In tank truck loading, overfill protection should be a loading permission system, not only a warning light. A modern controller checks whether the tanker, probe circuit and grounding condition are ready before product transfer starts, and it continues monitoring these conditions throughout loading.

This type of design is widely used in mature loading safety systems. The controller works as the safety decision point between the tanker and the loading rack: if all conditions are normal, loading is permitted; if any unsafe or unknown condition appears, loading is stopped or locked out.

Main Control Logic

A complete overfill prevention controller normally checks several conditions at the same time:

Whether the tanker plug is correctly connected to the loading socket

Whether each overfill probe circuit is detected correctly

Whether all monitored compartments are in dry status

Whether there is an open circuit, short circuit or abnormal signal

Whether static grounding is verified, if grounding protection is integrated

Whether the controller output is ready to provide loading permission

Only when these conditions are satisfied should the controller provide a permissive signal to the loading system.

Probe Monitoring and Self-Checking

The probe is installed at the high-level point of each tanker compartment. During normal loading, the probe remains dry. When liquid reaches the probe, its signal changes and the controller immediately removes loading permission.

For reliable protection, the controller also needs to check the health of the circuit. If the probe is disconnected, wiring is damaged, the plug/socket contact is poor, or the signal is outside the expected range, the controller should treat the condition as unsafe.

This fail-safe logic is important. The loading system should not continue simply because the controller cannot read the tanker probe correctly.

Grounding and Connection Verification

In hazardous liquid loading, overfill prevention is often combined with static grounding verification. The controller can confirm that the tanker is properly connected and grounded before allowing loading.

If grounding is lost during operation, or if the tanker connection becomes abnormal, the controller removes the permissive output. This helps reduce both spill risk and ignition risk at the loading rack.

Interlock with Pumps, Valves and Batch Control

The controller output can be connected to the site control system as a loading permissive. Typical interlock points include:

Pump start circuit

Loading valve control

Batch controller permissive input

Emergency shutdown or loading stop logic

Audible and visual alarm devices

When all safety conditions are normal, the controller allows loading. When a wet probe, wiring fault, poor connection or grounding fault is detected, the controller changes state and the loading process is stopped or prevented from starting.

Clear Status for Operators

A practical controller should give operators clear status information, not just a single alarm. Useful indications include:

Ready-to-load status

Wet probe alarm

Probe or wiring fault

Compartment/channel status

Grounding status

Interlock output status

These indications help operators quickly distinguish between a real high-level condition and a maintenance issue such as a damaged cable or poor socket contact.

ALPTEC Solution

ALPTEC provides overfill prevention and grounding verification solutions for tanker loading applications.

FlexPerm 2.0 is designed to monitor tanker probe status, verify loading conditions, indicate compartment status and output alarm/interlock signals to help prevent unsafe loading. ALPTEC also offers SLA-S-Y-EN and SLA-S-Y-API systems, optical sensors, sockets and grounding accessories for complete loading bay protection.

Conclusion

An overfill prevention controller is the core of the loading safety chain. It continuously verifies probe status, circuit integrity, tanker connection and grounding condition, then sends a permissive or shutdown signal to pumps, valves or the batch controller.

By combining detection, self-checking and interlock control, the system helps stop loading before overfill occurs and supports safer tank truck loading operations.