APPLIANCE DISCHARGE VOLUMES
The size of pipe required in any system is dependent on the volume of waste which is to be conveyed, which is in turn determined by the type, grouping and number of appliances.The Discharge Unit Method gives a method of determining the size of stacks and branch pipes where a large number of appliances are in use. This method is comparable to that detailed in BS EN 12056-2:2000 sanitary pipework layout and calculations.

A numerical value is given to each type of appliance: the table below gives typical values. On the basis of this information and the flow capacities of vertical and branch pipes, correct pipe diameters can be selected or their adequacy confirmed.

DISCHARGE RATES OF APPLIANCES



PIPE SIZING VERTICAL PIPE CAPACITY
Each diameter of pipe fixed vertically in a soil stack can convey a given number of discharge units. Alternatively this can be expressed as a flow capacity in litres per second, as is illustrated in the table below.

VERTICAL PIPE CAPACITY

* WC's should not be connected to vertical pipes of 50mm
and 65mm diameter.

BRANCH PIPE CAPACITY
The flow capacities of branch discharge pipes differ from those of vertical pipes, since this capacity is reduced by the pipe gradient. Gradients for waste pipes must not be below 22mm/m (1.25°).
The diameter of a branch pipe must not be less than that of the appliance trap outlet to which it is connected.

The table below illustrates the flow capacities of various unvented branch pipe sizes for different pipe gradients.

BRANCH PIPE CAPACITY


Where the conditions of the above table are not satisfied, the branch pipe must be ventilated, usually with a 25mm pipe connected not more than *750mm from the trap: the principle is illustrated in the diagram below.
*300mm max NI only



Note that a 32mm ventilating pipe must be used where pipe length will be in excess of 5m, or when it contains more than five bends. Alternatively anti-vacuum traps, WBT742 and WBT746 can be used. The branch ventilating pipe must terminate, or be connected to a ventilating stack which is similarly terminated.

SINGLE STACK SYSTEMS
Most sanitary waste disposal requirements, for buildings up to twenty storeys high, can be satisfied using a single ventilated effluent-conveying soil stack, as opposed to having two parallel stacks, one for waste conveyance and a second to provide ventilation to the first. One soil stack may be used for buildings up to five storeys high: the ground floors of buildings between five and ten storeys high, and the lower two storeys of buildings between ten and twenty storeys high should have their own soil stacks. External stacks are not permitted where building height exceeds three storeys.Those sections of stack in which effluent flows should not have any offsets, and pipe diameter must not decrease in the direction of flow.

BRANCH CONNECTION SPACING
Building Regulations and BS EN 12056:2000 place dimensional restrictions on the vertical spacing of branch pipe connections to single stack soil and ventilating systems, and on the lowest connection height.

These restrictions can be summarised as follows:
1. Branches of any diameter should not be positioned opposite each other such that effluent could discharge across between them, or potentially cause blockages. Waste branches may be connected in opposing directions if there is an adequate vertical spacing.



Opposed branches serving WC’s can be connected to a stack at the same level using a double branch fitting, in which the branch pipes are angled or swept into the main stack.

2. No other connection should be made less than 200mm lower than an opposing WC branch connection: a 50mm parallel branch connection can prevent this situation, or the connection can be at the same level if perpendicular to the WC connection.



A facility to make several waste connections at the same level, while avoiding cross flow conditions, is provided by the waste inlet manifold.This fitting accepts waste connections close to the WC branch connection, but discharges into the main stack below the 200mm restricted area.

3. The distance between the lowest connection to the stack and the foot of the stack is dependent on the building height and should be as indicated in the diagram below.