(a) Assume static conditions (reasonable
since water in the tanks moves very slowly). Also, assume the water mains
are buried 5 ft below grade.
For hydrostatic conditions,
p = γH
where γ = 62.4 lb/ft3 for water at 20 oC with H in ft, and p is in lb/ft2.
Divide by 144 to get p in lb/in2 and the reference pressure is 0 atm (gage pressure), so
p = (62.4 lb/ft3)(H ft)(1 ft2/144
For H = 100 + 5 = 105 ft: p = 45.5 psig
For H = 125 + 5 = 130 ft: p = 56.3 psig
Hence, the pressure range is
45.5 psig p 56.3