, its viscosity (thickness) de creases with temperature decrease. Water at 40°F (4°C) is twice as viscous as water at 90°F (32°C) and four times as much at 170°F (77°C). As a result, pumping energy and cost are higher when water temperatures are lower.

Volume Change with Temperature Change

Water is the only substance that can exist as a solid, liquid, and gas at ordinary temperatures. Like most substances, water expands when it's heated. Unlike most substances, the volume of water increases when it freezes. Below 39°F (4°C), water molecules begin to align themselves into the crystal structure of ice, causing its volume to increase slightly. Under normal conditions, water freezes at 32°F (0°C). It expands significantly when the molecules of water are pushed farther apart than they were in a liquid state.

Freezing

A phase change from liquid (water) to solid (ice) results in about a 10% increase in volume.

Example 4

Water in a 50-gal water heater cools to a temperature below 32°F (0°C) and freezes fully. Determine the volumetric change, in gallons, as a result of freezing.

delta_V _ 10% _ 50 gal _ 5.0 gal increase in volume

A pipe, fitting, or tank filled with water can burst if the water is exposed to below-freezing temperatures, even for a short period of time. A burst pipe will typically result in flooding that can cause catastrophic damage to the building and its contents. Flooding may occur as the water freezes and bursts the pipe, or after the frozen water thaws.

In most climates, provisions must be made in plumbing system design to ensure that pipes containing water don't freeze. Pipes must be located in a heated space or underground, where they are not exposed to freezing temperatures. Water supply lines located in unheated attic and crawl spaces are susceptible to freezing, particularly if they are located next to a crawl space or attic space vent, or make-up air (outdoor air) inlet. Water supply pipes should be located in interior plumbing walls when possible. Water pipes located inside exterior walls must be placed on the heated side of the wall insulation. Under ground lines must be adequately insulated by soil and should be placed well below the frost line.

Expanding Water

Liquid water expands above 39°F (4°C). Expansion is about 4.37% from 40°F (4.4°C) to 212°F (100°C). This volumetric change from expansion (?V) equates to about 0.0254% per °F (0.0457% per °C).

Example 5

Water in a 50-gal water heater is heated from a temperature of 50°F (10°C) to 125°F (51.7°C). Determine the volumetric change, in gallons.

1.91% _ 50 gal _ 0.96 gal

_ 1.91% increase in volume delta_V _ 0.0254% per °F(125°F _ 50°F)

In any piping system, provisions for expansion and con traction of liquid water must be considered. In an open plumbing system, such as a building water supply system, pressure buildup from thermal expansion of water is released each time a faucet or valve opens. Excess pressure is released into the surrounding air.

In a closed plumbing system, where water is contained fully within the system (e.g., a hydronic heating system), provisions are necessary. A temperature-pressure relief (T/P) valve is a safety valve installed in a system that remains closed at normal operating pressures yet is permitted to open to release excessive pressure.

They are commonly found as a safety feature on water heaters and boilers. Expansion tanks, installed in a closed system, provide additional volume in the closed system for expansion of water from temperature increase. See Imgs. 25 and 26.

IMG. 26 A temperature-pressure relief valve on a storage tank water heater.

IMG. 25 An expansion tank for potable water.

Steam

Under standard atmospheric pressure (14.696 psi, 101.04 kPa), the boiling point temperature of water is 212°F (100°C). As discussed in Section 12, the boiling point temperature varies with atmospheric or system pressure. Water boils and is converted to steam at its boiling point temperature. This phase change from liquid (water) to water vapor (steam) results in a 160 000% increase in volume (an increase of 1600 times).

Although steam is not found in building plumbing systems under ordinary conditions, a faulty water heating appliance such as a boiler or domestic water heater may result in the development of steam (e.g., a water heater with defective thermostatic controls), which can have catastrophic results. Reports of malfunctioning boilers exploding and water heaters rocketing through the roof of a building are not exaggerated. Provisions must be made in a system to release steam and reduce the buildup of pressure.

A T/P valve is a safety device that prevents the buildup of pressure in a closed vessel such as a tank, water heater, or boiler. It is designed to open to release excessive pressure if such a malfunction occurs. A T/P valve is installed in any system or component that heats water.

IMG. 27 Pipe insulation with protective sheathing.

Aging

As pipes in a plumbing system are used, their inner walls become increasingly rough. The effects of aging in a plumbing system are related to piping material, quality of water (e.g., hard versus soft), and water temperature. Buildup from calcium deposits (especially in high-temperature hard water) and corrosion (especially in ferrous pipe materials) reduces the inside opening in the pipe, which restricts flow.

