Cavitation is a phenomenon that occurs in industrial liquid-flow systems, damaging various components of the system, including the control valve. It’s a process that results in significant erosion of the valve plug and seat ring. If left unchecked, cavitation can damage the body severely, leading to premature wear and tear. It not only disrupts your operations but also diminishes the valve’s functional lifespan. The downtime caused by a faulty valve can be quite costly. Therefore, it’s critical to get professional valve repair services to avoid failures. This article provides valuable information to help you understand cavitation, how it affects the control valve performance, and the corrective measures to solve it.
What is Cavitation?
Cavitation is a type of damage you may notice on your control valves. It often looks like pockmarks or pitting on valve surfaces. Cavitation will require either valve repair or valve rebuild services to keep your valves functional.
When fluid flows through the constricted area around the plug and seat – also known as the vena contracta – its velocity increases. As velocity increases, pressure decreases.
If the pressure in the vena contracta falls below the liquid’s vapor pressure, vapor bubbles (also known as vapor cavities) start to form. As the liquid advances downstream to the wider outlet of the valve, it slows down and recovers some of the pressure above the vapor pressure, creating a high-pressure section. At this point, the vapor cavities collapse instantaneously. This rapid change from vapor bubbles to liquid causes cavitation.
If you notice the pipe and valve of your high-pressure boiler feedwater pump shaking vigorously with a noise similar to rocks passing through the valve, it’s a red flag for cavitation. If the flow pressure at the outlet remains lower than the liquid’s vapor pressure, vapor cavities persist downstream of the valve, a process called flashing.
How Cavitation Affects Your Valves & Why You Need Valve Repair Services for Cavitation
Cavitation is not a novel issue in industrial processes, it’s actually quite common. Control valves are engineered to withstand cavitation during its lifespan before natural wear begins. Nonetheless, if the phenomenon continues for an extended time, it damages both the valve and piping, leading to unacceptable process failures, including:
Valve Failure
As the vapor changes to liquid, the bubbles nucleate, grow, collapse, and rebound. That implosion damages the valve by creating pits on the valve’s internal passages components, such as the valve plug, seat rig, or body. As time goes by, cavitation-induced wear-and-tear continues. This extended exposure to erosion causes the valve to fail. For severe issues, valve repair services may not be an option and you may be forced to replace the valve entirely.
Reduced Fluid Capacity
Cavitation can also limit the capacity of fluid flowing through the valve. Rising levels of cavitation cause an increase in the amount of vapor in the wider valve outlet. This can lead to restricted liquid flow. If you haven’t received valve repair services at this point, the downstream vapor load becomes too immense such that it chokes the flow. That means no more fluid passes through the valve, no matter how low the downstream pressure is.
Exposure to Chemical Damage
Furthermore, cavitation increases the control valve’s susceptibility to chemical damage, especially if the fluid flowing through the piping is corrosive.
If caught in the early stages, you can get valve repair services to replace the damaged valve component. In other cases, you may need valve rebuild services. This involves disassembling the valve, replacing or refurbishing damaged components, reassembly, and testing to ensure it meets applicable standards. Refurbishing valve parts during valve rebuild services may include machining, welding, or applying valve coatings to restore functionality.
How to Detect or Predict Cavitation
It’s pretty challenging to anticipate the onset of cavitation, but it’s relatively easy to detect its severity. You can do that in two ways:
- Noise and rapid vibration – You can easily tell that your control valve is severely corroded when you hear a sound like gravel passing through it. That sound is usually accompanied by violent vibrations of the pipe and valve.
- Damaged internal components – If you notice irregular, rough, and cinder-like pits in the valve’s internal parts, which indicate severe cavitation in an application.
In its Recommended Practice 75.23-1995 (Consideration for Evaluating Control Valve Cavitation), the International Society of Automation (ISA) defines the cavitation index (σ) as the ratio between (P1-Pv) to (P1-P2). The severity of cavitation varies from the onset of cavitation to severe shaking or choking, depending on the value of σ.
The σ value doesn’t offer insights into a control valve’s performance in an application. The valve’s tolerance level varies based on the cavitation severity, and each application may be focused on different stages of cavitation. It could be noise, vibration, or damage.
Control valve manufacturers determine their product design based on their experience in each application and the design specs. Cavitation is highly likely to occur in high recovery valves. These have a lower vena contracta pressure, increasing the probability of bubble formation that causes cavitation.
The standard metrics used to measure cavitation damage potential include:
- Upstream and downstream pressures
- The fluid’s vapor pressure
- Fluid temperature
The damage potential depends on the duty cycle of the cavitation conditions. Suppose the phenomenon only occurs when there’s a rare upset in pressure; in that case, it won’t compromise the valve’s overall performance because the damaging effect is time dependent.
Tips for Solving Cavitation & Avoiding Frequent Valve Repair Services
You can solve cavitation in two ways. They include:
Avoidance
The best means of dealing with cavitation (and alleviating the need for frequent valve repair services) is to avoid it altogether. Incipient cavitation occurs when the pressure at the valve outlet is close to the liquid’s vapor pressure. Installing the valve at a point where the fluid approaches vapor pressure only increases the likelihood of cavitation.
On the other hand, if you move the valve to a different location, closer to the pump – where there’s high pressure and low velocity – the line losses are minimized. It increases outlet pressure above the liquid’s vapor pressure, significantly reducing the probability of cavitation.
Design the Control Valve for Cavitation
If there’s no way of avoiding cavitation, it would be best to design the valve to handle it to ensure minimal damage. You can achieve that in three ways:
- Robustness – Manufacturers engineer robust control valves using high-strength materials for cavitation-prone components. While the hardened materials will ultimately wear, they offer higher resistance. This can extend the valve’s durability despite extended exposure to cavitation.
- Elimination – Elimination minimizes cavitation by taking the pressure drop in a series of stages instead of taking all the pressure at once. A sudden surge in pressure results in a significant dip that drops below the vapor pressure. On the flip side, gradually taking the pressure in three or four stages eliminates the sudden dip, ensuring the pressure remains above the vapor pressure.
- Isolation – This involves diverting the vapor bubbles in the middle of the flow stream, where shock waves and microjet have a lower impact on the structural integrity of the valve components.
Cavitation occurs when the flow pressure leaving the vena contracta is closer to the vapor pressure. It can be challenging to anticipate erosion that potentially damages the valve’s internal components and the piping walls. Luckily, some formulae can help predict cavitation. Manufacturers and designers can redesign the control valve to mitigate cavitation. You can also work with manufacturers to choose valves with high tolerance to cavitation from the very start to avoid failures and unnecessary down time.
Expert Valve Repair Services at Leverage Mechanical Services
When you need specialty mechanical services, choose our team at Leverage Mechanical Services. Our experts offer a wide range of services to help you keep your industrial infrastructure at peak operating efficiency. We handle everything from hot taps to access components in large pipelines to repairing or rebuilding damaged valves. Our team travels throughout the Gulf Coast to provide top quality mechanical, machining, and repair services. Contact us now to request a quote for your service needs.