What is priming?

Short Answer:

Priming is the process of removing air, gases, or vapors from a pump and its suction pipe before starting the pump. This is mainly required in centrifugal pumps because they cannot pump air or vapor and need to be completely filled with liquid to start proper operation. Without priming, the pump will not lift the liquid and may get damaged due to dry running.

Priming ensures that the impeller and suction line are full of liquid, allowing the pump to develop the required suction pressure and discharge water efficiently. It is an essential step for the safe and efficient working of a centrifugal pump and helps avoid mechanical damage or loss of efficiency.

Detailed Explanation :

Priming

Priming is an important process in the operation of a centrifugal pump. Before a centrifugal pump starts working, all the air or gases present inside the pump casing and suction pipe must be removed. This removal process is known as priming. The pump and suction pipe are filled with the liquid to be pumped, which ensures that the pump can create the required suction pressure and lift the fluid from the sump.

Centrifugal pumps work based on the principle of converting mechanical energy into kinetic energy and then into pressure energy. However, this conversion can only happen when the impeller of the pump is surrounded by liquid, not air. Air or vapor cannot be handled effectively by centrifugal pumps because they cannot create the necessary vacuum in the suction pipe. Therefore, priming is a must before starting the pump.

Need for Priming

When a centrifugal pump is started without priming, the impeller rotates in air, and since air is much lighter than liquid, it cannot produce sufficient suction pressure. As a result, no liquid enters the pump, and the pump runs dry. This may cause friction, overheating, and damage to the internal parts of the pump such as the impeller, shaft, and bearings.

Therefore, priming ensures the following:

  1. The pump casing and suction line are completely filled with liquid.
  2. The impeller is surrounded by liquid, allowing smooth suction.
  3. The pump can deliver liquid without delay.
  4. It prevents damage due to dry running.

Methods of Priming

There are several methods used to prime centrifugal pumps depending on their design and working conditions:

  1. Manual Priming:
    In this method, the liquid is poured manually into the pump casing and suction pipe through a funnel or filling pipe until all the air is expelled. This is a simple method suitable for small pumps.
  2. Foot Valve Method:
    A foot valve with a strainer is fitted at the end of the suction pipe and submerged in the liquid source. The suction pipe and casing are filled with liquid before starting. The foot valve prevents the liquid from flowing back, keeping the pump ready for operation.
  3. Vacuum Pump Priming:
    A small vacuum pump is used to remove air from the suction line and pump casing. Once the vacuum is created, the liquid from the source rises and fills the suction pipe and pump casing automatically.
  4. Self-Priming Pumps:
    These pumps are designed in such a way that they can automatically remove air and fill the suction pipe with liquid. They have special design features like air separation chambers that help in automatic priming.
  5. Jet Priming:
    In this method, a jet of water under pressure is directed into the suction pipe through a nozzle, which creates a vacuum and helps draw the liquid from the source into the pump casing.

Importance of Priming

  • Ensures smooth starting of the pump.
  • Prevents air locking and cavitation.
  • Avoids mechanical wear due to dry running.
  • Increases the efficiency and lifespan of the pump.
  • Maintains continuous and steady flow of liquid.

Without proper priming, the centrifugal pump cannot develop the required discharge head and may fail to deliver liquid. Therefore, operators must always ensure that the pump is fully primed before switching it on.

Applications

Priming is commonly required in all centrifugal pumps used in water supply systems, irrigation, industries, and power plants. In large installations, automatic priming arrangements are often provided to save time and reduce human effort.

Conclusion

Priming is a vital step in the operation of a centrifugal pump that ensures efficient performance and prevents mechanical damage. It removes air or gases from the pump and suction pipe, allowing the pump to lift and deliver liquid smoothly. Without priming, the pump cannot function correctly and may suffer from overheating or reduced efficiency. Therefore, proper and timely priming should always be carried out before starting any centrifugal pump.