PLUMBING SYSTEM DESIGN Tall Building Drainage Society of Public Health Engineers 4th October 2011
PETER WHITE PRINCIPAL HOARE LEA PUBLIC HEALTH GROUP
CONTENTS • Dispelling Some Common Drainage Misconceptions • What Makes Tall Building Drainage Different? • Design Guidance And How Applicable Is It To Tall Buildings? • Latest Research • Control of Soil Stack Pressure Using Mechanical Devices
COMMON MISCONCEPTIONS • Annular flow • Fully developed within 3-5m of point of entry • Terminal velocity of 3-5 m/s • Water velocity at base unchanged between 3 and 100 storey building • No requirement for velocity breaks
COMMON MISCONCEPTIONS • Problems can occur at the base of a stack or at a change of direction, resulting in ‘blown’ trap seals • This is due to increasing air pressure, not increasing water velocity
GENERAL DESIGN PRINCIPLES Pressure falls below atmospheric immediately below top of stack Negative pressure increases down stack due to friction Further pressure drops where stack is restricted by branch flows Below the lowest discharging branch pressure gradually increases Pressure increases above atmospheric at base & can ‘blow out’ water seals
GENERAL DESIGN PRINCIPLES • The key issue is controlling air pressure, not controlling water velocity • Foul air is kept within the system via water seal traps, which are very sensitive to pressure changes • When water is discharged, air is entrained at 8-15 times that volume • The taller the building, the further the fresh air has to travel and the resistances generated result in increasing negative pressure • Surcharge will result in large positive pressure transients
DESIGN ISSUES FOR TALL BUILDINGS • For large buildings the traditional method of controlling pressure fluctuation is the secondary ventilating stack • BS EN 12056:2 states: - 100 SVP + 50 VP; Qmax = 7.3 l/s - 150 SVP + 80 VP; Qmax = 18.3 l/s - 200 SVP + 100 VP; Qmax = 27.3 l/s • BS makes no reference to maximum length of vent
DESIGN ISSUES FOR TALL BUILDINGS HEIGHT OF VENT versus Qmax • As the number of storeys increase, Qmax is likely to increase • As Qmax increases the volume of entrained air increases • As the number of storeys increase, the vent pipe length increases, so the negative pressure increases • Vent pipes must be correctly sized to reflect building height & Qmax of connected applications
DESIGN ISSUES FOR TALL BUILDINGS AVAILABILITY & SUITABILITY OF DESIGN GUIDANCE • Current BS makes no reference to building height • Superseded BS 5572:1994 suggested that a 30 storey residential block with a 150 SVP & 32 VP has a Qmax of 8.3 l/s • This is less than half the current BS figure of 18.3 l/s (but with no height limit)
DESIGN ISSUES FOR TALL BUILDINGS AVAILABILITY & SUITABILITY OF DESIGN GUIDANCE The Americans have been constructing skyscrapers for more than a century
American Society of Plumbing Engineers (ASPE) publishes a design table that relates Qmax to vent pipe size and maximum length
DESIGN ISSUES FOR TALL BUILDINGS
For the same 30 storey scenario as the 5572/12056 contradiction discussed earlier, US designers are directed to a 152mm soil pipe and a127mm vent.
DESIGN ISSUES FOR TALL BUILDINGS • All code guidance is based upon the translation of steady state empirical data into ‘safe’ design guidelines • There is no transparency as to how ‘safe’ was judged • The current BS is not robust enough for tall buildings due to lack of reference to vent length • US guidance addresses vent length but is very conservative
DESIGN ISSUES FOR TALL BUILDINGS In 2005, Hoare Lea was commissioned to design the 48 storey Pan Peninsula project in London Docklands Our design response to the various design guidance available for tall buildings was to develop a BS/ASPE hybrid To verify the venting of this design we approached Heriot-Watt University to use their AIRNET computer simulation to model a typical stack
LATEST RESEARCH - DESIGN TOOLS OF THE FUTURE? A BACKGROUND TO AIRNET • AIRNET is a Heriot-Watt University research tool which came to wider attention when it was used to investigate the SARS outbreak at Amoy Gardens in 2003 • Existing codes are based on steady state flow, but the reality of a drainage system is that the flows are inherently unsteady and flow rate, annular downflow thickness, entrained airflow and suction pressure all vary with time • AIRNET simulates the behaviour of a drainage system over a predetermined period of time, so it simulates unsteady state flow
LATEST RESEARCH - DESIGN TOOLS OF THE FUTURE? A BACKGROUND TO AIRNET AIRNET models the passive and active boundary conditions in a method of characteristics (MoC) simulation
AIRNET takes these boundary conditions and the driving functions which determine entrained airflow and together with data entry to describe the system and the connected appliances, simulates the system behaviour
LATEST RESEARCH - DESIGN TOOLS OF THE FUTURE? A BACKGROUND TO AIRNET
LATEST RESEARCH - DESIGN TOOLS OF THE FUTURE? A BACKGROUND TO AIRNET AIRNET is an academic research tool; it is not very user friendly AIRNET could be developed and distributed as a design software package AIRNET has already been used to develop the control of stack pressures using mechanical devices in place of secondary vents
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES Synthetic rubber bladder
200mm diameter by 750mm long vented containment vessel Self-priming – bladder evacuated by negative pressure of entrained airflow Positive pressure transient begins journey back up the stack Bladder begins to expand and branch becomes path of least resistance. Fully inflates in 0.2 sec
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES Air admittance valves allow air to be entrained at the point of need (PON) Positive air pressure attenuators (PAPA) act as simple air accumulators to absorb ‘excess’ air at PON
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES
CONTROL OF STACK PRESSURES USING MECHANICAL DEVICES – IS THE UK READY? • Current UK code of practice does not recognise the use of PAPAs • The PAPA is not BBA approved • Current BBA certification of AAVs limits them to maximum 10 storey building • The Studor AS/NZS PAPA design guidance is very broad brush – AIRNET modelling would be far better • Public Health Engineers have always been wary of incorporating mechanical devices into drainage systems
PLUMBING SYSTEM DESIGN A focus on drainage
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