A Systematic Guide to Choosing the Right Capacity for Your Chemical Storage Tank

Understanding Tank Capacity Fundamentals

Tank capacity selection is more complex than you might think. When done properly, it involves balancing operational needs with safety requirements, regulatory compliance, and economic considerations. The process requires input from process engineers, safety personnel, operations staff, and financial decision-makers.

Capacity calculations will also depend on whether a chemical tank is a storage tank or a process tank. Storage tanks hold inventory for extended periods and are refilled periodically based on supply schedules and consumption rates. Process tanks contain chemicals that flow continuously in and out during manufacturing operations. Each type requires different methods for capacity calculation, though both must account for peak-demand scenarios and safety margins.

Proper tank sizing begins with a thorough assessment of operational requirements, considering current needs while accounting for anticipated future growth and operational variability.

Calculate Daily Chemical Usage

Whenever possible, determine the average daily consumption of the chemical to be stored using actual operational data. For existing facilities, review historical consumption records over at least twelve months to identify patterns, seasonal variations, and trends. For new facilities, develop consumption estimates based on process flow diagrams, equipment specifications, and production schedules.

Determine Peak Demand Scenarios

Calculate your capacity needs based on peak demand, not average need. Peak demand is the maximum rate at which chemicals will be consumed during the highest-intensity operational periods. This is particularly critical for process tanks that must continuously supply chemicals during production.

Insufficient capacity during peak demand can force production slowdowns, compromise product quality, or create safety hazards.

Establish Inventory Turnover Requirements

For storage tanks, inventory turnover significantly influences capacity requirements. Industry turnover is the frequency with which tank contents are completely used and replenished.

The best practice is typically to maintain a minimum of four weeks’ supply in storage tanks. This provides a buffer against supply chain disruptions, delivery delays, quality issues with incoming shipments, and unexpected increases in consumption.

Account for Future Growth

One of the most common mistakes in tank sizing is failing to adequately plan for future growth. Installing a tank that’s sized precisely for your current needs may seem cost-effective initially, but it quickly becomes a constraint as your operations expand.

Adding tank capacity later is more expensive than installing the right capacity from the start. In addition, waiting until you need to upsize can put you in a time-constrained situation where a bottleneck in the purchasing process affects your production capacity.

Understand Working Volume vs. Total Volume

Tank capacity refers to total volume, typically measured in gallons, liters, or cubic meters. However, working capacity—the actual volume available for storing chemicals—is typically less than the total capacity. Industry practice recommends maintaining working volumes at approximately 85-90% of total tank capacity.

Tanks must never be filled completely to the top. Adequate freeboard (vertical distance between liquid surface and top of tank) is essential for thermal expansion, vapor space for chemicals that generate vapors or off-gas, and a safety margin against overfilling during tank filling operations.

Be sure to calculate both the working volume and the total volume when choosing the capacity of your chemical storage tanks.

Calculate Secondary Containment Requirements 

Secondary containment is a critical safety feature that captures and contains spills or leaks, preventing environmental contamination. Under EPA regulations, secondary containment must be sufficient to contain 110% of the volume of the largest tank, or 100% of the largest tank plus 10% of the aggregate volume of all other tanks, whichever is greater.

Secondary containment can take several forms, including concrete or earthen dikes that surround outdoor tanks, double-wall tanks with an interstitial space between inner and outer walls, or buildings with sealed floors and containment sumps.

Double-wall tanks provide built-in secondary containment but are more expensive than single-wall tanks. However, they are often less expensive than the cost of building and maintaining concrete berms. They also preserve the leaked chemical for later use, preventing expensive inventory loss.

Download the Poly Processing Secondary Containment Guide

Consider Regulatory Capacity Thresholds

Various environmental and safety regulations establish threshold quantities for chemical storage that trigger additional capacity requirements. The EPA's Risk Management Program (RMP) establishes threshold quantities for regulated substances. Facilities that store more than the threshold quantity must develop and implement comprehensive risk management programs.

Similarly, OSHA's Process Safety Management (PSM) standard applies to facilities that store threshold quantities of highly hazardous chemicals.

Be sure to review all of the environmental safety requirements that apply to your facility and application when sizing a chemical storage tank.

Apply Industry-Specific Safety Factors

Industry best practices recommend applying safety factors to tank capacity calculations. These factors account for uncertainties in consumption estimates, operational variability, and consequences of capacity shortfalls.

Assess Available Space

Conduct a detailed survey of the space that’s available for tank installation. Don’t include only the footprint of the tank itself; also consider any clearances required for access, maintenance, piping connections, and safety equipment.

For outdoor installations, consider setback requirements from property lines, buildings, and other tanks. Fire codes typically require minimum separation distances between tanks storing flammable liquids.

Consider Tank Material and Construction Limitations

High-Density Crosslinked Polyethylene tanks are popular for corrosive chemicals due to their excellent chemical resistance and relatively low cost, but are typically limited to vertical tank capacities of approximately 15,500 gallons. Many end users prefer to install multiple smaller HDXLPE tanks rather than a single large FRP tank. This gives the user greater flexibility with their tank farm.

Evaluate Economic Implications

If you purchase bleach at 12.5% and the tank capacity is too large, the chemical could sit for several months and dilute down to just 6% bleach. You would lose half of your chemical’s effectiveness, or you may need to count it as a loss and replace it entirely.

Perform a lifecycle cost analysis comparing different tank capacity options. Consider capital costs, inventory carrying costs, delivery costs, operational costs, and risk costs. Compare the total lifecycle costs of different capacity options over the expected service life of the tank.

Choose the Right Size Chemical Tank with Confidence

Selecting the right capacity for a chemical storage tank requires careful analysis of operational requirements, safety considerations, regulatory compliance, physical constraints, and economic factors. Following a systematic approach ensures that you’ll select a capacity that meets your needs safely and cost effectively.

Remember that selecting tank capacity is about understanding your operation, anticipating future needs, managing risks, and making informed trade-offs among competing objectives. By following this systematic approach, you can confidently select a chemical storage tank capacity that will serve your facility safely and effectively for many years to come.

Still need expert help in making capacity calculations? Consult with Poly Processing’s experienced tank engineers.