Introduction to Self-Priming Oil Pumps

I. Definition

A self-priming pump is a centrifugal pump capable of automatically drawing up water without requiring the suction pipeline to be initially filled with water.

II. Working Principle

Self-priming pumps come in various structural types. Among them, the working principle of an external-mixing self-priming pump is as follows: Before starting, the pump casing is filled with water (or water is retained within the casing). Upon startup, the impeller rotates at high speed, causing the water in the impeller channels to flow into the volute. This creates a vacuum at the inlet, opening the suction check valve. Air from the suction pipe enters the pump, travels through the impeller channels, and reaches the outer periphery. Meanwhile, the water discharged into the gas-water separation chamber flows back to the outer periphery of the impeller through the left and right return water holes. Driven by the pressure differential and gravity, the water flowing back through the left return hole is injected into the impeller channels, where it is shattered by the impeller, mixes with the incoming air from the suction pipe, and is flung into the volute, flowing in the direction of rotation. It then merges with the water returning from the right return hole and continues to flow along the volute. As the liquid continuously impacts the blade cascade within the volute and is repeatedly shattered by the impeller, it vigorously stirs and mixes with the air to form a gas-water mixture. The continuous flow prevents the gas and water from separating. The mixture is peeled off by the volute tongue at the volute outlet and enters the separation chamber through a short pipe. In the separation chamber, the air is separated and discharged through the outlet pipe, while the water continues to flow back to the outer periphery of the impeller via the return holes to mix with the air from the suction pipe. This cycle repeats, gradually exhausting the air in the suction pipe until water enters the pump, completing the self-priming process.

The working principle of an internal-mixing self-priming oil pump is identical to that of the external-mixing type, with the sole difference being that the return water flows to the impeller inlet rather than the outer periphery. During startup, the return valve located below the front of the impeller must be opened to allow the liquid inside the pump to flow back to the impeller inlet. Driven by the high-speed rotation of the impeller, the water mixes with the air from the suction pipe to form a gas-water mixture, which is then discharged into the separation chamber. Here, the air is vented, and the water returns to the impeller inlet through the return valve. This process repeats until all the air is exhausted and water is successfully drawn up.

III. Usage Guidelines

The self-priming height of a self-priming pump is influenced by factors such as the front sealing clearance of the impeller, pump speed, and the liquid level height in the separation chamber. The smaller the front sealing clearance, the greater the self-priming height; it is generally set between 0.3 mm and 0.5 mm. If the clearance increases, not only does the self-priming height decrease, but the pump's head and efficiency also drop. The self-priming height increases with the circumferential speed (u2) of the impeller; however, once the maximum self-priming height is reached, further increases in speed will only reduce the self-priming time without increasing the height. Conversely, a decrease in speed will reduce the self-priming height. Assuming other conditions remain constant, the self-priming height also increases as the stored water level rises (though it should not exceed the optimal water storage height of the separation chamber). To facilitate better gas-water mixing, the impeller should have fewer blades to increase the blade cascade pitch. Additionally, semi-open impellers (or impellers with wider channels) are recommended, as they allow the return water to penetrate deeper into the blade cascade.

Most self-priming pumps are paired with internal combustion engines and mounted on mobile carts, making them highly suitable for field operations.

IV. Operation and Maintenance of Self-Priming Oil Pumps

1. Pre-Startup Preparation and Inspection

1.1 Check whether the lubricating oil in the bearing housing meets the specified requirements. The oil level should be maintained between the two marks on the dipstick or at the center line of the oil sight glass. It should neither be too high nor too low.

1.2 Ensure the liquid stored in the pump body is above the edge of the impeller. If insufficient, inject the stored liquid directly into the pump body through the filling port. Never start the pump with an insufficient liquid level, as this will prevent normal operation and easily damage the mechanical seal.

1.3 Inspect the rotating components for any jamming or collisions.

1.4 Check the pump base and all connections for looseness.

1.5 Verify the coaxiality and balance between the pump shaft and the prime mover's main shaft.

1.6 Check the inlet pipeline for air leaks.

1.7 Open the valve on the suction pipeline (Note: Do not open it completely).

2. Startup and Operation

2.1 Start the oil pump and verify that the direction of the pump shaft's rotation is correct.

2.2 Monitor for any abnormal noise or vibration during rotation.

2.3 Observe the vacuum gauge reading. After startup, once the readings on the pressure gauge and vacuum gauge stabilize following a period of fluctuation, it indicates that the pump has successfully drawn in the liquid and entered normal working condition.

2.4 Before the pump enters normal oil delivery operation, pay special attention to the rise in oil temperature inside the pump. If this process takes too long and the internal oil temperature becomes excessively high, stop the pump immediately to investigate the cause.

2.5 If high internal temperatures cause self-priming difficulties, temporarily shut down the pump. Allow liquid from the discharge pipeline to flow backward, or directly replenish liquid into the pump to cool it down before restarting.

2.6 If the pump stops unexpectedly during operation, slightly open the discharge control valve when restarting. This facilitates the discharge of air during the self-priming process and ensures the pump starts under a lighter load.

2.7 Every 15–20 hours of operation, or before each use, check and replenish the No. 10 mechanical oil as necessary.

3. Shutdown

3.1 Close the gate valve on the discharge pipeline.

3.2 Stop the pump.

4. Maintenance of Wearing Parts

4.1 Rolling Bearings: After prolonged operation, if the oil pump experiences excessive wear, the bearings must be replaced.

4.2 Front and Rear Sealing Rings: Replace the sealing rings when they reach a certain degree of wear.

4.3 Mechanical Seal: Under normal conditions without oil leakage, the mechanical seal should not be disassembled for inspection. However, if severe leakage occurs at the lower drain port of the bearing housing, the mechanical seal must be disassembled for inspection. When installing or removing the mechanical seal, handle it with care. Ensure the mating surfaces are clean, strictly avoid damaging the surface finish of the stationary and rotating rings, and never strike or collide with the components.