Page 128
December 2009
Water System Design Manual
11.1.2 Conventional Tank Sizing Equations (bottom outlet)
Horizontally Oriented Tanks
Equation 11-1:
V
t
= [(P
1
+ 14.7)] 15
Q
p
(MF)
[P
1
- P
2
] Nc
Vertically Oriented Tanks
Equation 11-2:
V
t
= [(P
1
+ 14.7)] 15
Q
p
+ 0.0204 D
2
[P
1
- P
2
] Nc
Where:
V
t
= Total tank volume in gallons
P
1,
P
2
= Pressures selected for water system operation in psig (not absolute pressures). P
1
corresponds to the pump-off pressure and P
2
to the pump-on pressure
Nc = Number of pump operating cycles per hour. This number is either the current DOH
recommendation of six cycles per hour or a larger value that can be justified and
documented by pump or motor manufacturers' warranties
Q
p
= Pump delivery capacity in gallons per minute at the midpoint of the selected pressure
range. Determine this by examining pump curves or tables. If this value is not used,
the Q
p
that occurs at P
2
(pump-on) must be used
D = Tank diameter in inches
MF = A multiplying factor related to tank diameter to ensure a six-inch water seal at the
bottom of the tank. These values for several tank diameters are in Table 11-2. Use
this factor only for sizing a horizontal tank
Note:
The MF is the ratio of the total tank volume, V
t
, divided by the difference between the
nominal volume and the volume needed to ensure a 6-inch water seal at the bottom of the tank
(V
t
- V
6
). Because the ratios of the volumes for any tank at a given length are the same as the
ratio of the tank cross-sectional areas, MF can be determined with the following equation:
6
6
A
A
A
V
V
V
MF
t
t
t
t
-
=
-
=