Installing Supply Piping

To avoid these problems, water supply piping should be installed below grade as follows. First, make the burial trench at least 12 inches (30 centimeters) wide on each side. Shovel-backfill with clean sand 6 inches (15 centimeters) above and below each pipe. Cover the piping with resin paper or strips of 90-pound roofing felt on top of the sand backfill.

In meter-sewage systems, place the main above and to one side of the sewer. Never locate the sewer above the main, one above the other, or side-by-side. Sleeve the piping where driveways cross over it with conduit of at least 4 inches (10 centimeters) larger diameter; and do not pass piping under footings but sleeve it through the foundation wall above.

Finally, know that the only sure data gathering is onsite inspection, so retain plan records of exact dimensions where piping is laid.

Just inside the building, a supply water main typically requires: 1) a service valve, 2) an accessible strainer that removes any grit or other particles from the inflowing water, 3) an easily visible meter that measures the water flow, 4) a backflow preventer that keeps any water in the building from flowing backward and contaminating the public water supply, 5) a shut-off valve with a drain port for emptying the plumbing system if necessary, and 6) a protective air gap between the drain port and a nearby floor drain.

If these components are below grade, the floor should slope at least 1/8 inch per linear foot (1 centimeter per meter) from enclosing walls toward a central drain that empties into a sump pump connected to emergency power, and the walls should have continuous water sensing cable 1 inch (25 millimeters) above the floor that is connected to a 24-hour-a-day monitor/alarm upstairs.

If all service components are above grade, the drain port may flow via gravity to a spill tap and splash block just outdoors.

Approximating Piping Design

In meter/sewage systems (and a few well/septic systems), the water supply may be upfeed (water is pumped up from the lowest floor by initial pressure from the supply main or holding tank) or downfeed (water is pumped to a tank on or near the roof then flows by gravity down to the fixtures).

The lowest floors fed have the highest pressures. A water main's initial pressure must be great enough to overcome all reductions due to pipe length, wall irregularities, number of fittings, and net vertical distance traveled while still delivering the required pressure at the remote outlet. However, piping design is a highly inexact science due to the following:

The fundamental piping design parameter, fixture unit flow, is a rough estimate based on continuous vs. intermittent water demands in a great variety of occupancies, which is then used to estimate supply delivery rates, pipe diameters, and component capacities. Thus, using this value is a bit like rounding off your own weight to the nearest hundred pounds (50 kilograms).

Another important design variable, pipe flow friction loss, varies widely because it is too tedious to sum all the tiny increments of flow friction loss caused by each valve, tee, elbow, and other fitting in every system. Instead, an average loss is usually taken, based on "normal pressure drop and normal number of fittings."

Flow friction losses also increase unpredictably at sharp bends, in pipe constrictions such as valve thresholds and diameter changes, and in runs exceeding 80 feet (24 meters).

Initial water pressure often varies widely. Municipal water pressures are often higher in the morning than the afternoon and are often higher in October than July; and the initial pressure of any water tank varies considerably as its pump cycles between cutoff and activation modes.

As water flows through a pipe, a viscous shear occurs between the fluid and its enclosing walls that varies considerably with changes in temperature Water at 40 degrees Fahrenheit (4 degrees Centigrade) is twice as viscous as water at 90 degrees F. (32 degrees C.) and four times as much at 170 degrees F. (77 degrees C.).

In this era of increased environmental awareness, rising water use costs, and legal mandating of efficient plumbing fixtures, the amount of water traditionally equated with one fixture unit is becoming less, which could result in oversized system components and sluggish waterflow that could cause more stoppages.

The Art of Piping Design

Our technological age is seeing a proliferation of piping design
software that lulls the user into believing that all the above variables are nonexistent — that the perfect diameter for any pipe lies only a double-click away. On the contrary!

Not only does a successful designer need to be keenly aware of all the above variations, he or she must have a perceptive understanding of how all the subtle plumbing puzzle pieces fit together into complete and coherent systems.

From the above it is easy to see why supply piping design is highly inexact, why approximations are acceptable, and why all solutions should include a slight safety factor. Due to the vagaries involved, it usually pays to organize all initial data into an accurately scaled building schematic that shows floor-to-floor heights, piping dimensions, initial pressure ranges, remote pressure requirements, fixture unit values at every outlet, and conversion of any nonfixture demands to equivalent flow.

Every length of piping should also 1) be heated above freezing, 2) be pitched slightly, 3) have taps that allow drainage from peaks to valleys, 4) have enough chase and riser space to allow movement due to thermal expansion, 5) be supported closely enough to prevent pipe sags between supports, 6) have its contained weight added to structural calculations, and 7) be accessible for maintenance and future upgrading.

Hot and cold pipes should also be spaced at least 6 inches (15 centimeters) apart or have insulation placed between them to prevent heat interchange. And in top-quality work, both pipes should be insulated, hot water to minimize heat loss and cold water to prevent surface condensation.

Robert Brown Butler is an architect with over 40 years of experience in the field of architectural engineering.

This article is excerpted from Architectural Engineering Design: Mechanical Systems, copyright © 2002, available from McGraw-Hill Professional Publishing and from Amazon.com. The book includes a CD-ROM containing procedures for performing engineering calculations.