Answered by David Eddy, MWH

A customer requires a supply of 1 l/s maximum instantaneous demand, 1.5 km from the nearest water main. If the water company will only guarantee 15 metres static head what size supply pipe should the customer lay. The pipe size must be large enough to avoid unacceptable head loss and small enough to ensure that the water is turned over.

A pipe with 60 mm internal bore is suitable if there is negligible elevation difference between the connection to the main and the point of discharge at the customer's premises.

The customer will need to choose a commercially available material and diameter. 63 mm PE SDR17 (not SDR11) is adequate as this will give more than eight metres supply pressure at the specified flow rate. If SDR17 is not available locally or a higher pressure rating is required then 90 mm PE will be suitable. Remember that PE pipeline is normally specified by outside diameter, whereas it is the internal diameter that determines the head loss. Other pipe materials can be used if they are approved for conveying potable water.

The customer will also need to consider the potential for additional on-site head losses. For example if the customer has local storage, 60 mm is adequate but if there is signifcant on-site small bore pipework, a larger internal diameter, say 75 mm, is advisable. Assuming that peak draw of up to 1 l/s occurs daily, water quality will not be a problem with pipe diameters from 60 to 75 mm. If the design flow is a rarely experienced, for example if it is required for fire protection reasons, water quality may be affected and specialist advice is recommended.