BSEE


ABB’s Carl Turbi talks cavitaon, and how modern variable speed drives can oer one of the easiest and most cost eecve ways of prevenng it


suffering persistent failures, then it can be easy to place the blame on the pump supplier. However, one of the most common issues with pump systems is caused not by the pump itself, but by a phenomenon known as cavitation.


I What is cavitation?


The discovery of cavitation can be traced back to naval ships in the late 19th century. HMS Daring, designed to be one of the fastest vessels of its day, carried out sea trials in 1885 which were


unsuccessful due to an unexplained propeller failure.


Several years later, and after exhaustive investigations, it was discovered that Daring’s high propeller speeds had caused vapour bubbles to form on the propeller blades. These bubbles were caused by areas of low pressure forming behind the propeller blade. As the bubbles travelled past the propeller, they encountered areas of high pressure and quickly imploded. The resulting shockwaves not only


f a pump system is


impaired performance, but over time caused catastrophic damage to the propeller’s metal surfaces. As well as propellers, cavitation also affects pumps, making it a cause for concern in any HVAC system that involves pumping of fluids. A pump impeller creates a vacuum on the suction side/inlet of the pump. If the vacuum gets too large or if the pressure is too low the water starts to boil and transforms from liquid to steam. When the steam bubbles reach areas of high pressure, they collapse back into a liquid state again, producing a shockwave which impacts upon the impeller surface.


These shockwaves cause vibrations which damage bearings and valves. Over time, they will cause erosion and eventually failure of the pump system. Persistent cavitation can typically shorten pump lifetime by around half, but in extreme cases can reduce it by as much as 80 percent.


What causes cavitation?


In HVAC pumps, the main cause of cavitation is insufficient Net Positive Suction Head (NPSH), whereby the


water pressure on the suction side of the HVAC pump is too low. This can be caused by clogged filters, pipe blockages on the suction side, or poor piping design.


It can also be caused by air getting into pipelines, or backflow where a tank is full, yet the pump is still pumping. If pumps are incorrectly specified then they may run too far to the right of the pump curve, meaning the pump may be too small for the volume of flow required.


Symptoms of cavitation


The most noticeable signs of cavitation are vibrations in the pump system, and noise. The exploding bubbles of vapour make a crackling sound, as if small rocks are rattling around in the pipeline. Flow and pressure levels may also drift from what you would expect to see according to the pump manufacturer’s specifications, while power consumption may also need to be higher to achieve the same throughput.


When exposed to cavitation over prolonged periods, pump components will eventually erode and fail. The impeller, bearings, seals


PUMPS


Why cavitation needn’t cause you aggravation


or even the whole pump system, may well need to be completely replaced once the damage gets to this point.


Preventing cavitation


There are several ways to mitigate cavitation. When designing a new pumping system, the Net Positive Suction Head available (NPSHa) must exceed the Net Positive Suction Head required (NPSHr) as specified by the pump manufacturer. NPSHa is the fluid inlet pressure of the system after all losses and other factors have been considered, such as friction losses, velocity, temperature and atmospheric pressure. Ways to increase the NPSHa include reducing the temperature of the fluid or pump, decreasing the number of elbows and valves in the pipework, reducing the length of pipework, or increasing its diameter. Variable speed drives (VSDs) can also help at the design stage. By providing precision control of the pump motor, they can help to ensure a much smoother flow and reduce stress on system components. This can free


30 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2020


Read the latest at: www.bsee.co.uk


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50