What is Cavitation? - Valve World

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What is Cavitation? In my previous article, Valve World November 2011, I specified the problems of cavitation in process valves. In this issue I give you an explanation of cavitation – the physics behind it – for a better understanding and know how to avoid, or should I say control, it in the future. By Günter Öxler

Cavitation is a description of a physical process which can occur in piping systems handling fluid media with a negative influence on plants (assets) and components. This process occurs in 2 steps (see Fig. 1): • S tep 1: Conversion from fluid t vapour state. • S tep 2: Conversion from vapour to fluid state

These theoretical basics can also be directly implemented in other cases.

What are the results of cavitation? • Loud cracking noise • Strong vibrations • Material abrasion (Cavitational damage) – damage of assets!

When does water vapourise? The conversion of the two phases – fluid into vapour –depends on the two parameters: • Temperature • Pressure The correlation is illustrated in the vapour pressure curve – Fig. 3.

Typical cavitational damage Example shown – a butterfly valve (see Fig. 2)

Operating conditions • Upstream pressure: 1.2 - 1.4 bar • Back pressure: 0.1 bar • Flow velocity: 2.2 m/s (related to DN) • Working period: 2 years • Opening angle of the Butterfly: approx. 30° The above = wrong operating conditions as a butterfly valve is an ON-OFF valve and not a control valve.

What leads to cavitation? • A fluid media vapourises if there is not enough energy to hold the molecular structure.

Fig. 3 At atmospheric pressure (1 bar) water vapourises at 100°C With decreasing pressure the vapourising process begins at a lower temperature. Example: At a pressure of 0.02 bar, water vapourises at a temperature of 18°C (see Fig. 4).

Fig. 1

Where does cavitation occur? Cavitation is possible within any kind of fluid media where a conversion from fluid to vapour aggregate state is possible. Cavitation then becomes a possibility if the media is subject to high differential flow velocity. Moving / running parts: • Turbine shuffles • Pump propellers • Ship propellers

Fig. 4

Non-moving parts: • Cross-sectional reductions in pipelines • Constriction with vanes • Constriction on process valves

Fig. 2

Why & how is it possible to go below the vapour pressure?

• T  he molecules separate from each other. Vapour bubbles rise.

The following discussion concentrates on constriction procedures in process valves.

The following describes an example with water as the medium.

Streaming water in a pipeline is under pressure, held by a pump or geodatic difference in the height-level (gravity line) and is much higher, than the vapour pressure.

www.valve-world.net

March 2012

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Column To clarify why the vapour pressure at the constriction point of a valve can be lower we have to look at the energy balance of the streamline.

Energy of streaming media:

Also the losses increase because of the constriction. At the smallest diameter – because of the constant energy – the remaining pressure energy drops tremendously locally (see Fig. 7).

The energy of a streaming medium is generally composed of the following single energies (Fig. 5):

Energy flow at the constriction point: When the pressure in that area falls below the vapour pressure – the water vapourises. Vapour bubbles rise, ... ... with increasing pressure under deformation... ... and finally implode – (see Fig. 8).

Fig. 5

Fig. 8

Implosion of the vapour bubbles: In the middle of the pipeline (Fig. 9) the implosion of the vapour bubbles does not result in any damage, and the intensity is pressure dependant.

Fig. 6 The energy flow at the constriction point: With a reduced cross-section at the constriction point (vena contracta), the velocity increases and with it, the energy.

Fig. 9

Fig.7

On conversion from vapour into fluid, the bubbles collapse immediately (implosion), the water around the bubbles speeds up in less than 1/1,000 of a second which creates a so called “Microjet“ which has a velocity of more than 1,000 m/s. It directly leads to the body, pipeline wall resulting in pressure peaks of up to 10,000 bar, where this, in the molecular area, results in abrasion (See Fig. 10).

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When does cavitation occur? Main factors of influence: • high differential pressure • low backpressure • high flow velocity

How to avoid cavitation?

Energy flow in a pipeline: In the water reservoir, the existing energysum of the standing stream is stored as potential energy. This potential energy is converted by passing a horizontal line into (see Fig. 6): • Velocity energy • Pressure energy • Losses energy

Fig. 10

Cavitation is a physical effect, which occurs depending on the plant conditions. During the design stage and the selection of the process valves the process should be kept free of cavitational criteria. Where this is not possible, by means of appropriate process valves a plant can be made reliable – cavitation resistant. The following basics need to be considered: • Gate and butterfly valves installed only as ON – OFF valves • Needle valves considered in the specific design • Finally step-wise reduction or with aeration of extreme situation / processes, where they cannot be handled with standard designs, e.g.: orifice to increase the backpressure I hope this gives you an idea, at least, of what cavitation is, how it happens and how to avoid it which may help you in the future to achieve a proper process valve installation. I look forward to receiving additional questions from your side as I am available to support & guide you through the selection process of valves.

Günter Öxler [email protected] www.valve-world.net