INTRODUCTION TO SPACE GAINING DRAINAGE SYSTEM

TECHNICAL PAPER

SPACE GAINING DRAINAGE SYSTEM

Sanitary drainage systems in high-rise buildings are without doubt, one of the most dynamic systems within them. Data, water supply, electricity and air are all subject to ebb and flow, but nothing matches the action happening within the sewer systems of the growing number of residential and commercial towers being built within our cities.

Only the sewage system is expected to deal with an ever-changing mix of water, air and solids.

Unlike the building’s other systems, the loads and uses of sanitary drainage can change dramatically over its lifetime subject to its occupancy. As projects rise ever higher these systems come under greater pressure to accommodate the tasks expected from them.

Across the world numerous systems exist to efficiently drain the building of its so called “black” and “grey” waste water.

• Fully Vented: A system common to all early plumbing designs across the developed world where all plumbing fixtures were individually vented. Not so common now but still used substantially in the United States.

• Fully vented (Modified): A system where it is recognised that not all fixtures require their own vents and can share venting if sized to a set standard. A secondary relief vent pipe is connected parallel to the vertical stack to allow a greater load on the stack without increasing the diameter.

• Single Stack: All fixtures are individually connected to the vertical stack or via a common discharge pipe but have to be within a set distance from the stack.

• Single Stack (Modified): A parallel relief vent pipe is connected to the stack to allow an increase in fixture loading (numbers) and an increase in height.

• Twin Stack: Separate “Grey” and “Black” stacks with no secondary venting and the stacks venting to atmosphere.

• Triple Stack: A system used within the Middle East and some parts of Asia where the “Grey” and “Black” stacks are run separately with a central stack for combined venting.

• Other variations and versions of the above.

One system excluded from the above list is the system invented in the late 1950’s and used since then in various forms and materials utilising “Aerator Junctions”. In almost all parts of the world this is an engineered solution that sits outside most countries plumbing codes and standards.

Unique to the Australian & New Zealand Standards AS/NZS3500.Part 2, this system is recognised and accurately termed the Reduced Velocity Aerated Stack System (RVASS).

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TABLE 1

5 10 15 20 25 30 350

25

20

15

10

5

0

Height of fall in m = h [m]

Velo

city

(spe

ed) i

n m

/s

Theoretical fall = 2g · h

Water jacket withair column

In all systems of High-Rise Sanitary Drainage there are a few Universal Constants.

• Everything falls DOWN.

• All discharges down a stack will, over a certain distance, reach Terminal Velocity: A speed that once reached, will not be exceeded regardless of distance/height (Table 1)

• Sanitary discharges within a stack assume an annular form or flow:

• Water will fall to the inner wall of the stack

• Solids will gravitate to the centre

• Air will move up and down in the space between

• Discharges will both push air in front and pull or “entrain” air behind

• Air can be compressed.

• Water cannot.

Hydraulic pulses or “jumps” occur within the stack pipe when sanitary discharges transition from a vertical flow to a horizontal flow at offsets within the stack due to a changing floorplate or the base of the stack where it will connect to the main sewer. The annular flow is broken because the water on the inner wall of the vertical pipe collapses in the horizontal aspect forming a wall or barrier to the airflow within.

This causes the entrained air to compress and rebound against the water wall and travel back up the stack. If plumbing fixtures are connected near to the base of the stack or offset, bubbling or movement of the trap seals within the closest plumbing fixtures may occur, resulting in loss of trap seals and therefore ingress of sewer gasses to the living areas.

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FIGURE 1

Most plumbing systems and/or Codes and Standards have ”prohibited connection zones” or “foaming zones” noted within the stack where plumbing fixtures are not permitted to be connected to reduce the possibilities of this occurrence.

Dissimilarities between the above and RVASS is other systems can allow the flow to actually reach terminal velocity within the stack causing hydraulic pulses as described above and branch discharges on multiple floors discharging directly to the stack via “square” or 85° junctions can disrupt the stack’s annular flow creating a very active system of fluctuating pressures. (Figure 1)

Excessive velocity can also create low pressure areas behind stack discharges and again, cause trap seal loss within connected branch fixtures as external air pressure seeks to balance the pressure within the stack, usually through the weakest point, the trap seal.

Parallel relief vent pipes, cross-connection vent pipes, branch/group vent pipes and air admittance valves are all intended to combat this but can add to the complexity and consequently the cost of a system.

RVASS systems utilise specially designed Aerator Junctions at each branch connection.

This allows:

• Up to 6 connections on the one junction

• Limitation of discharge velocity within the stack

• Preservation of the stack’s annular flow by by-passing the branch connection

• Air pressure to be maintained within the stack

• Discharges from the branch to enter the stack vertically thereby limiting temporary blockages as per horizontal connections (Figure 1)

• Deletion of relief and cross vents as well as branch vents (to a prescribed length)

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FIGURE 2 FIGURE 3

FIGURE 4

Structure of the Geberit HDPE Sovent fitting d110

Unique requirements of these systems is the placement of a pressure relief or “de-aerator” assembly at the stack offset or base. This allows any positive pressure that may occur to be circulated within that area without migrating to any nearby fixtures. (Figure 3)

This can be used within other system types as can devices such as Pressure Attenuators.

“Velocity Breakers” or in-line offsets are also required where there are no branch connections or junctions over a set distance to limit discharge velocities. (Figure 2)

Almost all RVASS systems currently operate on the same principle of the same basic design from the late 20th century except for the Geberit Sovent system.

In 2014, after much research, Geberit upgraded their HDPE Sovent aerator junction.

