Stacks

"Stack" is a general term for any nominally vertical line of soil, waste or vent piping which receives soil and waste water from fixture drains and horizontal branch drains from more than one floor of a building. This vertical drain connects fixture and branch drains within each story of the structure to the building drain. The soil or wastewater comes from water closets, urinals, lavatories, bathtubs, showers, sinks and various other fixtures. A soil or waste stack collects wastes that contain fecal matter, such as from water closets and urinals as well as wastes which do not contain fecal matter, such as from sinks, lavatories, bathtubs, showers, and various other fixtures. The vents that are the continuation of these fixture drains and branches either connect to the continuation of the stack, the stack vent, or an individual, dual, branch, continuous vent, vent stack or header.

Stack Connections

    The connections to the stack from horizontal fixture drains or branches may made with Y fittings and other bends, or with a sanitary tee, which is a short radius tee-wye, or with a combination wye and one-eighth bend  tee-wye, which has a long radius. The tee-wye may be used in the vertical or horizontal position to connect branches. These fittings direct waste into the stack with an initial downward velocity that permits the stack to accept an increased rate of flow at any one level. The tee-wye gives the water a greater downward velocity than the sanitary tee and hence is more advantageous for greater stack capacities.

    The tee-wye or Y fitting and bends is not usable to connect fixture trap arms to a VERTICAL pipe due to a tendency to create self siphoning of the fixture traps. The sanitary tee must used to connect trap arms to a vertical drain that will also serve as the vent. The sanitary tee may connect a dry nominally vertical vent serving a horizontal branch or fixture drain. Y fittings and other bends or combination wye and one-eighth bend  tee-wye, may be used to connect traps to a nominally horizontal branch or drain.

Flow in Stacks

    The character of the flow of water in partially filled vertical pipes varies with the extent to which the pipe is filled. For small volumes of flow, amounting to little more than a trickle, the flow is entirely on the inner wall of the stack. With increases in volume, the water adheres to the wall of the pipe until frictional resistance of the air causes it to diaphragm across the pipe, temporarily forming a short slug of water which descends, filling the stack until the increased air pressure breaks through the water forming the slug. The water is either thrown against the wall or falls a short distance as separate streamlets in the center of the pipe. This  forming of slugs or diaphragming probably first appears in a 3 inch stack when the stack is from one-fourth to one-third full. This intermittent rate partially accounts for the rapid erratic oscillations of pressure in a plumbing system. A stack should never be designed for a capacity greater than one-third full because the pressure fluctuations in the system could greatly exceed the ±1" column of water pressure criterion and traps could possibly lose their seal by siphonage or blow-out (see Figure 2-1).

(Figure 2-1)

Cross-section of Stack Flowing at Design Capacity           

Terminal Velocity and Length

    When plumbing engineers are called upon to design the systems for a very tall building, they are invariably asked how they are going to accommodate the extremely high velocities developed at the base of the stacks. How are they going to prevent the base fitting from being blown out or broken? This is one of the oldest and most persistent myths in the plumbing profession and, unfortunately is still believed by too many uninformed designers.

    Depending upon the rate of flow the branch drain into the stack, type of stack fitting, the diameter of the stack, 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/second/second by the force of gravity and in a very short distance it forms a sheet around the inner wall of the pipe. 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. From this point downward, provided no flow enters the stack as the sheet passes a fitting, the sheet of water will fall at a velocity that 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 Systems, 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 BMS 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:

                   Where:

                    V_T = terminal velocity in stack, fps

                    L_T = terminal length below point of flow entry, ft

                    q = quantity rate of flow, gpm

                    d = diameter of stack, in.

HYDRAULIC JUMP

    At the base of the stack, waste enters the horizontal drain at a relatively high velocity when compared to the velocity of flow in a horizontal drain under uniform flow conditions.

    For a 3" stack flowing at capacity, the terminal velocity is 10.2 fps. For a 3" drain installed at a slope of one-quarter inch per foot, the velocity is 2.59 fps, at full or half-full flow under uniform flow conditions. When the water reaches the bend at the bottom of the stack, 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. The horizontal drain slope is not adequate to maintain the velocity of the water that existed when it reached the bottom of the stack. The velocity of the water flowing in the horizontal drain slowly decreases with a corresponding increase in the depth of flow until a critical velocity is reached where the depth of flow suddenly increases. This increase in depth is often great enough to completely fill the cross sectional area of the pipe.

    This phenomenon of 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 that may already exist in the horizontal drain when the new flow is introduced, roughness of the pipe, or the diameter and slope of the pipe. The distance varies from immediately at the stack fitting up to ten times the diameter of the 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 minimize the jump. After the hydraulic jump occurs and fills the drain, the pipe tends to flow full, 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 greater than 45 degrees can also cause a hydraulic jump.

See Figure 2-2 for an illustration of the hydraulic jump.

Figure 2-2

HIGH TEMPERATURE WASTES

    High-temperature wastes (above 140 degrees Fahrenheit) should never discharge directly into the drainage system. High temperatures can cause excessive expansion and contraction of the piping with resulting harmful effects. Joints may be pulled apart or loosened and solidly bedded pipe may be broken. The discharges from boiler blow-offs, steam exhaust, condensate, etc. must be cooled down to at least 140 degrees before connecting to the drainage system. This may be accomplished by piping the high temperature discharge to a water-supplied sump or a cooling tank.

DRAINAGE SYSTEMS BELOW SEWER LEVEL

    Where a drainage system is below the elevation of the house drain (which drains by gravity to the sewer) or the public sewer, the discharge should be conveyed to a sump or ejector and pumped or automatically lifted up into the gravity drainage system. Sumps are used to handle clear water and need not be air tight and vented. Ejectors are used for sewage and must be air tight and vented.

BACKWATER VALVES

    There is often the danger of backflow of sewage into a building when the public sewer becomes overloaded or is surcharged. To prevent backflow and possible flooding of the building, a backwater valve should be installed in the drainage piping from all fixtures which are at an elevation below the surcharge level of the public sewer.