Requirement of pressure / velocity breaker
One of the oldest-and most persistent myths in the plumbing profession is the belief that extremely high velocities develop in the stacks of high-rise buildings. The Plumbing Engineer is invariably asked how he is going to provide for these velocities at the base of the stack. How is he going to prevent the base fitting from being blown out or broken? No special provisions are required for tremendous velocities and no special precautions are required to protect the base fitting. Excessive velocities just do not occur! Depending upon the rate of flow from the branch drain into the stack, the type of stack fitting, the diameter of the stack, and the flow down the stack from upper levels, the ‘discharge from the branch may or may not entirely fill the cross-section of the stack at the point of entry. As soon as the water enters the stack, it is immediately accelerated at the rate of 32.2 feet per second, per second by the force of gravity, and in a very short distance it forms a sheet around the inner wall of the pipe. It can be simply described as a hollow cylinder of water. This sheet of water, with a core of air in the center, continues to accelerate until the frictional force exerted by the pipe wall on the falling sheet of water equals the gravitational force. The frictional force varies as the square of the velocity and thus resistance to flow is very rapidly increased. From the point where frictional force equals gravitational force, the sheet of water will continue to fall at a velocity which remains practically unchanged. This ultimate vertical velocity is called “terminal velocity,” and the distance in which this maximum velocity is achieved is called the “terminal length,” F. M. Dawson and A. A. Kalinske in “Report on Hydraulics and Pneumatics of Plumbing Drainage System” (State University of Iowa “Studies in Engineering” Bulletin 10, 1937) and R. S. Wyly and H. N. Eaton in “Capacities. of Plumbing Stacks in Buildings” (National Bureau of Standards Building Materials and Structures Report BM 132, 1952) have investigated terminal velocity and -derived a workable formula by treating the sheet of water as a solid hollow cylinder sliding down the inside wall of the pipe. The formulas developed for terminal velocity and terminal length, without going through the complicated calculus involved, are:
Vt = 3.0 (q/d)2/5
Lt = 0.052 Vt2
where: Vt = terminal velocity in stack, fps
Lt = terminal length below point of flow entry, ft.
q = quantity rate of flow, gpm
d = diameter of stack, in.
Applying the formulas for various size stacks it is found that terminal velocity is achieved at approximately’ 10 to 15 fps and this velocity is achieved within 10 to 15 feet of fall from point of entry. The importance of this research is that it conclusively destroys the myth that water falling in a stack from a great height will destroy the fitting at the base of the stack. THE VELOCITY AT THE BASE OF A 100-STORY STACK IS ONLY SLIGHTLY AND INSIGNIFICANTLY GREATER THAN THE VELOCITY AT THE BASE OF A THREE-STORY STACK! There is no scientific reason for limiting the height of a soil or waste stack of any size and the stacks can be run straight down, without offsets, for 1000 feet or more with the utmost confidence. So-called “velocity breaks” are absolutely unwarranted and, in fact, could cause excessive pneumatic pressure fluctuations in the stack, Most engineers are aware of the problem which exists whenever a stack offsets at an angle greater than, 45 degrees. At the point of offset, How enters the horizontal drain at a relatively high velocity when com-pared to the velocity of flow in a horizontal drain under uniform flow conditions. The terminal velocity for a three-inch stack flowing at capa- city is 10.2 fps. The velocity for a three-inch drain installed at a slope of one-quarter inch per foot is 2.59 fps under uniform flow conditions at full or half-full flow. When the water reaches the bend at the offset, it is turned at right angles to its original flow and for a few pipe diameters. downstream it will continue to flow at relatively high velocity along the lower part of the horizontal pipe. Since the slope of the horizontal piping is not adequate to maintain the velocity of flow that existed when the water reached the offset, the velocity of flow in the horizontal drain slowly decreases with a corresponding increase in the depth of flow until a critical point is reached where the depth of flow suddenly and sharply increases. This increase in depth is often great enough to completely fill the cross sectional area of the pipe. This sudden rise in depth is called the “hydraulic jump.” The critical distance at which the hydraulic jump may occur varies. It is dependent upon the entrance velocity, depth of water which may already exist in the horizontal drain when the new flow is introduced, roughness of the pipe, diameter of the pipe and the slope. The distance varies from immediately at the stack fitting up to ten times the diameter of stack downstream. Less jump occurs if the horizontal drain is larger in size than the stack. Increasing the slope of the horizontal drain will also the jump. After the hydraulic jump occurs and fills the drain, the pipe tends to flow full, with large bubbles of air moving along the top of the pipe with the water. Surging flow conditions will exist until the frictional resistance of the pipe retards the velocity to that of uniform flow conditions. Any offset of the stack, at any floor 0 the building greater than 45 degrees can cause hydraulic jump. When the hydraulic jump occurs and proper venting has not beer provided, tremendous pneumatic pressures are built up in the area behind the jump. There have been cases where this excess pressure (greater than a one-inch column of water) has extended 40 feet up the stack. It must be stressed that this excess pressure occurs only when adequate venting has not been provided. Under no circumstances should the fixtures on the floor directly above an offset connect to the stack before the offset. These fixtures should be piped and connected to the horizontal offset more than ten stack diameters downstream or prefer-ably connected to the vertical at least two feet below the horizontal offset. See Figure 35. Many high-rise buildings decrease the floor areas at certain specified heights. To accommodate the decreased areas, fixture layouts are changed and stacks must be offset to new locations. There is an acceptable method of sizing offset stacks which can result in substantial economies. Figure 36 illustrates a typical offset stack. The procedure for sizing is as follows:
1. Size the portion above the offset as for a regular stack based upon the total number of fixture units above the offset.
