Water Pressure
For the Estimating of Flow Rates and Water Usage

Water pressure available has a significant influence on the gallon per minute flow of the application. To illustrate this all important factor, the chart below provides evidence that the water pressure factor must be included in your sizing.

Variations in Flows with a 50 Foot Garden Hose
Water Pressure
PSI
Flow
 GPM
10
7
20
9
30
11
40
13
50
15
70
18
100
22

Due to the variation illustrated above, compensation must be applied when calculating the flow demand on any application. Multiplication factors must be applied upon completion of converting fixture value to probable GPM flow. The chart in Figure A should be used for this important adjustment. Example: A probable demand of 50 GPM was determined. The application has an inlet pressure of 60 PSI. Using the chart below, a multiple factor of 1.34 should be used. 50 GPM x 1.34 = 67 GPM compensated flow demand.

Figure A
Pressure
PSI
Factor
20
0.74
30
0.92
35
1.00
40
1.07
50
1.22
60
1.34
70
1.46
80
1.57
90
1.68
100
1.78

*Charts are based on inlet pressure of 35 PSI. For other pressures, adjust by use of Figure A.

Figure B
*Fixture Value Conversion Charts

Chart I
Country Clubs, Hospitals, Hotels,
Nursing Homes,
Office Buildings, Shopping Centers, Schools, Restaurants

Chart II
Apartments, Condominiums, Dormitories,
Trailer Parks,
Homes,
Motels

Fixture
Value

Probable
GPM Flow

Fixture
Value

Probable
GPM Flow
10
-
10
10
20
-
20
18
25
-
25
20
40
-
40
21
50
35
50
22
75
43
75
23
100
50
100
24
125
55
125
26
150
57
150
28
200
62
200
30
250
67
250
33
300
72
300
37
350
77
350
39
400
82
400
42
450
86
450
44
500
90
500
46
550
94
550
50
600
98
600
52
650
102
650
54
700
106
700
56
750
110
750
58
800
112
800
59
900
117
900
61
1,000
122
1,000
62
1,100
127
1,100
64
1,200
131
1,200
66
1,300
133
1,300
68
1,400
136
1,400
69
1,500
138
1,500
70
2,000
140
2,000
72
3,000
156
3,000
76
4,000
162
4,000
82
5,000
168
5,000
88
6,000
174
6,000
94
7,000
180
7,000
100
8,000
186
8,000
108
9,000
192
9,000
116
10,000
198
10,000
122
11,000
204
11,000
128
12,000
210
12,000
134
13,000
216
13,000
140

The following is an example of estimating the probable GPM demand for an apartment complex.
Customer: 160 unit apartment complex pressure at meter: 50 PSI

Fixture
Fixture
Value
Extended
Fixture
Values
205 tank water closets
x 3 =
615
259 lavatories: 3/8"
x 2 =
518
138 dishwashers: 1/2"
x 4 =
552
10 washing machines: 1/2"
x 5 =
50
165 kitchen sinks: 1/2"
x 3 =
495
162 bathtubs
x 8 =
1296
Total Fixture Value
3526

Fixture value: 3526 Conversion from Figure B, Chart II: 80 GPM

Adjustment to 50 PSI inlet water pressure: 80 GPM x 1.22 = 97.6 GPM or 98 GPM.

The probable peak demand therefore, in this example of a 160 unit apartment, would be 98 GPM.

 

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