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Why did the pump reverse after winter diesel tanker shut down?
In winter, maintenance staff often encounter issues where the dispenser pump appears to hang or rotate in reverse after being turned off. This problem is puzzling for repair crews, as they are unsure of the cause and worried it might interfere with the tanker’s normal operation.
So why does this happen? It all comes down to how the relief valve functions within the hydraulic system. The relief valve acts as a gate between the high-pressure chamber and the low-pressure chamber of the oil pump. Its main purpose is to regulate the maximum pressure in the system, protecting it from damage, while also helping control the dispenser's output.
When the motor starts and the pump is running, pressurized oil flows from the high-pressure side of the pump into the oil separator inlet and the left side of the relief valve. If the oil gun is open, the oil flows through the system, and the relief valve remains closed. However, when the oil gun is closed, the pump can’t release the oil from its outlet, causing pressure in the system to rise quickly.
At this point, the pressure overcomes the spring force inside the relief valve, pushing the valve spool to the right and opening the valve. This allows the high-pressure oil to flow back into the low-pressure chamber of the pump, creating a circulation loop.
But during cold weather, diesel fuel becomes more viscous—commonly referred to as "waxing." This increased viscosity slows down the oil flow, especially through the relief valve, creating higher resistance. After the pump stops, it may still spin due to inertia, but the oil can't escape through the outlet or pass smoothly through the relief valve. As a result, pressure builds up rapidly, and the high-pressure oil exerts a reactive force on the rotor via the blades, causing the pump to reverse slightly.
This phenomenon is more common in winter, and understanding the role of the relief valve is key to preventing or addressing the issue effectively.