BS 8490:2007
$167.15
Guide to siphonic roof drainage systems
| Published By | Publication Date | Number of Pages |
| BSI | 2007 | 36 |
This British Standard applies to roof drainage systems using circular-section pipes that operate under siphonic action. It does not apply to siphonic drainage systems using rectangular or trapezoidal section conduits for water conveyance.
It gives information and provides guidance on design parameters, installation, verification and maintenance needs of siphonic roof drainage systems conforming to the performance requirements specified in BS EN 12056‑3:2003, 6.2. Guidance is also given on carrying out checks of designs, for the use of specifiers and regulatory authorities.
Particular issues for site drainage systems that receive flows from siphonic roof drainage systems are also covered.
PDF Catalog
| PDF Pages | PDF Title |
|---|---|
| 3 | Contents 1 Scope 2 2 Normative references 2 3 Terms, definitions and symbols 2 4 General 6 5 Performance 7 6 Design parameters 7 7 Components of siphonic systems 10 8 Hydraulic design 10 9 Validation of designs 17 10 Installation 17 11 Testing and commissioning 19 12 Maintenance, inspection and cleaning 20 13 Information to be provided 21 Annexes Annex A (informative) Principles of operation of siphonic systems 23 Annex B (normative) Testing of siphonic outlets 26 Annex C (informative) Simplified checking procedure for siphonic systems 29 Bibliography 31 List of figures Figure 1 – Principal components of siphonic roof drainage systems 3 |
| 4 | Foreword |
| 5 | Introduction |
| 6 | 1 Scope 2 Normative references 3 Terms, definitions and symbols 3.1 Terms and definitions |
| 7 | Figure 1 Principal components of siphonic roof drainage systems |
| 8 | 3.2 Symbols |
| 10 | 4 General |
| 11 | 5 Performance 6 Design parameters 6.1 Design rainfall intensity and flow rates NOTE It is important to recognize that BS EN 12056-3 does not distinguish between siphonic and conventional gravity drainage systems in the selection of the design value of rainfall intensity. NOTE The factor SF provides a margin of safety for siphonic systems and outlets that is similar to the margin of safety incorporated in BS EN 12056-3 for the design of non-siphonic gravity drainage systems. |
| 12 | 6.2 Available head 6.3 Minimum pressure in pipes |
| 13 | 6.4 Minimum velocity in pipes 6.5 Balancing of systems 6.6 Roof drainage 6.7 Speed of priming |
| 14 | 7 Components of siphonic systems 7.1 General 7.2 Siphonic outlets 7.3 Pipework 8 Hydraulic design 8.1 General principles |
| 15 | 8.2 Siphonic outlets |
| 16 | 8.3 Roof drainage 8.4 Design of pipework |
| 17 | 8.5 Tailpipes 8.6 Collector pipes 8.7 Downpipes 8.8 Priming |
| 18 | 8.9 Primary and secondary systems |
| 19 | 8.10 Discharge arrangements 8.11 Overflows |
| 20 | 8.12 Site drainage |
| 21 | 9 Validation of designs 9.1 Outline checks 9.2 Design software 10 Installation 10.1 Preventing blockages during construction 10.2 Pipework jointing |
| 22 | 10.3 Fixings and support 10.4 Insulation/trace heating 10.5 Acoustic insulation |
| 23 | 10.6 Tolerances 10.7 Changes from design 10.8 Health and safety 11 Testing and commissioning 11.1 Inspection 11.2 Flow testing 11.3 Positive pressure testing |
| 24 | 11.4 Handover 12 Maintenance, inspection and cleaning |
| 25 | 13 Information to be provided 13.1 By the specifier a) location of building, and height of building above ordnance datum, drawing attention to any climatic peculiarities; b) required design storm return period, or category of storm and design life of building; c) roof plan, indicating areas to be drained; d) roof covering and height of potential leakage paths into the building; e) gutter positions and initial sizes; f) preferred outlet positions; g) overflow positions; h) preferred downpipe routes; i) temperature and humidity of rooms and spaces intended to be heated; j) decibel rating of rooms and spaces intended for quiet activities (e.g. sleeping, auditoria, studios) – generally available in room data sheets; k) building use, rooms and spaces intended to house valuable items or sensitive equipment, positions and types of existing trees; l) positions and types of proposed soft landscaping; m) levels of roofs and external ground; n) structural arrangements of roof, including any upstand or downstand beams; o) location of building movement joints; p) maximum allowable depth of water on roof; q) details of connections to site drainage system; r) CDM [5] risk assessment, highlighting any abnormal factors and proposed means of maintenance access on the roof; s) which party is responsible for the installation of the siphonic outlets; t) any other relevant information. |
