Storage Capacity and Fuel Inventory

A facility’s rated capacity is the quantity of fuel its storage system is designed or authorised to hold. Its actual inventory is the fuel physically present at a particular time, so capacity is a storage potential while inventory is a point-in-time stock.

Capacity and inventory compared

Rated capacity usually changes only when storage is commissioned, expanded, converted, derated or taken out of service. Inventory changes continually with fuel receipts, withdrawals, losses and transfers.

• Gross or nameplate capacity may exceed working capacity because safe filling limits require some empty space, called ullage.
• Usable or working capacity excludes space or fuel that cannot ordinarily be used, such as operational heel or inaccessible tank-bottom stock.
• An operational facility can therefore have substantial empty capacity while holding little actual fuel.

Why the two figures diverge

The gap reflects both physical constraints and operating decisions. Maintenance, product segregation, quality compatibility, minimum operating levels and the need to retain space for incoming supplies can prevent full use of nominal capacity.

• Installed tanks do not by themselves constitute a fuel reserve; the fuel must also be procured and stored.
• Total physical inventory may exceed deliverable inventory because some fuel cannot be withdrawn promptly or without affecting safe operation.
• A high occupancy level does not necessarily guarantee rapid supply if fuel is unavailable at the required location, grade or delivery rate.

How to interpret storage data

The denominator must be identified before comparing facilities: gross, rated and usable capacity are not interchangeable. Fuel volumes should also be compared on a consistent temperature, density or mass basis.

• The occupancy ratio is inventory divided by the stated capacity, but its meaning depends on which capacity measure is used.
• Days of cover should be calculated from deliverable inventory and an explicit daily consumption or drawdown assumption; it changes when demand changes.
• Capacity measures resilience potential, while inventory and deliverability indicate the fuel actually available during a disruption.