VFDs and their HAZOP Considerations
A VFD is an acronym for a “variable frequency drive”, and these devices are used to control the rotational speed of a motor. The VFD unit is connected to the motor’s electrical supply, and can vary the frequency of that electrical signal to ultimately control the rotational speed of the motor. The better speed control of pumps, compressors and fans provided by VFDs can lead to better machinery performance and can even unlock energy savings and improve operational margins. However, the benefits of VFDs are multifaceted.
Energy Savings: VFDs can reduce motor energy consumption by up to 90%
Reducing Wear: VFDs start motors slowly by ramping up the voltage rather than slamming motors with full line voltage to start. This reduces wear and potential overheating
Offset Oversizing: In many applications, centrifugal fans, compressors and pumps are oversized to account for various process uncertainties. VFD speed control minimizes energy wasted by oversizing motors that may never run at full speed
Although installing a VFD into a machine can have performance benefits, it’s important to recognize that it could also introduce new risks into the process, such as the potential to exceed the original design speed of the motor leading to machinery damage and dangerous pressures and flows. If poorly maintained, VFDs can lead to insulation breakdown and heating in a number of applications. It is therefore important to consider the possibility of over speeding pumps and compressors with VFDs as part of the HAZOP in order to ensure these consequences are captured and that the system design has the appropriate safeguards in place.
To keep VFDs functioning optimally, they should be kept clean, dry and be inspected regularly to ensure all connections are tight. Most VFDs have side vents to provide cooling airflow, however they make the VFD susceptible to dust ingress. This can easily reduce the effectiveness of heat circulation fans and cause the VFD to malfunction, so these vents should be cleaned periodically. In addition, if a VFD is installed in a location where there is a high chance of air moisture condensation, a thermostatically controlled space heater should be installed to prevent the corrosion of the circuit board.