| 26 | 13.2 By the system designer/installer a) confirmation of design rainfall intensity and factor of safety used (see 6.1.3); b) schematic pipework layout, including outlet positions; c) identification of primary and secondary systems; d) pipework lengths and levels; e) calculated maximum depth of water on the roof during the design storm; f) pipework type and strength classification; g) minimum flow velocity under design conditions; h) hydraulic head loss factors used in design; i) maximum amount of entrained air assumed in design; j) maximum imbalance in head loss between outlets; k) maximum positive and negative pressures (at both kP = 0.15 mm and kP = roughness of pipe when new); l) estimated time for priming of systems; m) agreed point of connection with site drainage system and details; n) builders’ work requirements for item m); o) CDM [5] risk assessment, CDM method statement; p) method of fixing outlet to gutter/roof membrane; q) methods of supporting and fixing pipework; r) builders’ work requirements for item q); s) structural loads imposed by siphonic systems; t) method of dealing with expansion forces; u) inspection and test certificates; v) maintenance manual, including as-built information. |
| 27 | Annex A (informative) Principles of operation of siphonic systems A.1 General A.2 Bernoulli equation a) Determine the value of the design flow rate entering each outlet of the system from the individual area drained, the design rainfall intensity and the selected factor of safety (see 6.1.3). b) Divide the pipework system into a series of convenient sections determined by changes in pipe diameter, positions of pipe fit… c) Start at the downstream end of the system (point 2) where the exit velocity, u2, and the static pressure head, h2, are known…. d) Determine the change in elevation, Dz12, in the first upstream section of pipework (to point 2), and find the corresponding head loss, Dh12, from A.3 and/or A.4. e) Use the known velocity, u1, at point 1 and equation (A.1) to find the unknown pressure head, h1, at this point. |
| 28 | f) Repeat the calculations for each pipe section in turn to find the value of static pressure head at each outlet in the system…. g) If the calculated value of h1 on the upstream side of the outlet is greater than zero, this implies that the siphonic system has insufficient flow capacity and will cause local surcharging at that point. h) If the calculated value of h1 is less than zero, this implies that the system has some reserve capacity. Limits are given in 6.5.2 for the values of the reserve capacities at individual outlets. i) Check the results to ensure that the recommendations for minimum pressure head and minimum velocity given in 6.3 and 6.4 are met at all points in the system. A.3 Colebrook-White resistance equation A.4 Head losses at pipe fittings A.5 Cavitation |
| 30 | Annex B (normative) Testing of siphonic outlets B.1 General a) to determine the relationship between the flow rate entering the outlet and the depth of water in the gutter or on the flat roof at the approach to the outlet; b) to check the effectiveness of the outlet at preventing entry of air and the speed of response to changes in flow rate; c) to determine the head loss coefficient for the outlet for use in the hydraulic design of siphonic systems in which such outlets are installed. B.2 Test method for determining rating curve of outlet |
| 32 | B.3 Test method for determining loss coefficient of outlet |
| 33 | Annex C (informative) Simplified checking procedure for siphonic systems C.1 General C.2 Calculation procedure |
| 35 | Bibliography [1] MAY, R.W.P. Design criteria for siphonic roof drainage systems. Report SR 654. Wallingford, England: HR Wallingford, 2004. [2] BARR, D.I.H., and HR Wallingford. Tables for the hydraulic design of pipes, sewers and channels – Volume 1. Seventh edition. London: Thomas Telford, 1998. [3] IDELCHIK, I.E. Handbook of hydraulic resistance. Washington: Hemisphere Publishing Corporation, 1986. (Distribution outside North America: Springer-Verlag, Berlin.) [4] MILLER, D.S. Internal flow systems. Second edition. Cranfield, England: BHR Group, 1990. [5] GREAT BRITAIN. Construction (Design and Management) Regulations 1994. London: HMSO. Statutory Instrument 1994 No. 3140. [6] MAY, R.W.P. Cavitation in hydraulic structures – Occurrence and prevention. Report SR 79. Wallingford, England: HR Wallingford, 1987. [7] KNAPP, R.T., DAILY J.W. and HAMMITT F.G. Cavitation. New York: McGraw-Hill Book Co., 1970. |
