What is priming of pump?

Short Answer:

The priming of a pump is the process of removing air, gases, or vapors from the suction pipe and pump casing and replacing them with the liquid to be pumped. This is done before starting the pump to ensure smooth and efficient operation. Without priming, the pump will not be able to lift the liquid from the reservoir because air does not create the required suction pressure.

Priming is essential for centrifugal pumps as they cannot handle air and require the impeller to be fully surrounded by liquid. If the pump is not properly primed, it may cause damage, noise, and reduced efficiency during operation.

Detailed Explanation :

Priming of Pump

The priming of a pump is an important preliminary step before starting a centrifugal pump or any other type of pump that cannot handle air. The main purpose of priming is to fill the suction line, impeller, and casing completely with the liquid to be pumped. This ensures that no air or gas remains inside, as the presence of air pockets can prevent the formation of suction pressure, making it impossible for the pump to lift fluid.

When a centrifugal pump is started without priming, the impeller rotates in air instead of liquid. Since air has a very low density compared to water, the centrifugal force generated is insufficient to create the necessary pressure difference between the suction and delivery sides. Therefore, the pump fails to lift the liquid.

Need for Priming

Priming is required mainly for centrifugal pumps because they work based on the principle of converting mechanical energy into kinetic energy and then into pressure energy. To perform this conversion, the impeller must be surrounded by liquid. Air cannot be handled by the impeller effectively, leading to cavitation, vibration, and loss of suction. Thus, proper priming ensures:

  1. Smooth flow of liquid into the pump.
  2. Prevention of air lock and cavitation.
  3. Safe start-up and efficient operation of the pump.
  4. Prevention of mechanical damage due to dry running.

Principle of Priming

The principle behind priming is simple. The pump and suction line must be filled with liquid so that when the impeller starts rotating, the liquid is thrown outward by centrifugal action, creating a low-pressure region (vacuum) at the impeller eye. This vacuum draws more liquid from the source into the pump through the suction pipe. If the pump is filled with air instead of liquid, no vacuum will form, and suction cannot occur.

Methods of Priming

Several methods are used to prime a pump depending on the type of installation and the height of suction lift. The most common methods include:

  1. Manual Priming:
    In this method, liquid is poured directly into the pump casing and suction line through a filling pipe or funnel until all air is expelled. It is used for small pumps where suction lift is low.
  2. Foot Valve and Priming Method:
    A foot valve is fitted at the bottom of the suction pipe, below the water level. This valve prevents the liquid from flowing back into the sump when the pump stops. Once the pump is primed for the first time, the suction line remains filled with water due to the foot valve, making subsequent priming unnecessary.
  3. Vacuum Pump Priming:
    A separate small vacuum pump is connected to the suction line to remove air from the pump casing and suction pipe. This method is used for large centrifugal pumps or systems with high suction lifts.
  4. Ejector Priming:
    In this method, an ejector or jet pump uses high-velocity water or air to create a vacuum that removes air from the suction line. It is efficient for large pumping systems.

Steps Involved in Priming

  1. Ensure the suction pipe and pump casing are properly connected and free from leakage.
  2. Fill the suction pipe and casing with liquid using a funnel or valve until air is fully expelled.
  3. Check that the foot valve is working properly to prevent the liquid from flowing back.
  4. Start the pump once the entire system is filled with liquid.

Importance of Priming

Priming is extremely important because failure to prime a pump can lead to several issues:

  • The pump will not lift liquid, resulting in dry running.
  • The impeller may overheat and get damaged.
  • Cavitation may occur, leading to vibration and noise.
  • Efficiency and discharge capacity are reduced.
  • Overall life of the pump decreases due to mechanical wear and tear.

Examples of Pumps Requiring Priming

  • Centrifugal pumps always require priming before starting.
  • Positive displacement pumps (like reciprocating pumps) do not require priming as they can handle air and self-prime.

Precautions During Priming

  1. Always ensure that the suction line is airtight to avoid air leakage.
  2. The foot valve must be checked regularly for proper sealing.
  3. Avoid overfilling the casing as it may cause overflow or pressure buildup.
  4. Never start the pump without confirming proper priming.

Advantages of Proper Priming

  • Ensures smooth and continuous operation.
  • Prevents mechanical damage and overheating.
  • Maintains desired flow rate and pressure.
  • Reduces noise and vibration in the pump system.
  • Improves the overall efficiency and lifespan of the pump.
Conclusion:

The priming of a pump is a vital process to prepare a centrifugal pump for operation by completely filling it with the liquid to be pumped and removing all air from the system. This allows the pump to generate the required suction head for smooth operation. Without priming, the pump cannot function properly and may get damaged. Therefore, proper priming is an essential maintenance step that ensures the reliability, efficiency, and longevity of pumping systems.