What causes a self-priming pump to suddenly lose suction?
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The pump is used to pump wastewater from underground tanks in the tank area. It has a head of 85 meters and a flow rate of 15 m3/h. The outlet pressure of the pump gradually decreases; it drops to around 8 kilograms before falling further to 5 or 6 kilograms, resulting in a vacuum condition. It is initially suspected that the filter is clogged, as this issue has occurred frequently before. Since the wastewater contains impurities, the filter was removed only to find that there weren’t many impurities present. After filling the pump several times and restarting it, it operated for an hour before developing a vacuum condition again. After operating for half an hour, the pressure dropped slowly to 5 or 6 kilograms, after which a vacuum condition arose once more. Subsequent attempts to start the pump by filling it again failed, and it only started after pressurizing it directly into the underground tank. Even after releasing pressure and trying to start it again, it still wouldn’t work
What could possibly be the reason for no liquid being drawn out?
The only reason is a clogged pump inlet, or air leakage. For reference.
I. Blocked inlet 1. Clogged filter； 2. The liquid level is low; excessive sediment at the inlet prevents liquid from being drawn out ； 3. The water temperature is high; the required net positive suction head at the inlet is insufficient, so cooling is achieved by watering ； II. Pump issues – Disassemble the pump for inspection: 1. Scaling on the pump impeller; 2. Excessive clearance in the mouth ring; 3. Debris present at the pump inlet; 4. Cavitation damage to the pump blades
I infer there might be four reasons: 1. Air leakage in the suction pipeline or pump (especially at static seal points or mechanical seals)； 2. The back pressure at the outlet decreases, resulting in an increased pump flow rate and a worse net positive suction head ； 3. The outlet valve is opened too wide, resulting in excessive pump flow and a reduced net positive suction head ； 4. Due to particle wear, the gap between the impeller and the pump body increases, resulting in a reduced self-priming capacity.
When a self-priming pump draws liquid from an underground tank, a certain amount of nitrogen is introduced to maintain pressure; otherwise, as the liquid level drops and the volume inside the tank increases, the pressure in the tank falls, causing the inlet pipeline of the pump to lose suction and the pump to empty out. It is necessary to check the instructions for the self-priming pump to find out what the suction capacity is. Otherwise, if the pipeline is long or large, the suction volume will not be sufficient to cover the volume of the tank. There is already a certain amount of static pressure due to the height of the water column, and even after prolonged operation, it is not certain that the pump will be able to draw liquid from the tank
Everyone’s views are similar; there are three main reasons for this: First, cavitation in the pump prevents liquid from being drawn in (1. Foreign objects blocking the inlet, 2. Scaling in the pump’s inlet pipes, which increases resistance, 3. High outlet flow rates resulting in fast inlet velocities and increased pipe resistance, 4. Rising liquid temperature)； II. Air entrapment: Air leaks into the pump through the feed pipe ； III. Problems with the pump itself (1. Large intermittent gaps in the sealing ring, 2. Corrosion and damage to the impeller, 3. Blockage by foreign objects).
The medium and design are relatively light, causing vapor cavitation in the pump, which results in an inability to draw fluid up. Solution: Change the inlet pipe to reduce resistance, or pressurize the tank; this requires redesign
Did this pump work properly before? If it was operating normally before and there were no changes in the material composition, it is possible to consider whether there is a fault with the pump itself. If the composition changes, it is necessary to recalculate whether the required net positive suction head at the pump inlet is sufficient. Furthermore, if it was already like this from the start, as the liquid level drops and the gas space in the container decreases, is a negative pressure created? Is it the insufficient suction pressure of the pump?
If the new pump components are worn out, check whether there is air leakage in the inlet pipeline; this can lead to an increasing amount of gas, thereby reducing the pump’s performance. Additionally, if the filter in the inlet pipeline is clogged and the flow rate is not adequate, it can cause more severe cavitation, which also results in a decline in the pump’s performance
The possibility of a problem with the remote level gauge was considered, but it was ruled out; everything is normal during dehydration in the dehydrator as well as when pressure is used to push the fluid out