What type of pump is used for the external liquid level control in underground tanks?

For underground tanks, I have only seen two types of pumps: one is called a submersible pump, and the other is called a self-priming pump. It seems that the use of self-priming pumps has been on the rise in our factory in recent years; however, their specific advantages and disadvantages are not very clear. The advantage of submersible pumps is the absence of the problem of NPSH; the disadvantage is that the pump shaft is long, and vibration issues are difficult to resolve. I have really never had hands-on experience with self-priming pumps; their advantages and disadvantages are likely the opposite of those of submersible pumps. I recently witnessed the first start-up of a self-priming pump: after filling the pump, it was started up, but no pressure was generated at the pump’s outlet for a long time. They said it takes about 10 minutes to fill the inlet pipeline with water; I don’t quite understand this situation.

Here, for underground tanks or ponds, we use submersible pumps as well as self-priming pumps! As for the advantages of submersible pumps, in my experience I’ve never encountered a situation where they failed to deliver the required flow rate; they haven’t needed repair yet, and I think repairing them is more complicated compared to self-priming pumps! It seems a bit troublesome the first time a self-priming pump is used. Here, with the manufacturer’s guidance, we put one in operation; it took almost an hour before it started working properly. The main issue was filling the water tank at the inlet! We suggested filling the tank with water first before starting the pump, but the manufacturer said their pumps did not require priming and could be started right away; however, after much hassle... it finally started working! When using the manufacturer to go out for food, we fill the water tank ourselves, close all the outlets, and the flow rate increases immediately; then we gradually open the outlet valves to ensure a normal flow rate! The same principle applies to the other units as well; it’s sufficient to ensure that the outlet pressure remains at an appropriate level, so that the outlet cannot be opened too wide!

Underground tanks generally use strong self-priming pumps to transfer liquids; ordinary centrifugal pumps are unable to draw up the liquid due to insufficient static pressure

Submersible pumps are simple to operate; there is no need to fill them with fluid, no concern is required regarding freezing, and they are less prone to leakage. However, they are difficult to repair. When operating a self-priming pump, it is necessary to fill the pump; when not in use, it must be protected from freezing. Attention should be paid to seal leaks, and the pump should be easy to maintain.

We don’t have underground tanks; after communicating with the design institute, they suggested connecting temporary hoses to temporary containers – but there’s nothing we can do about it.

If it’s an underground oil tank, the use of submersible pumps is prohibited; CNPC seems to have regulations regarding this!

We use long-shaft submersible pumps here; the advantage is that they are easy to start up and do not require priming. No disadvantages have been observed so far during use. However, when our company hired a third-party firm to conduct a HAZOP analysis, they submitted a report recommending corrective actions, stating that there have been numerous incidents in China where fires and explosions occurred as a result of long-shaft submersible pumps being used to transport dirty oil and wastewater. Does anyone know of similar cases?

Adding a buffer tank before the inlet of the centrifugal pump will solve the problem

Submersible pumps deliver fluid under positive pressure, while self-priming pumps deliver fluid under negative pressure. Submersible pumps have a low failure rate and require high sealing performance of the pipes ； Suction pumps have more faults compared to submersible pumps; they are prone to air blockages and an inability to draw up liquid. Additionally, submersible pumps generally have a longer delivery distance, while self-priming pumps have a shorter one.

Repairing submersible pumps is complicated, as the pump needs to be lifted out for maintenance