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BY ED HOLTGRAVER

If you fully understand the cause, effect and prevention of cavitation as related to the process and the hardware, feel free to move on to another article. However, if you are new to the industry or don’t have a strong understanding of cavitation, read on.

Definition
Cavitation in a pipe occurs only with liquids, since by definition cavitation is the result of a liquid temporarily being converted to a vapor state followed soon after by a return to the liquid state. But if we have a pipe with liquid flowing through it and the liquid is under pressure, how and when does the liquid change itself to a vapor and then back again to a liquid? And why is this a problem?

How and When
Figure 1 demonstrates the velocity and pressure within a pipeline when the flowing media, liquid in our example, flows through a restriction. Obviously, the same amount of liquid is flowing through all portions of the piping. The smaller flow area of the restriction requires that the media flows at a much higher rate when passing through the restriction. Likewise, the media flows at a faster than normal speed as it approaches and as it leaves the restriction.

A few hundred years ago, Daniel Bernoulli pointed out that, ignoring friction, the amount of energy within a flow stream is the same at all points in a pipe and is the total of the energy due to velocity + pressure + elevation. If we consider a horizontal pipeline, then for energy to remain the same at all points in the piping when velocity increases, the pressure portion of the energy equation must be less. Summation: When the velocity in a pipe increases, the pressure decreases, sometimes to very low levels.

Liquids
If we heat water above the boiling point, it will turn to steam, i.e., the vapor form of water. The temperature at which water boils decreases as the pressure on the water decreases. At sea level it requires a higher temperature to boil water than it does in the mountains. If a mountain is high enough, water would boil at room temperature, i.e., we can boil water at any temperature by lowering the pressure to below the vapor pressure of the liquid at that temperature.

In a pipe, if the flowing velocity is great enough, the pressure within the liquid may fall to below the vapor pressure of the liquid. If this occurs, the liquid will boil at this location and turn to vapor.

Restrictions and Cavitation
Figure 2 portrays the velocity and pressure values as a liquid flows through a restriction. Note that at the smaller sectional area, the velocity increases and the pressure decreases. In the first graph, the pressure does not decrease to below the liquid’s vapor pressure; therefore it remains as a liquid at all points in the flow stream. There is a lower downstream pressure caused by energy losses caused by the restriction.

This is pressure drop. The lower graph shows what would occur if the velocity were to be great enough to cause the liquid pressure to fall below its vapor pressure value. At the point where this first occurs, some of the liquid converts to vapor and this conversion process will continue as long as the pressure remains below the vapor pressure of the liquid. However, some short distance downstream of the restriction, the flow area increases and the velocity returns to normal, raising the liquid’s pressure to above the vapor pressure. When this “pressure recovery” occurs, the media returns to a liquid state. The vapor is said to implode back to a liquid – i.e., cavitation.

Why Should We Care?
First, if cavitation occurs, the flow volumes no longer act in accordance with established flow sizing equations, thus rendering inaccurate the data upon which the valve and piping sizes were selected.

Second, cavitation is accompanied by considerable noise levels. Described accurately as sounding like heavy gravel passing through the pipe, cavitation can create noise levels above those acceptable for nearby personnel.

Third, cavitation is accompanied by often severe damage to the valve and downstream piping. The implosion from vapor to liquid removes surface particles from contacted materials, eventually destroying the subject component.

Therefore, we want not only to understand what cavitation is, but what we might do to prevent its occurrence.

If the Restriction is a Valve
Figure 3 portrays the effect on velocity and pressure that might be expected when a valve is controlling flow.

As a liquid flows through the valve, the flow area is less in the valve than in the open pipe and the flowing velocity in these areas is higher than in the open pipe. Additionally, the valve may cause a distortion of the flow stream such that lesser and greater flow volumes occur across the open areas.

Shown for example and discussion is a butterfly valve and a globe valve. Each acts as a variable restriction to the flowing media but each has differing affects on the flow stream and the likelihood of whether cavitation may occur.

Butterfly valves with their relatively straight through flow path, allow very high velocities to occur within and downstream of the valve. Globe valves have a rather torturous flow path, thus the internal flowing velocities are less than in a butterfly valve, given the same eventual pressure drop. Is one valve type more likely than another to create a situation where cavitation occurs? Globe type valves are often referred to as “high recovery” valves, whereas butterfly valves would be said to be “low recovery” valves. What does this mean, and how does it relate to the onset of cavitation?

Earlier in this article it was stated that the pressure “recovers” as the velocity slows downstream of a restriction. The same applies when a valve is employed as the restriction. Internal to the valve there is higher velocity than there is both upstream and downstream.

When the velocity increases internal to the valve, the pressure decreases. Downstream, the velocity slows and the pressure increases, or recovers, from the lower values internal to the valve.

With its more torturous flow path, a globe valve creates an equivalent pressure drop to that of a high butterfly valve but with lower velocities within the valve. Therefore, given the same pressure drop, the pressures internal to the globe valve will not be reduced to as low of a level as they would be in a butterfly valve. Since there is a smaller difference between the internal pressures of a globe valve and the downstream pressure, the globe valve is described as a low recovery valve. Logically, butterfly valves are described as high recovery valves.

Given identical differences between upstream and downstream pressures (pressure drop) there will be lower pressures internal to a butterfly valve than in a globe valve, although again, each causes the same pressure drop.

Figure 3 shows three situations. One is where the internal pressure of a high recovery and a low recovery valve is equal. The downstream pressure will be greater, by definition, for a high recovery valve.

The second situation shown in Figure 3 shows that while the downstream pressure, and pressure drop, may be identical with either a high or low recovery valve, the internal pressure within the high recovery valve will fall to a lower value that it does within the low recovery valve, and may fall below that of the vapor pressure of the liquid. Thus one valve type might experience cavitation while another, though controlling the same pressure drop, may not experience cavitation.

The third situation shows that a high recovery valve may induce cavitation even though the pressure drop it creates is less than the pressure drop caused by a low recovery valve.

If using a valve to cause a pressure drop (as opposed to control of flow volumes), it is safe to say that a low recovery valve will resist causing cavitation more so than a high recovery type. Of course, all valves can cause cavitation and to a differing degree at different travel positions. To predict if a valve will experience cavitation, certain valve manufacturers have performed extensive testing and can be relied upon to provide prediction and prevention information to users.

Can We Design to Reduce Cavitation?
Absolutely. With the information available from your valve supplier, it is possible to select an optimum valve type. Occasionally, it may be possible in a pressure control application to fit two or more high recovery valves in tandem within the piping, taking an equal fraction of the drop across each valve. In addition, some valve manufacturers have developed extremely effective “trim” or valve internals that combat both the onset and effect of cavitation. Again, look to your supplier for assistance.

ED HOLTGRAVER is CEO of QTRCO, Inc. (www.qtrco.com), located in Tomball, TX. Reach him at 281-516-0277.

This entry was posted on Wednesday, September 26th, 2007 at 5:54 pm and is filed under Cavitation. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.