Category Archives: Self-priming Pumps

In many fluid pumping applications, pumps have to be placed above the level of liquid, allowing air into the line, which can lead to challenges such as overheating and pump failure. To avoid these issues, self-priming pumps are used to evacuate the air before moving liquids. At Thai Khuong (TKP), we offer a variety of self-priming pumps to suit the needs of various applications.

What Is Self-Priming?

Pumps that are partially or completely above the fluid level allow air into the suction line, which must be evacuated or displaced before the pump can move the liquid. Self-priming pumps are capable of removing air at startup before beginning its normal pumping functions.

Are Centrifugal Pumps Self-Priming?

These pumps have different working than standard centrifugal pumps. The self-priming pump has a built-in water storage tank. This tank allows the air to be removed from the pump and suction lines by recirculating the water in the pump during the priming process.

The water storage tank locates in front or above the runner. In this pump, the “self-priming” ability of the pump derives from the ability of the pump to hold water after the initial priming.

Centrifugal pumps do not have seals between the suction and discharge sides of the pump, meaning they are not self-priming and cannot evacuate gases and air if the pump is above fluid level.

To make a centrifugal pump self-priming, it can be surrounded by a tank to ensure that it is always immersed in a liquid and that it has the necessary lubrication and cooling to prevent the pump from overheating.

Self-priming Pump Working

To start the self-priming pump, water or other liquid is required in the pump housing. Dry operation of the pump (even for a short period) can break the mechanical seal and cause the pump to fail.

The self-priming pump works into two operating stages:

• Priming Mode
• Pumping Mode

In the priming mode, this pump basically works as a liquid ring pump. The pump impeller gets power by an electric motor or engine via a shaft. The impeller rotates according to the shaft rotation. As the impeller starts rotating, a vacuum creates in the impeller’s eye. Due to the creation of this vacuum, the external high-pressure air starts entering inside the pump from the suction line.

Simultaneously, it forms a cylindrical liquid ring in the pump housing. This process creates an air-tight seal and prevents air from flowing back into the suction line from the discharge line. These air bubbles are surrounded by the fluid inside the impeller blades and transferred to the outlet valve. There, the air expels, and the fluid goes back to the reservoir (“suction chamber”) in the pump casing under the action of gravity.

As the air expels, the fluid gradually rises to the suction line. This procedure persists until the fluid has replaced all of the air in the suction line and pump. At this point, the normal pumping mode starts, and the fluid discharges. If the connected discharge valve cannot deliver this detached air to the downstream discharge line system, it may be necessary to use a bypass line to empty it.

As the pump is turned off, the design of the priming chamber makes sure that sufficient fluid retains in the casing. Due to this remaining fluid in the casing, the next time, the pump will prime itself. This means that air and water will mix again in the pump housing to produce a pumping fluid until the pump is fully primed again.

If the pump has not been used for a long period of time, it is essential to check the pump housing for evaporation or leaks prior to operating it.

Types of Self Priming Pumps

The self-priming pumps have the following major types:

1. Liquid Primed Self Priming Pump
2. Compressed Air Primed Self Priming Pumps
3. Vacuum Primed Self Priming Pump

1) Liquid Primed Self Priming Pump

These pumps have their own built-in or separate chambers (called ” Priming chambers”) that fill with the liquid for the pump to be “self-priming.” If the priming chamber doesn’t have the initial liquid, the liquid primped self-priming pump can’t perform the priming or pumping process.

Priming decreases the risk of pump damage during start-up by preventing the pump runner to becomes air bound. The priming helps the pump to transfer the required fluid.

To ensure reliable working, firstly, the pump must be primed. That is, air or gas is ejected from the inlet port and the runner’s eye and replaced with the pumping fluid. If the fluid is not completely filled, the pump will not work correctly.

Methods of Priming

The priming of a pump start can be done by an external arrangement that makes sure priming, or by considering the design, or by using a Self Priming Pump. Following are some external arrangements that ensure the pump-priming:

1) Manual Priming

In the manual priming, the water pours manually in the inlet of the pump. In this method, the water directly pours in the pump inlet valve or by using other equipment such as a funnel and pump will manually prime by a gravity supply. As the priming process completes, all of the air escapes from the outlet valve, and normal pumping operation of the pump starts.

2) Priming with Vacuum Pump

In this priming method, a small self-priming pump or a positive displacement pump, or a vacuum pump uses to prime the primary centrifugal pump. The suction line of a vacuum pump connects with the pressure line of the main centrifugal pump. This vacuum pump sucks all the air out of the suction pipe and the primary pump.

3) Priming with Jet Pump

This method allows high headwater to flow via a nozzle. The nozzle design allows the jet pressure outside the nozzle to be less than atmospheric pressure, allowing water to be drawn from the sink.

