- Published on 02 March 2015
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By: Geoff Alder
In this article, originally published in RACA Journal March 1999, we take a look at pumps in parallel and series
Let's look at an undesirable situation called 'cavitation'. A pump curve expresses pressure versus flow generated by a specific pump with a given impeller diameter and operating at a fixed rotational speed. See Figure 1.
Each published curve cuts off at some point. What happens should you find that there is a pump operating in the area to the right of that cut-off? The answer is easy: you make very sure that you never go there. That may be easier said than done, so we will discuss the situation. As water enters and proceeds through the impeller, it becomes subjected to increasing centrifugal force. Its pressure increases. But before that happens, it has to make its way into the impeller.
Water approaches the impeller axially through the eye of the pump, and then is turned through 90° in order to flow radially through the impeller. As you can imagine, there is some unseemly pushing and shoving at this point. As flowrate is increased, this pushing and shoving gets worse.
It is represented by a pressure dip. Before centrifugal force makes itself felt, right at the entry to the impeller pressure initially plunges down. The greater the flow, the more pronounced this dip.
We have often spoken of the saturation pressure of a refrigerant, and of the temperature-pressure relationship at saturation. We have in fact often used water as our example of a refrigerant (remember, R718 is good old H2O, just as it comes out of the tap. Well, maybe after being distilled!) When this pressure dip takes the entering water down below its saturation point, some water flashes instantaneously into pockets of vapour. But far quicker than a wink, it proceeds deeper into the impeller to enter into a pressure zone which is again above saturation.
The pockets of vapour exist only for milliseconds. They again collapse with a crunching noise and with disastrous effects upon the impeller. These implosions take place where the inflowing water is pushing the vapour pockets against the material of the impeller, and the effect of this is to exfoliate fine layers of bronze or cast iron from the impeller. The sound effects are weird.
They sound to all the world as if there is a handful of small stones in the impeller. More than one pump has been stripped in order to remove these non-existent stones from the pump housing, leaving the guy with the spanner very perplexed after having found no stones, only then to find that they have mysteriously returned once the pump starts again.
- Published on 01 January 2015
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By: Geoff Alder
In this, and future editions, we will be rerunning various ‘Our Industry’ articles that are still relevant to the trade to ensure that Geoff Alder’s valuable knowledge isn’t lost. We thank Glenda Alder for allowing us to continue Geoff’s work posthumously.