Discharge Units

Discharge units were devised to accommodate a diversity factor into the flow from appliances through the drainage system. They were initially designed for use in sizing stacks, but their use has spread to the designing of sub-soil drainage, though generally in the smaller sizes of pipes (up to and including 225mm diameter).

The two types of DU's available are found in BS5572 (now superceeded) or the IoP design guide, and in the newer BS EN12056. Each system uses its own values, and its own method of calculating the derived flow. The two systems are NOT interchangeable. The program provides a facility for converting between the two, but this should be used with care, as the usage loading factors cannot be integrated into this function.

Sizing using flow rates is the preferred method for larger drains. The program contains a user-set variable containing the flow rate per head of population, and the population served by each section of drain is used to size the pipe, adding in any flows from upstream. This method can be used to size systems in mainland Europe, as BS EN12056 contains the (widely varying) data for average daily demand.

Sewers to be adopted by a local authority or water undertaker should use design parameters in accordance with the latest edition of 'Sewers for Adoption'.