Understanding Tank Overfill Protection Systems and Their Uses

Tank overfill protection systems are essential in preventing hazardous liquids from overflowing when loaded into cargo tanks. But as necessary as they are, they can lead to issues. Continue reading this article to learn about overfill system troubleshooting for common problems like pump failure and terminal fault.

What Is a Tank Overfill Protection System?

An overfill prevention system is designed to quickly and efficiently shut off flows when spillage is detected. These monitoring safety systems are ideal for both top-loading and bottom-loading tank trucks equipped with sensors.

Tank overfill protection systems work jointly with filling terminal mechanisms to provide a secondary means of shut-off when overfill is detected. Systems use pulsed electronic signals to gauge the liquid level in a tank and shut it off to avoid any dangerous and costly situations like pump failure, human error, or terminal fault.

Why Are Tank Overfill Protection Systems Essential?

Tank overfill prevention systems promote the safety of people, trucks, and cargo. They also lower commissioning times and costs and reduce maintenance costs by helping avoid serious problems with your truck.


Tank overfill protection systems are commonly utilized on tanker trucks to detect the overfilling of:

  • Petroleum products
  • Water and wastewater
  • Food grade dry or liquid bulk

The Elements of an Overfill Protection System

Overfill protection systems typically involve four essential components—an electronic sensor, bottom sensors, an electric socket, and a rack monitor. Typically attached to manhole covers, electronic sensors detect dry or wet conditions at the top of a storage tank.

The bottom sensors of an overfill protection system are used to recognize when a tank is empty or near empty. Electronic sockets are installed on the exterior bottom of tankers. They are electronically connected to the interior sensors and include a physical grounding for the prevention system.

The rack monitor component is connected to the tanker socket by cables. Monitors like Dixon Bayco’s continuously track each tank overfill protection system’s sensors to allow for liquid pumping. When an upper sensor detects wetness, it immediately sends a signal to shut down rack pumping equipment. Likewise, if bottom sensors are wet, they prevent loading while at a filling terminal.

Having trouble with your tank overfill protection system? Reach out to White Tank & Truck Repair for our testing and diagnostic services.

The oil truck tankers in the refinery background

Overfill System Troubleshooting

There are several reasons compartments can overfill, including:

  • Terminal fault
  • Metering device or pump failure
  • Terminal operators or drivers attempt to fill compartments already filled
  • Tankers return to terminals with partial loads
  • Electrical failures of the system and voltage issues

Let’s explore overfill system troubleshooting solutions:

Terminal Fault

Terminal fault is a common source of blame—the truth is, most often, it is not the cause of tank overfill protection system failures. If you believe terminal fault is the cause of any issues, you should contact the terminal directly.

If you or your drivers can’t get the green light needed to begin filling, it could be faulty monitors, flooded probes, sockets the rack plugs into, or wiring issues. Finding a repair shop with experienced technicians that use a sophisticated testing system and run diagnostics is a great way to narrow down the cause of the problem quickly.

If testing and diagnostics identify failed components, professionals can replace almost any. Top probes, monitors, and sockets used by loading rack equipment are easily worn or loosened from the weight of rack plugs. Insufficient voltage to monitors is another problem testing can pinpoint the cause of.

Metering Device or Pump Failure

If a metering device is the source of your tank overfill protection system’s problems, there are several likely causes. Often, a meter issue arises from the loss of connectivity to the truck cab’s printer, loss of constant power to the system, or memory failure on a control board.

If connectivity to the printer is the problem, tickets in the meter are likely shown as needing to be printed. This error stops the system because tickets are caught and need to be cleared out. The simple solution is fixing the wiring to the printer or on the printer itself. Though less common, mechanical problems are also possible. Mechanical issues include check valves, malfunctioning air eliminator parts, and leaking diaphragm solenoids.

Pump failures, whether PTO or hydraulic driven (from wet kits), can be diagnosed. A common problem that leads to pump failure is bypass reliefs coming out of adjustment. This causes the bypass to “back itself off” and lead the pump to relieve the pressure internally and bypass back into the pump.

Another pump failure cause is leaking mechanical seals, which can be quickly and efficiently repaired. Vanes can be worn, and housing can be grooved if a large amount of grit is not caught by a screen and pumped through the pump. While vanes can be repaired, badly grooved housing may need to be replaced.

If the pump runs too fast with no product to lubricate the vanes, there’s a good chance a PTO was engaged while driving. In most cases, this leads to grooved housing or overheating bearings. The pump has to be disassembled to determine if it is repairable.

Terminal Operators or Drivers Attempt to Fill Already Full Compartments

Virtually all loading racks require a tank to have a verified outage point. This ensures there is enough room in the compartment and it doesn’t run fuel over the tank and cause a spill. The typical outage is 60 gallons. That’s because tanks fill so quickly that 60 gallons are needed for a tank overfill protection system’s electrical signal to shut down the valves at the loading dock.

Tankers Return to Terminals With Partial Loads

Most carriers have not adopted the “retain” portion of the loading system. The probes of a tank overfill protection system relay if a partial load is still on the bottom of the tank. Often, carriers don’t utilize this adoption because of the maintenance costs associated with caring for a second system that loading facilities don’t require.

Electrical Failures of the System and Voltage Issues

This is really the first thing to confirm with any electrical component on a truck or trailer. Tank overfill protection systems are very sensitive to voltage going out of the ECU and to the probes. They are built to operate with less than 12 volts, but a steady power as close to 12 volts is always a target to consider when continuing to diagnose electrical failures.

Experiencing Tank Overfill Protection System Issues? Turn to White Tank & Truck Repair

For over 30 years, White Tank has been Missouri, Kansas, and southern Illinois’ go-to source for tank and truck services. We can efficiently diagnose and repair issues if you’re having problems with your tank overfill protection system. From pump failures to terminal faults, there’s no issue too complicated or complex for our experienced technicians. Reach out today to schedule an appointment and get your truck back on the road with minimized downtime.