A practical guide for engineers and technicians working with liquid control systems in industrial plants.

1. 1. What is cavitation?
2. 2. What damage does it cause?
3. 3. The K_C number — how to measure the risk
4. 4. Fix 1 : Choose better materials
5. 5. Fix 2 : Change the valve design
6. 6. Key takeaways

Control valves are workhorses in any industrial plant. They keep pressure, flow, temperature, and liquid levels in check. But there is one problem that can quietly destroy a valve from the inside — cavitation.

In some cases, cavitation can cause serious damage in just a few months. The good news: if you know what to look for, it is manageable. This guide explains what cavitation is, how to measure the risk, and what you can do about it.

1. What is cavitation?

When liquid flows through a control valve, it has to squeeze through a narrow gap between the plug and seat ring. That narrow gap speeds the liquid up — and when liquid speeds up, its pressure drops.

If the pressure drops far enough — below the liquid's boiling point at that temperature — small vapor bubbles form inside the liquid. This can happen even with cold water, as long as the pressure drops low enough.

A moment later, the flow opens back up and the pressure rises again. Those bubbles then collapse suddenly and violently. That collapse is cavitation — and it releases a huge amount of energy in a very small space.

2. What damage does it cause?

When those bubbles collapse near a metal surface, they hit it with enormous force — sometimes the equivalent of hundreds of atmospheres of pressure, over and over again. This gradually eats away at the metal. The technical term is cavitation erosion.

The parts most at risk are the plug, seat ring, cage, and valve body. Over time, the damage leads to :

• The valve leaking when it should be fully closed
• Loss of accurate flow control
• Complete valve failure — sometimes within months
• Damage spreading to the downstream piping

One tricky aspect: cavitation conditions are not always obvious at the design stage. A valve may run fine for a while, then conditions shift — and by the time damage is visible, it may already be serious.

3. The K_C number — how to measure the risk

Engineers use a simple number called the cavitation coefficient K_C to estimate how likely cavitation is. Here's the formula :

P₁ : Upstream pressure (absolute)

P₂ : Downstream pressure (absolute)

P_V : Vapor pressure of the fluid

The higher the K_C value, the more likely cavitation is to occur. Think of it this way: a high K_C means there is a large pressure drop across the valve, and the liquid is getting close to its boiling point inside the valve.

4. Fix 1 : Choose better materials

When some cavitation is unavoidable, the goal is to use materials that can take the punishment. The key parts to think about are the trim — that is, the plug, seat ring, and cage.

For the trim (internal parts)
When K_C ≥ 0.5, or when startup conditions are particularly harsh, hardening treatments are used on the trim. Common options include

• Hardened stainless (SUS630)
  A heat-treated steel with good hardness and corrosion resistance. Common in cage and rotary valve applications.
• Martensitic SS (SUS440C)
  Very hard stainless steel. Good for globe valves, hot water above 100°C, and mist-laden steam.
• CoCr-A Hardfacing
  A cobalt alloy welded onto the plug and seat surface. Extremely hard and resistant to wear and corrosion.
• Colmonoy Hardfacing
  A nickel alloy overlay, similar to CoCr-A Hardfacing. Works especially well where organic acids or alkalis are present.

5. Fix 2 : Change the valve design

For more serious cavitation conditions, changing the valve's internal structure is the most effective approach. The goal is either to stop the bubbles from forming, or to make sure they collapse away from metal surfaces.

