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Practical issues for centrifugal fans
Fans have been widely used in many facilities and plants to move gases. Supply and exhaust fans have also been used in different systems to avoid excessive pressure build-up in the equipment or service being served. Forced and induced-draft fans, too, have been used to maintain a specified draft over the intended application. A fan should be selected to match its downstream and upstream systems to provide a proper flow of gas.
Although one operating fan and another standby configuration has been used, multiple parallel operation is discouraged. Many experts have recommended using just one operating fan at a time (one operating and another standby) on each system, avoiding parallel operating fans. This is to reduce complexity and operational problems as parallel operation has always been complicated and is usually problematic. On the other hand, there have been various reasons for using multiple fans in parallel operation on some special systems. In some specific cases, such as some revamp and renovation projects two fans may be provided to working in parallel. Capacity control by various fan combinations may be more economical than other control methods in some specific cases. Multistage fans may be necessary when pressure requirements exceed the capabilities of a single-stage fan.
When two fans are used in parallel, they may be located quite remote from each other or close together. Sometime two parallel side-by-side fans are close enough to share some items such as lubrication system, shaft, bearings, and casing. Double-width, double-inlet fans are essentially two fans in parallel in a common housing. However, the above-mentioned sharing of items or parts should be avoided as it reduces the overall reliability. Two completely independent fans have always been preferred.
Parallel operation of identical fans has been studied and employed in many plants and facilities. The parallel operation of different fans is far more challenging. Parallel fans may have almost any amount of their operating resistance in common. At one extreme, the fans may have common inlet and discharge plenums. At the other extreme, they may both have considerable individual ductwork of equal or unequal resistance. Fans in parallel should all develop sufficient pressure to overcome the losses in any individual ductwork, as well as the losses in the common portions of the system.
When the fans do have individual ducts but are of equal resistance and joined together at equal velocities, they should be selected for the same fan total pressure. If fan velocity pressures are equal, static pressures will be equal. If the two streams join together at unequal velocities, there will be a transfer of momentum from the higher-velocity stream to the lower velocity stream.
Fans may also be in series but usually at opposite ends of a system. Those in series should handle the same amount of gas by weight measurements, assuming no losses or gains between fans. The combined total pressure will be the sum of the total pressures of individual fans. The volumetric capacities will differ whenever the inlet densities vary from one fan to another. In other words, because of compressibility, the volumetric capacities of the second fan will not equal the volumetric capacities of the first fan. With any fan, the pressure capabilities are also influenced by density.
Commissioning a large fan
The case study is for no-load test of a large, critical 2.3 MW fan. This no-load test was part of commissioning to observe and verify if operation and functionality of the fan was suitable for the next stage (load test). The fan was a fixed speed 1,485 rpm turbomachinery which was fed by 11 kV AC power electricity. The rated current was about 140 A and the estimated start-up current was around 750 A.
There were initially different trips on high vibration and some other issues. After resolution of different problems, the test started and operation was sustained. The first problem afterward was a relatively high vibration at the NDE (non-drive end) bearing. The recorded vibration was around 45 microns. This was still below limits published by the manufacturer. However, full investigations were done. Another concern was that this was about two or three times more than other measured vibrations on different bearings.
To start the investigation, the composition of this high vibration (amplitude vs. frequency) was checked. Details of balancing and alignment were also verified. The main part of vibration was related to the operating frequency (1,485 rpm). Therefore, it was assessed as harmless. The manufacturer confirmed that the provisions for re-balancing at site were provided. The key-phasor was provided for this fan and a contractor specialist provided a comprehensive vibration measurement and report that showed no developing problems and no concerning issues.
Relatively high axial vibrations were recorded by handheld vibration measurement devices on this large, critical fan. Permanent, axial vibration sensors should have been provided for such a large fan. The manufacturer, therefore, was required to install an axial velocity meter.
Another issue was the access to an inspection point of the lubrication oil return line. Sight glass for lubrication oil return piping lines of DE (drive end) side was not easily visible. An additional platform was provided to enable maintenance people to easily check these lines.
The lubrication oil was then found to be dirty. The source of dirt and debris was the system itself due to poor flushing. A complete lubrication oil flushing was done again and verified that all internal parts in contact with the lubrication oil were fully cleaned.