SRDA-The Siphonic Guide-v1-1305.pdf

7/27/2019 SRDA-The Siphonic Guide-v1-1305.pdf

1/18

The SiphonicGuide

A guide to Siphonic Roof Drainage

by the Siphonic Roof Drainage Association

Siphonic Roof drainage Association, 2011


2/18

SRDA Guide to siphonic roof drainage 2

Contents

Preface The Siphonic Roof Drainage Association 3

1.0 The history of siphonic drainage 4

2.0 Siphonic drainage explained 5

2.1 Self priming 7

2.2 Primary & secondary systems 7

2.3 Gutter design 7

2.4 Underground drainage 8

2.5 Maintenance 8

3.0 How to specify siphonic drainage 10

4.0 Recommended design practice 12

5.0 Materials for siphonic drainage pipework 15

6.0 Siphonic Glossary 16

7.0 Further reading 18


3/18


Preface What is the Siphonic Roof Drainage Association?

The Siphonic Roof DrainageAssociation (SRDA) was set up in2004 to promote a widerunderstanding of the principles ofsiphonic drainage, to promotegood practice among siphoniccompanies, and to provide arecognised platform for specifiersto seek advice.

Members have to comply with thefollowing requirements to gainmembership:

1. A suitable siphonic outlet.

2. A pipe and bracket systemcapable of accepting thepressure and dynamic loadingfrom siphonic flow.

3. Design software capable ofaccurately assessing the verycomplex flow regime in thesystem.

4. Not use misleadingadvertising, which makesfalse claims about benefits ofa particular product, ordenigrates another.

All the requirements must havebeen assessed as suitable by athird party organisation.

All members are subjected to anannual random check by theAssociation to confirm that theirdesign and installation proceduresare being properly implemented.

This helps to ensure that standardsare maintained, and that there is athorough understanding of theseimportant factors throughout thecompany structure.

By using a member of the SRDA, aspecifier can ensure that they aregoing to a company which has thetools and experience to do the jobproperly.

The Association can answertechnical questions about siphonicroof drainage specification, design,or problems with existing systems.Contact the Secretary, Jim HookerBSc FIoR (M 07908 789454):

SRDARoofing House31 Worship StreetLondon EC2A 2DY

Tel 0115 914 4500Fax 0115 974 9827Email: [email protected]: www.siphonic.org

The members of the SRDA are:

Blucher UKDallmerGeberitPolypipe TerrainSaint-Gobain PipelinesSapoflow


4/18


1.0 Thehistory of siphonic roof drainageThe principle of siphonic roofdrainage was first developed bythe Finnish engineer OlaviEbeling in the late 1960s. Thefirst commercial installation was ina Swedish Turbine Factory by aconsulting engineer, PerSommerhein, founder of the UV-System company. Siphonic roofdrainage then spread throughEurope, and arrived in the UK in

the 1980s, with Geberit andSapolite (UV-System) being theoriginal companies.

During the 1990s there wereserious problems with some roofdrainage designs. The rulingBritish Standard, BS6367:1983had been written whenconstruction methods andmaterials were very different, and

the move from fibre cement toplastisol coated steel roofingproducts, created much higherrun off into gutters than thestandard anticipated. BS6367was ambiguous in its designguidance with respect to suitablerainfall intensities, and thus manygutters were seriously underdesigned. Although this problemoccurred in both gravity and

siphonic drainage systems, in thecase of the siphonic installation,the method of drainage was oftenblamed, rather than the designguidance.

The siphonic industry today is aprogressive one, with 6 majorcompanies, all committed todesigning to the much improved

Standard BSEN12056-3:2000. Adedicated British Standard forsiphonic drainage, BS8490:2007was published in March 2007,which formalises standards alreadymet by the members of the SRDA.

A specifier and their client can besure, if they use a member of theSRDA, that they are dealing with acompany which has the design and

installation experience to do the jobproperly.

Furthermore, by specifyingSiphonic drainage they can use atried and tested method ofdrainage with over 30 years proventrack record in installations acrossthe globe.


