What is a Self Priming Pump?- How it Works And Uses

What is a Self-Priming Pump?

Self-priming pumps are a specialized category of liquid pumps, engineered to contain just enough liquid within the pump chamber or body to initiate the pumping action. This characteristic can be particularly beneficial in process plants where pumps are frequently required to operate on a stop-start basis across different tasks.

When a pump sits below the liquid level, gravity and atmospheric pressure work together to keep the pump and suction line continuously filled. In such cases, air cannot enter the pump or its suction line, ensuring uninterrupted operation.

In practice, however, it’s often necessary to position pumps above the liquid source—for instance, when removing fluid from underground storage tanks. At startup in these scenarios, air will inevitably be present in the suction line. Before the pump can begin transferring liquid, this air must be removed. While pumps are generally optimized for moving liquids, they are not naturally adept at handling gases, which presents a separate challenge.

There are a few strategies to address this issue. One approach involves installing an auxiliary pump to clear air from the suction line. Alternatively, a non-return (foot) valve or an evacuation tank may be used to prevent the liquid from draining away when the pump is turned off.

Of course, these workarounds add extra components and steps to the system. In situations like these, it’s preferable to use a pump that can automatically clear air from the suction side at startup and then switch seamlessly to regular operation. Pumps with this capability are referred to as self-priming.

How Do Self-Priming Pumps Work?

To prevent air from mixing with water, self-priming pumps rely on creating a partial vacuum that helps expel air while drawing water through the system. During the priming stage, the pump blends air and water, causing the air to naturally rise and the water to settle downward. As gravity does its work, the water—now free of most air—is drawn back toward the impeller. Here, it once again encounters any remaining air in the suction line.

This cycle continues, with the pump repeatedly separating and discharging air, until the suction line is cleared. At that point, the absence of air allows a proper vacuum to develop, and atmospheric pressure pushes water steadily into the suction line and onward to the impeller. Once this balance is achieved, the pump is able to operate normally and begin moving water as intended.

Common Applications

Self-priming pumps are used in various industrial and commercial facilities, from steel mills, power plants, and sewage treatment facilities to wineries, breweries, and more. Common applications include:

• Pumping water, fuels, clear or gray water, raw sewage, industrial wastewater, and more
• Liquid transfer systems
• Irrigation
• Basement floodwater pumps
• Bailing out boats
• Increasing water pressure
• Sewage treatment

Advantages Of Self-Priming Centrifugal Pumps

• These pumps are capable of managing a wide range of liquids, making them highly adaptable across various applications.
• They perform reliably when dealing with slurries, corrosive substances, and fluids containing suspended solids, which is often a challenge for other types of pumps.
• Unlike submersible pumps, self-priming centrifugal models maintain their pumping function even after they are no longer immersed in a tank or vessel. This feature significantly broadens their usability, especially in dynamic or changing environments.
• For situations that require either frequent or occasional pumping, these pumps prove especially practical. Since there is no need to prime the pump at each start-up, the overall process becomes much more efficient and less labor-intensive.

Disadvantages Of Self-Priming Centrifugal Pumps

• The pump requires an initial charge of priming liquid within its reservoir to begin operating; without this, it cannot function as intended.
• Because these pumps rely on a built-in liquid reservoir, they are often bulkier than conventional centrifugal pumps. This added size can present challenges in installations where available space is limited.
• It is essential to position the pump as near as possible to the production lines. This proximity helps prevent the priming liquid from running low during self-priming cycles, thereby maintaining efficient operation.

Are Positive Displacement Pumps Self-Priming?

By design, all positive displacement pumps have the ability to self-prime. This quality is evident in various types such as rotary gear pumps (both internal and external), lobe pumps, vane pumps, and diaphragm pumps.

What these pumps share is a reliance on components manufactured with precise tolerances, ensuring that fluid does not leak back from the discharge side to the suction side.

The effectiveness of these tight seals determines how well a positive displacement pump can remove air from its suction line. While these pumps can handle some air venting, they are not immune to challenges.

If a pump runs dry for too long, friction and heat can quickly build up, putting stress on the seals and potentially leading to premature wear or outright failure.

Reciprocating pumps introduce another issue worth noting: the risk of cavitation, especially as the pump transitions from air to liquid. When a mix of air and liquid is present, vapor bubbles can form and expand on the suction side.

Once these bubbles reach the discharge side, where the pressure is much higher, they collapse with force. This sudden implosion can cause noticeable vibration and may even damage internal pump components.

Because of these practical concerns, it is wise to consult the manufacturer before selecting a positive displacement pump for any application that involves self-priming or prolonged dry running. Their guidance can help avoid unnecessary damage and ensure the pump operates as intended.

