Pumps & Systems, October 2008
The Merchandise Mart in Chicago, Ill. is the world's largest commercial building, largest wholesale design center and a premier international business location. Encompassing 4.2 million gross square feet, the Mart spans two city blocks and stands 25 stories high. It welcomes more than three million visitors each year to its retail shops; boutiques; 11 floors of permanent showrooms for gift, residential, casual and contract furnishings; 10 floors of office space; dozens of trade shows and a myriad of special educational, community and consumer events.
When the Merchandise Mart began the task of obtaining LEED-EB certification, it was just the fifth building in Chicago to go "Green." Adhering to standards set forth by the LEED Green Building Rating System, a voluntary, consensus-based national standard for developing high-performance, sustainable buildings, the Merchandise Mart focused on five key areas of green building strategy: sustainable sites, water efficiency, energy efficiency, materials/resources and indoor environmental quality. Such ongoing improvements involved reevaluation of capital programs for replacement of windows, roofs, controls, plumbing fixtures, utility meters and lighting.
To better target energy efficiency, one of the many strategies directed by Merchandise Mart's Vice President of Engineering, Mark J Bettin, P.E., was replacing an outdated domestic water supply system that was a large consumer of energy and an inefficient supplier of water.
Original System
The original system consisted of five pumps ranging in size from 40-hp to 75-hp. The pumps ran at a constant speed intended for the highest demands even during low-flow periods such as the middle of the night. The system was designed to deliver over 1,250-gpm, churned at 290-psig. Pressure reducing valves on the discharge of each pump maintained a system pressure at 200-psig. At any flow less than peak flow, the pressure reducing valves would throttle flow and waste energy.
"With recent restroom renovations complete, the water consumption and demand reduced annually by more than 35 percent, resulting in an oversized, inefficient pumping system. It required large amounts of energy to run, and replacing it would help in the buildings' ongoing energy reduction efforts and contribute in maintaining the standards of LEED certification," Bettin said.
According to Merchandise Mart's Plumbing Foreman, John Geiss, prior to the retrofit and during heavy usage, the domestic water system would see momentary swings in the system as much as 40-psi at the top end of the system, resulting in noticeable fluctuations at the fixtures and excessive pipe movement.
During peak flows, which typically occur during a large tradeshow event or the workweek, demand can reach 750-gpm when up to 20,000 people populate the building during major exhibitions. However, during low flows, such as the early morning hours, demand can slow to a few gallons a minute. With such a wide range of flows and demanding show schedule where water consumption constantly changes, the new domestic water system would need to match not only extremely high flow demands, but also the low flow demands using the least amount of energy necessary.
During the bidding process, quality, service and experience were the criteria important to Bettin when choosing the companies to complete the work."We pre-qualified the companies such as engineer KGWW, Metropolitan Industries, Inc., Great Lakes Plumbing and Blaze Electric based on their experience and reputation," he said.
Chicago-based Great Lakes Plumbing was awarded the design/build contract based on the proposal they submitted. Great Lakes' Bob DeGuiseppe, project manager, served as the general plumbing contractor; pump and control system manufacturer Metropolitan Industries, Inc. (Romeoville, Ill.) designed the new system and pressure reducing zone stations; Blaze Electric (Broadview, Ill.) served as the installing electrical contractor and KJWW (Chicago, Ill.) served as engineer during the project.
The New "Green" System
The pump system and pressure reducing zone station design was based on two specific goals: reducing energy costs on day one of start-up and ensuring enough redundancy to prevent any future loss of service due to power or mechanical issues.
Working closely with DeGuiseppe, Metropolitan Chicago Sales Manager Mike Ponx suggested installing a variable-speed system. Since the system can precisely match demand using only the minimal amount of energy necessary, it would reduce energy costs by half.
The system supplied was a variable speed, quadraplex booster system using vertical, multistage pumps, total system rated at 1000-gpm at 200 psi. Since peak flows are 750-gpm, the system will still be able to handle the load without shut down if a single pump fails. To add to the redundancy, two separate power feeds are divided between the system. Feed A runs three pumps and Feed B runs two pumps. If either Feed A or Feed B is lost, the building will still have one electrical feed to supply water to the building without interruption or system shutdown. The electrical control system and plumbing design was also configured to one existing pump to operate as a manual standby pump.
The building is divided into four zones to stabilize pressure. At each zone, pressure reducing valves were downsized to maintain each of the zone's pressure requirements and the four bladder tanks were installed on the 6-in cold water express risers to sustain pressure when the system shuts off during low flow periods.
Using bladder tanks at the top of the express riser, pressure is stored so the system can shut off during low flow periods. As flow is consumed, the pump system will activate and supply water to the building at the desired pressure. Once the demand for water is reduced to a low flow period, the system will shut off once again. Significant energy savings are realized during low flow periods when using variable speed drives and bladder tanks in combination.
During installation of the pumps and controls, interruption of service did not occur to the building. "Since the existing booster system had two independent electrical feeds and the building could not be without water at any time, the electrical controls were built into two sections, which would be mated together during the second phase, DeGuiseppe said. DeGuiseppe spilt the new control panel into two sections. Blaze Electric installed the main control section first after disconnecting the first electrical feed, then installed the first two of four pumps using the new controls. Once the first two new pumps were online, Blaze Electric removed the old panel and installed the second half of the new control panel, which was designed to mate up to the first section. After that, the remaining two pumps were installed.
Green Savings
To commission the system and measure the savings, meters were installed to measure flow and energy consumption. Results appeared quickly with the installation of the first two pumps. Energy reductions in the first week were 34 percent. With completion of all the pumps, the bladder tanks came on-line, and the system was set for commissioning. Utilizing a system bypass and real-time flowmeter, various loads were created to allow for fine tuning of the drives and staging of pumps. With the removal of the top zone PRV, the system pressure was successfully lowered from 200-psi to 185-psi. Taking advantage of the bladder tanks, the nighttime mode was enabled, resulting in the cycling of pumps. Weekly evaluation has shown a reduction in energy by up to 50 percent with an anticipated annual energy reduction of over 125,000-kWh.
Within a decade, the system will pay for itself entirely on energy savings. The expected life of this system is more than 25 to 30 years, so eventually the system will not only pay the end user for the cost of the system but it will also pay the costs to replace itself in its lifetime.
Conclusion
According to Geiss, the completion of the retrofit work improved the overall performance of the system by lowering working pressure and reducing fluctuations throughout the system.
"For the first time we have experienced the most even flow and the best overall PSI performance at the top of the 10th, 18th and 25th floor zones," he said.
Reducing the size of the valves in the respective zones has tightened up the flow in conjunction with the smooth operation of the variable speed booster system and bladder tanks that feed and assist the entire system.
"Overall. . . all parties involved came through with the promise and commitment to a 2- to 4-psi swing, and an energy savings that will match the Merchandise Mart's committed effort to LEED providing for a greener tomorrow," said Geiss.