Defying gravity

South Boston's new convention center sits in a low-lying area, posing some unusual roof drainage hurdles and spurring innovation in siphonic systems
August 11, 2010

Faced with a difficult roof drainage challenge, the design team for the Boston Convention and Exhibition Center (BCEC) now under construction has elected to use a siphonic drain system that will be the first application of its type in the U.S., according to John Rattenbury, former senior engineer with Boston-based R.G. Vanderweil Engineers.

Installation of a conventional pitched pipe drainage system within the center's ceiling space was declared infeasible, given the mandated requirement for the disposal of roof drainage from the site. To avoid overflowing the antiquated storm sewer systems in the neighborhood, the City of Boston required that all roof-level rainwater be removed through an independent storm sewer pipe and discharged into the adjacent Boston Harbor.

The siphonic system was developed in Finland in 1968 and has been used extensively in Europe. Its use will eliminate the need for a 3.7-million-gallon retention tank and a pump station that would be required to lift collected water from the tank to the Summer Street sewer — thereby avoiding construction costs of more than $10 million.

Architect for the facility, which will cost approximately $450 million, is a joint venture of the Boston office of A/E HNTB, Kansas City, Mo., and New York City-based Rafael Vinoly Architects. The general contractor is a joint venture of the Bethesda, Md.-based Clark Construction Group, Indianapolis-based Hunt Construction Group and William A. Barry Co., Boston. The convention center is scheduled to open in March 2004.

The low level of the site, which is nearly the same as water in the harbor at high tide, prompted the need for an atypical drainage system, says Rattenbury, who recently left Vanderweil to form UV Ameridrain LLC, Hull, Mass., which specializes in siphonic systems. BCEC will ultimately have a roof area of 37 acres, with a runoff rate that is greater than that of the previously occupied site.

"The only way to properly handle the convention center drainage was to pitch it to Summer Street, an elevated roadway, Rattenbury says. Construction of a new drain line to the harbor from the grade level of the exhibition hall was impractical because it would need to be dead flat, subjecting it to harbor backflow.

Pitch versus pump

The new storm drainpipe elevation had to be about 12 feet above the elevation of the main floor at the convention center property line. This necessitated either pitching the roof drain piping over a distance of 3,000 feet within the convention center or pumping water from grade level up to Summer Street.

A conventional drainage system is designed to operate under atmospheric pressure in a partially filled mode, and requires pitched horizontal piping. Plumbing engineers are constrained by state or local codes that prescribe minimum required pitch and the maximum roof area that can be collected by horizontal and vertical pipes. Accordingly, ceilings may need to be lowered to accommodate the pitch of a horizontal pipe, or walls or columns fattened to hide vertical drainpipes. Obtained during schematic design, a variance from the Massachusetts Plumbing Code allowed the siphonic system.

"The key to a siphonic system, apart from the siphonic drain, is the downpipe," Rattenbury says. "A full downpipe pulls water from the horizontal pipe, causing pressure in that part of the system to fall below atmospheric pressure. As a result, water on the roof is literally siphoned into the connected drains."

UV-System is the trademark name of the roof drains to be used, which were co-developed by engineer Per Sommerhein. A U.S. patent was filed in 1983 by the original inventor, Olavi Ebeling of Helsinki, Finland.

The vertical drop of a siphonic system generates the flow. A piped gradient is not required, since the flow is driven by a pressure differential. Piping at the top of the system is under negative pressure, and piping at the bottom of the system is under positive pressure. The system operates at very high velocities, allowing for smaller piping and cleansing flows.

Smaller-diameter piping

A siphonic system also uses smaller diameter piping than a gravity system of equivalent capacity. The convention center's siphonic drains will each collect an area of up to 20,000 square feet, using 21/2-in. drains and collector pipes. An equivalent gravity system would require 5-in. drains and 8-in horizontal lines.

Water is carried vertically from roof drains to horizontal connector pipes that terminate in a vertical downpipe. As the storm builds, the normally empty piping starts to collect water, which flows horizontally to the downpipe. As the storm intensity increases, the design of the drain and the piping mixes air and water to allow the entrained air to run downstream. The horizontal pipe system exhibits a "plug flow" in which propagating wave crests contact the pipe crown, trapping air into pockets.

 
An integral baffle in roof outlets allows water to enter radially, preventing the ingestion of air.


"When it is fully primed, a sudden increase in velocity occurs in the whole system. That's when you're operating in a siphonic condition," Rattenbury says.




If the roof water buildup rate is less than the flow rate of the drain system, air will be drawn into the system through repressurization to inhibit the water flow. However, if the water volume exceeds the capacity of the drainage system, water will accumulate on the roof.

Because the convention center roof is designed to support a large snow load (the equivalent of 12 inches of water), the designers were able to downsize the piping system. The roof will be waterproofed to enable it to accommodate an 8-in. buildup of water. If the rain volume is greater than that of a 100-year storm, emergency drains at the side of the building will discharge water to the ground instead of directly into the storm sewer system. No matter how hard it rains, the convention center system will remove water at a rate of 11/2 inches per hour, which is equivalent to a two-year rain event.

The system's drain provider, Lidingo, Sweden-based Sommerhein AB, says conventional gravity-flow drainage systems cannot effectively dispose of the water received by large and/or complex roofs without costly multiple discharge points inserted throughout the building.

Rattenbury notes that these systems are used throughout the world, and are gaining favor over conventional systems because they reduce construction costs and can accommodate complex architecture. Airports, warehouses, convention centers and factories are among the types of buildings for which the systems are particularly suited, he says.