5/18


2.0 Siphonic drainage explained

The basic theory behind siphonicroof drainage is very simple andall systems work in the same way.Water dropping down thedownpipe creates a negativepressure at the highest point, in asimilar way to the action of asimple siphon, such as would beused to drain a fish tank. Thisnegative pressure is harnessed todraw water along a horizontalcollector pipe, removing the needfor many downpipes in thebuilding. This gives a number ofbenefits compared to a traditionalsystem:

o Internal underground drainagecan be eliminated in thebuilding, and significantreductions can be made inexternal undergrounddrainage. This can provideconsiderable cost savings andenhance the constructionprogramme on all sites, andparticularly on contaminatedones.

o Pipe sizes are reducedoverall, reducing the loads onthe structure, when comparedwithy lateral gravity drainage.

o There will be a significantlyreduced number of downpipesfor each gutter, which can belocated at the end of thegutter. This can free floorspace and allow columns tobe omitted.

o The collector pipe, whichruns horizontally, can bevery close to the roof orgutter, allowing full use ofinternal space.

o For sites with arequirement for asustainable drainagesolution (SuDS), siphonic

drainage will allow water tobe delivered to a specificpoint on the site, at ashallow depth. This cansignificantly reduce thestorage construction costs,especially for pond baseddesigns.

Water is collected from the gutterin small diameter tailpipes,

which fill with water (or prime)very quickly, and then fill the restof the system. Once the wholesystem is primed, full siphonicaction occurs, and flow ratesachieve the design level.

There are four vitally importantconcepts to ensure in design of asystem, which mean that it is atask for a specialised company.

These are:

o The system must becarefully balanced, so thatthe friction losses in thepipework ensure that thecorrect amount of waterpasses through eachoutlet. This is normallyachieved by making the


6/18


tailpipes on the outlets closestto the downpipe a smallerdiameter than those furtheraway. Another alternative isto space the outlets unevenly,so the outlets closest to thegutter take more flow.Whichever system is used, itis very difficult to undertakethese calculations without aspecialised computerprogram. All SRDA membersutilise sophisticated (andindependently proven)

analytical software to properlybalance the designedsystems, eliminating any riskof air being drawn into thesystem therefore retaining thefull integrity and effectivenessof the system at all times.

o The system must be designedso that the peak negativepressure in the system is

within the negative pressurecapability of the pipe system,and above the thresholdwhere cavitation (air comingout of suspension in bubbles)occurs. To ensure this, themaximum negative pressurein the system should be 8.8m(863mbar), or less negative ifpipework type dictates it. Toaccurately assess the

pressure, software mustevaluate friction, bend and

junction losses.

o The system must fill quicklyenough (or prime) to operatein a 2 minute storm. Allsiphonic systems must beable to fill the collector pipeusing only the flow from the

primed tailpipes. This flowwill be much smaller thanthe fully primed capacity ofthe system. If this doesnot happen withinapproximately 50-60seconds, the gutter maybe overcome before thesystem operates. Theonly exclusion to thiswould be gutters or flatroofs where storage isacceptable, and thus thesystem does not have to

function as quickly.

o The system must havesufficient drop betweengutter and collector pipe toallow enough flow to begenerated to fill thedownpipe. VDI 3806, (theGerman siphonicstandard) contains aspecific design clause

which relates the tail dropto the maximum downpipediameter. If the collectorpipe is too close to thegutter sole, there will bevery little energy gainduring the early part offilling process, and it ispossible that the systemwill not function.

2.1 Self-Priming

All siphonic systems operate inexactly the same way, and sotitles such as self-priming


7/18


apply to all systems. Comparisontests carried out at HR Wallingfordin 1996 (SR463) showed thatpipework design was the keyfactor in performance, and theoutlets tested all behave in theway predicted by the theory. Ifsmall diameter outlets are fitted toa large carrier system, thesiphonic pipework will tend toprime slowly, and so water depthin the gutter may rise above thefinal steady running level. Asystem which has been better

matched at design stage will notovershoot, provided it uses anyfunctional siphonic roof drainageoutlet.

2.2 Primary and secondarysystems

The introduction of BS EN12056-3:2000, and the steady increase in

building size, has lead to asituation where greater andgreater volumes of water arebeing drained from roofs. Thismeans that for a single siphonicroof drainage system to drain agutter, the collector pipes can bevery large. This has a number ofdisadvantages:

o The time for the system to fill

can become longer.

o The larger pipes imposesignificant static and dynamicloads to the building structure.

For these reasons some UKcompanies favour a primary &secondary approach to design.

The primary system will cater forthe 1-2 year event, and thesecondary system will drain anysurplus water in longer termevents. In most cases there isnot enough capacity inunderground drainage to copewith above ground requirements,and so this secondary drainagecan be discharged harmlesslyonto external surfacing.

