Major infrastructure upgrades handle the pressure from a Mexican drainage system.
Two big wastewater pump stations with high performance submersible motor pumps relieve flooding during the rainy season to protect thousands from damages. They form part of the millennium infrastructure project in the Valley of Mexico. State-of-the-art hydraulic engineering tools were used to ensure reliable and efficient operation.
The Mexico City Valley experiences increasing population and rapid urbanization, and the demand for water and sanitation climbs constantly. This demand has been met by a growing system of wells drilled into the soft aquifer underlying the region. This intense exploitation has significantly lowered the ground—up to 50 centimeters per year in some areas.
In addition, spreading urbanization has caused the loss of forests in the surrounding mountains, which has led to decreasing rates of storm water percolation. It also increases the amount of wastewater to handle and treat. Soil subsidence and storm water surges threatened to collapse the existing infrastructure.
In February 2010, a sewer canal ruptured during heavy rainfall. The Chalco neighborhood was flooded. The National Water Commission (CONAGUA) is combating the devastating consequences with several major infrastructure upgrade measures. The pump station, La Caldera, forms an essential part of one project—the tunnel “Río de la Compañia”—that will help avoid flooding in the municipalities of Chalco (Valle de Chalco, Solidaridad and Ixtapaluca) in the eastern part of Mexico City Valley.
Inaugurated on March 8, 2011, by President Felipe Calderón Hinojosa, the pumping station is intended to lift the water arriving through the tunnel approximately 30 meters in an open drainage channel. From there, it flows by the ancient Texcoco lake area. The pumping plant consists of one sump where mechanical screens are installed and two pump sumps, each accommodating 12 pumps inside a cylinder of 20 meters in diameter and around 35 meters deep.
The Caracol and La Caldera Pump Stations
A similar design concept was applied on a second pumping station. The Caracol pump station is being constructed in the federal zone of the Texcoco Lake and is projected to discharge 40 cubic meters per second (m3/s) of combined sewage from the Tunnel Emisor Oriente to the Gran Canal open sewer channel. Twenty large submersible pumps will lift the fluid more than 40 meters. The pumps were supplied by a submersible motor pump company and are specially designed for heavy-duty wastewater applications.
In the La Caldera plant, eight of the 24 units (four per sump) supplied, handle a 1 m³/s of municipal sewage each, totaling the 8 m³/s that system designers consider to be the maximum dry-weather flow. Sixteen pumps with a capacity of 2 m³/s each were projected to join in operation during storm water flow, raising the total pump capacity to 40 m³/s.
In the Caracol pump station, each of the 20 units will pump 2 m³/s. The dry weather flow will be handled by four pumps per sump. Twelve units will take care of peak flows during the rainy season.
The pump design engineers faced several challenges. The huge medium-voltage motors had to develop up to 1,600 horsepower and still fit in the general series design concept. Moreover, the standard guide system for lowering and lifting the pumps in and out of the sump had to be adapted to site conditions.
The engineers de-signed a device that would allow automatic connection or disconnection of the lifting crane’s chain without the need to see any part of the pump in the up to 50-meter-deep sumps or maintain the chain connected to it.
The greatest challenge was to find a sump design that would allow the end user to operate both plants in an efficient and reliable way. The pump company was contracted to create a basic design that would fit in the given dimensions and optimize the same flow through computational fluid dynamic simulations, as well as a physical model test. The goal was to reduce the energy of the incoming flow and to provide the pumps with favorable suction-side conditions.
The flow was intended to approach each pump with limited and equally-distributed velocities, as well as without pre-swirls. Surface and submerged vortices should not occur in any part of the sump. Air entrainment had to be avoided.
Some wastewater-specific issues had to be observed. For example, flow velocities were not expected to fall below a certain value to avoid sedimentation of the solid parts within the pumped medium. No dead zones were desired. All this had to be combined with the demand for a separation between dry weather and rainy season pumps.
Figure 3. Snapshots of the CFD simulation (top, La Caldera PS; bottom, Caracol PS)
Computational Fluid Dynamics
The use of computational fluid dynamics—CFD—(numerical flow simulation) is becoming more important. The software specially developed for this application was effective and allowed for relatively precise predictions of the flow conditions.