Storage Facilities, Tanks, and System Pressure Management
Core concepts
Storage serves three jobs: fire/emergency reserve, demand equalization, and pressure maintenance. Know tank types - elevated, ground-level, and standpipe - and how each provides pressure via elevation head. Understand tank level controls, altitude valves, and the relationship between tank water level and system pressure (pressure at a point = elevation difference ÷ 2.31). Grasp turnover and storage volume concepts: usable storage excludes the fire reserve and dead storage below the outlet. Understand pressure zones and how pressure-reducing valves (PRVs) and booster pumps maintain service pressure across elevation changes. Know the purpose of overflow, vent, and altitude/level control valves and the need for tank mixing to avoid stagnation and stratification.
Traps to watch
Questions confuse total storage with usable storage. Another trap: forgetting that a full tank gives maximum pressure and a low tank gives minimum - pressure at a customer depends on the water surface elevation, not the tank bottom. Also watch for stratification and turnover issues that degrade water quality in large tanks.
Worked example
A tank's high-water surface is at elevation 320 ft; a customer is at elevation 210 ft. Static pressure = (320 − 210) ÷ 2.31 ≈ 47.6 psi. If the low-water level drops to 290 ft, pressure falls to (290 − 210) ÷ 2.31 ≈ 34.6 psi - still above the 20 psi minimum, so the design is adequate.
Rule of thumb
Pressure comes from water-surface elevation; usable storage = total minus fire reserve and dead storage.