Over several decades of use, aging galvanized steel and iron pipe can result in a capacity loss of up to 80%. Copper piping experiences about half the capacity loss from aging as do steel and iron pipes. Plastic pipes experience little loss of capacity as they age.

Pipe Insulation

Pipe insulation is applied to the outer walls of piping to reduce heat loss from the pipe or prevent condensation on the outside pipe walls. Foam and covered fiberglass insulation are common pipe insulation materials. Hot water supply lines, especially on hot water recirculating systems, should be insulated to reduce heat loss and save energy. In most commercial buildings, heat loss increases the cooling load and the costs of air conditioning. In residential applications in temperate to cold climates, insulating hot water lines is less cost-effective because heat lost from hot water pipes contributes to space heating during the heating season; that's , the heat lost is extremely small. Building codes generally re quire pipe insulation to be 1/2 to 1 1/2 in thick in commercial installations, depending on pipe diameter, and 1/2 in thick in residential installations. When the pipe is exposed, insulation is usually cased with a protective plastic sheathing. See Img. 27.

Under high humidity conditions it's necessary to insulate cold water lines to keep condensation from forming. Cold water in a pipe cools the outside pipe walls, which lowers the temperature of the air surrounding the pipe. If the temperature of air surrounding the pipe drops below the dew point temperature, moisture condenses out of the air on to the pipe surface.

Dripping water from condensation can cause cosmetic or, over time, even structural damage. The potential for forming condensation increases when water is supplied from an under ground water source where water temperatures are significantly lower than indoor temperatures and flow is great, and when water flows through a pipe located in a cool unheated space (e.g., a crawl space or garage).

Testing

The water supply system should be tested for leaks before it's covered with finish materials to determine if it's watertight.

Tests commonly run on water systems require that it be water tight under a hydrostatic water pressure of 125 psi for a mini mum of 1 hr. Any leaks that occur should be repaired with the joint compound originally used.

Leaks

Surprisingly, plumbing leaks contribute significantly to water consumption in operating plumbing systems. Leaks account for about 12.7% of household per capita water use in a typical U.S. home (AWWA). A leak of just one drop per second will waste about 2700 gal (10 200 L) of water a year. Leaks not only waste money and water, they can cause damage to walls, flooring, ceilings, furniture, and electrical systems. Leaking pipes also create an environment for mold and mildew to thrive. Water damage claims may result in a building owner's insurance premiums being raised or nonrenewal of policies. Leaks can develop from substandard piping, improper use of materials, poor workmanship, and improper design.

Pinhole leaks in copper plumbing affects property owners throughout the United States and elsewhere in the world. Al though much research has been conducted around the issue, no definitive cause has been determined to date. Potential causes include the following: galvanic (dissimilar metal) corrosion; changes in water chemistry and chemical water compositions; the improper addition of chemicals during the water treatment process; aggressive water, which includes factors such as pH content, presence of chlorides, metal ions, and dissolved gases in the water; and excessive water velocity in water pipes (74 ft/s), which may erode or work on weak points in the materials.

TBL. 5 APPROXIMATE FLOW RATES AND TYPICAL CONSUMPTION BY TYPE OF PLUMBING FIXTURE AND LOCATION. FLOW RATE VARIES SIGNIFICANTLY BY RESIDUAL PRESSURE (WATER PRESSURE BEFORE THE FIXTURE).

Approximate Flow Rates by Fixture Type:

  • Bathtub faucet Clothes washer (laundry)
  • Dishwasher (domestic)
  • Drinking fountain; Hose bibb (1/2 in.)
  • Kitchen sink Lavatory faucet Lawn sprinkler (domestic)
  • Showerhead (low flow design)
  • Showerhead (standard flow design)
  • Toilet, flush tank Toilet or urinal, 1 in flush valve (15 to 30 psi)
Typical Consumption for Urban or Suburban Residence Per person per day, for all purposes Per shower, per use Fill bathtub, per fill; Fill sink, per fill Flush toilet, per flush Irrigate with 1/4 in of water, per 1000 ft2 of lawn Dishwasher, per load Washing machine, per load Domestic water softener regeneration cycle Typical Consumption for Rural Residence Per person per day, for all purposes Per horse, dry cow, or beef animal per day Per milking cow per day Per hog per day Per sheep per day Per 100 chickens per day.
















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Updated: Tuesday, February 21, 2012 12:00