Lesson 10 - Control of Fluid Flow 2
This lesson will conclude our study of pumps. It will focus on two topics that deal with the unwanted generation of gas or vapour from the liquid being pumped – cavitation. It will then discuss the means by how we can ensure that cavitation does not occur when operating pumps.
Point of Reflection – Reconvene
Welcome back. So, what were your thoughts?
The best way to identify preventative and contributory cavitation factors is to consider the various pressure terms:
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Vapour pressure – for any given set-up, if the vapour pressure was reduced, then the available NPSHA would increase. In practice, this would require the wort temperature to be reduced before the wort is cast, which may not be possible. Therefore, vapour pressure (and hence wort temperature) is a contributory cavitation factor. This explains why cavitation is a key consideration when transporting hot liquids.
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Atmospheric pressure – if this increased it would increase the NPSHA as atmospheric pressure pushes down on the pump suction via the wort in the system. In practice, there is nothing that we can do about controlling atmospheric pressure. But, it is a parameter that we should be wary of when comparing systems at vastly differing altitude, where atmospheric pressure could vary by as much as 15-20 kPa (1.5 to 2 m water). Atmospheric pressure is a preventative cavitation factor.
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Vessel wort depth – in a similar way as atmospheric pressure, an increase in wort depth would increase the pump suction pressure and increase NPSHA. Wort depth would be a preventative cavitation factor.
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Suction line pressure drop – any loss of pressure (we rely on keeping pressure high enough through having a high value of NPSHA to prevent cavitation) is a bad thing from a cavitation perspective. This is where good line design using the Darcy-Weisbach equation and loss coefficient approach comes to the fore. Suction line pressure drop would be a contributory cavitation factor.
To summarise, cavitation preventative factors which increase NPSHA include:
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Increased atmospheric pressure
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Increase vessel headspace pressure
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Increase vessel wort depth
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Reduced suction line pressure drop
Having looked at available NPSH, let us now look at the second NPSH parameter – the NPSH required by the pump, NPSHR.