Freeflo System Design



Performance Requirements

The primary aim of any drainage installation is that it must be functional and stable, make use of the minimum piping required to remove sewerage from sanitary fixtures in a building: rapidly, quietly and without nuisance or risk to health.

Design Parameters


Sizes must not unnecessarily limit the outflow from sanitary fixtures. The probability of simultaneous discharge of sanitary fixtures must be taken into account when determining the pipe size.

Containment of water and air:

The pipework and fittings must prevent leakage of contaminated water and foul air into buildings.

Limitation of noise:

Discharge pipes must be correctly vented to prevent noise and maintain the environmental quality in buildings.

Resistance to blockage:

The discharge pipework must be so designed to minimise the risk of blockage.


The discharge system, including pipes, fittings, joints and holderbats. must have a life expectancy of at least thot of the building without risk of leakage.

Access for maintaining and testing:

The system must be designed and installed to allow easy access for repairs, maintenance, clearing of blockages and testing.

Aeration and venting:

The system must be so designed and installed to enable the passage of air out (release pressure build up) and air in (relieve vacuum build up).

Flow Characteristics
Branch Lines:

The flow characteristics in branch lines can be effected by a number of factors:

  • Size, length and gradient of the branch pipe.
  • Type of trap and outlet fitting.
  • Adequate branch venting.
  • Volume of water being discharged.
  • Design of pipe fittings.

Trap seals in branches can be broken by self syphonage which is caused by a single branch running full, or by induced syphonage which is caused by a full flow in a common branch.


Flow in stacks causes pressure fluctuations which can effect the trap seals through induced syphonage - negative pressure in stacks, or back pressure. Factors effecting flow in stacks are:

  • Hydraulic load on stack, i.e. the number of sanitary appliances connected to the stack.
  • Height, diameter and changes in the direction of the stack.
  • Design of pipe fittings.
  • Venting.
  • Wind effects over the top of the vent.

Conventional Drainage Sytems

Conventional systems have an open gull/ and make use of manholes and or access fittings on the underground drainage system. These systems employ either 50mm or 110mm vents.

Closed Drainage Systems

In closed systems, also termed gullyless systems or "stub stack", the open gully head Is replaced by a stub stack which incorporates an overflow cap and at least one waste pipe connection. No access fittings are provided in the underground drainage system, as rodding takes place from the full bore access provided at the above ground junction. However, on long underground runs, provisions should be made for intermediate rodding eyes. Long radius bends are used both vertically and horizontally to ensure ease of rodding, Advantages:

  • Less wet weather flow in sewerage networks.
  • Elimination of foreign objects and materials in the drainage system.
  • Elimination of wind blown sand, a problem In coastal resorts.
  • Cost effectiveness - the elimination of manholes and access fittings provides savings.
  • An aesthetically pleasing appearance.

Vent Valves
What is a Vent Valve?

They were developed as a replacement for vent pipe used to ventilate drainage systems in order to reduce the expense of the vent pipe and roof flashing or the unsightly offset bends used around the eaves.
Freefio® 400 vent valves bear the SABS 1532 mark of approval and are categorised like the pipes that they replaced.

  • Soil pipe vent valves - SPW.
  • Waste pipe vent valves - WPW
    They are available In 50mm and 110mm in one-way and two-way configurations.
Vacuum relief (air In)
Internal or external use
Vacuum and pressure relief
(air in and out) External use only

Two-way valves are generally used externally, however there are occasions where one would use a 400 l/min one-way vent valve externally, bearing in mind that it only allows vacuum relief (air In). It would be ideal on a branch line near the front door or patio where the emission of foul air would be undesirable.

Waste Pipe Vent Valve

A waste pipe vent valve shall be connected to the crown of the fixture discharge pipe on the outlet side of the trap, at a point at least 150mm above the flood level of the fixture.

  • Rated at 1001/min.
  • For Internal use only.

Soil Pipe Vent Valve

Any soil pipe vent valve shall be connected to the open end of a ventilation pipe at a point at least 150mm above the flood level of:

  • Any sanitary fixture served by such a ventilation pipe.
  • The highest sanitary fixture served by such ventilation pipe.
  • Rated at 400 l/min,
  • One-woy: external and internal use.
  • Two-way: external use only.

Any ventilation pipe shall not have a reduction in diameter, i.e. 110mm vent valve on a 110mm vent pipe at the head of a drain and on branch drains. The vent valve provides for this by design, that is provided the valve is rated as a 400 l/min valve, it can be used in this situation.

This is where our Freeflo® 400 valves are beneficial. The 50mm and 110mm valves are the same. An adaptor is simply added to increase the size to 110mm.

Bracketing for Expantion and Contraction

  • Expansion and contraction in PVC Is six times that of steel.
  • Expansion and contraction on most domestic installations does not need to be provided for specifically. The runs are generally short with enough rubber ring fittings to cater for it.
  • Long runs typical of shopping complexes and commercial installations where industrial cleaning is carried out in food preparation areas, are installations that need special attention.
  • Steam cleaning followed by a hose down could see a 30° C temperature change within minutes.
  • The expansion and contraction factor is 0.06mm per metre per "C rise or fall in temperature. For example, a 6 metre length of pipe will expand and contract 10.8mm for a 30"C change in temperature.

Wall Mounted

The correct way to provide for expansion and contraction when using lipseal rubber ring fittings is to
anchor the fitting and allow the pipe to expand and contract between the fittings.

The maximum space between fixed points is 6 metres.

For this to work when the installation Is being done. use the depth of entry mark method. Measure the depth of the socket of the fittings that are being used and subtract the value in the adjacent table and mark the pipe.
The other way is to insert the pipe fully and then withdraw it by the value in the adjacent table. This is more difficult when working on a scaffolding.

Fitting Withdraw Pipe
Size From Socket
(mm) (mm)
50 8
75 10
110 12
160 12

Suspension drains

In instances where pipework is suspended from beneath floor slabs on pipe hangers, care must be taken to ensure that fixed points are provided at 6 metre spacings.
The ideal is a fabricated triangular steel bracket and fixing holderbat bolted to the underside of the floor slab and fixed to a fitting allowing the pipe to expand and contract between fittings.
Suspended from floor slab


Our standard range of holderbats are designed to fit pipes and not fittings, special holderbats will need to be used to fix fittings in this instance.

Spacing for holderbats
Pipe Size Horizontal Spacing Vertical Spacing
(mm) (mm) (mm)
40 400 1200
50 500 1200
75 800 1600
110 900 1800
160 1000 2000