2. Size the horizontal offset as for a building drain.
3. The portion of the stack below the offset shall be at least the size of the offset or based upon the total number of fixture units on the entire stack (both above and below the offset), which- ever is the larger.
What’s high rise
In the search to overcome ever-escalating cost of land, as well as the scarcity of appropriate sites, builders have adopted the philosophy of building vertically rather than horizontally. Some ecological visionaries have already proposed vertical cities to maintain our open spaces and halt the devastation of urban spread. The proposal of Frank Lloyd Wright , many years ago, to build a mile high building is no longer viewed as an amusing dream. Buildings are now going higher rather than wider. During the 1930s and ’40s a six-story building was considered high-rise, and the few buildings .-which exceeded this height were romantically classified as “skyscrapers.” During the ’50s 20 and 30-story buildings became commonplace and during the ’60s, the 100-story building was no longer an oddity. The Empire State Building, built in the ’30s, was once a serious contender for status as one of the wonders of the world. It is now in third place, merely as contender for the tallest building in the world.
The term “high-rise” is very ambiguous, meaning one thing to one person and something else to another. Some codes delineate criteria for buildings higher than three-stories. Does this mean that any building higher than three-stories is thus a high-rise? On the basis of pure engineering principles, only when 25 stories is exceeded do design concepts require modification for the plumbing systems. Standard material for plumbing systems is designed and manufactured for a working pressure of 125 pounds per square inch. A 25-story building, or approximately 250 feet in height, is the highest limit that can be built without exceeding the safe working pressure of standard piping and equipment. It might therefore be logical to classify only those buildings which exceed 250 feet in height as high-rise. Some codes which have recently been revised and updated to cover high-rise construction have established any height above 75 feet as high-rise. This figure was proposed on the basis of the fire protection requirements of a building and is fundamentally sound. It is suggested that this criterion of 75 feet be universally accepted so that when an engineer in any other locale does not have to Wonder what the frame .of reference is.
In the design of plumbing systems for any building, including high-rise, the Plumbing Engineer has three basic functions to perform:
The primary function of the Plumbing Engineer has always been, and always will be, the protection of the health of the public. Incorrectly designed and installed plumbing systems are a major healthy hazard and should never be tolerated.
The second function of the Plumbing Engineer is to help alleviate two major crises facing our nation today: water pollution and energy.
The third function, and many people erroneously think this is really , his only function, is to design the most efficient plumbing systems for his client at the minimum cost. It is extremely important to recognize that minimum cost does not necessarily mean the lowest capital expenditure, Factors that must be evaluated before arriving at the most economical cost are life expectancy, replacement, and, of the utmost importance, maintenance and operation.
All three of these functions of the Plumbing Engineer should be scrupulously adhered to in the concept and design for any building. In fact, the secondary function can lead to some innovative approaches which in turn can result in substantial savings and thus fulfill the tertiary function.
The confidence and assurance of the average Plumbing Engineer seem to be shaken when he is called upon to design the domestic water system for a high-rise building. The following statement is made without any reservations and with the utmost confidence: any good Plumbing Engineer can properly design the domestic water system for any high-rise building.
You would probably not have purchased this book if the subject had been “Water Systems for Buildings.” And yet, that is exactly what is going to be explored in this book. Mysteries and myths surrounding high-rise plumbing are going to be destroyed. Water has a most peculiar characteristic of slavishly conforming to the laws of hydraulics, whether flowing in Chicago or Miami, whether flowing at the lowest levels of Death Valley or the highest altitudes of the Rockies, whether flowing in a three story building or a hundred-story building. Basic engineering – principles are applicable in all cases. Of all the plumbing systems in a building it is probably in the; design of the domestic water system that the Plumbing Engineer is most Free to express his creativity, originally, inventiveness and expertise. Before design can begin, two basic problems must be resolved:
The method or system employed to obtain and maintain adequate pressures at the highest fixtures and throughout the building.
The method or system employed to adequately handle exceedingly high pressure. When these two problems – adequate pressure and excess pressure – are resolved, design reverts to the average standard job. To reiterate, if basic engineering principles are followed, high- rise design is no more difficult or involved than for any similar low raise design building.
When the pressure in the public mains is not great enough to satisfy the requirements of a building, some means must be provided for boosting the pressure to acceptable levels. There are presently three basic systems available for boosting the available pressure to the required pressure: 1. Gravity tank system; 2. Hydro pneumatic tank system; 3. Tankless or booster pump system. They can be used singly or in various combinations.
No one of these systems is the best for all high-rise buildings, and it is probably because this fact is ignored that owners may, on occasion, end up with inferior and costly water distribution systems.
Gravity tanks were used almost exclusively until the early 1960`s, with hydropneumatic tanks being used for some of the smaller high-rise buildings. In the early sixties, variable speed pumps and pump controls were developed to a point where booster pump started to replace the gravity tank, and today, engineers specify the booster pump system almost exclusively. All three system, when properly designed, are good and each has its application. Engineers have been oversold on some of the supposed advantages of the booster system and the time is long overdue for a careful analysis and evaluation of all systems.