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WATER SUPPLY PIPING FOR BUILDINGS. PLUMBING OBJECTIVE: The correct method of properly sizing plumbing pipes.
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PLUMBING OBJECTIVE: The correct method of properly sizing plumbing pipes. Piped in water supply systems have become an essential part of society. Think as you got up this morning what it would have been like if you didn’t have water. Perhaps you couldn’t get a drink, brush your teeth, or flush a toilet. Indeed life would be a little different. Thank goodness for the folks who pioneered plumbing. It probably is the most convenient, yet most taken for granted part of our modern technology. Truly this is a system we don’t notice when it is there and works. But abruptly do without plumbing, and life gets miserable. There are two primary parts of plumbing. Water supply piping Piping for waste – getting rid of all that water Supply piping has evolved with modern technology. In the earliest times, pipe was made of wrought iron, and was not easily worked. Piping then changed to steel, then galvanized steel with a coating of zinc to retard rust. Installation was difficult because pipe joints had to be threaded and joined with fittings to make the system water-tight.
Installation of steel piping was slow and tedious, because lengths between fittings had to be cut exact, then fastened together with threaded joints, one following another in progression. Copper pipe was developed, along with a new type of fitting to connect joints. Pipe and joints no longer needed to be threaded. Copper piping is joined together with brass fittings, in a connection called a “sweated Joint.” A brass fitting is heated with a gas torch so the material expands. While it is still hot, the fitting easily slides over the outside of the end of the pipe, and when it cools, the fitting contracts against the pipe to make a tight fit. The connection is then soldered. Plastic piping, such as polyvinylchloride (PVC) was developed into an economical and efficient system, but is not suitable for water piping under pressure inside buildings. The fittings and methods of joining are not dependable enough to trust against a leak that might occur inside a wall or an attic. But PVC piping is used extensively where piping is installed outside of habitable buildings.
Within the past several years a high strength plastic pipe has been developed that is suitable for high pressure water systems, and is approved by most building codes. The fittings are made of brass, and the connections by steel crimps. The major advantage of the material is that it is manufactured in long rolls, making installation easier because it eliminates many joints in a system. Another advantage is that the material for cold water use is translucent white, and the material for hot water use is translucent red. The pipe also comes in blue. Material is manufactured under the name “Wirsbo-pex.” Wirsbo is a manufacturer’s name, and “pex” is a chemical acronym for “polyethylene cross-linked,” hence the X. Water is transported through a system of piping by pressure. A common pressure available from a municipal system may be in the vicinity of 60 pounds per square inch. In early times, the most economical way for a municipality to provide water pressure was to raise a water tank to a height of 100 feet or so, and the weight of the water would cause the pressure.
Consider that water weighs 62.4 pounds per cubic foot. Imagine a volume of one cubic foot, a block 12” x 12” x 12” high. Then divide the block into one inch x one inch columns, each 12 inches tall. The cubic foot would consist of 144 of these 1” x 1” x 12” columns of water. How much would each column weigh? 62.4 divided by 144 = 0.433 pounds. Since the cross section area of one of these columns of water is one square inch, it follows that the weight of water transposes to 0.433 pounds per square inch, PER FOOT OF HEIGHT. Since unit pressure, or stress, is in terms of weight per unit of area, and the cross section of the column is one square inch, water pressure equals 0.433 psi per foot of height. So what is the pressure of a one-square-inch column of water ten feet high? 0.433 x 10 = 4.33 pounds. What is the pressure of a one-square-inch column of water one hundred feet high? 0.433 x 100 = 43.3 pounds.
Say you have a vertical pipe, 2” in diameter, 10’ high. What is the pressure at the bottom of the pipe? 4.33 psi. What if the pipe were 4” in diameter, what would be the pressure in the bottom of the pipe? 4.33 psi. It doesn’t matter the volume of water per foot of height - - - unit pressure is in pounds per square inch. If the ocean were only one foot deep, the pressure at the bottom would be 0.433 psi. Pressure is the force that pushes water through a system, from the point of origin, to the fixture that is farthest away from the source. Available pressure diminishes within the system for a variety of reasons; first, it takes pressure to make the water meter work, so some of the available pressure is used to operate the water meter. Then, if the water piping rises in height, say from the water meter that may be 3’ below the ground, and the pipe extends upward to the attic space within a building, a portion of available pressure must be used to raise the water upward.
Then a portion of the available pressure must be used to operate a fixture, such as a sink, lavatory, or water closet. And finally, there is pressure lost in the system because of friction. Water moves against the walls of the pipe, and water movement must negotiate through bumps at fittings and valves. All these contribute to the reduction of pressure from the source to a point of use. Another aspect of properly sizing the pipes in a plumbing system is the speed at which water moves through the system. If water is allowed to move faster than about 8 feet per second, the movement will cause turbulence against the walls of the pipe, and through fittings. Turbulence in water flow creates NOISE. In areas like West Texas, where the water contains a large amount of particulate matter, (calcium, magnesium, etc.) the turbulence will cause the particles to fasten themselves to pipe walls due to a difference in electrical charge. Over a period of time, particles build up in the pipe and reduce the effective diameter of the pipe.