4) Priming with Separator

In this priming method, an air-water separation chamber is placed on the pump outlet side, and a curved suction tube is placed partially at the pump inlet side. The curved section of the suction tube keeps some water in the pump at all times. The air separates and discharges from the outlet port of the pump. But the water has more weight than air, and it returns to the separation chamber.

This design is also part of some self-priming centrifugal pumps. In the case of a self-priming centrifugal pump with a separation chamber, the pumped medium and the air bubbles entrained in it are pumped into the separation chamber by the action of a runner. During this, the air expels by the outlet port of the pump, but the water recedes and is again accompanied by the runner.

This design has two major disadvantages:

• One is to decrease the efficiency of the pump
• This pump needs a separation chamber that increases the size of the pump.

Why use a Self-Priming Pump?

The common factors due to that you should use a self-priming pump are given below.

• Because the self-priming pump installs above the water source, it requires less infrastructure to install.
• Since they install on the ground level, they are easily accessible and easy to repair and maintain.
• Wet well has no mechanical equipment, so there is no need to open the well cover, hoists, or cranes (as with submersible pumps), making it a safer option for operators.

Why your Self-priming Pump won’t prime? 

Following are some reasons due to that your pump won’t prime:

1) Air leakage in the suction line 

When the liquid in the pump recirculates and pushes the air out of the delivery chamber, it tries to create a ​​low-pressure area. But if the suction line has leakage problems, the air will continuously enter the pump. Due to this leakage, the pump will not release the air completely and also will not produce a low-pressure area. In this way, the leakage at the suction line creates priming problems.

2) Debris in the impeller 

If there is debris in the impeller eye, this debris eliminates the hydraulic capacity of the impeller to produce a low-pressure area for priming.

3) Pump is air bound 

A pump may have air bound problems in the following conditions:

i) No Pipe for air discharge:

• If the pump doesn’t have any pipe for air discharge, the air can’t be discharged into the atmosphere and instead gathers on the outlet section. Because of no air discharge pipe, the pump can’t generate a low-pressure area, due to which the priming process will start.

ii) Pressurization of the discharge line:

• If the valve on the air discharge pipe is shut, and the valve in the discharge air line closes again and provides no space for air to drain from the pump.

iii) Highest impeller clearance:

• If the gap between the wear plate and the impeller is too large, it will be difficult for the pump to create a low-pressure zone. This is usually due to wear and tear, but it can also be due to improper assembly.

Applications of Self-Priming Pump

The pumps are used for the following purposes:

• These pumps for pumping gray or clear water, industrial wastewater, raw sewage, water, fuels, and more
• Transfer the product to the production line
• Pump clean-in-place (CIP) fluid to clean and disinfect pipes and parts
• Sewage treatment systems
• Increasing water pressure
• Bailing out boats
• Basement floodwater
• Irrigation systems
• Liquid transfer systems

Advantages and Disadvantages of Self-priming Pump 

Advantages	Disadvantages
These pumps can handle numerous fluids	It can’t work without the provision of initial priming.
These have self priming capability.	It can’t run dry. If you operate it in dry condition then the pump impeller may damage.
Self priming pump can also work with suspended solids, corrosive liquids, and slurries.	Due to the reservoir requirement with a pump, the size of this positive displacement pump becomes more than a standard pump, which can create problems in applications that have space limitation problems.
These pumps are best for intermittent and frequent pumping functions.	The pump tank should be as close as possible to the production line so that it does not empty during the self-priming process.
Compared to a submersible pump, a self-priming pump can deliver liquid continuously even the pump is no longer immersed in a liquid reservoir or container.	These pumps require a larger clearance to control larger solids. In addition, they have a larger volute that helps with the self-priming process.

FAQ Section

Can self priming pump run dry?

A self priming pump can’t run dry. Even for a short time, dry running may cause its failure.

What are the uses of self priming pumps?

A self priming pump uses for many applications such as power plants, irrigation systems, steel mills, sewage treatment systems, and more.

Which pump does not need priming?

A positive displacement doesn’t need priming because a positive displacement pump has very short clearance inside the pump. This helps the pump to make a vacuum and expels the air out of the pump until the water replaces the whole air. Therefore, it has the capability of self-priming.

Which pump must always be primed?

A centrifugal must always be primed before operating it. A positive displacement pump has the self-priming capability.

How long does it take for a self priming pump to prime?

If your pump gets more than 4 minutes for priming, you must turn off the pump and check the problem due to that pump is not priming and fix the reason of your pump problem.

What happens if pump is not primed?

Priming regulates the proper operation of the pump because water works as a coolant that stops the overheating problems of the pump. If you do not prime the pump with liquid, it will operate dry, which can cause pump failure.

Which Pump cant achieve priming itself?

A centrifugal pump has no capability of self-priming. Therefore, it cant achieve priming itself.