Are Centrifugal Pumps Self-Priming?

In centrifugal pumps, the process of moving fluid is driven by the transfer of rotational energy from the impeller to the liquid. Since there are no seals separating the suction and discharge sides, these pumps are not effective when handling gases and cannot remove air from the suction line if the liquid level falls below the impeller.

Under such circumstances, the pump becomes air-bound, which can lead to overheating. This is primarily because most centrifugal pumps depend on the fluid being pumped to lubricate and cool their bearings.

Despite these limitations, certain modifications to the standard design allow a centrifugal pump to become self-priming. In these models, the impeller and volute casing are enclosed within a chamber that holds enough liquid to enable the pump to initiate operation, as long as sufficient time is taken during priming. This design ensures that lubrication and cooling are available from the outset.

Proper filling of the reservoir with liquid after installation is essential for a self-priming centrifugal pump to function correctly. Here, “self-priming” refers to the pump’s ability to use the liquid stored in its casing to generate a vacuum on the suction side.

However, it is important to note that even a self-priming centrifugal pump cannot function when completely dry. Although pumps equipped with suitable bearings and seals may tolerate brief periods of dry operation, this should be avoided over longer durations.

Why Your Self-Priming Pump Won’t Prime?

1. Air Leak in Suction Line

When a pump is recirculating fluid, its goal is to expel air from the discharge chamber and establish a region of low pressure. If there happens to be a leak somewhere along the suction line, though, air keeps seeping in. As a result, the pump is never quite able to clear out enough air to create that crucial low-pressure area.

2. Debris in The Impeller

Any debris lodged at the impeller’s eye essentially strips away the impeller’s hydraulic capability. When that happens, the pump can’t generate the low-pressure zone it relies on for proper function.

3. Pump Is Air Bound

There are a few situations where a pump might become air-bound:

• No Air Release Line: Without an air release line, air has nowhere to escape except into the discharge side, where it accumulates.
• Pressurized Discharge Line: If both the air release and discharge line valves are closed, air is once again trapped inside the pump, unable to vent.
• Excessive Impeller Clearance: When there’s too much space between the impeller and the wear plate—often due to wear or reassembly mistakes—the pump struggles to form a low-pressure region.

4. Plugged Recirculation Port

During priming, the pump recirculates fluid through the volute casing. If the recirculation port becomes clogged, the impeller’s eye loses its ability to create the necessary low-pressure zone to draw liquid up the suction line.

5. Lift Too High for Pump Speed or Impeller Diameter

When the pump is undersized relative to the suction lift requirements, it simply cannot establish the low pressure needed for priming. It’s worth emphasizing how important it is to factor in suction lift specifications before choosing a pump. Tools like Gorman-Rupp’s Pump Selection Guide are quite helpful for working through those calculations.

Ultimately, successful priming in self-priming pumps depends on a few non-negotiables: the pump needs to evacuate air efficiently, create low pressure at the impeller’s eye, and be matched appropriately to the system’s NPSH requirements.

What are the Common Problems with Self-Priming Pumps?

Several factors can influence whether a pump is able to self-prime effectively. To begin with, it’s crucial that the discharge line remains free from pressure or blockages; any restriction here can interfere with the priming process.

Across all pump types, maintaining an air-tight suction line is non-negotiable. If air is able to leak in, the pump cannot lower the pressure as needed, and as a result, fluid won’t rise through the suction line as intended.

Another key point is the volume of the piping on the suction side. Keeping this volume as small as possible helps to shorten priming time. When priming takes too long, there’s a real risk that the liquid charge will evaporate before the pump finishes priming, potentially leading to dry-running—which can, in turn, cause damage to the pump itself.

Centrifugal pumps, in particular, have their own sensitivities. Anything that diminishes the impeller’s efficiency will also limit the pump’s self-priming capacity. For example, if the liquid contains solids, debris might gather in the recirculation port.

This build-up can obstruct the flow of fluid and disrupt the formation of the liquid ring that’s necessary for priming. Likewise, debris settling directly on the impeller can prevent it from creating the low-pressure region needed at the eye of the impeller.

As pumps age and their internal components wear down, the gaps between the impeller and the volute casing widen. These increased clearances make it harder for the pump to generate an adequate low-pressure area. Sometimes, these clearances may also widen due to incorrect reassembly after maintenance.

Lastly, when pumping water in cold environments, it’s wise to either drain the pump or ensure some form of heating is available if there’s any chance of freezing temperatures. Frozen water within the pump or pipework can lead to damage, so a bit of preventive action here goes a long way.

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