Handle with Care: Precision Lowering of a Large Tank
Successful completion of a unique tank fabrication process requires precisely lowering the almost-finished tank from a working height of about four feet unto its pad. By analogy, think of balancing a tall stack of poker chips in the palm of your hand. Similarly, although the tank is empty, it must be lowered at all support points in a precise and uniform manner in order to avoid undue stresses or even distortion.
Cobra Tanks Inc., located in Blaine, WA near the Canada border, offers an unusual method of on-site tank fabrication. “The tank is built top-to-bottom,” says Cobra’s Phil Cudmore. He explains that the four year-old company builds tanks from continuous stainless steel strips in a spiral manner similar to the way some pipes are made. The strip material is fed in from the bottom, and the tank slowly rises as it rotates and successive turns are added to the bottom edge.
Cudmore further explains that a “corrugator” machine puts a “chair bend” on the strip material as it unwinds from an as-delivered coil. The next process step is welding, where adjacent upper and lower bends of the strips are welded together on a track system. Rollers guide the revolving shell, which moves upward as a helix.
“Because the tank is built top-first, we can easily install high-level connections, platforms, and such while that part of the tank is still at a convenient height,” says Cudmore, adding that “When the spiral shell reaches the desired height, the top and bottom edges are plasma-cut square with the base.”
Synchronization is Key to Lowering
A computer-synchronized hydraulic jacking system from Enerpac Integrated Solutions met Cobra’s needs and was first used by the tank fabricator for the large double-wall wastewater tank at a New Jersey manufacturing facility described here. Near the end of the on-site fabrication process, the almost-completed tank had to be lowered uniformly from the approximately four-foot height used for the spiral fabrication process.
Enerpac representative Mike Storch of Metro Hydraulics explains that his company helped design the “walking” (pin-post) jacks incorporating computer-controlled hydraulic cylinders. The jacks, which allowed 12-in.-stroke cylinders to be used for the four foot jacking distance, included fine-adjustment screws. “Each of the jacks had to work at a different height because of the tank spiral,” says Storch.
The tank lowering proceeded smoothly because of the “Synchronous Lifting” digital control system using position sensors . As configured for this job, the system used four jacks, each accompanied by a pressure gauge, a flow control valve, a solenoid valve, and a position sensor.
The digital hydraulic control system maintained vertical position accuracy to within 1 mm at each lifting point. The position sensors, with mechanical construction analogous to that of a tape measure, unwind a fine wire from a spool to a fixed reference point. Rotation of the spool is monitored with high angular resolution, and the resulting signal goes to the digital controller, which operates solenoid valves at each jack as needed.
In addition to accurate position monitoring by the controller, the force at each cylinder was readily observable via the pressure gauges installed on each outgoing line from the distribution manifold. With both force and displacement monitored at all lift points, on-site personnel could have confidence that the big tank would be lowered as intended.
One might think of computerized equipment as adding complexity to a task. However, for multi-point lifting, the opposite is often the case. Manually operating the four jacks used for the job would have required intense coordination, and the possibility of human error is always a concern. Pleased with the results, Cobra Tanks continues to use the Synchronous Lifting system.