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In my March 2022 column, I wrote about anticipated water demand for a new project and how to estimate how much you might need.
This month, we’re talking about emergency water storage tank systems. Granted, there are many ways to design such a system, but I will walk you through one method I have used a few times.
It is important to remember this is an early design estimate method that can be carried forward as the design progresses and built into the final design.
The project is a six-building hospital campus consisting of a two-story central utility plant; a two-story research building; an eight-story, 145-bed hospital; two five-story clinic buildings; and a five-story administration building.
We have six buildings, but which ones will receive emergency water and continue to operate for four days? Which buildings are operational 24/7 and key to the functionality of the campus during emergency conditions? What will be the average hours of peak operation where the highest usage will be 8 hours, 12 hours or 16 hours?
You will want to evaluate how much water will be required to serve all six buildings for four days.
Most likely, the decision will be that only the 145-bed hospital will need emergency water for that long, but a thorough discussion with the client is needed to determine how it wishes to operate in an emergency.
Other pieces of equipment to consider during an emergency are cooling towers; how much water will they need? Will the facility cut back on the use of cooling towers to reduce storage needs?
The hospital cooling tower requires 100 gallons/minute of makeup water, which is 6,000 gallons/hour and 144,00 gallons/day (gpd). If we stored water for four days, we would need 576,000 gallons stored for the cooling tower only. This water could be captured under a mechanical water storage tank to address its needs.
After everything has been discussed with the client and evaluated for emergency usage, we start with the gallons per day per bed measurement you used when sizing your water service. In this case, we will use 300 gallons/day:
145 beds x 300 gpd = 43,500 gallons/day/bed (gdb)
Does the facility plan to expand in the future? Let’s say this facility will expand by 30% over the next 10 years:
43,500 + 30% = 56,550 gdb
Next, we want to store water for 96 hours (four days):
56,550 gdb x 4 = 226,200 gdb stored, including 30% for expansion.
We need to store 226,200 gallons, but how do we keep it turned over as a viable water source? We need to figure out what a good buffer use will be to turn the tank over. Do we use a device at the bottom of the tank to keep the water moving, or do we do a different style of water entry into the tank?
We did use a four-way nozzle at the bottom of the tank to fill it. There were three 4-inch conical-shaped nozzles, one pointing upward toward the center of the tank at a 45-degree angle. A 2-inch nozzle points to the backside behind the fill nozzles to move water that will be caught there. We added a turbine at the bottom of the tank that turns the tank slowly, keeping the water moving.
To drain the tank down to within 6 inches of its bottom, we created a sump recessed in the bottom of the tank by two feet. We placed a stainless-steel anti-vortex device into the 90-degree elbow under the floor. The anti-vortex sits level at the tank floor. The top of the tank includes a dome vent to allow air to move.
The water in the tank should be turned over at least twice a day, more if we can get it. We know the gallons used per day as 56,550 gallons (and includes 30% expansion). The tank should always have at least 226,200 gallons of water for the four-day storage requirement; it could be more if an emergency occurs shortly after a refill cycle. How fast the tank can fill will determine how many times you can turn the tank over.
Now we need to add the floats to operate the fill valve and automatic bypass. City water fills the tank based on the float levels, but during a water line break, the pressure will drop significantly, and the automatic valve will put the system into emergency mode. Once this occurs, the facility is alerted to begin closing nonessential departments and buildings to maximize the water in the tank (see Figure 1).
You will need a water flow test to have a better understanding of water main capacity to better determine possible fill times.
So, how big of a tank do we need?
This tank is 30 feet tall with a domed structure; the radius is 20 feet. The tank holds 262,208 gallons of water when filled. Maximum capacity to overflow conditions is 282,008 gallons. The tank is 28 feet to the filled depth. The difference between 262,208 gallons and 226,200 gallons is 37,008 gallons to work with to turn the tank over.
3.1417 x tank radius in feet squared x tank filled depth x 7.48024 = tank filled volume in gallons
Domestic Water Tank Sequence of Operation
• Normal Tank Operation:
A. Normal mode (starting from a full tank of 263,208 gallons)
1. Solenoid valve No. 1 will open when the tank level falls to the 19-foot level (132,301 gallons).
2. Solenoid valve No. 1 will close when the water level in the tank has reached the 28-foot level (263,208 gallons).
3. A high-water alarm will trigger if tank water rises to the 29-foot level. Water will overflow at 30 feet.
B. Bypass mode
1. Bypass solenoid valve No. 2 will open if the tank level falls to the 14-foot level (94,003 gallons). At this point, solenoid valves No. 1 and No. 2 are both open to fill the tank and serve the facility.
2. When the tank level rises to the 19-foot level, solenoid valve No. 2 closes and solenoid valve No. 1 remains open.
3. Solenoid valve No. 1 will close when the water level in the tank has reached the 28-foot level (263,208 gallons).
4. A high-water alarm will trigger if the tank water rises to the 29-foot level. Water will overflow at 30 feet.
• Emergency Tank Operation:
1. If the pressure sensor located upstream of the water meter senses a loss in city water pressure, this will send a signal to the building management system (BMS) to switch into emergency mode.
2. Solenoid valves No. 1 and No. 2 close. The hospital is now completely served by the storage tank.
3. The BMS closes the control valve serving makeup water to cooling towers.
4. A low-water alarm will trigger when the tank level reaches 8 feet 6 inches.
5. The low-water alarm will send a signal to the BMS that the water level is critical and the booster pump is approaching shutdown level.
6. If the tank has reached 4 inches above the bottom of the tank, this will send a signal to the BMS to shut down the booster pump, and the facility will be without water.
7. As city water pressure returns, solenoid valves No. 1 and No. 2 open.
8. After the solenoid valves have opened, the BMS will send a signal for the booster pump to start.
9. When the tank water level reaches 19 feet, solenoid valve No. 2 closes.
10. When the tank level reaches 28 feet, solenoid valve No. 1 closes.