This article is a continuation of Water Towers: Critical Assets for Quality of Life and Fire Safety.
Whether water is going to a storage tank or to consumers, it takes pressure to get it there. This pressure can come from pumps or water towers. When using the latter, the taller the structure is, the more water pressure is available.
Water tower with a 40′ head range and 100′
from top of foundation to low water level. (click to enlarge)
For every 1 psi, 2.31 feet of elevation is required. In other words, each foot of height provides 0.433 psi of head pressure. This is the static pressure “pushing down.” Only the tank’s height, called the head range, matters for psi / feet of range calculations – not its diameter.
Calculating psi / feet of range
Also called the altitude range, psi / feet of range is how the water and wastewater industry refers to the available water pressure. The available pressure of any given water tower will vary according to how much water is currently in the tank. To calculate this head pressure, let’s assume that our water tower has a tank with a 40′ head range and a 100′ support column. An empty water tower means that the water column is 100′, whereas a full water tower has a 140′ water column. Knowing that 1′ in elevation equals .433 psi, we can make the following calculations:
.433 psi x 100′ = 43.3 psi (empty)
.433 psi x 120′ = 51.96 psi (half full)
.433 psi x 140′ = 60.62 psi (full)
So, this 140′ water tower has an altitude range of 43.3 psi at 0% capacity (empty) to 60.62 psi at 100% capacity (full). Municipal water supplies usually require 45 to 80 psi of head pressure, enough to reach about the sixth floor of a building. This is why skyscrapers and other tall buildings have their own elevated storage tanks and/or pumps.
Measuring Pressure, Level, and Temperature in Water Tower Applications
Weak water pressure means more than just unsatisfying showers. Appliances like washers and dishwashers require 20–30 psi for efficient operations. What’s more, when the water in pipelines is sufficiently pressurized, it creates a barrier that prevents contaminants from entering. These are some of the critical points for pressure measurement in municipal water delivery systems:
- Pumps – pumping stations, booster stations, booster pumps within water tanks
- Suction nozzle (inlet)
- Discharge nozzle (outlet)
- Water tower
- Inlet pipe
- Outlet pipe
Pressure instruments located at ground level, installed on the outlet (discharge) pipe of a water tower, can also measure the water level in the tank. By using the above psi / feet of range calculation, if we know the pressure value, we can determine the water level with a simple calculation. Let’s use the same example of a water tower with a 40′ head range and 100′ support column.
measured pressure / .433 psi = water column
water column – height of support column = water level in tank
If the pressure sensor measures 50 psi, then the water level in the tank is about 15½ feet deep, or roughly 39% full:
50 psi / .433 psi = 115.47′
115.47′ – 100′ = 15.47′
The critical points for pressure measurement in municipal water delivery systems include a pump’s suction and discharge nozzles, and a water tower’s inlet and outlet pipes. Click to enlarge
Measuring pressure at ground level to determine the elevated tank’s water level is preferable to using a level indicator, which would have to be as tall as the head range – an impractical and expensive solution. A submersible pressure sensor for level measurement would have to be located at the base of the tank, which wouldn’t work since that’s usually where the inlet, outlet, and water evacuation pipes are located.
Temperature is another important parameter to measure, for these reasons:
- Temperature compensation for accurate pressure measurement
- Switching function, such as to turn on a heater to prevent water from freezing inside a pipe
- Monitoring the health of high-pressure pumps, as an elevated temperature in the bearings could indicate a change in loading or that the equipment is starting to fail
This series continues next week with an overview of the specific measuring instruments used in water tower applications.