Home > Blog > Centrifugal Water Pump >

Why is priming necessary in centrifugal pumps?

Published on 2019/12/27

Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping.

So energy impart on air is much lesser. So impeller CANNOT impart enough energy to air to go out of casing and suck water so priming is compulsory for in case of centrifugal pump. In case of reciprocating pump, it can push out all air by itself as it has suction and delivery valves to displace “fluid” positively

How do you prime a centrifugal pump?

Consider using a foot valve and an outside source of liquid in order to prime the pump.

  1. Close the discharge isolation valve.
  2. Open the air vent valves in the casing.
  3. Open the valve in the outside supply line until only liquid escapes from the vent valves.
  4. Close the vent valves.
  5. Close the outside supply line.

Priming a pump is probably the first and one of the most important things you should do before operating it.

Not priming a pump or not doing it properly makes up 80 percent of centrifugal pump problems. While centrifugal pumps are relatively inexpensive, the downtime of your system due to a malfunctioning pump might be costly.

WHAT IS PUMP PRIMING?

Priming simply means preparing or getting something ready for operation.

For a centrifugal pump to work properly, you need to fill it up with water.

For a centrifugal pump to work properly, you need to fill it up with water. For a centrifugal pump to work properly, you need to fill it up with water.

When everything is right, a standard (non-self-priming) centrifugal pump looks like this.

The pump will resume operation once the air is removed.

WHAT ABOUT SELF-PRIMING PUMPS?

A self-priming centrifugal pump is able to overcome the problem of air binding by mixing air with water. After it gets rid of the air, the pump will continue to move water like a standard centrifugal pump.

Self-priming pumps can’t operate without water.

HOW TO PRIME A CENTRIFUGAL PUMP

Different pumps and different system setups require different steps to prime. Refer to your pump manual for more information.

WHEN THE SUCTION SUPPLY IS ABOVE THE PUMP

THE SUCTION SUPPLY IS ABOVE THE PUMP

1: Discharge isolation valve

2: Check valve

3: Suction isolation valve

Following are the steps to priming the pump:

Slowly open the suction isolation valve.

Open the air vents on the suction and discharge piping until the pumped fluid flows out.

Close the air vents.

WHEN THE SUCTION SUPPLY BELOW THE PUMP

THE SUCTION SUPPLY BELOW THE PUMP

1: Discharge isolation valve

2: Shutoff valve

3: from outside supply.

4: Foot valve

5: Check valve

Consider using a foot valve and an outside source of liquid in order to prime the pump.

Close the discharge isolation valve.

Open the air vent valves in the casing.

Open the valve in the outside supply line until only liquid escapes from the vent valves.

Close the vent valves.

Close the outside supply line.

Priming of a centrifugal pump

Priming of a centrifugal pump is the process of filling the liquid at the suction pipe and the impeller. Priming is done to put pump into working order by filling or charging with water.

Why Priming is required ?

Pressure developed by the impeller of centrifugal pump, is proportional to the density of the fluid in the impeller. If the impeller is running in air, it will produce only a negligible pressure. This pressure will not suck water from its source through the suction pipe. To avoid this, pump is first filled up with water.

Priming is done as follows :

1: First of all the suction pipe and impeller is completely filled with water.

2: The delivery valve is closed and the pump is started.

3: The rotating impeller pushes water in the delivery pipe, opens the delivery valve and sucks water through the suction pipe.

Useful information on Self-Priming Pumps

What is Self-Priming?

If a pump is located beneath the level of the liquid to be pumped, gravity and air pressure ensure that it is constantly filled with the liquid and there can be no ingress of air into the pump or suction line.

In many applications, a pump has to be placed above the level of the liquid, for example, when emptying an underground storage tank. At start-up, there will be air in the suction line and before the pump can discharge the liquid, this air must be evacuated or displaced. Pumps are optimised to move a particular liquid; evacuating a gas is a very different challenge.

Various methods can be used to get around this problem. A secondary pump can be used to evacuate the suction line. A non-return (foot) valve or evacuation tank can be used to stop fluid draining from the suction line when the pump is stopped. However, these solutions all involve extra equipment, piping and processes. Ideally, for these applications, a pump is required that can evacuate air from the suction side at startup before commencing its normal pumping mode. Such a pump is considered to be self-priming.

What Limits a Self-Priming Pump?

As a pump evacuates air from the suction side, liquid is forced into the suction line by the pressure of the surrounding air. This process can only continue until the head of liquid balances the local air pressure. With water, for example, it is theoretically possible for a perfectly efficient pump to self-prime to a height of only about 10m from its source. The precise limit is affected by altitude and temperature and will be different for other liquids.

Are Centrifugal Pumps Self-Priming?

With centrifugal pumps, the pumping action is generated by the transfer of rotational energy from the impeller to the liquid. There are no seals between the suction and discharge sides of the pump. This means that centrifugal pumps are ineffective with gases and are not capable of evacuating air from a suction line when the liquid level is below that of the impeller. In such cases, the pump is said to be air-bound and there is a danger of overheating: generally, pumps rely on the pumped fluid to lubricate and cool the pump’s bearings.

However, with a few modifications to the basic design, a centrifugal pump can be self-priming. The impeller and volute casing is essentially surrounded by a tank so that it can always be immersed in a liquid sufficient to get the pump started and provide the pump with lubrication and cooling – provided the time taken to prime the pump is not excessive.

It is important that a self-priming centrifugal pump’s reservoir is filled correctly with liquid after installation. “Self-priming” in this context means that the pump has the ability to use liquid stored in its housing to generate a vacuum on the suction line. Even a ‘self-priming’ centrifugal pump will not operate when dry. With appropriate bearings and seals, a centrifugal pump can tolerate dry running for a limited time but this is not recommended for extended periods.

How does a Self-Priming Centrifugal Pump Work?

Self-Priming Centrifugal Pump Work Progress

A self-priming centrifugal pump has two phases of operation: priming mode and pumping mode.

In its priming mode, the pump essentially acts as a liquid-ring pump. The rotating impeller generates a vacuum at the impeller’s ‘eye’ which draws air into the pump from the suction line. At the same time, it also creates a cylindrical ring of liquid on the inside of the pump casing. This effectively forms a gas-tight seal, stopping air returning from the discharge line to the suction line. Air bubbles are trapped in the liquid within the impeller’s vanes and transported to the discharge port. There, the air is expelled and the liquid returns under gravity to the reservoir in the pump housing.

Gradually, liquid rises up the suction line as it is evacuated. This process continues until liquid replaces all the air in the suction piping and the pump. At this stage, the normal pumping mode commences, and liquid is discharged.

When the pump is shut off, the design of the priming chamber (normally involving a ‘goose-neck’ on the suction piping) ensures that enough liquid is retained so that the pump can self-prime on the next occasion it is used. If a pump has not been used for a while, it is important to check for losses from the casing due to leaks or evaporation before starting it.

 

[elementor-template id='1074']