Pipe Sizing (AS/NZS 3500

School of Plumbing Services

Pipe Sizing

AS/NZS 3500.5:2000

Or

AS/NZS 3500.1:2003

Cold Water Supply

Pipe Sizing (AS/NZS 3500.5:2000) (AS/NZS 3500.1.2003)

General Requirements

• Pressure

Pressure is determined by vertical distance a supply is above the outlet. For every 1 meter of vertical height 9.8 kPa is obtained. Therefore if a supply tank were 35m above an outlet the static pressure (when nothing is in motion) would be 343 kPa.

• Available pressure.

The minimum and maximum static pressure can be obtained form the local authority. Pipe sizing is to be based on the minimum pressure. Or can be taken by reading a pressure reading at point of installation. When pipe sizing, always use the minimum pressure available.

• Pressure and delivery flow rates.

AS/NZS 3500.1.2003 states that the pressure within buildings is not to exceed 500 kPa while the minimum dynamic pressure at the most disadvantaged point needs to be at least 50 kPa at the flow rates dictated in Table 2.8 (3.1).

• Velocity

Velocity is the speed at which water travels through the piping system. If the velocity is too great, water hammer, noise, life of appliances and pipe (copper life expectancy) problems are likely to occur. AS/NZS 3500 states the max. velocity is not to exceed 3 meters / second (m/s).

• Available water supply.

If it is determined the water supply will not be sufficient to meet the demand, supply tank/s will need to be installed.

• Frictional losses

Frictional losses occur in the piping system with the end result being a drop in pressure and flow rates. Frictional losses are caused by:

• Type of pipe used

• Length of pipe

• Amount of fittings

• Type of fittings

• Velocity

• Pressure

• Valves

Table 2.11 (C 1) makes an allowance of 50% for frictional losses.

Two Methods

Using 2.22 (Appendix D)

• Flow rates / load units,
• Probable Simultaneous Flow Rates (PSFR)
• Length of piping system
• Pressure

2.21 (3.5)

Criteria:

• 20mm

• 18mm

• 15mm

• 10mm

Fig 1.

Note:No branches are greater than 3m.

* SA Water directive

Exercise 1

Size the diagrams below.

Fig. 2

Note:All branches less than 3m unless indicated.

Exercise 2

Fig. 3

Note:All branches less than 3m unless indicated.

Other exercises are in Cold Water 2 notes.

Using 2.22 (Appendix D)

Steps to take:

1. Determine Pressure Drop (Hm - Hx - or + Hs)

• Take static pressure test at meter. Fig. 3 shows the pressure at the meter is 375 kPa. This equates to 37.5m head. (Variable, Hm)

Fig. 3

• Minimum pressure must be no less than 5m head. (This is a constant, Hx)

• Determine elevation difference between meter and highest outlet. This can be a negative or positive figure. (Variable, Hs)

Fig.4

Fig. 4 shows several outlets are 2m above the meter therefore pressure at highest outlet is 2m less than 37.5, i.e. 35.5 m head.

Fig. 5

Fig. 5 (above) shows the highest outlet is 4m below the meter therefore the pressure at the most disadvantaged outlet is 4m greater than 37.5, i.e. 41.5 m head.

Pressure Drop, Fig. 4:37.5 – 5 – 2 = 30.5m head

Pressure Drop, Fig. 5:37.5 – 5 + 4= 36.5 m head

1. Determine “Index Length”. The length from the meter to furthermost point.

1. Sectionalise drawing of installation. Start from the meter and work down the main line then work back doing the branches as shown in Fig. 6.

Fig. 6

1. Determine the load units for each section using Table 2.8.

Load Units take into account the flow rate requirements, length of time a fixture is used and the frequency it is used.

Fig. 7

Table 2.8 (Part)

Flow Rates and Load Units

Fig. 8

1. Draw a pipe sizing table as shown below. Transfer the load units to that column.

1. Convert the load units to Probable Simultaneous Flow Rates (PSFR) using Table 2.9.

As all fixtures are very unlikely to be used simultaneously, load units are converted to a PSFR giving an estimate of the actual flow requirements.

Pipe Sizing Table

1. Determine nominal pipe size using Table 2.11. For the “Pressure Drop” and “Index Length” use the next most disadvantaged section. Using Fig. 8:

Pressure Drop:37.7 – 5 – 2 = 30.5Use 30 m Head (next table down)

Index Length:43mUse 45 (next column up)

15mm can handle upto 0.17 PSFR (inclusive)

18mm can handle upto 0.33 PSFR (inclusive)

20mm can handle upto 0.56 PSFR (inclusive)

25mm can handle upto 1.24 PSFR (inclusive)

Starting from 15mm, size any section that has 0.17 or less PSFR, then do18mm to 0.33 and so on.

1. Convert nominal pipe size to that of the actual pipe-line material being used, using Table 2.1.

STUDENT EXERCISES

Exercise 3

Using 2.22, pipe size the following.

Fig. 9

Pressure Drop:

Index Length:

Exercise 4

Using 2.22, pipe size the following.

Fig. 10

Pressure Drop:

Index Length:

ANSWER SHEET / STUDENT EXERCISES

Exercise 3

Using 2.22, pipe size the following.

Fig. 9

Pressure Drop:

Index Length:

15mm0.12

18mm0.22

20mm0.39

25mm0.87

ANSWER SHEET / Exercise 4

Using 2.22, pipe size the following.

Fig. 10

Pressure Drop:

Index Length:

15mm0.12

18mm0.22

20mm0.39

25mm0.87