PUMPS


up vital NPSHa while reducing heat and energy use.


In calculating NPSHa it is always wise to build a margin of safety into the equation to further reduce the chances of cavitation occurring.


Retrofitting cavitation prevention


For equipment already in place, or where space or layout issues are a challenge, making significant alterations to the system design can be difficult. One option is to mount a pressure sensor at the inlet and monitor the actual pressure. The sensor feeds back to a PLC or similar which instructs the motor to reduce its speed, preventing the build ups of high pressure. This will likely require additional time and expense in installing and testing new control equipment. There is also a chance that latency may result in a delayed response between the sensor picking up a disturbance and the motor correcting it.


Realtime prevention


As well as helping to prevent cavitation at the design stage, modern VSDs also include functions that can detect and prevent it as it occurs. For pump systems incorporating VSDs this represents an easy and cost-effective cavitation solution requiring no additional equipment. Some ABB VSDs include anti-cavitation software that uses patented algorithms to look for specific patterns in the motor and drive operating data, and automatically reduces pump speed to decrease the risk of cavitation occurring. The algorithm is based on measurement of pump torque and speed, comparing results with normal torque and adapting accordingly to slow down the pump, prevent it from increasing speed, or stop the motor from running.


This means that cavitation can be solved automatically in real-time. Because the VSD is measuring directly from the motor shaft, any


BSEE


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potential flow issues can be picked up immediately with zero latency, and no need for additional sensors, controllers or interfaces. No extra components are needed for the VSD nor is any additional installation work required as the software is already pre-packaged within the drive. The effects of cavitation can take some time to become apparent, and unfortunately this can mean that by the time they are identified it is too late to save the pump system or its components. Rather than immediately destroying equipment, it will chip away gradually over the course of months or years. Nevertheless, ignoring cavitation


The most


uWhen cavitaon occurs over a prolonged period it can cause damage to pump systems. (Image via Shuerstock)


noceable signs of cavitaon are vibraons in the pump system, and noise. The exploding bubbles of vapour make a crackling sound, as if small rocks are raling around in the pipeline


’


can end up being an expensive mistake if it means having to replace systems more frequently. With VSDs now providing the functionality to detect and prevent it in real-time, pump lifetimes can be significantly increased.


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


BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2020 31


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