Overfill protection systems on tank trucks are legally mandated, routinely inspected, and widely trusted. But trust is not the same thing as verification. Across hundreds of fleet audits and field assessments, a consistent pattern emerges: operators make assumptions about their overfill protection that the evidence does not support. Here are five of the most common.
Assumption 1: "The float valve was tested at installation, so it's still working."
Mechanical float valves drift. The float mechanism is subject to wear, corrosion, and fouling from product residues, particularly in multi-product operations where different fuel grades leave different deposits. Industry data suggests that after 36 months of active service, a measurable percentage of float-type overfill prevention valves will not trigger at the correct fill level. Some will trigger too late. Some will not trigger at all.
The problem is compounded by the fact that many fleets treat overfill protection as a "fit and forget" system. The valve was certified when installed, it passed its last inspection, and there has been no incident. But the absence of an incident is not evidence that the valve would prevent one. Testing protocols exist (EN 13616 specifies performance requirements), but the frequency and rigour of in-service testing varies enormously between operators and between countries.
Assumption 2: "Our system handles all products the same way."
It does not, or at least it should not. Different fuel products have different densities, and density affects float behaviour. A float calibrated for standard diesel (density approximately 0.835 kg/l at 15 degrees C) will behave differently when the same compartment carries premium diesel, biodiesel blends, or heating oil. The density difference between winter-grade and summer-grade diesel alone can be enough to shift the trigger point by several centimetres in a large compartment.
For fleets operating multi-product vehicles (which is the majority in European fuel distribution), this is not an edge case. It is the normal operating condition. If the overfill protection system does not account for product density variations, it is operating outside its calibrated parameters on every delivery where the product differs from the calibration baseline.
Assumption 3: "The sensor is protected inside the tank, so fouling isn't a concern."
Sensors inside tank compartments are exposed to the product they measure. Over time, they accumulate residues. Paraffin wax from diesel, sulphur compounds, water condensation, microbiological growth in biodiesel blends: all of these coat sensor surfaces and affect measurement accuracy. Optical sensors are particularly susceptible to fouling because even a thin film on the lens can attenuate the signal enough to cause false readings or missed triggers.
The fouling rate depends on the products carried, the ambient temperature cycle (condensation is worse with large temperature swings), and the cleaning regime. Operators who carry biodiesel blends above B7 report noticeably faster sensor degradation compared to those carrying only mineral diesel.
Assumption 4: "Our drivers would report a near-miss."
Research across multiple safety-critical industries consistently shows that near-miss reporting rates are a fraction of actual near-miss occurrences. In fuel logistics, the gap is even wider because many near-miss events are invisible to the driver. A float valve that triggers 50 litres later than it should is still a successful stop from the driver's perspective. The delivery is completed, the tank did not overflow, and there is no visible indication that the safety margin was thinner than designed.
Even when drivers do notice something unusual (an unexpectedly slow valve response, a fill level that seems higher than normal, a sensor warning that appears and then clears), reporting is inconsistent. The reasons are familiar: time pressure, paperwork burden, concern about being blamed, and a culture where "it worked out fine" is treated as equivalent to "the system is working correctly."
Assumption 5: "Compliance equals safety."
This is perhaps the most deeply held assumption, and the most misleading. Regulatory compliance means your equipment meets minimum specified standards at the time of inspection. It does not mean the equipment is functioning optimally between inspections. It does not mean the system accounts for your specific operating conditions. And it does not mean the overall safety architecture is adequate for the combination of products, routes, sites, and conditions your fleet actually encounters.
Compliance is the baseline. It is necessary but not sufficient. Operators who treat compliance as the ceiling rather than the floor tend to discover the gap during an incident, not before one.
