Science Alert describes cavitation as the formation of the bubbles or voids in the liquids. These are caused by quick changes in pressure. Cavitation is a phenomenon where rapid pressure changes lead to the formation of small vapor-filled cavities in places where the pressure is relatively low. When subjected to higher pressure cavities, called “bubbles” or “voids”, collapse. The result may be shock waves and rapid weakening on or near the bubble. Hydropower plants are especially susceptible to cavitation and cavitation repair and continued maintenance is important to keep plants operational and costly repairs low.
Cavitation damages create a need for maintenance. Additionally, cavitation damages even if repaired creates a specific need for ongoing maintenance as it has the potential of severely disrupting plant operations. In addition, an unforeseen cavitation issue highlights the need for continuous oversight of items such as:
- Monitoring the costs of cavitation repair
- Keeping budgets in check
- Examining the distortion of materials due to excessive welding
- Identifying the intensity of plant outages
How Does Cavitation Work?
Early research by Avellan and Farhat discovered that bubbles imploded, producing emission of large amplitude shockwaves. Bubbles discharged in higher pressures areas implode. The result is high shockwaves on the metal surface. Tiny imperfections are the site of greater erosion.
In a low-pressure area, vapor bubbles begin in higher-pressure locations. These bubbles collapse on the solid walls, increasing the pressure. Vapor bubbles are seeded to the machine’s metal surface defects. Common cavitation locations are the impeller of a centrifugal pump and the piston or gear of the displacement pump. Cavitation also occurs on such surfaces as turbine blades.
New pumps and rotors are more resistant to cavitation. Metal on the new pump has fewer surface imperfections where bubbles could form. However, as the metal ages, surface defects appear and cavitation damage will be more evident.
Types of Cavitation
Classic cavitation occurs when a pump is fluid-starved. Filters get clogged restricting pipes and partially closing valves. Classic cavitation occurs in a centrifugal pump at the impellor’s eye. In a positive displacement pump, cavitation often occurs in a piston, or plunger or the chamber of a gear pump.
Vane syndrome cavitation happens when the spacing between the vanes of a centrifugal pump’s impeller and its housing is too small. This restricts airflow. Internal recirculation occurs in centrifugal pumps that operate at reduced flow. If a valve is only partly open or the pump is operated at too low a flow, fluid may be ejected from the vanes. Pressure pulses may result in serious cavitation. Leaking valves may cause sucking of air into a pumping system. Air bubbles may form and create shock waves through the pump. Inertial or transient cavitation, the diameter of the bubble doubles during a single cycle. The bubble collapses driven by the inertia of the fluid.
In non-inertial cavitation, a bubble in a fluid may be forced to spin, changing its size or shape.
When Does Cavitation Occur?
Cavitation occurs when the pressure of the liquid is reduced in the machine operation. Again, cavitation occurs when the liquid in a pump turns to a vapor at low pressure. It occurs because there is not enough pressure at the suction end of the pump, or insufficient Net Positive Suction Head available (NPSHa). When cavitation takes place, air bubbles are created at low pressure. As the liquid passes from the suction side of the impeller to the delivery side, the bubbles implode. This creates a shockwave that hits the impeller and creates pump vibration and mechanical damage, possibly leading to complete failure of the pump at some stage.
Cavitation should be mitigated in order to keep costs of cavitation repair low, plus other cascading issues that may impact a plant’s output.
When cavitation happens, air bubbles collapse on solid walls causing higher pressure. Other indicators of cavitation include increased noise, metal pitting, erosion, vibration, parts damage, and a decrease in the machine’s efficiency.
Cavitation causes cyclic stress. Surface fatigue is an indication of cavitation.
Signs of Cavitation
- Look for noise and vibration. Vapor bubble implosion makes crackling sounds like gravel rattling around.
- Vibrations not usually in the pump may be a sign of cavitation.
- Discharge pressure may also be reduced.
- In positive displacement pumps, cavitation you may not reduce flow.
- Power consumption may also be evident.
How Can Cavitation Be Prevented?
British engineer, Michael Smith, notes that cavitation in pumps can be prevented through such action as matching the pump to the fluid, the system, and its application. This needs to be discussed beforehand with the pump supplier.
Preventative measures suggested by Enggcyclopedia.com include:
- Lowering temperature
- Raising suction vessel’s liquid level
- Changing pump
- Reducing the motor’s revolutions per minute
- Increasing impeller eye’s diameter
- Employing an impeller inducer
- Setting up two parallel lower-capacity pumps
- Regular checking of all valves and joints, O-rings, and mechanical seals.
- Looking for leaks or erosion
What is Cavitation Repair?
Cavitation repair involves regular checking of machines. Checking uncovers damages that need immediate attention. Failure to identify and repair signs of cavitation can result in increased deterioration of the equipment, costly shutdowns, and equipment replacement.
Cavitation Repair Techniques
Thanks to modern technology, there are many techniques available for cavitation repair.
- Welding of damaged parts can be done onsite. 3D replacement parts, once created, can be welded immediately.
- Reinforced Epoxy Overlay and Coating—In repairing and reinforcing hydroelectric pumps and turbines, epoxy overlay or coating lasts six months to a year. Cavitation repair is done very quickly.
- Thermal Sprays—One of the best ways to prevent or slow cavitation is to operate pumps the way they were intended. Purchasing a pump made of material that withstands cavitation is very costly. However, using a thermal spray can protect a more economical pump that protects surfaces that are susceptible to cavitation damage.
- Use of a portable scanner—This technique produces a three-dimensional image of machines like the runner blade surface of a turbine blade. The data, downloaded to a computer, can, then, be analyzed.
- Three-D print equipment can build a replacement piece on site and install it.
Why Does Cavitation Repair Need to Occur?
If cavitation damage is not repaired, it will result in machines like pumps or rotor blades becoming less efficient. The equipment may become noisier and less efficient. The equipment may become incapable of doing its job. Costly shutdowns to do repair or replacement will result in lost workdays.
How does CEO Machining Help with Cavitation Repair Needs?
Every minute wasted in waiting for machinery to be repaired is money lost. Cavitation repair specialists like CEO Machining boast some of the most experienced and technically-advanced machinists in the business.
CEO Machining uses state-of-the-art machining equipment. Knowledgeable and experienced cavitation repair specialists are adept at doing on-the-spot repairs and replacements safely, quickly, and efficiently. Moreover, CEO machining technicians are unmatched in problem analysis, solution diagnosis, and quality repair.
About CEO Machining & Cavitation Repair
Based in Onalaska, Texas, CEO Machining uses 6000sf fabrication in their shop. They can customize machines for projects anywhere in the world.
CEO Machining has a history of long-term relationships with companies. Trained technologists understand customers’ unique needs. Whether you are looking at a simple maintenance project or you want a significant cavitation repair CEO Machining is confident that we can handle your maintenance, repairs, and project resumption concerns. Our company understands the debilitating costs of machines that have to be shut down for repairs. That’s why we specialize in the fast, efficient diagnosis of the problem and on-site repairs.
Our onsite machining brings state-of-the-art equipment to your worksite. We save you the cost of moving expensive equipment to a repair site with our in-place machining.
Our well-trained, highly committed, experienced field machinists have the expertise and tools to do cavitation repair and maintenance right at your worksite. Our onsite specialists bring to the job a wide variety of skills including problem diagnosis, assembling, disassembling, welding, cavitation repairs, sprays, epoxy coating, and troubleshooting.