Utilising historical data, computer modelling and their own in-house 10 storey drainage tower, Geberit achieved a 40% increase in flow rate through their junction. With the addition of a flow divider at the inlet and an offset at the bend, the annular flow is set in rotation, stabilising it through the junction and continuing through to the floor below. (Figure 4)

From 8-9 Litres per second, Geberit could now achieve up to 12 litres per second in an 110mm HDPE pipe. This allowed much greater loads through a Geberit stack than “traditional” RVASS designs.

Figure 1: Geberit Sovent system

Main ventilationthrough roof

Expansion socketSovent

Pressure reduction

Underground pipe or collector pipeEnd of Geberit

Sovent system

25 m

25 m

Expansion socketSovent

1 Stack connection2 Branch pipe connection, 6-way,

sealed at the factory3 Flow divider4 Offset

5

TABLE 2

FIGURE 5 – 1

FIGURE 5 – 2

Using the European Standards DIN EN 12056-2 & DIN EN 1519 where stack system loads are based on Discharge Units (DU), a value based on the flow rates through the various fixtures, a Geberit stack can assume a load far in excess of that normally attributed to 110 dia pipe. (See Table 2)

Four years later Geberit upgraded the system again and named it SuperTube with the addition of two new fittings. The BottomTurn Bend (1) and the BackFlip Bend (2) (Figure 5)

164

334

d

d

334

117

418

304

d

d

Sink laboratory LS 50 – 1 1.0

Sink (pot or utility) PS 50 – 5 1.0

Slop hopper SH 100 –6 (F. valve)

4 (Cist)2.5

Trough–• ablution• laundry (single or double)

Tr.(A)Tr.(L)

4040

– 35

1.02.0

Urinal–• wall-hung (including waterless)• stall, or each 600 mm

lenght of slab

Ur4050

3211

0.50.8

Water closet panWC 80 –

6 (F. valve)2 (Cist)

2.02.0

Water closet panWC 100 –

6 (F. valve)4 (Cist)

2.52.0

Bathroom group in a single room (basin, bath, shower, water closet)

– – – 6 3.5

Combination pan room sink and flushing bowl

PRS 80 –6 (F. valve)

4 (Cist)2.5

Combination pan room sink and flushing bowl

PRS 100 –6 (F. valve)

4 (Cist)2.5

Fixture Fixture abbreviations

Min. size of trap outlet and fixture discharge pipe DN

Fixture unit rating DU

NZ (only) AU/NZ

6

ive

Once more, using historical data from the improved junction, next generation computer modelling and the real-life testing on the in-house drainage tower, Geberit was able to eliminate the pressure relief line/aerator assembly that was previously required at stack offsets and bases.

Rather than fighting the pressure dynamics at these points, the specially designed BottomTurn Bend with its inbuilt Flow Divider provides a smooth transition from a vertical annular flow to a horizontal laminar flow. The central air column from the stack is maintained within the upper portion of the offset and the kinetic energy previously lost at that point is now harnessed to propel the flow up to 6 metres from the bend with no fall or slope required in the pipe. After 6 metres or a deviation in the layout of the pipe, local standards of size and grade apply. (Figure 6)

Through the elimination of the Foaming/ No-connection zone, plumbing fixtures can now be connected at any point along the offset.

At the offset’s end, regardless of length, the BackFlip Bend is applied to allow the annular flow to reform within the vertical stack. Its unique design uses the same principle as the offset within the Sovent junction (Figure 4) to set the flow into rotation as it transitions from horizontal to vertical. (Figure 7)

Velocity breakers/In-line offsets (Figure 2) are also no longer required. “Express” stacks (Straight stacks with no branch connections) can now be used to further simplify plumbing design and installation.

FIGURE 6: TRANSITION OF A STACK WITH GEBERIT SUPERTUBE TO THE COLLECTOR PIPE1 Geberit HDPE BottomTurn bend2 System boundary3 Slope in accordance with local standard or in accordance with DIN EN 12056-2:2001-014 Dimensioning in accordance with local standard or in accordance with DIN EN 12056-2:2001-01

FIGURE 7: ZONE WITHOUT CONNECTIONS GEBERIT SUPERTUBE WITH A STACK OFFSET WITH CONNECTIONS INTO THE STACK OFFSET1 Geberit HDPE BottomTurn bend2 Geberit HDPE BackFlip bend3 Zone without connection: upper edge Geberit HDPE Sovent

fitting up to pipe axis

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SUMMARY

The space within these buildings becomes more precious and every square centimetre is fought over between the architects with their vision and the contractors with their reality.

These two bends in conjunction with the previously upgraded junction now provide Hydraulic and Sanitary System Engineers with an efficient, cost effective sewage system that allows a clean, uncluttered design without superfluous vent piping whilst utilising the full capabilities of the pipe without arbitrary upsizing.

Plumbing contractors benefit from simpler installation allowing more efficient use of labour and less material costs.

The 6 metre flat offsets also gives architects increased ceiling heights and the lack of Pressure Relief Lines decreased duct sizes. HDPE piping is also one of the “greenest” materials that can be used within a new building.

It appears plumbing drainage design has finally joined us here in the 21st Century.

Marc Williams Geberit Australia

REFERENCES:1. ‘Geberit Sovent Installation Guide’, Geberit Australia.

https://www.geberit.com.au/local-media/files/geberit-sovent-installation-guide.pdf?lang=en

2. ‘Geberit SuperTube Planning Manual’, Geberit Australia. https://www.geberit.com.au/local-media/files/planning-manual-geberit-supertube-geberit-sovent-australia-final.pdf

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