However, the primary &secondary approach has its

drawbacks, the main one beingthat the gutter needs anadequate depth for it still tofunction as a gutter with waterrunning into the secondarysystem. With gutter sizesgradually getting smaller due toincreased insulation thickness,this can often be hard to achieve,and so a primary only system canoften be the best solution. Gutter

depths will be covered in moredetail in the next section.

2.3 Gutter design

Although siphonic roof drainagesystems work in a fundamentallydifferent way to a gravity system,the gutters which they drain workin exactly the same way. When a

siphonic system is designed,calculations should be carried outusing either the free or restrictedflow analysis procedures in BSEN12056-3:2000, using waterdepth information from outlettesting.


8/18


Various industry factors, such asthe introduction of Part L of theBuilding Regulations, have led toinsulation thicknesses increasing,and consequently gutters gettingsmaller. This can make it verydifficult to make aprimary/secondary siphonicsystem work properly. Be verycautious of companies who claimthat water depth in gutters is notimportant with a particular outletor gutter type. For all siphonicsystems, gutter calculations

should be undertaken to show thatthere is adequate capacity, basedon outlet water depth informationfrom a third party source (BBA orindependent testing).

It should be noted that problemscaused by a lack of gutter depthdo not only apply to siphonicdrainage. Many gravity guttersare very hard to design because

of lack of available depth.

2.4 Underground drainage

In UK design practiceunderground drainage is typicallydesigned to carry approximatelyhalf the flow of the roof drainage.This is for two reasons:

o Roof drainage is designed toprotect the building for itslifetime (usually 90 years plus)whereas below grounddrainage can flood in eventsover 30 years.

o Water concentrates veryquickly on a roof, and so roofdrainage design is based on a

2 minute storm asopposed to the 3-5 minutestorm used forunderground drainage. Itis a general rule that thelonger the event, the lowerthe intensity of rainfall.

Most siphonic systems use thefull building height to generatetheir flow, and thus it is vitallyimportant that when the systemdischarges it has no restriction. Itis therefore recommended that a

vented manhole lid be fitted atthe discharge point to allow airdrawn in during the primingprocess to be released, and anysurplus water to overflow withoutany risk to building. SRDAmembers can provide standarddetails for this on request.

2.5 Maintenance

Many specifiers are concernedthat siphonic roof drainagesystems need much higher levelsof maintenance, but the fact isthat all roof drainage systemsneed adequate maintenance,gravity or siphonic. All guttersshould be inspected at least bi-annually, and cleaned whenappropriate. Inspections and

cleaning may need to be carriedout more regularly where there isa high density of trees or othersources of airborne debris.

Siphonic outlets are morecomplex than conventional ones,and so it should be explained togutter maintenance staff that they


9/18


Must be re-assembled with alltheir original parts if they have tobe removed for maintenance.

Debris should never be swept intoany roof drainage system, but thisis particularly important forsiphonic drainage, where tailpipesmay be reasonably smalldiameter.


10/18


3.0 Specifying Siphonic Drainage

When specifying siphonicdrainage there are a number ofkey factors which must becovered. These are:

o Rainfall intensity - The rainfalllevels should be determinedfrom BS EN 12056-3:2000,using the projected buildinglife, and a suitable factor ofsafety.

o It is specifically stated in BS8490:2007, that there shouldbe no differentiation betweensiphonic and conventionaldrainage when calculatingrainfall intensities for design.

o The contents of the buildingshould be considered as wellas building type. The moreyears specified the lower therisk to the building, but themore expensive the system,so it is always a balance tosuit the acceptable level ofrisk.

o The minimum drainagerequirements set out in BSEN12056-3:2000, stipulate a1.5 or 4.5 factor of safety tobe applied to building life.4.5 is normally used forbuilding where theconsequences of floodingwould be more significant,though these are neverexplicitly defined.

o Many large developersconsider that for distribution

buildings, the 25 yearstandard building life is tooshort to adequately protectthe building's contents, andso a factor of 4.5 is oftenused (Category 3) giving anoverall protection life of112.5 years.

o Office buildings often have alonger building life such as60 years, and so the factorof safety of 1.5 (Category 2)gives a much greater levelof protection.

o Specification of rainfallintensity is the responsibilityof the architect, and theSRDA can give no absoluteguidance, but it should benoted that if the overallprotection life includingsafety factor is 30 years orless there is a serious riskthat water will flow into thebuilding during its lifespan.

o Most importantly 75mm/hrshould never be specifiedfor internal gutters as it willlead to over flow into thebuilding every 1-2 years,depending on location.

