Turbo Tips: Practical issues with large fans in turbomachinery

Fans have been widely used in many facilities and plants. They have been employed to move gases, been used in different systems to avoid excessive pressure build-up in the equipment or service being served, and to maintain a specified draft over the intended application.

A fan should be selected for the match with its downstream and upstream systems to provide proper flow of gas. This article discusses fans and some practical issues associated with them. The focus is on practical notes and useful guidelines on fans. A case study is also reviewed.

Fan Configurations & Arrangements

Most experts discourage multiple parallel operation, instead recommending the use of just one operating fan at a time (one operating and another on standby) on each system. This reduces 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 special systems. In some revamp and renovation projects, two fans may be provided to work in parallel. Capacity control by various fan combinations may be more economical than other control methods in specific cases. Multistage fans may be necessary when pressure requirements exceed the capabilities of a single-stage fan.

Parallel Operation

Sometimes, two parallel side-by-side fans are close enough to share some items such as lubrication system, shaft, bearings, casing, etc. 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 reliability. Having two independent fans is 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, the fans 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 fans have individual ducts but are of equal resistance and joined together at equal velocities, they should be selected with the same total pressure. If fan velocity pressures are equal, static pressures will be equal. If the two streams join at unequal velocities, there will be a transfer of momentum from the higher-to the lower-velocity stream.

Fans in Series

Fans in series are usually at opposite ends of a system. Fans 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 pressure 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. With any fan, 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 commission to verify if operation and functionality of the fan was suitable for a load test. This fixed speed 1,485 rpm fan 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.

Initially, there were issues, namely different trips on high vibration. After resolving these problems, the test started and operation was sustained. The first problem afterward was a relatively high vibration at NDE (non-drive end) bearing with a recorded vibration of around 45 microns. While this was still below limits published by the manufacturer, it called for further investigation.

To start the investigation, the compositions of this high vibration (amplitude vs. frequency) were checked. Also details of balancing, alignment, etc., were checked and verified. The main part of vibration related to the operating frequency (1,485 rpm). Therefore, it was assessed as harmless. Also, the manufacturer confirmed that the provisions for re-balancing at site were provided. The key-phasor was provided for this fan and the contractor specialist provided a comprehensive vibration measurement and report including “amplitude – time”, “amplitude – frequency”, graphs, spectrum, etc. The review of the report showed no developing problem and no concerning issue.

Relatively high axial vibrations were recorded by handheld vibration measurement devices. Note that this was a large and critical fan. Yet permanent axial vibration sensors were not provided. The manufacturer was asked to investigate if a kind of permanent axial vibration sensor can be installed on this fan. The manufacturer proceeded to install an axial velocity-meter.

Another major issue was access to an inspection point of the lubrication oil return line. Sight glass for lubrication oil return piping lines on the DE (drive end) side was not accessible. Sight glasses on lubrication oil return piping lines should be accessible by the operator. This major fault was left unchecked from design and installation. It should have been corrected during commissioning. An additional platform was provided for this purpose.

The lubrication oil was found to be dirty. The source was poor flushing. This can cause major difficulties and reliability issues over time. A complete lubrication oil flushing was performed and all internal parts in contact with the lubrication oil were fully flushed and cleaned.

It was observed that the noise was extraordinarily high at the start-up. Noise measurements based on analysis of noise data were performed and no problem was found. A complete noise measurement was planned for the load test.