Nautical engineers design propellers for submarines, and mechanical engineers design control valves for industrial plants. These engineering tasks have more in common than you might think.
Let Off Some Steam
As a submarine propeller turns, it creates a low pressure area at the trailing edge of each blade. Now, at sufficiently low pressures, water becomes steam. Yes, water becomes steam at sufficiently high temperatures, but it also becomes steam at sufficiently low pressures. In other words, you can boil water with low pressure just as you can boil it with high temperature.
So, if the spinning propeller produces sufficiently low pressure on the trailing edges of the blades, steam bubbles are created in the water.
These bubbles are called cavitation. That is, they are essentially cavities of vapor (the vapor in this case being steam) inside of a liquid. These bubbles are perfectly happy as long as they stay in low pressure, but they don’t stay long. The blades keep moving, taking with them the low pressure and stranding the cavitation bubbles in the high pressure of the deep ocean– where they pop.
Run Silent, Run Deep
These pops produce noise, and since a submarine must run silently to avoid detection, noise is a submarine’s enemy.
Now About Those Control Valves
Just like low pressure on the trailing edges of submarine blades, low pressure inside a control valve can produce cavitation.
As liquid traverses a control valve, it flows through an area called the vena contracta, which is the narrowest part of the valve.
The liquid’s velocity increases through the vena contracta just as water’s velocity increases when you put your thumb over the end of a garden hose. Your thumb creates a vena contracta.
By Bernoulli’s Principle, increasing a liquid’s velocity decreases its pressure. Now, liquids have a “pressure of vaporization,” the pressure at which the liquid becomes a vapor. If you increase the liquid’s velocity enough to drop the pressure below the pressure of vaporization, the liquid will cavitate. In controls valves, this is not uncommon.
This graphic shows the relationship between the pressure and velocity of a liquid. Velocity (V2) through the vena contracta (the narrow part of the pipe) is increased from velocity (V1). This velocity increase causes a pressure decrease, which is why the liquid’s pressure (P2) at the vena contracta is lower than the pressure (P1) at the wider part of the pipe. If the velocity through V2 has increased sufficiently high to decrease the pressure below the liquid’s pressure of vaporization, the liquid cavitates.
Like the cavitation bubbles created by propeller blades, the cavitation bubbles created by a control valve’s vena contracta are perfectly happy as long as they stay in low pressure, but they don’t stay long. The liquid keeps moving through the valve, bringing the cavitation bubbles with it, and once they leave the low pressure of the vena contracta, the bubbles are exposed to pressures above the liquid’s pressure of vaporization– and they pop.
Can You Turn That Down Please?
Like with the submarine, these pops produce noise. Cavitation noise can sound like gravel flowing through control valves, and it can be very loud.
Maritime engineers design submarine propellers to minimize cavitation, thereby minimizing noise. Likewise, mechanical engineers design control valves to minimize cavitation, thereby minimizing noise.
But noise is not the only unwanted effect of cavitation. When cavitation bubbles pop, they release destructive energy. In a future article we will examine the damage caused to propeller blades and control valves by cavitation.
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