Built from the Top Down
In October of 2006, Alliance Constructors received an order to erect a water standpipe in eastern Missouri. There was nothing unusual about the circumstances or the assignment. Tank Connection, the company that placed the order, told Kevin Hill, general manager at Alliance, that it was a “plain, vanilla standpipe.”
Selecting water standpipes has become a very basic process: if there is time and money, a field-welded standpipe is an option; if either of those commodities is in short supply, a factory-coated bolted standpipe is specified. Shop-welded standpipes are available in a narrow capacity range but outside of that range, the choices remain field-welded or bolted.
Tank Connection recommended bolted construction for its rapid installation and economy.
However, the project manager for the development had examined a bolted standpipe in the area and it was evident that the structure leaked, even though it was less than three years old. Based on that, he was understandably skeptical of choosing a bolted design. Still, field-welded construction would be limited by inclement weather and would jeopardize the schedule.
Filling a rush order
The bolted standpipe that Alliance would be erecting was for a new housing subdivision. The 225,000-gal-plus capacity requirements eliminated the shop-welded fabrication option. The job was a rush order with a requested completion date of Dec. 1.
Hill, a veteran of domestic and international tank installations under every conceivable condition, had just one question: “Is it a new style tank or one of the old ones?”
“I’ve erected flat panel tanks and tanks with flanged panels, and there’s just no comparison,” said Hill. When the customer showed me the flanged tank a few miles away, I saw why he had doubts.”
Originally used for petroleum storage applications, API 12B tanks were adapted for other liquids in the 1970s. The original API design was for crude oil tanks no more than 24 ft high with steel panels no thicker than 3?16 in.
As manufacturers increased their fabrication capabilities, they pushed these limits with heavier gauge steel and taller structures.
In oil applications using the API 12B design, the thicker viscosity of crude oil minimizes leaks. With water storage, though, the problem is persistent. As the pressure produced by the stored liquid increases, so does the pressure against the space between panels. Increasing the height of the tank increases pressure; so does increasing the diameter. A heavier plate allows the tank wall to withstand the pressure, but the space between the plates remains vulnerable.
Tank Connection had recommended a bolted tank manufactured by BOSS Tank. Their flat panel (FP) standpipes have been designed for hydrostatic loading. Increased head pressure reinforces the overlapping panel seal instead of forcing the panels apart.
Glass-coated bolted tanks and stainless steel bolted tanks use these same design principles. In the last two years, the modular FP design has begun to replace the API 12B design and industry observers believe that in less than five years flanged panel tanks will be largely obsolete.
The coating systems on FP standpipes are applied in a factory-controlled environment, which is an advantage over field welding. Their automated line has three times the capacity of their closest competitor, with continuous electronic monitoring. The BOSS Tank powder coating system has uniform shell coverage and superior edge protection. The coatings and contact components are NSF-approved.
Tank Connection specified an 18 x 114 ft FP standpipe with 226,000-gal capacity. BOSS Tank offered another distinct advantage to both the end user and the erection crew: This standpipe could be built from the “top down.”
Top-down assembly begins by forming the topmost ring of the structure. The second ring is then built under that, using a proprietary jacking system to raise the first ring to a height where the second ring can be added underneath. Flanged tanks employ gasket strips between the chimes, but the FP design uses a mastic between the overlapping joints that bonds the panels to each other. The deck is added after the first two rings are complete.
“Our crews are sensitive to the risk factors of construction, just like the end user and everyone else involved in the project. Keeping crews closer to the ground for the assembly has tremendous appeal, especially on a 114 ft tall standpipe,” Hill said.
Tank Connection provided the foundation design and turnkey services on the job. Erection of the tank began in October, with a projected completion of 35 days. If there were any lingering doubts about whether a field-welded standpipe would have been better, they disappeared with the November weather. An early season snowstorm caused a delay that disrupted the schedule.
A strong water supply allowed the crew to fill the tank in five hours. Bolts were tightened in two locations and the tank was functional. The first houses in the subdivision were opened early in December 2006. The standpipe supplies potable water for the residents and has sufficient capacity to provide water for fire protection. The tank had been commissioned, inspected and in service by the time the first residents moved in.
“I can’t say that everything was perfect,” said Hill. “When the tank was commissioned, there were a couple of minor leaks. We returned and tightened the seal on a manway cover and replaced a bolt. Imagine, one bolt in the whole tank. On the old API 12B bolted tanks, you could never get a lasting seal on the chimes.”