Make Success With Centrifugal Blowers a Breeze

Consider a variety of factors to achieve efficient and reliable operation.

By Amin Almasi, rotating equipment consultant

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Diverse applications rely on blowers to supply gas at relatively low pressure. Process plants usually opt for centrifugal blowers; they use rotating impellers to increase the speed of gas streams. These blowers most often have a single sophisticated three-dimensional impeller — but special blowers sometimes use two or three impellers.

Centrifugal blowers often have variable frequency drives (VFDs) to optimize efficiency as operating conditions change, provide overcurrent protection, reduce power consumption, and facilitate soft startup and shutdown. The most important aspect of VFDs is preventing surge over a wide operating range.

Capacity turndown and delivered head variation are both important features of a centrifugal blower. Speed variation usually serves as the fundamental means of capacity and head control. In many cases, a system of variable inlet guide vanes (VIGVs) also is installed as another way to regulate the blower operation to give the required operational flexibility. Using both methods, a blower can provide a minimum flow as low as 20–30% of the normal flow rate.

Design And Operation

Overhung designs are popular for centrifugal blowers. In some designs that use a gear unit, the impeller is mounted on the gear unit flange. An overhung design offers many benefits such as simplicity and easy access for servicing the diffusor vane assembly or other parts while the casing is off. For safety reasons, a vent valve allows the volute to be purged with inert gas prior to startup.

The impeller usually is three-dimensional semi-open (unshrouded) design. Its vanes typically are machined out of a solid forging. Material options include aluminum alloys, stainless steel and alloy steels — and even titanium alloys for some special applications. The impeller is fixed to the shaft by different methods — a popular one is a cylindrical polygon fit, with its unique torque transmission capability. A key-less option generally is preferred.

Spare parts and redundancy are important for critical components such as lubrication oil pumps, oil filters, oil coolers, etc. Storing individual parts of a component in a warehouse is fine but an entire unit, such as a spare lubrication oil pump, should be installed as a standby rather than kept in a warehouse. Sometimes, installing a redundant blower is wise (Figure 1). Standby units require regular monitoring to ensure they will work when needed.

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Balancing and spin testing are important for impeller assemblies of high-speed blowers. Each element of the rotating assembly, i.e., impeller, shaft, high-speed gear, etc., should be dynamically balanced to close tolerances. The complete rotating assembly once assembled then should undergo dynamic balancing before installation. Finally, during the mechanical test of the complete unit in the shop, vibration probes should verify lack of vibrations due to unbalance.

For many high-speed blowers (say, above 9,000 rpm), balancing the whole high-speed rotor assembly to ISO 1940 Grade G1 is desirable. However, in some cases, G1 isn’t possible and, for many other cases, assembly balancing to ISO 1940 Grade G2.5 will suffice. Blower impellers usually undergo a spin test in a vacuum chamber at 1.2 times the nominal speed (or 1.15 times the “maximum continuous speed”) together with dimensional checks before and after the test.

Shop performance tests have been specified for many process blowers, with ASME PTC-10 used for many critical units. However, for low-pressure blowers, vendors often propose testing with compressed air in an open loop configuration to reduce the cost of the performance testing. For some of these blowers, such a performance test can’t accord with ASME PTC-10 because air can’t provide aerodynamic similitude. For example, if a large process gas blower must comply with the ASME PTC-10 performance test, using air as the test medium would require the blower to reach 124% of design speed, which may be impossible.

Vibration And Reliability

Radial vibration sensors and monitoring commonly are specified for blowers. X-Y vibration measurement probes for each bearing and a keyphasor transducer for each shaft usually are provided. Sometimes axial probes aren’t specified for low-pressure blowers. This is a major error. While some engineers think axial sensors only are useful for high-pressure applications, this isn’t true. Many mechanical issues and damaging mechanisms, such as misalignment, bent shafts, etc., do create axial vibration. In addition, axial loads and axial vibration (movements) can be affected by machinery process load changes, variation in operating conditions, surge, other operational issues, etc. Axial displacement probes can identify all these very effectively. Moreover, axial vibration measurement will help avoid or predict issues with the thrust (axial) bearing/collar (or similar device), which often is a vulnerable part in such a machine and the cause of many difficulties and failures. High axial vibration also can result in seal problems. Protecting both the thrust bearing and seal requires axial displacement probes. This is particularly true for any blower above 250 kW or in a critical process service.

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