o Filling time and guttercalculations - It is vitallyimportant that the siphoniccontractor providescalculations to show that thesystem will fill within therequired time period, andthat the gutter will function


11/18


correctly i.e. will not over-top.In the UK the design rainfallevent (the most intense periodof a storm) is 2 minutes, andso a siphonic system mustbegin to function within halfthis time unless roof storageis provided, or the roof mayflood. In the past somecompanies have claimed thattheir systems do not need tofill to operate, but this issimply not correct. Guttercalculations should be to

BSEN12056-3:2000, usingoutlet data from a BBAcertificate or other third partysource.

o The majority of siphonicdrainage systems in Britainuse high density polyethylenepipework. HDPE can beconnected using eitherelectro-fusion couplings,

which are heated by internalelements, or by butt-jointing,where the cut ends of the pipe

o are melted and then forcedtogether under pressure tomake a joint. Butt-jointsshould only be made using amachine incorporating a jigand control system to monitorthe temperature, time andpressure required, as is

required by BS8490:2007.

o Metal pipe systems (cast-iron,galvanized or stainless steel)can also be used for siphonicdrainage. The specificationshould detail that installationshould be according to theirmanufacturers

recommendations fornegative pressure.

o BS8490 sets out thefollowing information to beprovided by specifiers tosiphonic contractors:

o Location of building andheight above ordinancedatum,

o Required design stormreturn period, or category ofstorm and design life ofbuilding.

o Roof plan indicating area to

be drained.o Roof covering and height of

potential leakage paths intothe building.

o Gutter positions and initialsizes.

o Preferred outlet locationso Overflow positionso Preferred downpipe routeso Temperature and humidity

of rooms intended to be

heatedo Decibel rating of noise

sensitive areaso Building useo Position of soft landscapingo Levels of roof and external

groundo Structural arrangements of

roof, including beam heightso Location of movement jointso Maximum depth of water on

roofo Details of connection to site

drainage systemo CDM risk assessment for

installion of outletso Any other relevant

information


12/18

4.0 Recommended Design Practice

However complex the calculation program used by a siphonic company, thefunctionality of the system relies in the first instance on the pipes becomingfull of water. In some cases this may not occur because of badly detailedpipework. The following are examples of recommended details which shouldbe used to prevent problems in pipework design.

4.1 Expansion below the outlet

If the tailpipe below the outlet is too large, the outlet will not be able to fill it,leading to the situation where the outlet will be working at a tiny fraction of its

design flow (All UK siphonic outlets will only drain 3-5 l/s when notconnected to a primed pipe system). Expansion of one pipe size in thevertical is usually acceptable (i.e. 75 up to 90), but at figures beyond that thetail should be deemed not to be functional, unless it has been provedotherwise by third party physical testing. It should be noted that it is perfectlyacceptable to increase the size of the horizontal component of the tailpipe. Itis acceptable to have these non-functional tailpipes as perhaps one or two atthe end of a large system, but they should be excluded from any fill timecalculations.

Good Tail filled in vertical Bad Tail may fail to fill


13/18


4.2 Sloping tailpipes

If tailpipes slope, there is a significant risk that water will accelerate down the

slope in gravity flow such that the pipe will not run full and it will be leftrunning in gravity. This will lead to the same effect as above, that the outletswill stay in gravity mode, reducing their effectiveness, and probably stoppingthe system priming and thus reaching full effectiveness.

4.3 Sloping lateral pipes (Greater than 1:100)

For the same reasons that it is best to avoid sloping tailpipes, the lateral pipein a siphonic system should be installed horizontally. If the laterals areinstalled with a slope then care should be exercised to detail the pipeworkdownstream such that the system will prime. The best way is usually toreduce the diameter of the vertical pipe downstream of the sloping section,to force it to prime the sloping section.

4.4 Configuration of the downpipe

Expansions in diameter of the downpipe should be avoided whereverpossible, as there is a risk that the siphon will break at this stage. This willmean that if the system has been designed to be full to ground level, therewill be much less capacity as the system will only have a proportion of the

Good Siphonic actionPromoted by smallerdownpipe

Bad System may stay ingravity

Good Tail filled in vertical Bad Tail may fail to fill


14/18


available drop. The other assumption that can be made is that the pipe willnot fill lower down, and that the system becomes gravity at the expansionpoint. Unfortunately, the process is not reliable, and the pipe will sometimesfill and sometimes not, and so neither assumption can be made withconfidence. There are two approaches that can therefore be pursued:

o Make the expansion in anoffset which will ensure thatthe pipe primes all the waydown

o Expand the pipe such that thefilling degree in the lowersection is only 20% which willinhibit the ability to prime


15/18


5.0 Materials for siphonic roof drainage systems

Many different materials can be used for siphonic roof drainageinstallations, the key thing being that they can resist the dynamic andpressure loadings the pipework will be subject to. Most pipe manufacturersdo not provide negative pressure test data, and so manufacturers mustcarry out tests on pipe to ensure it is strong enough.

