Why Is It Necessary to Bleed Air Before Starting a Multistage Pump?
As a professional brand in the fluid transfer industry, Omron Tech Pumps has always focused on the efficient operation of industrial pumps. In the field of industrial fluid transfer, multistage centrifugal pumps are favored for their ability to provide high head and high efficiency, and their working principle follows the general laws of centrifugal pumps. However, many on-site operators often have questions: Why is it imperative to bleed air before starting a multistage pump? What are the risks of skipping this step?
I will deeply analyze this seemingly simple yet crucial operational specification from four dimensions: physical mechanism, equipment structure, operational safety, and actual working conditions.
Core Principle: Why Is Air the "Natural Enemy" of Centrifugal Pumps?
The working foundation of a centrifugal pump is centrifugal force. When the impeller rotates at high speed, it drives the medium in the pump cavity to produce centrifugal motion, thereby forming a low-pressure zone at the center of the impeller, which sucks liquid in by utilizing atmospheric pressure.
Air Binding: The specific gravity of air is much smaller than that of the medium to be transported (about 1/800 of water). According to the centrifugal force formula F=mω2r, at the same rotational speed, the centrifugal force generated by air is negligible. If air accumulates in the pump cavity, the vacuum degree formed at the center of the impeller is insufficient to suck in liquid, and the pump will be in an "idling" state, unable to fulfill the transfer task and thus losing its pumping function.
This phenomenon where the pump cannot suck liquid due to the presence of air inside is known as "air binding" in the industry. For multistage centrifugal pumps, due to their long internal flow channels and multiple stages, air is more likely to be trapped between the impellers of each stage. Incomplete air bleeding will directly lead to pump startup failure, failing to reach the rated pressure and flow rate.
It is worth noting that in addition to the common centrifugal multistage pumps, there are also rotary positive displacement vacuum pumps in industry that use oil to seal the gaps between moving parts and reduce harmful space. Such multistage pumps are usually equipped with a gas ballast device to ensure the integrity when extracting materials or increasing pressure. However, this article mainly discusses the air bleeding problem of multistage centrifugal pumps before startup, whose core lies in overcoming "air binding" to achieve liquid transfer.
In-depth Analysis: The Fatal Harm of Starting Without Bleeding Air
Forcing the startup of a multistage centrifugal pump without bleeding air will not only fail to transport the medium but also cause irreversible damage to the precision parts inside the pump body.
1. The "Dry Running" Nightmare of Mechanical Seals
The mechanical seal of a multistage pump relies on the transported medium for cooling and lubrication. If the pump is filled with air, the sealing surfaces will be in a state of dry friction under high-speed rotation. The instantaneous high temperature will cause the dynamic and static rings to burn out, the seal to fail, and even trigger the risk of medium leakage.
2. Failure of Balancing Devices and Axial Movement
A multistage pump generates enormous axial thrust during operation. To offset this thrust, the design usually includes a balance disc or balance drum device. The operation of these devices fully depends on the pressure support of the liquid inside the pump. The presence of air will lead to an imbalance in the balancing torque, causing severe axial movement of the rotor components and resulting in collision and wear between the impeller and the guide vane.
3. The Chain Reaction of Cavitation and Vibration
When residual air enters the high-pressure zone along with the liquid, it will collapse rapidly due to the sudden increase in pressure. This microscopic "implosion" will generate extremely high impact pressure, eroding the surface of the impeller and forming cavitation. The accompanying severe vibration and noise will accelerate bearing fatigue and shorten the service life of the entire machine.
How to Ensure the Air Bleeding Operation Is Foolproof?
From the perspective of operation and maintenance management and information retrieval, standardized operating procedures can not only protect equipment but also serve as an important part of enterprise technical precipitation. The following is a comparison table of the core steps for air bleeding of multistage pumps:
| Operation Stage | Key Actions | Purpose |
|---|---|---|
| Pump Priming Stage | Open the inlet valve and introduce water using liquid level difference or a vacuum pump | Expel most of the air inside the pump casing |
| Air Bleeding Stage | Gradually open the air release valve (air vent plug) on the upper part of the pump body | Eliminate trapped air in dead corners of the multistage flow channels |
| Confirmation Stage | Observe the exhaust hole for continuous, bubble-free liquid flow | Ensure the pump cavity is completely filled with the medium |
| Turning Stage | Manually turn the rotor 3-5 times | Check for jamming and assist in discharging residual air |
Special Case: "Flooded Suction System" That Does Not Require Priming
It is worth mentioning that not all multistage pumps need active priming and air bleeding before startup. If the multistage pump is located below the liquid level in the tank, i.e., in a flooded suction system, the medium will naturally flow into the pump body under the action of gravity, keeping it filled with liquid at all times. In this case, additional priming is usually not required during startup. However, even for flooded suction systems, after initial installation or long-term shutdown, it is still recommended to check and ensure there is no trapped air in the pump cavity to prevent accidents.
Summary: Professional Maintenance Is the Key to Extending Pump Life
Whether through active priming or leveraging the natural advantages of a flooded suction system, ensuring the pump cavity is completely filled with liquid before starting a multistage pump is the cornerstone for the safe and efficient operation of multistage centrifugal pumps.
If you have more questions about industrial pump selection, operation and maintenance, or fluid system optimization, welcome to follow our technical column. We will continue to provide you with data support and practical experience based on front-line scientific research to help your fluid transfer system operate more stably and efficiently. Choose Omron Tech Pumps, choose professionalism and trust!