What is a self-priming pump?
A self-priming pump is a centrifugal pump designed with a mechanism that helps remove air from the pump before normal pumping starts. The pump derives its name from its ability to remove air from a pump known as “priming.” The priming process in non-self-priming centrifugal pumps involves filling the pump and suction pipe with the liquid being pumped to help remove any air or gas from inside. Why is priming necessary? Okay, a pump must be primed to help create a vacuum when the pump is started. When there is air/gas in a pump, the pump will not be able to run its impeller and create enough vacuum to force fluid from its storage tank into the pump.
Self-priming pump manufacturers design this pump with a larger casing to help hold enough fluid for priming. This pump helps relieve the user of the tedious priming process common in non-self-priming pumps.
How does a self-priming pump work?
An electric motor powers a self-priming pump. When the pump is turned on, the pump impeller starts rotating. Due to the impeller spinning, a vacuum (a pressure lower than atmospheric pressure) is created at the center of the impeller (eye of the impeller). Due to the vacuum, the liquid moves from storage into the pump to enhance pressure balance. The difference in pressure (between the atmospheric pressure and vacuum) pushes the air in the suction pipe and inside the pump, forming a liquid ring around the impeller. The ring creates an air-tight seal, which prevents air from the discharge end going back into the suction side.
Air and liquid mixture move up the pump towards the discharge end, where the air exits while the liquid moves back to the reservoir via gravity. The liquid again mixes with any remaining air, and the mixture moves up the pump towards the discharge port, where the air moves out and the liquid goes back to the reservoir. The process repeats until all the air is removed from the pump. After all the air is removed, the pump starts the normal liquid transfer. When the pump is turned off, it retains some of the liquid, which is used to prime the pump the next time it starts.
Figure: Working of a self-priming pump.
Advantages of a self-priming pump
- They are easy and quick to prime. A self-priming pump does not need the user to do manual priming like in non-self-priming pumps since the pump is designed to prime automatically. This feature becomes necessary, especially where the pump has to be turned on and off frequently. It also becomes an inevitable pump when the suction lift is too large to fill with the liquid.
- It handles air, gas, and vapor. This pump can move out any air/gas/vapor, eliminating any worry of air build on the impeller.
- Unlike the non-self-priming pumps, these pumps are suitable for installation where there is no continuous fluid flow.
- Versatile in application. These pumps are designed for use in various fluids, such as wastewater, chemicals, and water.
- Cheaper to maintain. The pump’s impeller does not run dry, unlike in other pumps. This helps to reduce chances of wear and unnecessary costs on replacement.
- They are portable pumps. These pumps are easy to transport from one place to another. This makes their use very common in agriculture and construction sites.
- Ability to handle viscous fluids. They can transport viscous fluids without suffering cavitation and other performance challenges.
- Variety of sizes. Self-priming pump manufacturers design these pumps in different sizes and dimensions. This makes it possible to have pumps for different flow rates and heads.
- They are highly reliable and durable. They are not designed to rely on other accessories like suction valves to prime, which may fail, and are thus highly reliable. These pumps also have a long service life, mainly if used according to the manufacturer’s instructions.
- They are highly efficient. These pumps convert most of the power to pressure and mechanical energy necessary for transporting the fluid to be pumped. The high efficiency also helps to reduce operating costs and lifecycle expenses.
Disadvantages of self-priming pumps
- Priming. The pump has to be primed manually when it is new or if it has stayed for long without being used.
- Some of these pumps cannot handle solids and are thus only suitable for clean fluids.
- They are more expensive than the non-self-priming pumps of the same capacity.
Maintenance tips for a self-priming pump
- Perform a visual inspection of the pump to check on leakage and unusual noise. Also, check if the impeller is rotating freely.
- Check the bearings are adequately lubricated.
- Check on alignment and vibrations. Ensure the pump shaft is aligned correctly to prevent overheating and breakdown.