Applications: Domestic Water Service and Distribution

Introduction

Throughout history, many materials have been used to carry water to, and through, homes. Wood, clay, iron and steel have all had their day - and their problems. Wood rots, clay collapses, iron and steel rust, and water and food vessels made of lead may have contributed to the fall of the Roman Empire, according to historians. Until the late 1920’s galvanized steel pipe was also used as the primary means of water distribution, but that material, too, had inherent problems, mainly the propensity to corrode internally, plug up and become ineffective in conveying the water it was intended to.

With that shortcoming in mind, an alternate material became preferred to carry the water for domestic water service and distribution – copper. This noble metal in addition to being eminently recyclable is also an ideal material for transporting the water we require for our daily existence for drinking, cooking and sanitary purposes. Copper forms a protective coating (patina) when in contact with the water passing through it that renders it extremely resistant to corrosion. It is also biocidal – germs don’t like copper - which also makes it a perfect material for transporting our potable water supply.

All copper water tube made in the U.S. is made to an industry standard developed by the American Society for Testing and Materials, a consensus standards writing body. That standard is ASTM B88, the manufacturing standard for seamless copper water tube. There are three (3) types of copper water tube, Type K - has the heaviest wall and is used for high pressure applications, Type L – used for medium pressure applications and Type M - a tube for low pressure applications.

There are two types of systems involved in bringing water to us. The first type is defined as Water Service and is that pipe or tube that brings the water from the water main in the street or your private well to the dwelling. This line is usually underground and for that reason copper is an excellent material for use as a water service line. Copper has excellent resistance to corrosion when buried in the soil. It can be provided as a coil that allows the use of longer runs that don’t require intermediate buried joints and it is easy to work with. State and local codes usually prescribe Type K or Type L copper tube for underground water service.

After entering the dwelling the water service line now becomes the Water Distribution system. At this point it will be divided to direct water to a water heater or boiler for hot water supply and./or hydronic heating and a cold water supply to all of the familiar fixtures and appliances that we utilize for our daily existence. Also, depending on state or local plumbing codes, usually either Type L or Type M copper tube will be used for the water distribution system.

Copper tube has been the material of choice for domestic water service and distribution for 75 years. Architects, builders, plumbers, and homeowners have recognized its quality and its use in domestic applications.

Designing Copper Tube Water Distribution System

Designing a copper tube water supply system is a matter of determining the minimum tube size for each part of the total system by balancing the interrelationships of six primary design considerations:

• Available main pressure
• Pressure required at individual fixtures.
• Static pressure losses due to height.
• Water demand (gallons per minute) in the total system and in each of its parts.
• Pressure losses due to the friction of water flow in the system.
• Velocity limitations based on noise and erosion.

Design and sizing must always conform to applicable codes. But in the final analysis, design must also reflect judgment and results of engineering calculations. Many codes, especially the model codes, include design data and guidelines for sizing water distribution systems and also include examples showing how the data and guidelines are applied.

Small Systems

Distribution systems for single-family houses usually can be sized easily on the basis of experience and applicable code requirements, as can other similar small installations. Detailed study of the six design considerations above is not necessary in such cases.

In general, the mains that serve fixture branches can be sized as follows:

• Up to three 3/8-inch branches can be served by a 1/2-inch main.
• Up to three 1/2-inch branches can be served by a 3/4-inch main.
• Up to three 3/4-inch branches can be served by a 1-inch main.

The sizing of more complex distribution systems requires detailed analysis of each of the sizing design considerations listed above.

Pressure Considerations

At each fixture in the distribution system, a minimum pressure of 8 psi should be available for it to function properly except that some fixtures require a higher minimum pressure for proper function, for example:

Local codes and practices may be somewhat different from the above and should always be consulted for minimum pressure requirements.

The maximum water pressure available to supply each fixture depends on the water service pressure at the point where the building distribution system (or a segment or zone of it) begins. This pressure depends either on local main pressure, limits set by local codes, pressure desired by the system designer, or on a combination of these. In any case, it should not be higher than about 80 psi (pounds per square inch).

However, the entire water service pressure is not available at each fixture due to pressure losses inherent to the system. The pressure losses include losses in flow through the water meter, static losses in lifting water to higher elevations in the system, and friction losses encountered in flow through piping, fittings, valves and equipment.

Some of the service pressure is lost immediately in flow through the water meter, if there is one. The amount of loss depends on the relationship between flow rate and tube size. Design curves and table showing these relationships appear in most model codes and are available from meter manufacturers.

Some of the main pressure will also be lost in lifting the water to the highest fixture in the system. The height difference is measured starting at the meter, or at whatever other point represents the start of the system (or the segment or zone) being considered. To account for this, multiply the elevation of the highest fixture, in feet, by the factor 0.434, the pressure exerted by a 1-foot column of water. This will give the pressure in psi needed to raise the water to that level. For example, a difference in height of 30 feet reduces the available pressure by 13 psi (30 x 0.434 = 13.02).

Friction losses in the system, like losses through the water meter, are mainly dependent on the flow rate of the water through the system and the size of the piping. To determine these losses, water demand (and thus, flow rate) of the system must first be determined.

Water Demand

Each fixture in the system represents a certain demand for water. Some examples of approximate water demand in gallons per minute (gpm) of flow, are:

• Drinking fountain - 0.75
• Lavatory faucet - 2.0
• Lavatory faucet, self closing - 2.5
• Sink faucet, WC tank ball cock - 3.0
• Bathtub faucet, shower head, laundry tub faucet - 4.0
• Sill cock, hose bibb, wall hydrant - 5.0
• Flush valve (depending on design) - 3.5
• Shower head - 2.2

Adding up numbers like these to cover all the fixtures in an entire building distribution system would give the total demand for water usage in gpm, if all of the fixtures were operating and flowing at the same time - which of course does not happen. A reasonable estimate of demand is one based on the extent to which various fixtures in the building might actually be used simultaneously. Researchers at the National Institute of Standards and Technology studied this question some years ago. They applied probability theory and field observations to the real-life problem of simultaneous usage of plumbing fixtures.

The result was a system for estimating total water demand which is based on reasonable assumptions about the likelihood of simultaneous usage of fixtures. Out of this study came the concept of fixture units.

Each type of fixture is assigned a fixture unit value which reflects:

1. Its demand for water, that is, the flow rate into the fixture when it is used
2. The average time duration of flow when the fixture is used
3. The frequency with which the fixture is likely to be used

Assigned fixture unit values vary by jurisdiction. Consult local plumbing codes for values used in your area.

Totaling the fixture unit values for all the fixtures in a system, or for any part of the distribution system, gives a measure of the load combined fixtures impose on the plumbing distribution and supply system. This fixture unit total may be translated into expected maximum water demand following the procedure prescribed by your local code.

Keep in mind the demand calculations just described apply to fixtures that are used intermittently. To this must be added the actual demand in gpm for any fixtures which are designed to run continuously when they are in use; for example, air-conditioning systems, lawn sprinkler systems and hose bibbs.

For additional details, please click here for the Copper Tube Handbook.