Where are self-priming pumps used, and where are submersible pumps used?

Generally, it is better to use a self-priming pump whenever possible, as submersible pumps are prone to problems and are difficult to maintain. Self-priming pumps are commonly used for media such as wastewater and rainwater. For slurry fluids that tend to clog pump casings and the liquid storage tanks of self-priming pumps, submersible pumps are the best choice. When NPSHa is insufficient and it is not convenient to lower the pump height, a submersible pump can also be used, taking advantage of its feature of having an impeller at a low position.

Whether it’s above or below the liquid level, the principle is the same; there is no fundamental difference. The only thing is that submersible pumps do not require manual filling – they can operate using their own liquid level

We have just installed one self-priming pump in our facility to replace the original submersible pump; its installation is not yet complete. This process will basically no longer be needed, so there won’t be any opportunity for trial operation. There are still several submersible pumps left.

Self-priming pumps are generally used; it seems there was an accident involving submersible pumps at Sinopec, so it’s better to opt for self-priming pumps

I. Long-shaft submersible pump (API pump type VS4) Working principle: The working pump head is submerged in the liquid; the motor is connected to the pump shaft via a coupling, which drives the impeller to rotate. The blades force the liquid to spin, and the spiral flow channels in the pump body convert kinetic energy into pressure energy, allowing the liquid to flow out through the outlet. Advantages: Suitable for high head, high temperature, and space-constrained applications. Disadvantages: Long pump shaft, making maintenance difficult ； There are many failure points, making them difficult to detect ； The long shaft is supported by sliding bearings, which provide wear resistance in multiple areas; external or self-flushing is possible. Due to damage to these sliding bearings, flash explosions occurred at Liaoyang Petrochemical and Dagang Petrochemical. As a result, CNPC prohibits the use of long-shaft submersible pumps in locations where safety hazards exist and flash explosions are likely to occur. II. Ordinary self-priming pump: Working principle – It belongs to the external mixing type of centrifugal pump. Before the first start-up, the pump chamber needs to be filled with liquid; as the impeller rotates, it creates a vacuum in the liquid storage chamber, which draws out the air from the pipeline and allows the fluid to be drawn in. Advantages: High efficiency, easy to maintain, high reliability. Disadvantage: Leakage is likely to occur at the shaft seal of the auxiliary impeller ； It has a long self-priming time and low efficiency; heating is required in winter, or the fluid in the storage chamber must be drained. III. Internal recirculation self-priming pump – Working principle: When starting the pump, it is necessary to open the recirculation valve located at the front and lower part of the impeller, allowing the fluid inside the pump to flow back to the inlet of the impeller. Under the high-speed rotation of the impeller, this fluid mixes with the air coming from the suction pipe, forming a mixture of air and fluid that is then sent to the separation chamber. There, the air is discharged while the fluid returns to the inlet of the impeller through the recirculation valve. This process repeats until all the air is removed and the fluid can be drawn in. Advantages: The medium needs to be filled in for the first start-up of the pump, after which it is suitable for frequent start-stop operations. Disadvantages: long self-priming time, low efficiency, high energy consumption; outdoor use in winter requires heating or removal of residual liquid from the chamber. IV. Self-priming pump: Working principle – Before the pump is started, there is a certain amount of liquid medium inside the pump. Once it starts operating, the rotation of the impeller causes the liquid to be thrown into the pump body under the effect of centrifugal force; this liquid then returns to the inlet of the impeller through the return chamber and enters the circulation process. As the liquid continues to circulate, the air present in the suction pipe is gradually incorporated into the circulating liquid stream, forming a gas-liquid mixture. This mixture is then carried by the impeller to the inlet of the gas-liquid separation chamber within the pump, where it is recycled. This process continues until all the air in the suction pipe has been removed, after which the pump can operate normally. Advantages: High efficiency, good cavitation resistance and self-priming capability, few failures, easy maintenance, and simple operation. Disadvantages: Fluid needs to be filled in before first use. V. High-self-priming pump: Working principle: When starting the pump, the seal flushing valve is opened first to prevent dry grinding. The vacuum pump evacuates, allowing the medium to enter and fill the pump inlet tube and the pump chamber ； The float inside the floating chamber rises, closing the suction pipe; the clutch mechanism disengages automatically, the vacuum pumping device stops working, while the pump continues to operate normally. Advantages: Easy to operate ； Easy to maintain. Disadvantage: The pump operates, and the vacuum suction device runs simultaneously, resulting in high energy consumption ； Sealing requires external flushing, is prone to freezing, and the seal can easily fail ； Poor safety performance. VI. High-efficiency explosion-proof self-priming pump – Working principle: When the pump is started, the diaphragm pump creates a vacuum in the inlet pipe and within the pump chamber ； The medium enters and fills the pump inlet pipe and the pump chamber ； When the designed start-up liquid level is reached, it switches automatically, the pump starts up, the outlet valve is adjusted, and the pump operates normally. Advantages: Strong self-priming capability ； High efficiency ； Easy to operate ； Once the pump starts operating, the self-priming pump stops automatically, resulting in low energy consumption ； The pump stops operating, allowing for automatic drainage; no fluid remains in the pump chamber, so no anti-freezing measures are required ； The medium or gas does not come into contact with the rotating parts of the self-priming pump, and it can return to the medium tank, ensuring safety and reliability.

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In CNPC, several explosion accidents occurred due to malfunctions with self-priming pumps; as a result, CNPC’s headquarters ordered that submersible pumps be used instead. This is why submersible pumps are no longer used in explosion-proof areas. High-self-priming pumps work well for liquids that are clean and have a low density; however, they perform poorly when used with crude oil-like substances that are dirty, viscous, and have a high density. Several attempts to use high-self-priming pumps in such cases yielded unsatisfactory results.

Simply put, self-priming pumps are rather troublesome; I’ve used them several times, and one of the issues is the need to prime them, and sometimes they fail to draw in liquid due to blockages. Submersible pumps are more convenient and less prone to blockages, but since they are constantly exposed to water or acidic or alkaline liquids, they are more likely to suffer from corrosion and damage. There are advantages and disadvantages to each type, so it’s up to you to make the choice

In many applications, using long-shaft pumps is very dangerous; moreover, their maintenance is complicated, requires a lot of labor, and frequent repairs are needed. They are gradually being replaced by self-priming pumps

Self-priming pumps are unreliable and difficult to operate