Good design vital to odour control
The key issue when designing the drainage system in high-rise buildings is to control the effect the falling water has on the air pressure within the stack, says ROD GREEN, technical manager of Polypipe Gulf*, while highlighting an alternative ventilation solution to the problem.
01 March 2012
WHEN not in use, a drainage pipework system is filled only with foul air; and this foul air is prevented from escaping into the surrounding built environment by installing air-tight traps with a water seal to individual sanitary appliances.
To date, national drainage codes have provided guidance on drainage and ventilation pipework sizing based on steady state flow calculations (constant flow) and deal primarily with negative air pressures.
It should be noted, however, that a soil or waste stack is in a steady state only when it is at rest. The flow of fluid within the system constantly changes with time. Once a WC is flushed or a kitchen sink is discharged, the pipework system is an unsteady state (transient flow).
A minimum 50 mm of water is all that protects the built environment from potentially harmful sewer gases.
Therefore, it is essential to have the correct design for a drainage system in order to ensure protection of the water trap seal. Pipes are designed to run at a maximum of 33 per cent full to restrict the negative and positive air pressure fluctuations to +/-375 N per sq m (+/-375 Pa or +/-37.5 mm per wg). Water flows over two litres per second can entrain eight to 15 times more air than water volume, which can create air fluctuations in excess of 500 Pa (50 mm wg, minimum size of trap).
Pioneering research is currently being carried out at Heriot Watt University in Edinburgh, Scotland, based on transient state flow of fluid in drainage systems for buildings up to and beyond 50 storeys.
Current research has shown that in high-rise buildings, air fluctuations are continually oscillating between positive and negative while the system is in use and is proving that the flow of air within the drainage pipework system is equally as important as the flow of water in maintaining a safe and hygienic drainage system.
The acceleration of waste water down a discharge stack continues until the frictional force exerted by the internal wall of the drainage stack equals the force of gravity. This is known as the maximum velocity and is termed terminal velocity. The distance required to reach terminal velocity is known as “terminal length”.
New research has proved that terminal velocity is attained between 3 m to 5 m from the point of entry into the stack, travelling at a maximum velocity of 5 m per second. The terminal velocity at the base of a 100-storey stack is only marginally greater than the velocity at the base of a three-storey stack. Therefore, waste water will reach terminal velocity if it enters any stack above 5 m, be it a three-storey villa or a 100-storey apartment block.
It is the entrainment of air in the waste water discharge that causes negative pressure in stacks to act on waste branches and traps. Likewise, at a transition area, such as the base of a stack bend, it is the velocity and volume of water hitting the bend and becoming turbulent that will cause a reflected positive air pressure wave to propagate through the stack. Traps are sensitive to both the positive and negative air pressure fluctuations.
The height of the building is not relevant to the velocity of the water in a stack, provided the height of this stack is greater than 5 m.
Designers of drainage systems design within flow rate limitations for different sizes of stacks. Therefore, the key issue when designing drainage stacks is not to control the velocity of the falling water but to control the effect that the falling water has on the air movement within the stack – that is, the generation of both positive and negative pressures. This requires a strategy of introducing air at the point of need for dissipating negative pressures and dissipating, or attenuating at the point of need for positive pressures.
To maintain a state of equilibrium in a drainage pipework system, it is necessary to respond to an increase or decrease in air pressure. This response time is critical in protecting trap water seals.
Negative pressure fluctuations are low-amplitude, high-speed air pressure waves and can reduce the local ambient pressure in drainage pipework below 500 Pa.
Typically positive air fluctuations are low-amplitude, high-speed air pressure waves and can travel up to 320 m per second within a drainage stack.
Traditionally, a secondary ventilation stack and branch pipework system (Figure 1) has been incorporated into drainage design and installations to overcome air fluctuations.
Secondary ventilation pipework is costly to install and, more importantly, can be an inefficient solution, as the time lag to communicate an increase or decrease in the ambient pipework air-flow may result in an unsafe drainage pipework system. BS EN 12056-2:2000 gives guidance on the minimum size of vent pipework to be utilised in a drainage system, based on a maximum designed flow rate for a particular sized drainage pipe.
Terrain’s Pleura range is an alternative ventilation system offering a unique solution for high-rise buildings. The range consists of Pleura 50 and Pleura 100 valves and a positive air pressure attenuator (PAPA) unit.
The Pleura valves allow air to enter the drainage branch/system at the point of need (PoN), when negative pressures are created.
The valves open at -75 Pa and will balance the internal air pressure between 0 to 250 Pa.
The PAPA has been designed to accommodate positive air pressure fluctuations. Working like a bladder, the unit can react in 0.2 seconds to attenuate low amplitude, high-speed air pressure waves. As the internal air pressure begins to balance, the PAPA will release air back into the system at 12 m per second. It acts like a water hammer arrester, only for air.
The Terrain Pleura range provides a plumbing ventilation solution that improves the balance of positive and negative air pressures that are generated within the pipework system. This, in turn, mitigates loss of trap seals, a major symptom of unbalanced air pressures.
The Pleura valves and Papa replace extensive vent piping (Figure 2) resulting in:
• A considerable reduction in pipework;
• Reduced labour costs associated with installation and time;
• Fewer spatial requirements within risers;
• Less risk to public health with an engineered system;
• Performance benefits due to improved ambient air pressures; and
• Major cost savings.
* Polypipe Gulf, part of the UK-based Polypipe, offers a unique combination of above and below ground drainage, hot and cold water, heating, ventilation and water management solutions, suitable for commercial buildings such as hospitals, hotels, sports stadiums, retail complexes and offices, as well as residential apartments and villa developments.
Polypipe acquired Terrain in 2007 merging its commercial sector activities into the Terrain business and thus forming Polypipe Terrain.
- Good design vital to odour control
- Poloplast systems on display at Big 5 Saudi
- Al Osais tackles major projects in Jubail
- Wade makes inroads with gratings range
