How do you design and implement effective anti-cavitation and anti-flashing control valve solutions?

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Cavitation and flashing are two common phenomena that can damage control valves and reduce their performance. They occur when the pressure drop across the valve causes the liquid to vaporize and then collapse or escape. This can create noise, vibration, erosion, corrosion, and leakage. To prevent these problems, you need to design and implement effective anti-cavitation and anti-flashing control valve solutions. In this article, we will explain the causes and effects of cavitation and flashing, and how to select and apply the best control valve strategies for your application.

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1 What is cavitation and flashing?

Cavitation and flashing are related but different processes that involve the phase change of a liquid to a vapor. Cavitation happens when the liquid pressure falls below its vapor pressure at the valve vena contracta, creating vapor bubbles. These bubbles then collapse as the pressure increases downstream, releasing energy and creating shock waves. Flashing happens when the liquid pressure remains below its vapor pressure after the valve, creating a continuous flow of vapor. This vapor can then condense as it meets a higher pressure zone, causing a rapid change in volume and velocity.

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Hossein TooToonchy

Chief AI Officer @ Epic labs | Digital Transformation Expert | Senior Generative AI Strategist | Ex-Amazon
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When a fluid passes through the constrictive passageways of a control valve, its average velocity increases. This is predicted by the Law of Continuity, which states that the product of fluid density (ρ), the cross-sectional area of flow (A), and average velocity (v) must remain constant for any flow stream: ρ1A1v1 = ρ2A2v2

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2 Why are cavitation and flashing harmful?

Cavitation and flashing can have detrimental effects on the control valve and the piping system, such as noise levels surpassing 100 dB, vibration-induced mechanical stresses, erosion of the valve trim, body, and seat, and corrosion of the valve surface. The high-frequency sound waves created by the formation and collapse of vapor bubbles can cause hearing loss and communication difficulties for nearby personnel. Pressure fluctuations and flow turbulence can lead to cracks, leaks, and failures. Furthermore, vapor bubbles and jets can strip away protective coatings or films on the valve surface, making it vulnerable to corrosion. All of these consequences can reduce the valve’s ability to control the flow, seal leakage, and resist corrosion.

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Bay Nguyen

Snr Lead Instrument & Control Engineer, Oil & Gas and Wind Energy
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The mechanism of damage of cavitation and flashing are different and they should be clearly defined: 1) For cavitation, the vapor jets produced by the implosion of vapor bubbles will continuously erode the metal surfaces nearby and also the downstream piping. 2) For Flashing, the high speed vapor bubbles will continuously "shoot" the metal surfaces and can defect the surfaces over time, making the flow coefficient and the flow curve bad and unacceptable later on.

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Hossein TooToonchy

Chief AI Officer @ Epic labs | Digital Transformation Expert | Senior Generative AI Strategist | Ex-Amazon
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No substance known is able to continuously withstand this form of abuse, meaning that cavitation will destroy any control valve given enough time. The effect of each microjet impinging on a metal surface is to carve out a small pocket in that metal surface. Over time, the metal will begin to take on a “pock-marked” look over the area where cavitation occurs.

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3 How to prevent cavitation and flashing?

The best way to prevent cavitation and flashing is to avoid or reduce the pressure drop across the valve. To do this, select a valve with a high flow coefficient (Cv) and low pressure recovery factor (FL). Additionally, choose a valve with a multi-stage or multi-path trim design, which has multiple orifices or channels that divide the flow and reduce the pressure gradually. Furthermore, make sure to size the valve properly according to the flow conditions and desired control range. Lastly, position the valve close to the downstream pressure source or device, so that it is located near the pump, tank, or heat exchanger that maintains or regulates the pressure. This will minimize the pressure recovery distance.

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Bay Nguyen

Snr Lead Instrument & Control Engineer, Oil & Gas and Wind Energy
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Additionally, the valve trim should be designed in such a way that the exiting fluid flow will be as far from metal surfaces as possible, so that the damage of surface will be minimized.

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4 How to mitigate cavitation and flashing?

If cavitation and flashing cannot be avoided, mitigation techniques can be employed such as selecting a valve with a high resistance to damage, choosing one with a noise reduction feature, installing a bypass line around the valve, or adding a backpressure valve downstream. The valve should have robust and durable material, coating, or lining to withstand erosion and corrosion caused by vapor bubbles and jets. Additionally, special trim design, cage, or silencer should be used to absorb or dissipate sound energy generated by cavitation or flashing. A portion of the flow can also be diverted or returned to the upstream or downstream side of the valve to reduce the flow rate and pressure drop across the valve. Finally, a valve, nozzle, or restriction can be used to increase the downstream pressure and prevent flashing.

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5 How to monitor cavitation and flashing?

To ensure that your control valve solutions are effective and reliable, you need to monitor cavitation and flashing regularly. This can be done by measuring the pressure and temperature upstream and downstream of the valve, which can help you determine the pressure drop, vapor pressure, and degree of cavitation or flashing. Additionally, measuring the flow rate and valve position can evaluate the valve performance, efficiency, and stability. Measuring noise and vibration levels can detect abnormal or excessive sound or movement caused by cavitation or flashing, while inspecting the valve visually or with a camera will identify signs of wear, damage, or leakage on the valve trim, body, or seat.

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Bay Nguyen

Snr Lead Instrument & Control Engineer, Oil & Gas and Wind Energy
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A much better way to prevent the cavitation and flashing is to focus on the valve sizing calculation. Valve datasheet will be used as the standard input data for the sizing calculation by the software. Standard softwares such as Intrucalc or developed by valve manufacturers (Fisher, Materflo, CCI ...) can be used, and one of the core features is to predict the cavitation/ flashing/ noise level on each operating case. If any abnormal issues related to cavitation/flashing/choked flow ..., a discussion with valve expert or valve manufacturer is highly recommended. Final solution might be a more suitable valve model that designed for such application.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you design and implement effective anti-cavitation and anti-flashing control valve solutions?

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1 What is cavitation and flashing?

2 Why are cavitation and flashing harmful?

3 How to prevent cavitation and flashing?

4 How to mitigate cavitation and flashing?

5 How to monitor cavitation and flashing?

6 Here’s what else to consider

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