Vertical turbine and horizontal split pumps control pressure, overcome height obstacles.
by Todd Weir
May 15, 2018

The $1.35 billion Wilshire Grand Center opened in 2017 to international acclaim for its sophistication and luxury, a skyline-changing icon of modern design and a catalyst for the economic renaissance of downtown Los Angeles. Built entirely in the footprint of the former 1950s-era Wilshire Grand hotel—itself a cultural icon—the 73-story glass and steel skyscraper with its distinctive rooftop spire is the tallest building west of Chicago.

Wilshire Grand CenterImage 1. The Wilshire Grand Center. Photo by Gary Leonard (Photos provided to Pumps & Systems by Xylem Inc.)

Fire Codes

The roof design breaks with LA’s traditionally flat-roofed skyscrapers that are required to have helipads by city code. Wilshire Grand architects obtained an exemption from this 1970s rule reportedly because of the addition of other fire safety features that would exceed the city’s fire code requirements, such as the reinforced concrete central core that contains a staircase solely for the use of firefighters in the event of an emergency.

Stringent building codes for fire safety and National Fire Protection Association (NFPA) 20 Standard for the Installation of Stationary Pumps for Fire Protection—as well as project parameters—drove the design process. One of the challenges was to design a system that met the city of LA’s requirement that total pressure shutoff can not exceed 600 pounds per square inch (psi). The fire pump designer worked closely with sprinkler contractor XL Fire Protection on the design.

Brian Callahan, president of XL Fire Protection, is an accredited Certified Fire Protection Specialist (CFPS) by NFPA and a level 4 certified National Institute for Certification in Engineering (NICET) designer. He sketched out a two-zone system that met the requirements for the high rise, worked within the space restraints of the fire pump room and did not exceed maximum pressure requirements. One pump services the low zone, but the high zone portion requires two pumps in series to create the required 600 psi. Vertical turbine pumps and horizontal split case fire pumps were selected for the job. Codes require an exact set of redundant fire pumps on each level, so that meant six fire pumps in total.

“By getting the number of pumps down to six, there was no need for an additional emergency generator, saving project cost,” Callahan said.

Once the design was conceived, the team had to be sure the system would work as planned. The team provided theoretical planning on what the pump curves would look like, especially in the high zone where the vertical turbines pump into the split case pumps and were approaching 600 psi. It also became more complicated because of NFPA 20 requirements for fire pump impellers, which state the pressure at shutoff cannot exceed 140 percent of the rated pressure at the rated flow and cannot be below 65 percent at 150 percent of the rated flow.

Pump Pressure

The team looked at not just the design point flow and head system needs, but also the pressure on the pump when it is at churn. The impeller was designed to limit that pressure so it does not overpressurize the system components downstream of the pump. Even at shutoff, the static pressure in the turbine pumps was 77 psi, which required a pressure-reducing valve on the standpipe systems.

“Controlling the pressure was our biggest challenge,” Callahan said.

The Wilshire Grand fire protection system design is also unusual in the low zone pump room, with the vertical turbines sitting atop a 120,000-gallon, three-story water tank in the second of five basement levels. Space is at a premium on the urban site. Therefore, there was no room for the vertical turbines outside the building. Designers had to be sure this setup would meet the NFPA 20 requirement on how tanks are constructed and that the fire pump would be capable of providing demand for the whole building in an emergency.

Once the fire protection system was in place, the next steps were to test the system and train building personnel on their operation. The fire pump acceptance test was an essential step in obtaining the occupancy permit for the building. In testing the equipment, there was some fine-tuning in the time between turning on the pump and it feeding into the split case pump. By adjusting sequencing with the fire pump controller, the team was able to close the gap from 10 to 15 seconds at startup to 7 seconds. The system also met the challenge of the 600 psi requirement.

“In the end, everything worked as designed,” Callahan said. “It’s not just putting in products. Everything had to perform properly as a system and meet code.”

The building stands tall as a symbol of the new way of thinking about commercial building, using an integrated design approach involving the fields of architecture, engineering and construction to achieve safety, efficiency and sustainability goals. The interdisciplinary collaboration among engineers, architects, mechanical contractors and others in the design-build process resulted in innovative techniques that maximize technology to ensure the safety of occupants.