Lesson 10 - Control of Fluid Flow 2

The specifics of dissolved gases will be covered in the Gases Unit. For the moment, all you need to be aware of is that there is a direct relationship between the pressure (and temperature) that a liquid-dissolved gas system is held under and the maximum quantity of dissolved gas which can be kept in solution.

This pressure, called the equilibrium pressure, is a higher value than vapour pressures encountered in brewing, e.g. at 5 °C, the equilibrium pressure to hold 2.5 v/v of CO₂ in water is approximately 1.8 bar abs.

The importance of cavitation

So what’s the big deal with cavitation? Why would we be concerned about it enough to design and operate our pipeline systems to avoid it? The following reasons highlight these concerns:

• Impaired pump performance as cavitation creates vapour. This vapour can impair the performance of a pump by gas-binding the pump (discussed in the lesson Control of Fluid Flow 1).

• Slugs of unwanted vapour can adversely affect pipeline instrumentation, such as flow meters, which would register values vastly different from the actual flow rate.

• Physical damage to pumps, valves, pipes, and the liquid being transported. It might be difficult to imagine that a liquid could physically damage the metal of a pump or valve, but think of storms and shipwrecks, the impact of the sea on concrete breakwaters, and even when you get a dive into a swimming pool wrong – a belly-flop that knocks the wind out of you. The cause of cavitation damage is much more than merely the simple force of a mass of water, it involves implosions. Let us look at this in a little more detail.

Cavitation micro-damage

In general, it is not so much the flashing of vapour in a liquid line which is problematic. Instead, it is what happens on a micro-scale during the collapse of the vapour bubble. At the time of bubble collapse, there is a small void (or cavity) in the liquid, which the bubble once occupied. The surrounding liquid rushes in to fill the void. When this liquid, which is moving from all directions around the cavity, reaches the centre, there is a collision which leads to an implosion, where extremely high local pressures (up to 1,000 MPa) can occur for an instant. Commonly, cycles of bubble formation and collapse can occur many thousands of times in a second.

If cavitation occurs near a liquid-solid interface, e.g. a pipe wall or a pump impeller etc., then these surfaces are exposed to what is, in effect, thousands of miniature implosions. The consequence of these can include:

• Severe surface pitting and erosion, also known as spalling, which can lead to a reduced component life cycle. (A spall is a small piece or flake of material broken off of a larger piece of material).

• Permanent loss of performance due to the loss of close tolerance geometry – prone in valves and pumps.

It is not just equipment which may be affected by cavitation, but also the liquid which we are transporting. The high velocity gradients created during cavity in-filling and collision result in high shear stresses. So, if the liquid is sensitive to high shear stresses, then associated shear damage can occur. The damage of yeast cells due to cavitation is an example of this.

Signs of cavitation

Before we look at the analytical (calculation) means of ensuring that our liquid transport systems do not cavitate, let us first consider some signs of cavitation and the process areas to be particularly focused on when reviewing a system for the potential to cavitate:

• The classic indicator of cavitation occurring is the audible sound of rapid cycles of bubble formation and collapse. As much as you may think that super-human hearing might be required to hear such bubble behaviour, it is not. Such behaviour sounds like gravel is running through the pipe/valve or pump.

• Transport of very hot liquids, either by gravity or pumping, which typically are just below their boiling point should be treated with caution. The vapour pressure of the liquid is at its highest, thereby reducing the margin by which the pump suction line pressure needs to drop to cause cavitation. Classic examples of brewery situations where very hot liquids are transported are:

  • Wort being pumped over to the whirlpool from the wort boiler

  • Wort being pumped from the whirlpool to wort cooling

  • Hot caustic solution being pumped from a CIP plant

  • Hot water systems, such as those used to sterilise fillers and plate pasteurisers

• Liquid systems which have a substantial pressure loss within them, e.g. throttle valves, substantial pipe section changes, orifice meters, pressure drops across filters, and tank outlet lines where the tank runs empty.

We shall move on to explore the analytical means by which we can ensure that our liquid transport systems, and in this case specifically pumps, do not cavitate. We will be looking at the rather oddly named concept of ‘net positive suction head’. Do not worry, the name is actually quite useful when trying to understand the concept. Let us have a look at this now.