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Level Measurement in Oil Storage Tanks: Inventory, Custody and Overfill Are Three Different Jobs
An oil-storage project should specify inventory, commercial measurement and overfill prevention separately. Inventory describes what is in the tank. Custody transfer requires an approved measurement system where applicable. Overfill prevention requires a reliable response before containment is lost. One accuracy number does not define all three duties.
Separate the three jobs
| Duty | Result needed | What must be specified |
|---|---|---|
| Inventory management | Available stock in volume or mass | Required uncertainty, tank calibration and temperature or density inputs |
| Custody transfer | Commercially accepted quantity | Applicable metrology approval, installation conditions and verification procedures |
| Overfill prevention | Timely warning or termination of receipt | Risk assessment, response time, architecture, testing and responsibilities |
Do not assign universal millimetre tolerances to these duties. A terminal's inventory objective differs from a legal-metrology requirement, and a high-high alarm must have adequate setpoint margin, repeatability and response, not just a fine display resolution.
API 2350 is a prevention programme, not a transmitter certificate
API 2350 addresses overfill prevention for petroleum storage within its scope. Its applicability, tank category and the required safeguards must be established for the facility. It is not a rule that every liquid tank, including LNG storage, has the same instrumentation arrangement.
Where an independent high-high function is required, a second value from the inventory transmitter is not an independent sensor. Review the complete path, including power, logic, alarm or shutdown action, common-cause failures and the means of testing it.
Set alarm and action levels from available capacity, maximum credible receipt rate and the time needed to detect, respond and stop flow. Include communication and valve travel where relevant. A fixed percentage of tank height does not demonstrate adequate response time.
Proof-test procedures and intervals belong to the applicable standard, equipment instructions and documented risk assessment. Avoid inventing one annual or quarterly interval for all tank gauges. Retain the test scope, results, defects and corrective actions.
Select a gauge for its actual role
Free-space radar, servo gauging, guided-wave radar and other technologies serve different tank arrangements. A radar advertised for process level is not automatically approved for custody transfer or a safety-instrumented function. Verify the exact approval, options and installed system.
Servo systems have moving measuring components and their own maintenance needs. Hydrostatic systems depend on the pressure reference and representative density. Interface-capable guided-wave radar adds contact with the product and has limits on layer properties, thickness and probe arrangement.
Low-reflection oils do not always require a stilling well. Check the chosen gauge's application limits and echo performance. If a well is used, its diameter, straightness, slots, joints, liquid exchange and roof clearances must match the gauge's requirements; an arbitrary pipe can introduce new errors.
Floating roofs need a fixed measurement reference
Conventional tank gauging uses a defined fixed reference and an approved arrangement that can measure the liquid while avoiding interference from the roof. A suitable stilling pipe may provide access through the roof. Mounting an ordinary gauge on a moving roof and measuring the small distance below it does not, by itself, give absolute tank level.
Roof position is not always identical to liquid level: a landed, tilted or mechanically constrained roof is an obvious exception. Account for floating-roof displacement in the capacity calculation where required. Check that the gauge path remains clear throughout roof travel, including low-level conditions.
Oil-water interface and emulsions
First define the required boundary: free water below oil, an emulsion transition or a water-content profile. Guided-wave interface measurement can work when the upper layer transmits enough energy and the interface produces a usable response. A single universal dielectric-contrast threshold is not sufficient to select the instrument.
A rag layer does not have one sharp boundary. Some specialised instruments can characterise emulsions or profiles, but the quantity to report and its acceptance method must be agreed. Sampling may be part of that validation; it is not proof that every continuous method is impossible.
Build a system uncertainty budget
| Contribution | Evidence or control |
|---|---|
| Gauge and reference height | Installation survey and verification against an accepted reference |
| Tank capacity | Current calibration table and relevant structural corrections |
| Product temperature | Representative temperature measurement and applicable volume correction |
| Density or composition | Suitable measurement or analysis for mass calculations |
| Roof effects | Correct displacement treatment and unobstructed gauging arrangement |
| Data processing | Verified units, scaling, calculations and retained records |
Combine uncertainties using the applicable methodology rather than adding unrelated millimetres and percentages. State whether the result is observed volume, standard volume or mass. A level-sensor specification alone cannot establish the uncertainty of the delivered quantity.
Provide METRAVON with tank dimensions, roof type, products, operating limits, required quantity and the governing measurement and protection requirements.
FAQ
Can radar be used for custody transfer?
Yes, suitable approved gauging systems can be used where permitted. Verify the actual approval, tank installation, supporting measurements and required verification, not just the sensing principle.
Does API 2350 mean every tank needs the same two devices?
No. Establish the standard's applicability, tank category and required protection architecture. Where independence is required, two outputs from one transmitter do not provide it.
Can a gauge mounted on a moving roof report tank level?
Not from a short roof-to-liquid distance alone. Absolute level needs a fixed reference or a specifically engineered system that accounts for roof movement.
Can radar measure an oil-water interface?
Some guided-wave instruments can, within their interface-measurement limits. Surface radar does not automatically locate a submerged boundary, and emulsions need a defined measurement objective.
Is a stilling well mandatory for gasoline?
No. Its need depends on the gauge and installation. Both open-space and well-based arrangements require verification for the actual product and operating range.
How often should gauges and alarms be tested?
Use the applicable approval, equipment instructions, operating experience and documented risk assessment. Define the complete test scope and retain the results rather than adopting a generic interval.
Technical references
Related applications
Radar Level Transmitter for Chemical Tanks: Materials, Dielectrics and Traps | Cryogenic Level Measurement: LNG, Liquid Nitrogen and Liquid Oxygen Tanks