• Cage-type globe valve
  In a standard globe valve, bubbles often collapse right next to the valve body wall — causing direct damage. A cage design guides the flow through small holes in a cylindrical insert, steering the collapse zone away from the wall and into the open flow stream.
• Multi-hole cage valve
  Many small holes in the cage split the flow into lots of tiny jets. These jets meet in the middle of the cage, away from the walls, and the energy is released in open space. This helps protect the cage walls from erosion.
• Labyrinth trim valve
  A labyrinth is a series of small chambers that the fluid zigzags through. Each chamber gradually slows the fluid down and reduces its pressure step by step. The result: the dangerous bubble-collapse zone moves downstream, away from the seat and plug. As a bonus, if the first seating surface wears out, the seat ring can shift to expose a second one — giving the valve a longer service life.
• Multi-stage pressure reduction
  Instead of one big pressure drop, this design splits the drop into several smaller stages. By the time the fluid reaches the narrowest point, the pressure has already fallen gradually — so it never drops far enough to form bubbles. This is the most direct way to prevent cavitation from occurring at all.
• Downstream orifice plate
  A simple orifice plate installed in the pipe after the valve shares the total pressure drop between two points instead of one. Less pressure drop across the valve means less risk of cavitation. The effect is strongest when the valve is open wide.

6. Key takeaways

Cavitation is complex, and no formula can predict it perfectly. But these four steps cover the essentials :

• Check K_C early. If it is above 0.5, plan for protection — don't wait until damage appears.
• Use harder materials on trim parts. The plug and seat ring take the worst of the impact energy.
• Design so bubbles collapse away from metal. Cage valves, multi-stage designs, and labyrinth trims all do this in different ways.
• Inspect regularly. Even well-protected valves need periodic checks. Internal trim is replaceable — the valve body is much harder to fix.

Have questions about cavitation, or want to find the right control valve for your application? Contact Azbil Thailand — our technical team is happy to help.

7. Frequently Asked Questions: Control Valves & Cavitation

Basics & Identification

• What does cavitation sound like?
  It typically sounds like gravel or small rocks rattling through the pipe, or a distinct crackling or popping noise. It differs from aerodynamic noise (which sounds like a constant hiss or roar) and explicitly indicates a liquid service valve handling a sharp pressure drop.
• What is the difference between cavitation and flashing?
  Both involve vapor bubbles forming at the vena contracta (the point of lowest pressure just past the plug and seat ring). With cavitation, downstream pressure recovers and drops the bubbles back into liquid, releasing destructive shock energy. With flashing, downstream pressure remains below the boiling point, so bubbles continue down the pipeline as vapor without collapsing inside the valve body.
• Can cavitation happen at room temperature?
  Yes. Cavitation is driven by pressure changes, not high temperatures. If the local pressure inside the trim falls below the fluid’s vapor pressure—even at 25°C—cavitation will occur. This is why cold water processes with high pressure differentials suffer heavy erosion.

Damage Signs & Lifespans

• How quickly can cavitation erosion cause catastrophic damage?
  It depends entirely on the K_c severity and the selected trim metals. In mild environments, wear takes years. In harsh, high-pressure setups utilizing non-hardened alloys, serious structural erosion can destroy critical sealing components in under 3 months.
• How do I know if my valve has internal cavitation erosion?
  Look for these primary indicators:
  • A gravel-like rattling noise coming from the valve body.
  • Gradual loss of flow control accuracy.
  • Increased seat leakage when the valve is fully closed.
  • Visible pitting, cratering, or a "sponge-like" texture on the plug or seat ring during routine teardowns.

Maintenance & Troubleshooting

• How often should a valve operating in cavitation conditions be inspected?
  For systems with a K_c ≥ 0.5 in continuous operation, an annual internal inspection is a safe baseline. If early signs of pitting are found, or if you are running highly severe pressure drops, tighten the schedule to every 3 to 6 months until you establish a predictable wear profile.
• My valve ran perfectly fine for two years and suddenly started making a loud rattling noise. What changed?
  This is almost always caused by a shift in plant process conditions. If production scales up, seasons change, or upstream/downstream equipment is modified, the flow rates, inlet pressures, or process temperatures shift. A valve perfectly sized for original design specs can easily shift into a destructive K_c range when process parameters drift.
• Can a valve tolerate low levels of cavitation, or is it always unacceptable?
  A small amount of cavitation does not cause immediate failure; it depends on the duration, frequency, and structural integrity of your trim. However, cavitation is never completely benign—it causes gradual cumulative wear. The Azbil standard of K_c = 0.5 serves as the explicit engineering threshold where active countermeasures should be implemented.