Water Supply: Pipe Size Calculation

  

PAUL HAY Capital Projects

www.phcjam.com

Pipe Size Calculation

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

Problem:-  Find service pipe size given the following:

                       Street main pressure [E]               = 350 kPa

                       Height of uppermost fixture

                       (above street)                              = 10 m

                       Uppermost fixture                       = WC with flush valve
                       Total fixture units in the system^a   = 85

a. See table 1 for supply fixture units of individual plumbing fixtures.

                       Developed length (DL) of piping (to

                       highest and most remote fixture)   = 30 m

                       Predominant flushing mechanism   = Flush valves

Table 1: Water Supply Fixture-Unit Values [source:- International Residential Code]

Solution:-  Calculation of Service Pipe Size

                       1.            Find the pressure required in the system to provide the minimum fixture pressure [A] for uppermost fixture^b:

b. see value for Aflush valve for closet in table 2

                     A         = 103 kPa

Table 2:  Minimum Pressure & Flow required for typical plumbing fixtures   [source:- Mechanical & Electrical Equipment in Buildings]

                      2.            Calculate the Static Head [B]:

                   B         = 10 kPa/m x Height of uppermost fixture

                  = 10 x 10 m

                  = 100 kPa

Table 3:  Minimum pipe sizes for typical plumbing fixtures  [Source:- National Building Code of Jamaica 1983]

Figure 1:  Demand Load for predominant (1) flush valves & (2) flush tanks  [source:- Mechanical & Electrical Equipment in Buildings]

Figure 2:  Flowchart for typical pipes  [Source:- Mechanical & Electrical Equipment for Buildings]

                      3.            Using total fixture units, determine the demand load^c for the relevant flushing mechanism from figure 1.

c. 3.78 L/s demand, for 85 FU.

                       4.            Using demand load [3], determine an approximate pipe size^d from fig. 2 that is closest to the flow velocity of 3 m/s.

d.  40 mm dia. pipe, for 3.8 L/s demand.

                     5.            Using demand load [3] and approx. pipe size [4], determine the pressure loss in the water meter [D] from fig. 3:

                 D         = 62 kPa

Figure 3:  Pressure losses in Water Meters  [Source:- Mechanical & Electrical Equipment in Buildings]

                    6.            Calculate the maximum frictional loss [C] that can be tolerated in the service pipe:

                 C         = E - (A + B+ D)

                 = 350 - (103 + 100 + 62)      = 350 - 265

                = 85 kPa

                    7.            Calculate the Pipe length equivalent of fittings

                 [DL'] (estimated at 20 % of DL):

                 DL'      = 0.2 x DL = 6 m

                    8.            Calculate the total equivalent length (TEL) of the piping:

                TEL      = DL + DL'    = 30 + 6          = 36 m

                    9.            Calculate the unit-frictional loss of the pipe:

                 100 x C/TEL              = 100 x 85/36            = 189 kPa

              10.         Using the demand load [3] and the unit frictional loss [9], determine the pipe size^e from fig. 2

e.  50 mm dia. service pipe size

Table 4:  Maximum allowable pipe sizes  [Source:-  National Building Code of Jamaica 1983]