Custom Isolation Plug Makes Dewatering Possible at Charleston's Hanahan Water Treatment Plant

When routine maintenance turned into an unprecedented engineering challenge at Charleston’s Hanahan Water Treatment Plant, a diverse team—engineers, divers, manufacturers, contractors, and Petersen Products designers—collaborated to deliver a “first of its kind” solution (for treatment plants, that is) that may end up making future plant maintenance safer and more efficient for treatment plants everywhere. 

Hanahan relies on huge five-million-gallon clearwells (about 220-foot diameter by 17-foot depth) to store and process finished potable water. Periodic maintenance is a fact of life; every ten to fifteen years, baffle curtains—crucial for ensuring minimum chlorine contact times and thus water safety—must be inspected and replaced. This cycle was supposed to be routine until contractors discovered a serious issue: the 20-year-old, 84-inch butterfly valves intended to isolate and dewater Clearwell 3 were no longer able to achieve a reliable shutoff, with “leak-by” flows far too great for any ordinary bypass pumping system to handle.

Plant Engineer James Wilson says difficulty with valves is something of a known problem in water plants. “A lot of times, especially with older valves, you don’t get a full shut due to sediment or mineral buildup.” And in this case, “The leak-by from the valves was significant—so much so that temporary pumps couldn’t realistically keep up. That was a showstopper for safely dewatering and working in the tank.

So, how does one safely and securely dewater a five-million-gallon tank with an 84-inch inlet? 

An Intensively Customized “Mechanical Plug with Inflatable Seal”

“This was an unusual and challenging project—outside our usual experience and outside the comfort zone of industry experts,” explains Black & Veatch Project Engineer, Charles “Chip” Hall, P.E. “Normally, a dewatering issue would be up to contractors to figure out. But that wasn't an option here—the safety and financial risks were just too great. This one came down to engineers working out a method that we could sign off on. And we weren't sure at first what that method would be.” And that's saying something; Black & Veatch employs over 12,000 engineering staff worldwide, has more than 100 offices on six continents, and is the longtime engineering partner of Charleston Water System—so if they didn’t immediately know what to do, who would?

Well…there was one idea worth considering. The use of inflatable isolation plugs and line stops is not unusual in water treatment plants: the problem, though, was scale. “Prior to this project, the largest inflatables I had worked with were around 42 inches,” says Black & Veatch Project Engineering Manager Hunter Stewart, P.E. “Doubling that, and installing a plug deep in a buried steel pipe was an extremely challenging prospect.” 

But not an insuperable challenge; when Stewart and Hall began to consult Black & Veatch’s immense talent pool they quickly learned of a firm that specializes in the custom design of large and unusual isolation plugs meant to be installed in difficult scenarios. “Petersen, Petersen, Petersen. That’s the name we kept hearing.”

So Black & Veatch had found their vendor—Wisconsin-based and family-owned since 1916, Petersen Products is the world’s most experienced and successful maker of custom inflatables. Two examples demonstrate the range of custom solutions they've deployed in extreme situations:

  • A custom-made, inflatable tunnel seal from Petersen Products was precisely positioned and inflated 420 meters underground in the Cigar Lake uranium mine using remotely operated vehicles and sonar, successfully sealing a flooding fissure and allowing the mine to resume development. The seal was filled first with water for alignment, then with grout to create a permanent barrier, demonstrating innovative deployment in dangerous, inaccessible mining conditions.
  • Petersen designers demonstrated exceptional custom engineering in an inflatable annular seal intended specifically to make “dry welding” possible for a deep-sea tension leg platform repair in the Gulf of Mexico, adapting materials and construction to withstand both saltwater immersion and high-pressure welding environments. The success of the project—achieved through bespoke sizing, material selection, and innovative deployment methods—verified Petersen’s ability to solve unprecedented challenges in extreme industrial settings.

Specifying an Unusual Solution

Since potentially millions of dollars—not to mention lives—were at risk if pipes or valves were damaged by ill-considered intervention, Black & Veatch engineers knew they had to put in the work necessary to confidently guarantee that a dewatering solution as new to the water treatment industry as a relatively huge inflatable isolation plug would perform perfectly in the difficult, dangerous situation presented at Hanahan's Clearwell 3. This work began with a thorough investigation of the exact materials and conditions of the pipes affected by the butterfly valves failure. “We asked Salmons Dredging Corporation to inspect the underwater valves and pipes, and took detailed measurements,” explains Stewart. “This detailed inspection was necessary to verify that installing a large inflatable plug would not cause pipe damage.” 