The following pipe types have successfully been used in installations in UKor in other parts of the world:

High DensityPolyethylene (HDPE)

HDPE is the most commonly used pipe type inthe United Kingdom industry.

Unplasticised PolyVinylChloride (uPVC)

uPVC for use in siphonic systems must besolvent welded pressure pipe, and suitable fornegative pressure applications.

Cast Iron Plain ended cast Iron to BS EN877, is often usedin areas of buildings where a decorative feature isrequired, or to provide fire protection measures.

Galvanised Steel Galvanised steel to EN 1123 with push fit joints islighter than cast iron, less expensive thanstainless steel, and has been used extensively onthe continent.

Stainless Steel Stainless steel with clamped push-fit seals issometimes used in areas where a decorativefeature is required.

Copper & othermaterials

In a similar way copper and other materials canbe used, but must be checked for negativepressure resistance


16/18


6.0 Siphonic Glossary

Baffle plate A flat or shaped device (sometimes part of leafguard)which prevents air entering the siphonic system.

Butt joint A joint in HDPE pipework generated by heating pipeends and forcing together under pressure.

Cavitation Air bubbles coming out of suspension in water, causingdamage to pipe material. Occurs when velocity veryhigh and pressure very low, and will not occur if pipesystem designed correctly.

Downpipe Single pipe which drops down from lateral to ground

level. Water flowing down this downpipe creates thenegative pressure which drives a siphonic system.

Fill time The time taken for the tail pipes (when running full bore,but by themselves) to fill the system and start overallprime.

Friction loss The energy loss associated with the process of waterrunning along the inside wall of the pipe.

Full bore A pipe running full of water or primed

HDPE High Density Polyethylene a light, robust, impactresistant plastic pipe system used in the majority of UKsiphonic installations.

Implosion Pipe failure due to insufficient strength in material towithstand internal vacuum. Will not occur if pipecorrectly specified and design undertaken correctly.

Minor loss The energy loss associated with water passing through ajunction, bend, or fitting.

Negativepressure

A vacuum created in pipe system by full bore flow downdownpipe.

Priming The process of the pipe filling with water. Once full ofwater it will be primed, and siphonic action will occur.

Secondarysystem

An additional pipe system, with outlets located at a smalldistance above the roof level or gutter sole which willdrain larger storms, often onto hard surfacing outside


17/18


the building

Self-cleansing Flow in pipe systems needed to shift sediment. Flowvelocities in priming siphonic systems are very high soself cleansing could be expected at figures as low as10% of full design flow.

Self-Priming A description of how all siphonic systems operate.

Siphonic Outlet An outlet which has been specially designed toencourage full bore flow in a tail pipe at a shallow waterdepth.

Rail system A metal rail connected to HDPE pipe using circularbrackets, and fastened to structure at typically 2mcentres. This rail is designed to restrain thermalmovement of pipe.

RainfallIntensity

The design level of rainfall in a 2 minute storm which thesystem will have to drain from roof.

Tail pipe Small diameter pipes connecting the roof drainageoutlets into the horizontal collector pipe.

Ventedmanhole

A manhole close to building with a vented cover to allowair to be vented during priming process, and water toescape in the event of lack of capacity in undergrounddrainage.

Water depth The depth of water at the outlet when it is primed. Thiswill vary with flow rate and for some outlets with gutterwidth.


18/18


7.0 Further Reading

BS8490:2007 Guide to siphonic roof drainage systems

British Standards Institution September 2000

BSEN12056-3:2000 Gravity Drainage Systems Inside buildings layoutand calculation.

British Standards Institution September 2000

RP 463 Performance of Siphonic Drainage Systems forRoof Gutters

HR Wallingford Report Sept 1996,

Wearing et al., 2005 Flow into modular plastic box structures fromsiphonic and other high flow drainage systems

Proc of 3rd national Conference on SustainableDrainage, Coventry, - June 2005.

SRDA-The Siphonic Guide-v1-1305.pdf

Documents

Transcript of SRDA-The Siphonic Guide-v1-1305.pdf