To remain primed, the vertical drop must not be angled or increased in diameter.

The vertical drop's potential flow rates are reduced by friction losses in the connected pipe work but losses are minimised by connecting the vertical drops to larger horizontal pipes.

This is easy to calculate and is explained with examples in the PIPES & FITTINGS page.

*The simple adage is**: Larger pipes = less friction loss = higher flow rates.*

Polypipe 19mm vertical drops are often connected to 25mm (78% greater volume) horizontal polypipe, depending on intended usage. Polypipe has the advantage of low cost, ease of fitting and is easily altered.

This is easy to calculate and is explained with examples in the PIPES & FITTINGS page.

Polypipe 19mm vertical drops are often connected to 25mm (78% greater volume) horizontal polypipe, depending on intended usage. Polypipe has the advantage of low cost, ease of fitting and is easily altered.

For the recommended class 9 & 12 PVC pressure pipe, the 20mm vertical drops can be connected to 25mm, 32mm, 40mm and even 50mm PVC pressure pipe and fittings. PVC pipes maintain higher flow rates and are more suited to permanent connections than polypipe. Class 9 PVC pipe is cheaper than class 12. Poly and PVC pipe is easily mixed and matched.

For example, 32mm class 12 PVC pipe has an ID of 37.5mm and the 32mm class 9 PVC pipe has an ID of 38.5mm.

Air flushing requires a flow velocity of .8 metres per second.

Sediment flushing requires a flow velocity of .6 metres per second.

Nominal | ID | Wall | litres/metre | litres/min @ 0.8m/sec |

CLASS 9 PVC | ||||

25 | 30.5 | 1.5 | 0.8 | 35 |

32 | 38.5 | 1.9 | 1.2 | 56 |

40 | 44.1 | 2.1 | 1.5 | 73 |

50 | 55.2 | 2.6 | 2.4 | 115 |

CLASS 12 PVC | ||||

20 | 23.65 | 1.5 | 0.4 | 21 |

25 | 29.75 | 1.9 | 0.7 | 33 |

32 | 37.45 | 2.4 | 1.1 | 53 |

40 | 42.75 | 2.7 | 1.4 | 69 |

50 | 53.65 | 3.3 | 2.3 | 109 |

POLYPIPE | ||||

19 | 19 | 0.3 | 14 | |

25 | 25.4 | 0.5 | 24 |

Litres per minute flow rates with 20 metres of friction loss | |||||

HEAD | 20mm | 25mm | 32mm | 40mm | 50mm |

0.25m | 12 | 23 | 42 | 51 | 108 |

0.50m | 18 | 33 | 60 | 87 | 156 |

1.0m | 26 | 48 | 88 | 126 | 227 |

1.5m | 33 | 60 | 110 | 157 | 283 |

2.0m | 38 | 70 | 128 | 184 | 330 |

Litres per minute flow rates with 30 metres of friction loss | |||||

HEAD | 20mm | 25mm | 32mm | 40mm | 50mm |

0.25m | 10 | 18 | 33 | 47 | 86 |

0.50m | 16 | 27 | 49 | 69 | 125 |

1.0m | 21 | 39 | 71 | 100 | 182 |

1.5m | 26 | 48 | 88 | 125 | 227 |

2.0m | 31 | 56 | 103 | 146 | 265 |

OF INTEREST:Doubling a pipe's internal diameter increases the volume x4 and if the same head pressure is maintained, the flow rate increases roughly x6.

A single 19 mm poly pipe vertical drop connected to a 25 mm horizontal poly pipe can also generate impressive flow rates.

A Supadiverta fitted with a single 19 mm poly pipe vertical drop joined to a 13 metre horizontal length of 25 mm poly pipe diverted to a tank’s bottom inlet was found to flow at 25 litres per minute (1,500 litres per hour) with a 1 metre head during flow testing. This is equivalent to a steady rainfall of nearly 43 mm per hour on a 35 sq m roof area harvested by one downpipe.

Substituting the 25 mm horizontal poly pipe with 19 mm poly pipe in the same situation and head pressure still produced a flow rate of 15 litres per minute.

A Supadiverta fitted with a single 19 mm poly pipe vertical drop joined to a 13 metre horizontal length of 25 mm poly pipe diverted to a tank’s bottom inlet was found to flow at 25 litres per minute (1,500 litres per hour) with a 1 metre head during flow testing. This is equivalent to a steady rainfall of nearly 43 mm per hour on a 35 sq m roof area harvested by one downpipe.

Substituting the 25 mm horizontal poly pipe with 19 mm poly pipe in the same situation and head pressure still produced a flow rate of 15 litres per minute.

Shorter or larger internal diameter horizontal pipes have higher flow rates.