Plumbing fixtures have evolved through improvements in design required by efficiency of use, and by some governmental regulations with purpose of conserving water. Plumbing fixtures; sinks, lavatories, closets, faucets, etc., are rated by water use in terms of FIXTURE UNITS. A fixture unit once was defined as one cubic foot of water, but that definition has no specific meaning in sizing piping. Fixtures defined by fixture units is a comparison of the amount of water used – and these comparisons have led to the development of a chart defining gallons per minute demand, based on quantities of fixtures, and a reasonable assumption of frequency of simultaneous use of fixtures. In other words, the more fixtures that are installed within a system, the less likely that all fixtures are used at the same time. So demand in gpm is less per fixture unit as the number of fixture units increase.
Standard plumbing fixture charts through history of use define the amount of fixture units per fixture, based primarily on their demand for water. Two pages of the supplementary packet contain charts and tables that are useful as the basic components of water piping design. First Chart (next slide) Upper left gives maximum pressure required for fixtures. Lower right gives fixture unit value for various fixtures. Lower left gives pressure required to operate water meters. Upper right gives the length of a straight piece of pipe that is equal to the amount of friction loss for various fittings.
FITTINGS FIXT PRESSURE FIXTURE UNITS WATER METER
GLOBE VALVE GATE VALVE CHECK VALVE ANGLE VALVE
There are two charts on the opposite side: Chart One is the conversion from fixture units to quantity of water in gallons per minute. Notice the chart has two double columns; one labeled at the top for Predominantly Flush TANKS, and to the right, Predominantly Flush VALVES. Valve types refer to the method by which water closets and urinals expel waste. Flush tanks are the domestic type, like in a residence, where water for flushing is stored in a tank at the back of the fixture. Flush valves are the commercial type where there is no tank, and water for flushing must all come from the water supply pipes. This type of fixture requires more pressure and larger pipes to flush. Notice in the conversion chart that as the number of fixture units INCREASE, the quantity in gallons per minute decreases proportionately. This is simply an indication that the more fixtures in a facility, the less likely that all will be required to flush at the same time.
The second chart on the page is one that shows the relationship between available water pressure in p.s.i. per 100 feet, the flow of water in gallons per minute, the flow velocity of water, and the pipe diameter. First, limit the flow of water to 8 ft. per second. That is indicated by a slanted line from upper left to lower right. Then determine the available water pressure in p.s.i. per 100 feet from the piping layout. That will be a straight line upward from the bottom of the chart. Where the two lines intersect will determine if the size of the pipe is based on available pressure, or by limiting the velocity of water to 8 fps. Diameter of pipe is indicated by a slanted line from lower left to upper right, and where this pipe diameter line FIRST intersects the pressure line or the velocity line, READING HORIZONTALLY TO THE LEFT, will be the maximum amount in gpm that particular pipe diameter will supply.
An example problem follows, and shows a simple step by step procedure for determining the proper pipe size.
In the process, two assumptions must be made, and will be verified later in the calculation. First, a meter size is not known until pipes are sized, so an assumed meter size must be selected. At the water meter chart, the total gpm for the system is known ( 20 gpm ) Find 20 gpm at the bottom of the chart and draw a straight line upward from the bottom. Probably the line will cross 2 or 3 slanted lines (that indicate meter size). Select the middle one and read to the left to see a pressure to operate the meter is 9 psi for a ¾” meter. Second, since pipe sizes are not known, the equivalent length of fittings must be assumed. The equivalent length of fittings is the addition of equivalent lengths for each fitting that is in the pipe that extends from the source of water to the fixture that is farthest away. Since this cannot yet be determined, ASSUME AN EQUIVALENT LENGTH OF ½ THE MEASURED LENGTH. In this case, the measured length is 90 feet; ½ of 90 = 45; 90 + 45 = 135’ which is the CALCULATED length.
After subtracting the pressure loss of meter, rise, and fixture, an amount remains as the pressure that pushes the water through the system. But the pipe size chart needs a number that is the available pressure per 100 feet of length. Since an available pressure remains of 31.67 psi, it must push the water a distance of 135 feet. So pressure per 100 feet equals (pressure / calculated length) x 100. In this case, 31.67 x 100 = 23.46 psi/100 feet 135 On the pipe size chart, draw a line from 23.46 upward from the bottom of the chart and stop it at the 8 fps velocity line.
From this little chart that is made to show the limit in gpm of water for each pipe size, go to the plan layout and write the sizes of pipes for each segment. When you get to the maximum size of pipe at the meter, notice if it is larger than the meter you assumed. Chances are, the meter will need to be larger, which will result in LESS pressure required to operate. So the first assumption will be OK. Last - - - list the types and sizes of fittings along the pipe that extends from the meter to the farthest fixture. Add the equivalent lengths to see if it exceeds 45’, which was the assumption made for equivalent length of fittings. If the added numbers are less than 45’, then the second assumption is OK. If the number is larger than 45’, then go back and recalculate the available pressure in psi per 100 ft., and recalculate the little pipe size chart.
