Putting a Lid on Tanks

Kyle Hankinson, vice president of KCH Engineered Systems (Forest City, NC), likes to compare mechanical covers on chemical processing tanks to the coffee cup in his hand.

 

“The substances evaporate from the surface of the tank, then condense on the inside of the cover, and eventually drip back into the tank,” he says.

 

The company says the benefits of adding a covering for tanks are many. They reduce overall exhaust requirements, fugitive emissions, heat loss and evaporation rates, energy consumption and calculated surface area for exhaust rates.

 

Hankinson says the cost savings through energy reduction is clear: Adding a cover will reduce the demand on an exhaust fan by 45 horsepower, going from 75 to 30hp in a typical installation, assuming the fan is running 24/7.

 

At 0.7467 kW per horsepower, that’s more than 294,000 kWh per year, and at roughly $0.04 per kWh, that adds up to almost $13,000 a year just in energy savings on a fan. KCH Services, Inc. also says there will be a reduction in use of the scrubber pump motor ($1,400), as well as in tempered makeup maintenance ($31,500) and operation and maintenance ($17,000), for a total annual savings of $63,000.

 

Of course, those numbers were from a 2002 validation study done by the U.S. EPA, so the cost savings have dramatically risen in the past decade. But even 10-year-old numbers are still impressive.

 

“The test results show that the KCH technology, when placed on a tank system with ventilation and heating requirements, results in a smaller load demand for power and a reduced need for ventilation to meet ACGIH standards,” the EPA report says. “Consequently, a smaller scrubber, scrubber pump motor, and induced draft fan are needed when the KCH technology is used.”

 

Those are beautiful words for KCH, which presented a report at a recent National Association of Surface Finishing winter symposium. In addition to energy savings, the covers are said to make coal-fired electrical production 35–40% more efficient and reduce CO2 emissions by 869 tons.

 

KCH says the covers can be used in various chemical processes, including sulfuric acid tanks in anodizing, sulfuric/phosphoric in electropolishing, sodium hydroxide in electrocleaning, nitric/phosphoric in brightening, nitric/HF in precleaning and etching, and in most tanks in electroplating.

 

In addition, controlling airborne contaminant exposure to employees occurs when covering a tank reduces the open surface area, thus limiting employee exposure to the contaminants evolving from the liquid surface of the tank.

 

Hankinson says there are several factors that affect chemical emissions in process tanks, including type of process solution; concentration of chemicals in the tank; amount of exposed surface area to open air; electrification of the solution in the tank; operating temperature; vapor pressure of the liquid; and part agitation (air vs. eductor).

 

But there are a few things to take into consideration when thinking about adding a cover for a tank. “They must not be removable,” Hankinson says. “Employees will permanently leave them off.”

 

Other considerations include mechanical operation of the lids, movement control, and either stainless steel or polypropylene material selection.

 

These requirements led to KCH’s development of the SmartCover, a mechanical cover permanently mounted on the sides or back of a tank. Depending on space constraints, it can be single or double hinged, similar to a horizontally mounted bedroom closet. Connected to the cover and mounted to a side wall bracket to raise and lower the cover is an actuator that can be electromechanical, pneumatic or hydraulically powered.

 

“When an operator needs to add or remove a part to be processed, they simply press the up or down push button control mounted on the operator side of the tank, which triggers the actuator to open or close the cover,” Hankinson says.

 

Many existing tanks were designed without the idea of a cover, he says, so bussing, fixtures and utilities can often make covering a tank very difficult. KCH says a tank can be retrofitted by cleaning up the bussing, fixture and piping configurations. A taller side wall and rim can be welded on top of the existing rim, extending the overall height of the tank, and the freeboard inside the tank, and allowing for bussing and utilities to be installed through the new sidewall. A new cover can be mounted over the new rim along with the exhaust hoods.

 

KCH says tanks with covers incorporate a lateral, low-profile type exhaust hood for contaminant control and removal. When a tank cover is in the closed position, the amount of exhaust required to control tank emissions is only a fraction of that when the cover is in the open position.

 

A volume damper in the exhaust hood is interlocked with the cover. When the cover is in the open position, the volume damper is open, increasing the capture velocity through the exhaust hood to properly control contaminants from the open tank surface.

 

“When the cover closes, the volume damper closes, allowing only minimal exhaust at the lateral hood to prevent fugitive emissions from escaping the enclosed tank area,” Hankinson says. “A slightly negative air pressure is maintained inside the enclosed tank area. This also eliminates the risk of hydrogen or other combustible gas explosions, which can form during a plating process.”

 

For most automatic lines that have one hoist transporting parts from station to station over open surface tanks, KCH says Smart Covers with lateral exhaust hoods can be installed to cut down on the total volume of air that is exhausted from the line. The covers are linked to an automatic volume damper in the exhaust hood, which will open and close in tandem with the cover. All covers on the line will be closed, except when parts are being lowered into or lifted from the tank.

 

Hankinson says a ventilation system on a one-hoist automatic line with 10 covered tanks will be sized based on only one cover being open at any one time. That one open tank will be exhausting at full ventilation rate, while the other nine covered tanks are exhausting at a small percentage of the full rate.

 

The covers also limit workers’ exposure to certain chemicals, which helps to meet various OSHA standards.

 

 “Incorporating a properly designed exhaust system with automatic covers and volume dampers operating in tandem can create tremendous pollution prevention and energy savings,” Hankinson says, “especially today where energy costs alone could make the difference between profit and peril.”