Self-Priming Pumps V Non-Self-Priming Pumps: The Difference

What is pump priming?

When a pump is turned off, its interior becomes full of air and water. When the pump starts again, the air and water inside will move together, leading to a low density of the fluid. As such, creating a vacuum inside the pump becomes impossible. The air inside the pump must be removed to ensure a vacuum is made; thus, it is possible to suck the fluid from the storage tank into the pump. The air is usually removed by filling the pump with liquid being pumped. The process of filling the pump with water/or liquid to be pumped to remove any air/gas/vapor is called pump priming. Priming is usually done before the pump starts regular fluid pumping.

In case the pump is started before priming, it will malfunction. Pump malfunctioning from not being primed will cause an overall delay in production. If the pump is operated long without being primed, it will cause unpredicted damages and, eventually, pump failure.

Figure: Pump priming.

What is a self-priming pump?

A self-priming pump is designed with unique capabilities that allow it to prime itself until all the air/vapor/gas is entirely removed. One of the critical characteristics of a self-priming pump is a relatively large casing with a reservoir, unlike other pumps.

Figure: Self-priming pump.

How does a self-priming pump work?

A self-priming pump removes air/gas/vapor by rotating the impeller. Due to the impeller rotation, a vacuum is created at the impeller eye. Due to this vacuum, air is sucked from the suction line. Also, a cylindrical ring of the fluid being pumped is sucked into the pump from the storage tank. This leads to a gas-tight seal, which stops air/gas/vapor from moving from the discharge port to the suction line. The air/gas/vapor is then trapped in the fluid within the impeller’s vanes and transferred to the pump exit port while the liquid returns to the pump housing via gravity. The process repeats until all the air is removed from the pump and suction line. Normal fluid pumping starts once all the gas/air/vapor is released.

Centrifugal pump manufacturers have designed self-priming pumps such that when the pump is turned off, the priming chamber stores some fluid, which is then used to prime the pump when required to transfer liquid.

Figure: Working of a self-priming pump.

What is a Non-self-priming pump?

A non-self-priming pump is a pump that needs the user to remove air/gas/vapor from the pump and suction line by filling the pump with liquid to be pumped. Without removing the air, the pump cannot generate the vacuum that the pump needs to transfer fluid to the required destination. This type of pump is used primarily where there is steady flow of liquid, such as in circulation, water treatment, and chemical processing systems. One main characteristic of this pump is that it lacks a special chamber or mechanism like in self-priming pumps.

Figure: Non-self-priming pump.

Difference between self-priming and non-self-priming

  • Self-priming pumps have a priming chamber that ensures the pump primes itself when turned on. On the other hand, the non-self-priming pumps lack this chamber or mechanism. As such, the user must always prime the non-self-priming pumps; otherwise, the pump will not create a vacuum enough to initiate routine pumping.
  • Non-self-priming pumps lack the chamber for self-priming. This design makes the pump much cheaper relative to the self-priming type. The pump becomes complex in design due to the priming mechanism in a self-priming pump.
  • Non-self-priming pumps are easier and cheaper to repair and do maintenance. Self-priming pumps are more expensive to repair and do maintenance due to the priming chamber and mechanism.
  • Self-priming pumps are more user-friendly, unlike the non-self-priming type. This is so especially where frequent priming is necessary, meaning more effort and time are required. This also makes the self-priming type more efficient than the standard centrifugal pump.
  • Unlike the non-self-priming pumps, a self-priming pump does not need a foot valve since the fluid reservoir does not allow air removal from the pump during the priming cycle.
  • Self-priming pumps are suitable for use where the pump is mounted above the fluid to be pumped since it can prime itself. On the other hand, the non-self-priming pump must be primed before normal water pumping if installed above the pump fluid.
  • Self-priming pumps take up to 45 seconds to prime. On the other hand, non-self-priming pumps take longer depending suction distance