NOTES ON THE PLASTIC WATER PIPE SIZING CALCULATOR

Using the Australian Plumbing Code 'AS/NZS 3500 part 1 Water Services'
These programs are designed to work on a mobile device and also on a PC.
Once opened the pages/programs will work off line.

The free version will calculate Plastic water pipe sizes up to 50mm diameter.
The Proversion will calculate sizes up to 200mm diameter.

The free version is based on the number of dwellings. Although fractional dwellings can be entered.
The proversion allows the user to enter any combination of individual fixtures, dwellings, and known flows.

Plastic Pipes
Plastic pipe sizes are based on Polyethylene Pipes manufactured to AS/NZS 4130:1997 Polyethylene Pipes for Pressure Applications. Strength PE100, Pressure Pn16, Standard Dimension Ratio SDR11.

Note: The Australian standard does not take into account Water Efficiency Rated tapware for pipe sizing.

Pipe sizes. The internal diameters used in the programs are shown below

Free Version Plastic
Nominal Dia
Dn (mm)
Actual I.D.
(mm)
16 13
20 16
25 20
32 26
40 32
50 40
Pro Version Plastic
Nominal Dia
Dn (mm)
Actual I.D.
(mm)
63 57
75 61
90 73
110 89
125 101
140 114
160 130
200 162

Maximum Allowable Velocity
The worlds Major Plumbing codes limit the velocity in pipework to 2.4 m/s, and sometimes even less for hot water pipes.
At present in Australia it is 3 m/s; but this will soon change to 2.4 m/s
There are three reasons for setting the maximum allowable velocity.
  • To reduce piping noise
  • To reduce ware and tare on fittings
  • To reduce water hammer.
The free version of the program uses 2.4m/s. The Proversion allows the user to enter any value. However increasing the velocity above 3m/s should really only be done on underground pipes of straight runs with no bends or fittings, especially valves. But this will be outside the plumbing code. It may be justified in irrigation, fire flows, etc, or even in an area of high pressure when the dwelling is a long way from the source. But still not allowed under the plumbing Codes.
Lowering the velocity on the otherhand, would be advantageous.
Lowering the allowable velocity increases the pipe size, increasing the allowable velocity reduces the pipe size.

Minimum Residual Head
This is the pressure that we want to end up with at the last fixture (the worst case).
The Australian Code stipulates 5m. But this is likely to increase in the future.
Manufacturers recommend a minimum of 11m for most mixing and tempering valves. The Uniform Plumbing Code used in the United States also uses approximately this figure.
The free version of the program uses 15m as the residual head.
The pro version allows the user to enter any value.
Water will always flow from a point of higher head (pressure) to a point of lower head (pressure) even if the difference is only mm's. It may not flow very fast under these conditions, (a flat hydraulic grade) but it will flow.
So, if we have a house tank flowing under gravity to a kitchen sink (or similar fixture) with a standard tap (faucet), then we could (or may have to) lower the residual pressure to get water without having to rely on a pump. Provided we don't care how fast it comes out.

Pro Version
The proversion also has a table that displays the capability of every pipe size, for the claculated Hydraulic Grade, for both Copper and Plastic (Polyethylene).
That is :-
  • How many dwellings each size can service
  • What flow (L/s) each diameter can supply
  • And what is the velocity in each pipe diameter.
The advantage of this, is that it is not necessary to keep changing the number of dwellings to find the pie size. because it is obvious from the table.
For example, it might show that a 25 dia can service 3 dwellings, and the next size up (32dia) can service 8 dwellings. So no need to calculate the pipe size for all the dwellings from 3 to 8, as the answer will be 32mm dia.
Also, showing the flow capacity of each size will help in cases where the flow is known. For example, irrigation, some fire services, and some mechanical systems.
If the velocity shown is the maximum allowable, then we know that velocity is controlling the pipe size, not the pressure.
Which means that we have plenty of pressure, which means that we should end up with more pressure at the end point than our desired minimum.

The Proversion also allows input of the number of individual fixtures. This is handy for large houses, or projects that have sheds, or something else as well as houses, or projects that don't have dwellings at all, eg industrial buildings, office buildings, shopping centers etc.

Proversion allows the user to change the maximum allowable velocity. This is useful for some other plumbing codes, or for hot water, or for some 5 star hotels that specify different velocities for different locations in the building.
Also allows the user to change the minimum residual head. Handy for tanks (lower residual head), fire hose reels or hydrants (higher residual head), or other fixtures where the manufacturer requires a different minimum head for proper operation eg mixing valves.

Pro version also calculates Pump Duty. If the hydraulic grade is so flat that a 32mm dia pipe is required for one dwelling (along with most of the internal pipework) then a pump size is calculated. This is also useful when pumping from tanks etc where the start pressure is zero, or it could be less than zero if the pump is above the tank.
When the pump duty is calculated, use the pump kPa pressure, (shown at the end of the page) as the 'Start pressure' to calculate the pipe sizes.

Screen Shots
Free Version screen
free version
Pro Version screen 1
Pro version
Pro Version screen 2
Pro version
Pro Version screen 3
Pro version
Pro Version screen 4
Pro version

Maximum Allowable Velocity
The free version of the program uses 2.4m/s for best design Practice.
(Note: Australian Plumbing Code at present allows this to go to 3m/s. However this is not recommended for cold or heated water supplies).

The Proversion allows the user to enter any value.

Minimum Residual Head
This is the pressure that we would like to achieve at the last fixture (the worst case).
The free version of the program uses 15m (approx 150kPa) as the residual head (Note Plumbing Code is 5m, but this is too low to work modern devices).
The pro version allows the user to enter any value.

Pro Version
The proversion also has a table that displays the capability of every pipe size, for the calculated Hydraulic Grade.

That is :- Proversion allows the user to change the maximum allowable velocity, and change the minimum residual head.

Pro version also calculates Pump Duty.


Author : Ken Sutherland BTech MIEAust RPEQ CPEng.