Mainline and submain design

The goal of the design is to apply the required water, at the required pressure, with the minimum system (upfront) and ongoing (running) costs. The system should also be designed to allow for future expansion if it is expected.

As mainlines can often be 1/3 of the cost of the entire system, it can be very tempting to reduce the diameter of the pipe and to increase the velocity through the pipe. However, caution must be advised as this will increase your operating costs.


Increased velocity = greater friction = greater head (pressure) loss = higher pumping cost.

While meeting m3/hr (gpm) requirements


The recommended maximum velocity is 1.5–2.5 m/sec (5.0–7.5 ft/sec). Above this velocity, in addition to high friction, there will also be increased risk of water hammer in the mainline.

Pipe manufacturers publish head loss charts that allow you to calculate the total head loss based on the diameter, velocity, and flow rate, like the example below.

Other considerations for mainlines and submains

Where possible, placing the valve to the midpoint of the submain will reduce the size of the submain you require, as there is a shorter total length for the water to travel.

Remember that on sloping ground, and when using a submain with opposing laterals extending from each side, the required inlet pressure of both sides needs to be considered.


If it is possible for the laterals to run the full length of the block, place the submain at the end of the row at the high side. If not, you can have a submain making a center-fed system as illustrated.

Submains should be placed in an acceptable location, meeting the maximum pressure range of the drip laterals and the pipe grade.

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