Selecting Correct Pipe Size

In determining the best size of pipe to install for a specific water reticulation project the proposed system must firstly be fully evaluated. The purpose of evaluation stage is to gather what physical conditions exist that will affect the way your system is finally established.  From all of these questions, interpretations can be made to enable the designer of the system to choose the correct components from an array of differing options and install a system which will work in the most efficient and cost effective manner and deliver the desired outcome. For the purpose of this discussion, we will consider the use of Polyethylene (PE) Pipe, which is virtually universally used. Having said that, the same basic rules apply whether you are using galvanised pipe, copper pipe, cement pipe or PVC pipe.  It is just the friction loss factor varies between each.

It is necessary for the “Total Head” to be calculated which the proposed system is required to operate. Total Head is the sum of the addition of three elements, they being Static Lift + Static Height + Friction Loss.

Static lift

In the case of using a pump to supply the water into the system, Static Lift is the vertical distance of the suction pipe from the ground level at the pump site to the lowest water level in the Bore, Well, Dam or Creek.

If the water supply is from a Storage Tank adjacent to the pump site, there is no Static Lift factor.

Similarly, where the water supply is from a reticulated Mains Supply, quite obviously there is no Static Lift, and hence this factor in this sense again is ignored.

Static height

This is the maximum height to which the water has to flow, and is the vertical distance from the ground level at the water source to the highest point in the proposed delivery pipeline. (When flowing downhill, water acquires a Static Height equal to the height of the fall.)

Friction loss

Just as it takes power to propel an aeroplane through the sky because of the friction and turbulence generated as it passes through the air, so does it take power to push water through the pipe.  The faster the aeroplane, the greater the power needed to overcome the friction.

And the same applies to pipes.  If we double the quantity of water passing through a pipe, it must obviously travel at twice the speed.  This then means a greatly increased friction factor, but doubling the speed of the flow doesn’t mean the friction is doubled too.  The increase in friction is almost 4 times.

Friction loss is best described as being the additional pressure, expressed as height of delivery, due to the drag and turbulence between the moving water and the inside wall of the suction and delivery pipelines.

Friction loss is the most important element in the determination of any reticulation system, because it is the only variable that can be changed.  For example, friction loss can be reduced by an increase in the size of the delivery pipe.

Designing the system

Once all the information required to design your system has been gathered and evaluated, the next process is to use this information to select the correct pump (if required) and pipe size.

We now have to refer to Friction Loss Tables which show the Friction Loss Factor for every class and type of pipe according to the size and the capacity of water passing through it.  This Friction Loss is designated exactly in the same form of measurement which is used for the calculation of the Static Suction Lift (if required) and the Static Delivery Height.

So if we add the individual measurements of Static Lift, Static Height and Friction Loss together, the total sum obtained is the “Total Head” against which the system has to operate against.  In other words, the “Total Head” is the physical barrier which either the water supply pressure pump or the minimum Reticulated Mains Pressure is required to overcome in order to push the required amount of water to the highest delivery point.

When determining the size and Class (Pressure Rating) of the Polyethylene Pipe you intend to install, careful considerations must also be afforded to both the temperature of the reticulated water and the possibility of high ambient temperature.

In the instance of a water temperature increase, this results in the decrease in the viscosity of the water with a corresponding increase in discharge (or reduced friction loss) through the pipe line. 

High ambient temperature can significantly reduce the rated pressure capability of Polyethylene Pipe.

Take for example the widely used and popular Class PN12.5 Metric Blue Stripe Polyethylene Pipe (the physical size is immaterial here and makes no difference).  With the pipe and its contents at a temperature of 20 degrees C, which we would suggest as being normal for well installed and buried pipe, it has a pressure rated working capability of 125 metre head or 1228 Kpa or 178 Psi.

Should that pipe and contents reach say 45 degrees C, which it can quite easily do on a hot day, the pressure rated working capability of that same pipe reduces to 865 Kpa or 125 Psi.

Again, take particular note and include these possible variants in determining the actual Class or Pressure Rated Capability for the pipe you ultimately install.

When all these factors have been considered and evaluated, you are then in the best position to make the best decision of pipe size choice to well meet your water reticulation requirement and at the same time being a fully acceptable and economic investment in the further long term development of your property.

For further information contact Warren Dickie on 868202711 or your local supplier.