Tp2 siphonic roof drainage systems the road to priming(dr)
Siphonic roof drainage systems: the road to priming Marc Buitenhuis Hydraulic research engineer Akatherm International BV, Panningen, The Netherlands 16-01-2009AbstractIn this article the start up of a siphonic roof drainage system is described. It is intended to givebetter insight in the behavior of the system and help design systems that will work optimallyand siphonic more often.The development of the hydraulic jump and the elimination of air from the system by thesuction power in the downpipe are key items in the development of siphonic functioning.To reach siphonic drainage as quickly as possible a longer vertical tailpipe as well as a quickdeceleration in the horizontal pipe is preferable. 1. Introduction 2. Start up phaseSiphonic roof drainage systems are When the rain starts the roof drainagedesigned to operate with full bore flow. system slowly starts up. At first the waterThe computations made for siphonic roof flows into the roof outlet at a low rate anddrainage systems therefore focus on single shallow water level. The water flows alongphase water flow. For the proper design of the vertical walls of the tail pipe creatingthe maximum capacity of a siphonic an annular flow. At the bend to thesystem this is sufficient. horizontal tail pipe or collector pipe theHowever it is necessary to have a good flow collects at the bottom of the bendinsight in the start up phase to siphonic resulting in a separated flow in thefunctioning of the system as well. Most of horizontal pipe. In the downpipe behindthe times the rain intensity will not be high the collector pipe the water is forming anenough to get the system to work siphonic. annular flow again.It is preferable to have the system worksiphonic once or twice a year at least toself clean the system of debris. Also whenthe system works siphonic the noiseproduction is lower than with two-phaseflow (combined water/air flow).In this article the start up phase will be fig 1. flow regimes in vertical pipes: annular, slug and bubbledescribed to have a better understanding of flowthe phenomena and develop measures tostimulate full bore flow.
Figure 3. hydraulic jumpfig 2. flow regimes in horizontal pipes: straight, wavy and slugflow The above explains 2 things: first of allThis is independent of the water velocity why an increasing length of vertical tailstreaming into this pipe. The flow can pipe leads to earlier priming, second whyfollow the inner contour of the bend or an increasing resistance in the collectorsplash onto the opposite wall. The point at pipe leads to this same result.which the annular flow is reinstalled will An increasing length of tail pipe leads todiffer, as will the pressure distribution more length to accelerate the fluid comingwhen the pipe is closed off by the from the roof, thus to higher velocities insplashing water. the bend to the horizontal pipe. This willAt some higher flow rates the separated lead to a higher hydraulic jump when theflow in the horizontal pipes will become flow is decelerated in the horizontal pipe.wavy. Also the more the flow is decelerated in the horizontal pipe, thus the higher the resistance downstream of the bend, the 3. Hydraulic jump higher the hydraulic jump will be. Eventually the hydraulic jump will closeWhen the water streams down the vertical off the whole pipe diameter, leading totailpipe it is accelerated by gravity. When below atmospheric pressures in the systemit flows into the horizontal pipe the flow is behind the closure and priming will start.decelerated forming a hydraulic jump.The principle can be compared to the 4. Start of primingstream of vehicles on highways or racetracks. Vehicles can accelerate optimally When the hydraulic jump closes off theon roads that are straight and keep on whole periphery of the pipe the air behindbeing straight for miles. As soon as there is the jump has only one way to leave thea curve in the road the vehicles have to system and that is through the downpipe.slow down. When the first vehicle To transport the air through the downpipedecelerates the one behind him has to the friction forces between the water anddecelerate also and the distance between the air have to overcome the buoyancythe vehicles is decreasing. This is very forces of the air. In other words the wateroften the moment for accidents to happen: has to drag the air along against itsthere is an increasing chance for collision. tendency to rise. To make this happen theExactly this is the case for fluid particles in flow rate has to increase further.a stream. When particles are redirectedfrom the vertical downfall to horizontal 5. Measures to enhance primingflow the fluid is decelerated. As fluidparticles have no brakes they will collide As stated above to enhance the priming ofand the only way they can go is up, the system a longer tail pipe can be chosencreating height and thus a hydraulic jump. or the deceleration of the flow in the horizontal pipe can be increased in order to
make the hydraulic jump close off the The development of the hydraulic jumpperiphery of the pipe as quickly as possible and the elimination of air from the systemleading to earlier priming by the suction power in the downpipe are key items in the development of siphonic 6. Conclusions functioning. To reach siphonic drainage as quickly asIn this article the start up of a siphonic roof possible a longer vertical tailpipe as well asdrainage system has been described. It is a quick deceleration in the horizontal pipeintended to give better insight in the is preferable.behavior of the system and help designsystems that will work optimally andsiphonic more often. 7. References 1. Fox, Robert W. , McDonald, Alan T. , Introduction to fluid mechanics, third edition, 1985, School of Mechanical Engineering Purdue University, John Wiley & Sons 2. Scott Arthur, John A. Swaffield, Siphonic roof drainage: current understanding, 2001, Water research group, Department of civil and offshore engineering, Heriot-Watt University, Edinburg, Scotland (UK) 3. G.B. Wright, S. Arthur, J.A. Swaffield, Numerical simulation of the dynamic operation of multi-outlet siphonic roof drainage systems, 2005, Drainage and water supply research group, School of the build environment, Heriot-Watt University, Edinburg, Scotland (UK) 4. Scott Arthur, The priming focused design of siphonic roof drainage, drainage research group, School of the built environment, Heriot-Watt University, Edinburg, Scotland (UK) 5. S. Arthur, G.B. Wright, Siphonic roof drainage systems – priming focused design, 2006, School of the built environment, Heriot-Watt University, Edinburg, Scotland (UK) 6. S. Arthur, J.A. Swaffield, Siphonic roof drainage system analysis utilizing unsteady flow theory, 2000, Department of building engineering and surveying, Heriot-Watt University, Edinburg, Scotland (UK)