Mains Water Pipe Sizing

Pipe Sizing Procedure

1.         Reference the pipe section.

2.         Calculate flow rates from Table below. 

3.   Estimate flow rates in each section.

Keep velocity below 2 m/s. See also CIBSE Guide G (2003) part 2, Table 2.19.

4.   Estimate pipe diameter from pipe sizing tables in CIBSE Guide C.

5.   Measure the pipe run from drawings.

6.   Calculate length of pipe equal to resistance of fittings.

The Total equivalent length of a fitting = Equivalent Length x Pressure Loss factor z(Zeta).

 

7.   Calculate effective pipe length.

8.   Determine pressure loss due to friction from CIBSE Tables.

9.   Calculate pressure consumed due to friction (Pa)   =  effective pipe length (m)  x pressure loss due to friction (Pa/m)

10.  Calculate total pressure consumed  =  Friction loss + Static pressure loss

11.  Determine pressure at start of section.

12.  Calculate pressure available at end of section  =  Pressure at start of section  -   Total pressure consumed

      If pressure available at end of section is less than the maximum allowable pressure drop then we can accept this pipe size.

13.    Determine pressure required at end of section, this can be the minimum pressure that is required for terminal equipment.

14.    If the pressure available at the end of the section is more than or equal to the pressure required at the end of the section then the pipe size is correct.

 

Water Flow Rates

Cold water flow rates for sanitary appliances for small installations may be found from the table below.

Approximate hot or cold water demand

Flow rate (l/s)

Basin (spray tap)

0.05

Basin (tap)

0.15

Bath (private)

0.30

Bath (public)

0.60

Flushing cistern

0.10

Shower (nozzle)

0.15

Shower (100mm rose)

0.40

Sink (15mm tap)

0.20

Sink (20mm tap)

0.30

Wash fountain

0.40

 

In larger more complex buildings where many sanitary appliances are installed simultaneous demand should be considered from tables CIBSE Guide B (1986) B4.20 and B4.21.

 

Notes:-       

An alternative method of pipe sizing is to use a nomogram.

This can be found in CIBSE Guide G (2004) Public Health Engineering Figure 2.21.

 

Pipe Sizing table

 


Pressurised Cold Water Pipe Sizing Table

 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

 

Ref

Demand

Units if required

Flow

Rate

 

(l/s)

Estimated

Pipe Dia.

 

(mm)

Measured

Pipe Run

 

(m)

Length of

Pipe Equal to Fittings Resistance’s

(m)

Effective

Pipe Length

Col . 5 + 6

(m)

Pipe Pressure Loss

(Pa/m)

From CIBSE Tables

Pressure

Consumed due to Friction

Col.  7 x 8

(Pa)

Friction loss + Static pressure loss = Total

Pressure

Consumed

(Pa)

Pressure at Start of Section

 

(Pa)

Pressure

Available at End of Section

(Pa)

Pressure Required at End of Section

 

(Pa)

Final Pipe

Size

 

(mm)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Example 1

 


Calculate an appropriate pipe size for the system shown above.

Use Copper Table X  pipework for water at 10oC..

 

Answer

 

The maximum allowable pressure drop along the length of pipe = 300,000 Pa   –   250,000 Pa  =  50,000 Pa

 

Pressurised Cold Water Pipe Sizing Table

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Ref

Demand

Units if required

Flow

Rate

 

(l/s)

Estimated

Pipe Dia.

 

(mm)

Measured

Pipe Run

 

(m)

Length of

Pipe Equal to Fittings Resistance’s

(m)

Effective

Pipe Length

Col . 5 + 6

(m)

Pipe Pressure Loss

(Pa/m)

From CIBSE Tables

Pressure

Consumed due to Friction

Col.  7 x 8

(Pa)

Friction loss + Static pressure loss = Total

Pressure

Consumed

(Pa)

Pressure at Start of Section

 

(Pa)

Pressure

Available at End of Section

(Pa)

Pressure Required at End of Section

 

(Pa)

Final Pipe

Size

 

(mm)

A

none

0.8

22

50

none

50

3500

175,000

175,000

300,000

125,000

250,000

Too small

A

none

0.8

28

50

none

50

1000

50,000

50,000

300,000

250,000

250,000

28

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

           

Pipe Sizing Procedure

1.       Reference the pipe section -  section A.

2.       Calculate demand units or loading units from Tables in CIBSE guide (attached). – not required, see No.3 below.

3.       Estimate flow rates in each section. Keep velocity below 2 m/s. -  given

4.       Estimate pipe diameter from pipe sizing tables in CIBSE Guide C. – 22mm (velocity too high at approx 2.4 m/s) or 28mm (velocity is 1.5 m/s).

