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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's test machine hangs down after the pump stops. The pump sometimes reverses rotation before coming to a complete stop, which worries the repair teams as they are unsure of the cause and concerned it might affect the tanker's normal operation.
So why does this happen? It all comes down to how the relief valve functions. The relief valve acts as a gate that connects the high-pressure chamber of the oil pump to the low-pressure chamber. Its main purpose is to limit the maximum pressure in the hydraulic system, protecting it from damage, and also helps regulate the dispenser’s output.
When the motor starts and the pump is running, pressure oil from the high-pressure side of the pump flows into the oil separator’s inlet and the left side of the relief valve. If the oil gun is open, the pressure oil moves into the oil separator, and the relief valve remains closed. However, when the oil gun is closed, the pump can’t release the oil through its outlet, causing the system pressure to rise quickly. This increased pressure overcomes the spring force inside the relief valve, pushing the valve spool to the right and opening the valve. The high-pressure oil then flows through the opened relief valve back into the low-pressure chamber, creating a circulation within the pump.
During cold winter temperatures, diesel oil becomes more viscous, commonly referred to as "waxing." This increased viscosity makes the oil flow more slowly through the relief valve, increasing resistance. After the pump stops, the inertia of the pump keeps it rotating for a short time. But since the oil can't exit the pump outlet or pass smoothly through the relief valve, the pressure builds up rapidly. This high pressure exerts a reactive force on the rotor through the blades, causing the pump to reverse direction.