Water System Simulation
We developed this exhibit for use in Science Technology Engineering and Math (STEM) presentations. The intent is to show grade school and middle school students a simplified model of how public water supply systems work. Going deeper, it demonstrates how civil, structural, mechanical, electrical and control system engineers come together to get clean drinking water to people’s homes.
The exhibit consists of an aquarium, fish tank pump, variable frequency drive, control panel, elevated storage tank and three houses.
The aquarium represents the aquifer where a majority of El Paso’s water comes from. The fish tank pump simulates a groundwater well that pumps water from the aquifer into an elevated storage tank. The elevated tank stores the water and also provides nominal pressure to distribute the water to the houses. The valves on the houses are used to vary the demand in the distribution system which makes the tank drain faster or slower depending on how far open they are. This portion of the exhibit demonstrates the civil, structural and mechanical engineering parts of the system.
Moving to the control panel, you can see the parts of the system that depend on the electrical and control system engineers. The components of the control system include the heart of the system, a programmable logic controller or PLC for short. The PLC is the brains of the control system and makes decisions on the operation of the automated system without human intervention. The touchscreen display is referred to as a human machine interface, HMI for short. The HMI allows the operator to see the status of the system including the water level in the tank and the pump speed. The operator can make changes to the system set points through the HMI. The pump speed is decided by the PLC, but in order to control the speed of the pump, a variable frequency drive or VFD for short is used to interpret the PLC speed into an electric output to the pump that makes it run slower or faster depending on the water demand. A level sensor in the tank provides the feedback to the PLC on the tank level.
To help emphasize how helpful the control system is, the simulator has a manual mode that lets the pump speed be controlled by the kids during the simulation. While other kids can change the demand on the system by turning the valves on the houses, the pump control needs to be varied to maintain the water level in the tank. In the short term, this is fun, but it soon becomes a chore and the person controlling the pump speed soon realizes that any lack of attention to the level on their part results in an alarm and the potential for the tank to run dry or overflow. Leaving the system unattended for any period in manual mode can result in unwanted deviation from the set point.
“A mechanical engineer would design and come up with the inner-workings of the pump. The piping that you would see in the street or underground would be the civil engineer along with the site work of the ground. The structural work would be done by a structural engineer to build or to design the structure of the tank, and then the control panel we would be the electrical and control systems engineering. So, all of that put together would require a lot of coordination between the different engineering fields.”
-Keith Rutherford, PE