5.       Measure the pipe run from drawings. – 50m

6.       Calculate length of pipe equal to resistance of fittings. – no fittings

7.       Calculate effective pipe length. -  50m

8.       Determine pressure loss due to friction from CIBSE Tables. See Table 4.18 in Guide C (CD version).

9.       Calculate pressure consumed due to friction (Pa)   =  effective pipe length (m)  x pressure loss due to friction (Pa/m). Column 7 x 8 in Pipe Sizing Table.

10.     Calculate total pressure consumed  =  Friction loss + Static pressure loss. There are no vertical pipe sections and therefore no static pressure loss.

11.     Determine pressure at start of section. Given in drawing as 300,000 Pa.

12.     Calculate pressure available at end of section  =  Pressure at start of section  -   Total pressure consumed. 

          300,000 – 175,000  =  125,000 Pa. (22mm) …………..300,000 – 50,000  =  250,000 Pa (22mm).

If pressure available at end of section is less than the maximum allowable pressure drop then we can accept this pipe size.

13.     Determine pressure required at end of section, this can be the minimum pressure that is required for terminal equipment. Given in drawing as 250,000 Pa.

14.     If the pressure available at the end of the section is more than or equal to the pressure required at the end of the section then the pipe size is correct.

28mm pipe is correct, 22mm is too small since there is not enough pressure available at the end of the section and the water velocity is also too high.


 

Example 2

 

 

 

 

 

 

 

 

 

 

 

 

 

Calculate an appropriate pipe size for the system shown above.

Use Copper Table X  pipework for water at 10oC..

 

Answer

 

The maximum allowable pressure drop along the length of pipe = 120,000 Pa   –   90,000 Pa  =  30,000 Pa

 

Pressurised Cold Water Pipe Sizing Table

 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

 

Ref

Demand

Units if required

Flow

Rate

 

(l/s)

Estimated

Pipe Dia.

 

(mm)

Measured

Pipe Run

 

(m)

Length of

Pipe Equal to Fittings Resistance’s

(m)

Effective

Pipe Length

Col . 5 + 6

(m)

Pipe Pressure Loss

(Pa/m)

From CIBSE Tables

Pressure

Consumed due to Friction

Col.  7 x 8

(Pa)

Friction loss + Static pressure loss = Total

Pressure

Consumed

(Pa)

Pressure at Start of Section

 

(Pa)

Pressure

Available at End of Section

(Pa)

Pressure Required at End of Section

 

(Pa)

Final Pipe

Size

 

(mm)

 

A

none

0.5

22

14

1.6

15.6

1500

23,400

23,400

120,000

96,600

90,000

22

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

           

Pipe Sizing Procedure

1.       Reference the pipe section -  section A.

2.       Calculate demand units or loading units from Tables in CIBSE guide (attached). – not required

3.         Estimate flow rates in each section. Keep velocity below 2 m/s. -  given

4.         Estimate pipe diameter from pipe sizing tables in CIBSE Guide C. – 22mm (velocity is 1.5 m/s).

5.         Measure the pipe run from drawings. – 14m

6.         Calculate length of pipe equal to resistance of fittings. – 2 bends.

The Total equivalent length of a fitting = Equivalent Length x Pressure Loss factor z(Zeta).

See Pipe Sizing Heating Section  - page 4 - pipe fitting losses.

Copper pipe elbow z(Zeta)  = 1.0 x 2 bends = 2.0

Determine equivalent length from CIBSE table C4.18,   le  = 0.8

Total equivalent length of fittings =  0.8  x  2.0  = 1.6 metres.

 

7.         Calculate effective pipe length. -  15.6m

8.         Determine pressure loss due to friction from CIBSE Tables. See Table 4.18 in Guide C (CD version).

9.         Calculate pressure consumed due to friction (Pa)   =  effective pipe length (m)  x pressure loss due to friction (Pa/m). Column 7 x 8 in Pipe Sizing Table.

10.       Calculate total pressure consumed  =  Friction loss + Static pressure loss. There are no vertical pipe sections and therefore no static pressure loss.

11.       Determine pressure at start of section. Given in drawing as 120,000 Pa.

12.       Calculate pressure available at end of section  =  Pressure at start of section  -   Total pressure consumed.  120,000 – 23,400  =  96,600 Pa.

 

   If pressure available at end of section is less than the maximum allowable pressure drop then we can accept this pipe size.

13.        Determine pressure required at end of section, this can be the minimum pressure that is required for terminal equipment. Given in drawing as 90,000Pa.

14.        If the pressure available at the end of the section is more than or equal to the pressure required at the end of the section then the pipe size is correct.

 22mm pipe is correct.