To commemorate those who died at New York City’s World Trade Center on September 11, 2001, a memorial entitled “Reflecting Absence,” was erected. The site consists of a 16-acre plaza with 400 trees that surround two recessed reflecting pools. Each pool is close to an acre, and they are in the footprints of the Twin Towers. Both reflecting pools hold 600,000 gallons of water, and they are the largest man-made waterfalls and reflecting pools in the world.
The contractor chosen by the architects, Delta Fountains, faced a series of challenges from the start. The reflecting pools required massive plumbing, pumps, filters, water chemistry tanks and precision automation controls to keep everything operating smoothly 24/7 throughout New York City’s four seasons. The memorial challenged the contractor’s engineers to design a method to keep the water in the fountains flowing in clear streams from a 29-foot fall, while maintaining the required reflective and acoustic properties year-round, no matter the weather (see Image 1).
They also had to design an automated control system that would allow the fountains to be operated and monitored remotely from the contractor’s headquarters almost 1,000 miles away in Jacksonville, Florida, to help reduce operational costs. Remote operation of the fountains and keeping the water in the pools sparkling clean with its acoustic and visual properties consistent minute-to-minute was going to be no small task. And since visitors from all around the world would visit the memorial daily, at all times of day and night, it was also critical that there be no equipment failures resulting in any unexpected downtime.
Fountain Equipment Design
Delta Fountains called on Siemens to provide them with sophisticated automation and control systems, having two levels of redundancy, to remotely operate and monitor the two memorial fountains. Multiple controllers were installed to provide the precision control required for the fountain’s critical functions. Variable frequency drives (VFDs) with additional control units were chosen to provide precision flow control for the powerful pumping system needed to transport the water in two applications, circulation and cascade. The VFD systems included the motor protection and pilot devices necessary for control and annunciation of the pump systems, while also providing an energy cost savings by keeping the pump motors operating at optimal speed at all times.
The pumping system for each fountain includes eight Flygt/Xylem 480 volt, three phase, 12-inch, 60 horsepower, vertically installed, dry-pit submersible centrifugal pumps used to provide the water circulation (See Image 2). Combined, they have the ability to deliver up to 32,000 gallons of water per minute to each fountain. The vertical cascade of the waterfalls is powered by four additional submersible pumps, while four more dry-pit electrical submersible pumps take care of the stormwater drainage for each fountain.
A massive filtration system was designed and installed to filter the water in each fountain at a rate of 6,000 gallons per minute through ultraviolet light filters as well as mechanical filters.
Remote Pump Operating & Monitoring Problem
The mechanical rooms housing the critical water pump systems are 50 feet below plaza level, directly beneath the pools. The contractor’s engineers needed to find a method to safely start and stop these pump motors from their remote location in Florida. At any given time, the pumps could be exposed to moisture or water leakage, which is a common cause of electric pump motor failures.
They needed a monitoring system with the ability to automatically test the electrical insulation inside the pump motor to determine if any breakdown occurred. Insulation breakdown can be caused by moisture, caustic chemicals, contaminants, thermal breakdown and vibration. And since the most common time for an electrical failure to occur in a motor is during startup, the operators needed to have this test information every time before a pump motor was called upon to operate.
AIRT’s Provide Solution
The solution was to install automatic insulation resistance testers (AIRT) on each of the 32 total memorial pumps to continuously test and monitor the insulation resistance (I/R) condition of the motors whenever they are offline (see Image 3). The status of the I/R test would then be sent through the programmable logic control (PLC) system to the contractor’s operators in Florida, which would allow them to determine if a motor was safe to start or if it required maintenance first.