Chemical tank

BASF Chemical Plant Portsmouth Site in the West Norfolk area of Portsmouth, Virginia. The plant is served by the Commonwealth Railway.

Chemical tanks are storage containers for chemicals. They come in a variety of sizes and shapes, and are used for static storage, processing, mixing, and transport of both raw materials and finished chemical products.

Design

The Chemical tank represents the first line of control for some of the most aggressive chemicals, control consists of selecting the chemical tank most able to cope with the application parameters and capabable of preventing the negative impact of the chemical finding its way into the environment.

A chemical tank will of necessity be made of a material as resistant to the chemical stored as design and economics allow. The design will further integrate the mechanical parameters (pressure and temperature, erosive and corrosive) of the chemical storage application, within an the applicable design timeline.

Compatibility is critical for chemical tank design. Chemical tanks/containers may also be impacted by external considerations, heat, cold, vacuum, pressure and the potential aggressive nature (acidic - caustic) These external effects may impact the tank/vessels ability to hold and maintain the viability of both the chemical and the container. (Example: liquid freezing or thickening due to ambient temperature, removes the use of the liquid and the tank.)

There are short term and long term goals associated with the engineering and specification of the correct materials for a chemical tank / container / vessel. While economic considerations are a factor, the design of chemical storage containers and how the chemistry may impact the environment is crucial.

Typically, there will be a chemical profile and information report known as a MSDS (Material Safety Data Sheet) which is provided by most chemical manufacturers or distributors and should be the starting point for chemical tank design. Chemical tanks should be designed in conjunction with the chemical manufacturer, providing some historic experience regarding tank materials compatibility to the chemical.

Design tips

A few design tips of successful additions that promote long term security of the chemical tank system.

1. Expansion joints are devices that isolate the stress forces that would otherwise be transferred to the chemical tank, i.e. vibration, torsional and tangential movement, expansion and contraction originating with the tank itself. The expansion joint absorbs stress from the tank wall, process connections and piping.

1. Emergency venting by rupture disc, a sacrificial device that prevents vacuum or pressure from destroying the chemical tank and release uncontrolled chemicals into the environment. Located on the tank top, the device vents air into or out of the chemical tank.

1. Valves with lockout capability prevent unauthorized movement of liquids from chemical tanks, compatibility is always critical.

1. Extreme attention to tank heaters and their negative impact on tanks materials, tank coatings and process liquids, (example; heaters have caused coating failure (melting), tank wall failure and changed the chemical compound being stored. Level-heater integrated controls help maintain integrity.

1. Level indication is critical to tank/vessel operation, clear tube level gauges, level switches, level transmitters are a few ways to accomplish, many times they are redundant design elements.

1. Venting is crucial to maintaining the integrity of a tank, so chemical tank venting and overflow control should be given consideration, as loading the tank requires venting air out of the tank, and unloading the tank requires air to fill the space voided by the removal of the stored chemical. A rule of tank / thumb is the vent needs to equal the largest process inlet or outlet.

1. Foundations for the tank, protect the container from damage and maintain relative relationships between tank and plumbing (pumps, pipe, valves and tank fittings) High wind and seismic considerations should always be integrated into the foundation design math.

Secondary containment provides for 110% capacity of largest tank in containment area. Containment prevents escape of chemicals into the environment, by providing a holding area for chemicals that may leak from the tank farm.

References

• NACE - National Association of Corrosion Engineers
• NACD - National Association of Chemical Distributors
• AICHE - American Institute Of Chemical Engineers
• ASME - American Society of Mechanical Engineers