Determining a section of pipe sufficiently restrained and of the right material to withstand the immense closing forces—up to “an 18-wheeler’s worth” of pressure on the plug, says Stewart—without structural failure was a key to success. And fortunately, dive inspections and as-built records identified a suitable 30-foot steel section near the butterfly valve, allowing the team to confidently specify the plug location.

With Salmons’ report on precise pipe dimensions and conditions in hand, Black & Veatch engineers approached Petersen Products with the concept of using a typical inflatable plug (albeit much larger) to isolate the clearwell, as this was a common solution for smaller-scale pipe work. But as the team consulted with Petersen experts, it became clear that a standard inflatable would not be sufficient for this unique application—the plug would need to hold for up to six months and withstand significant pressures and leak-by rates in an 84-inch pipe. Instead, Petersen designers proposed a custom mechanical plug that incorporated a smaller inflatable seal around the edge. This solution offered both the strength and longevity necessary for the lengthy dewatering project.

Engineering input flowed both ways. Petersen Products, working alongside Black & Veatch engineers, Salmons Dredging staff, and plant personnel, developed a semi-mechanical, semi-inflatable plug that was:

  • Foldable for entry into the tight confines through a clearwell hatch.
  • Outfitted with adjustable wheels for maneuverability, and a ratcheting pulley/jacking system to unfold and position it.
  • Built of steel and EPDM (after a change from Kevlar, based on diver handling in dry-run shop tests), and coated with NSF-approved materials due to contact with potable water.

     

The design and installation approach were thoroughly rehearsed in Petersen’s Wisconsin facility—a field trip described by Salmons’ topside dive master, John Lares, as akin to being on the Mythbusters set: meticulous, capable, imaginative, and bottom-line focused on safety and workability. 

One result of these hands-on trials was Petersen’s rapid reengineering of an installation procedure that in practice proved to be unwieldy. The lighter weight and simpler transport and jacking system a critical improvement for diver safety and real-world installation.

In the Field: Diver-Driven Success

Installation fell upon the expertise of Salmons Dredging veteran inland diving crew, who considered the Hanahan dewatering a career highlight.

  • Logistics: The plugs, each split into two 900-pound halves, was craned onto the Clearwell roof, lowered through the access hatch, then rolled or guided into the pipe—265 feet in some cases—over a 16- to 34-foot vertical drop, before being unfolded and jacked into place.
  • Conditions: Commercial, surface-supplied divers worked in cool potable water under, in dry suits sterilized to protect water quality, with days of bottom time—4 hours underwater was a routine day. Salmons Dredging used Nitrox, an oxygen-enriched breathing gas, to safely lengthen the allowable bottom times. This approach reduced nitrogen buildup in the divers’ bodies and lowered the risk of decompression sickness during the prolonged underwater work sessions.
  • Safety & Precision: With cramped quarters (each plug half occupying half the pipe diameter), diver safety and control were paramount—any pinched hose or movement error could have been catastrophic. Divers avoided trouble  with “slow and methodical” teamwork, real-time video, and disciplined planning.

Once jacked open and inflated (using an air supply monitored from the surface), the plugs provided the absolute isolation required—allowing nearly six months of secure, leak-free shutdown. Backup pumps and pressure monitoring added safeguards, but these were ultimately not needed: “Leakage was minimal, well within expectation… it was impressive how well the plugs worked,” Stewart summarized.

Underwater Infrastructure Heroes

With Clearwell 3 reliably isolated, contractors were able to pour a new concrete baffle wall (solving persistent hydraulic issues), replace curtains, perform comprehensive cleaning, and deliver a longer-lasting, more reliable water facility. Equally impressive was the collaborative spirit among project stakeholders:

  • Partnership: All parties described Petersen Products as true partners, open to discussion of both strengths and limitations during the rapid cycles of design and field testing.
  • Ingenuity: Everyone involved highlighted the dry run at Petersen’s shop as a pivotal moment important to project safety and success.
  • Legacy: The plugs—now proven in service—are in storage and likely to be used again, and the project itself has set a template for other municipalities facing similar “routine maintenance” emergencies.

Charleston’s clearwell isolation project stands as a testament to American infrastructure ingenuity; when the ‘routine’ becomes anything but, teamwork and ingenuity can keep clean water flowing.

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Angus W. Stocking, L.S. is a licensed land surveyor now writing full time about infrastructure topics