Kent Clark | Feb 1, 2018
The strange relationship between pressure and state
You probably already know that the boiling point of water is 100°C (212°F) but what you might not know is that that only holds true when water is at 1 atmosphere of pressure. If you’re brewing at the top of the Mt. Everest, where the atmosphere is thinner, your water would boil at ~73°C (163°F). Pressure and temperature are closely related. At different pressures water boils differently and less pressure requires less heat for your fluids to approach a phase change. In fact, at 0 atmospheres water will boil at just ~20°C deg (70°F).
This may seem little more than a thought experiment but your brewing system changes pressure all the time and the resulting water vaporization can be massively damaging to your pumps and equipment.
- When fluids move, particularly when pushed by a propeller or impeller, areas of high and low pressure occur.
- This also occurs when fluids compress through a restriction, like a valve. In this scenario high pressure builds at the mouth of the opening, and low pressure just inside.
- When fluids change directions - such as in an elbow - high and low pressure zones can occur.
Most of the time these scenarios are not a problem. However, when you combine them, particularly while pumping something at a high temperature (like hot wort) these pressures and/or temperatures result in cavitation - the formation of bubbles within a liquid.
Cavitation can be recognized by a familiar boiling sound, sometimes described as “gravely” or “like marbles”. It results from bubbles that form and implode throughout the system. These bubbles are carried to a section of the system with increased pressure (or lack of vacuum) where they compress back into a liquid. The vapor state of materials - especially water - occupies many thousand times more volume than the liquid state, so the collapse of a vapor bubble results in a spectacular shock wave that can produce pressures of up to 1000 PSI.
These bubbles collapse like a supernova, first assuming a torus shape, then firing “microjets” outward with significant power. Both the imploding action and the microjets can damage equipment, commonly the face of the impeller, removing large chunks of material and/or many very small bits, creating sponge-like damage. This also means you now have metal floating around in your product!
There are two common types of cavitation that affect pumps: suction cavitation and discharge cavitation.
Suction cavitation happens due to a low-pressure at the eye of an impeller due to a lack of flow - maybe due to a poor coupling or too small a hose or a problem in your line. These implode against the impeller causing both large chunks to be broken off, as well as the sponge like damage texture.
Discharge cavitation occurs when the pump has trouble pushing fluids out - such as if the output is too small or there is a jam in the line. This forces the fluid to circulate inside the pump body. With little clearance between the impeller and the pump housing, fluids move very quickly, vacuum develops, and cavitation occurs. This causes premature wear of the impeller vane tips and the pump housing. It can also result in failure of mechanical seal and bearings.
Preventing cavitation means thinking about where pressure is building within your system and then preventing anything that would increase pressure or increase heat. However, there are a few simple tricks to keep in mind:
- Keep your suction side hoses large and short.
- Never try to suck through a filter, heat exchanger or other piece of equipment.
- Your suction hoses and lines should always be the same size or larger than the inlet of your pump. Keep them as short as possible.
If you’re already experiencing cavitation:
- Look at your hoses and fittings to make sure that the path the liquid takes to get to and from your pump is not getting choked.
- Check your filters and strainers, as clogs on either side of the pump can cause a pressure imbalance.
- Pay attention to your pump's curve using a flowmeter or pressure gauge.
- Avoid hard angles just before your pump’s inlet.
- Consider the distance from your tank or kettle to your pump, to make sure your fluids sufficiently cool.