#1 Water Towers & Pressure

You don’t know what you don’t know. (Duh, right?) I started working for the City of Leon Water/Wastewater Department in October of 2009. I felt like I knew a fair amount about how the world works, for goodness sakes, I have a degree in chemistry and a minor in physics. Yet, as the son of school teacher, I have been instilled with the trait of being a lifelong learner, and for sure there were more things to learn regarding how water systems work.

One of the things that I believed that I had a grasp on was how water pressure was developed in the water pipes. I mean, I knew that the water pressure came from the water in the water tower. I recall a time when my cousin and I were at Iowa State University attending a summer wrestling camp. One evening a summer thunderstorm popped up over Ames, and all of us wrestlers staying in the high-rise dorms were instructed to go to the first-floor commons. While we were waiting out the storm, the power went out. Sometime during the outage, I approached the water fountain to get a drink. My cousin, who always enjoyed making me look silly/stupid said, “Hey, you can’t get a drink of water. The power is out, and there won’t be any water.”

I responded by telling him that it would be fine, the water would flow because there was water in the water tower. I was right and I got my drink. I don’t know if my cousin was surprised, but he was wrong. Ah, the sweet victory of being correct. Science, right?

The thing is, I grew up in town, I was used to the water working without fail. My cousin grew up on a farm with a well and a pump that wouldn’t keep the plumbing pressurized when the power was out. His experiences being different than mine, he didn’t have a reason to believe that I would shortly be putting him in his place as I approached that drinking fountain.

Foreshadowing? Maybe, but when I began working in the water department with this background knowledge/experience, there were things that I would learn that would enlighten me further. As you work with a system for years, you gradually become familiar with its intricacies and tend to forget that there was a time that you didn’t possess this knowledge. That can make it challenging to deal with the general public, which is in the same boat that you were in some number of years ago when you didn't possess the knowledge of the details with which you are now so intimate.

So now you know, I had some knowledge of water towers, but what you don't know is what I still had to learn. I didn’t understand why water towers are generally the same shape and height. These are subjects that we can talk about now, and the two are intertwined a bit.

An occasional issue is to have a customer having a problem with the water pressure appearing to be low at his/her house. Now if there were a significant water leak in the area of this person’s home, that would make sense, but if not, what is the explanation for what this customer is experiencing? The amount of water coming out of the faucet isn’t actually pressure. The amount of water coming out is flow. Flow is determined by a number of variables. One of those variables is pressure. Pressure can be thought of as the water pushing against all of the internal surfaces of the plumbing. If there is a hole in the plumbing for the water to escape, either a hole that we don’t want (a leak), or a hole that we want to control (a valve), the water will begin to flow out of the opening. The higher the pressure, the higher the flow. When the water begins flowing the pressure on all of the other internal pipe surfaces is reduced and the flow coming out of a second hole in the pipe will be reduced also. It’s like having a family member flush a toilet when you are taking a shower.

Back to the customer with a reduced amount of water coming out of the faucet. Where does the pressure that is pushing the water out of that faucet come from? Now we are talking about water towers.

Water towers are generally in the 100 to 150 foot range of height. This is what creates the pressure that is in the water system. How can we explain this? As anyone who has carried a couple of 5-gallon buckets full water knows, water has some significant weight. Water towers use this weight as an advantage by “stacking” water up to increase the weight/pressure that is exerted on the bottom of the “stack”. It turns out that the amount of water in a tank doesn’t matter. What matters is how high the water is from the piping delivering the water to each customer.

This can be illustrated mathematically. Assume that you have 2 different tanks. Each tank is cylindrical in shape and 125 ft off of the ground and has a diameter of 30 ft. The difference in the 2 tanks is the design. One tank is sitting on the ground and is 125 ft tall. The other tank is only 20 ft tall, on top of a supported pipe that is 1 ft in diameter, yet the top of the tank is still 125 ft off of the ground. To figure the pressure that is exerted on the interior of a pipe at ground level a mathematical calculation can provide the answer.

The cross-sectional area of each tank at ground level can be calculated using the formula for the area of a circle:

A = pi*r2

For the tank that has a 30 ft diameter footprint at the ground level, the answer is:

706.5 ft2 or 101,736 in2

For the tank that has a 1 ft diameter footprint at the ground level, the answer is:

0.785 ft2 or 113.04 in2

The next question to answer is, "How much weight is over that area of footprint?”

For the tank that is 30 ft in diameter at the ground, the weight over the ground level is:

5,501,868.75 lbs

For the tank that is 1 ft in diameter at the ground, the weight over the ground level is:

6,113.19 lbs

Using the answers that we got from these calculations we can use the following equation to calculate pressure:

psi = pounds ÷ sq. in.

So, for each tank, simply divide the number of pounds by the footprint in square inches to obtain psi.

For the tank that is 30 ft in diameter at the ground, the weight over the ground level is:

54.08 psi = 5,501,868.75 lbs ÷ 101,736 in2

For the tank that is 1 ft in diameter at the ground, the weight over the ground level is:

54.08 psi = 6,113.19 lbs ÷ 113.04 in2

Ok, I get it. I just got all “mathy” in my narrative. As Hans and Frans used to say, “You can hear me now and believe me later.” The main point being, even if the calculations don’t make sense at the time that you first review them, understand that professional engineers use these calculations when designing new towers for water systems. If they don’t make the calculations correctly, then a system can end up having zones within the system that have different pressures and that creates issues that have to be dealt with by more engineering. What we should have learned, and what I learned by working in a water system, is that the essential measurement for determining the pressure in a water system is the height of the water in the storage tank.

Well, what about the shape? If you look at the different designs of our 2 tanks, you will find that they both provide the same system pressure, but they have a significant difference in the volume of water necessary to fill them. The tank that is sitting on the ground holds a little more than 660,000 gallons of water and the elevated tank holds slightly more than 106,000 gallons. In a town like Leon, that means that the smaller elevated tank will turn over the water about once a day. If Leon had a ground level tank as described, it would take over 5 days to turn over the water in the tank. In that length of time the water quality begins to suffer, most notably the chlorine begins to dissipate and disinfection is reduced.

So, now you know that the main factor to pressure is height and that the elevated tank is more efficient at controlling water quality. Now back to our customer having a problem with the water pressure appearing to be low. If there is water in the tower, the pressure of the system is stable. What explains the lack of water coming out of the faucet? It could be an unidentified leak on the pipe coming to the house. It could be the condition of the pipe coming to the house. Is it a pipe that is shared by other houses? Is it a pipe that is too small? Is it a pipe that has corroded on the inside surface making it harder for the water to flow? Is there some kind of obstruction blocking flow? Each location has its own set of circumstances that influence the flow of water to all of the faucets. Internal and external plumbing all influence the flow. Assuredly, if there is water in the water tower 100 plus feet above the streets, there is pressure in the system.