Rotor thrust

Back-to-back impellers allow for a balanced rotor thrust and minimize overloading the thrust bearings. 

An aeeurate determination of rotor thrust balanee losses eould be made by measuring the flow, pressure, and temperature. However, in keeping with the aeeuraey of item 2, Elliott reeommends the use of a ealeulated design value of the horsepower loss. 

Rotor Thrust Problems Thrust loads in compressors due to aerodynamic forces are affected by impeller geometry, by pressure rise through the compressor, and by internal leakage due to labyrinth clearances. The impeller thrust is calculated by using correction factors to account for internal leakage, and a balance piston size is selected to compensate for the impeller thrust load. The common assumptions made in the calculation are... 

Thrust disc Thrust disc fits between the thrust bearing assemblies. Thrust disc and bearing arc desisted to absorb the rest 25% of the rotor thrust. 

Overhaul pump reposition individual impellers as needed. Reposition whole rotor by changing thrust collar locator spacer. 

Rotor Seals To balance the thrust on the rotor, usually there are one or two labyrinth-type seals on the rotor. These seals often are damaged if there is dust in the incoming fluid or gas, and wear on the backside seal causes serious upsets in thrust-bearing loads. Provisions are available for coUecting and disposing of the dust which tends to accumulate in the seal so as to protect the seal from serious erosion. 

The reaction turbine, shown schematically in Figure 2-2, is generally more efficient. In its primary (stationary) nozzles only half the pressure energy of the gas stream is converted to velocity. The rotor with a blade speed matching the full-jetted stream velocity receives this jetted gas stream. In the rotor blades the other half of the pressure energy is used to jet the gas backward out of the rotor and, hence, to exhaust. Because half the pressure drop is taken across the rotor, a seat must be created around the periphery of the rotor to contain this pressure. Also, the pressure difference across the rotor acts on the full rotor area and creates a large thrust load on the shaft. 

Figure 2-11 illustrates the potential severity of three problems that tend to eorrelate turboexpander inlet pressure thrust bearings, erosion of rotor and nozzles, and radial bearings. 

The reaetion turbine avoids this U-turn and its effieieney penalty. In the reaetion turbine half of the pressure energy is spent aeross the rotor, so there must be a seal around the rotor. With the rotor inlet at its periphery, the diseharge from the rotor may now be ehosen at a redueed diameter radial, or quasi axial, shaft-eoneentrie position. Sinee the diseharge is obviously smaller in diameter, the rotor seal will also be smaller in diameter beeause it only needs to suiTound the diseharge portion of the rotor. As a eonsequenee, seal loss is redueed and shaft thrust deereases as well. Likewise, the diseharge or seeondary nozzle losses are redueed beeause the gas exits at lower veloeity. 

Two or four lobe bearings are used in these expanders, determined by the funetion of the rotor-dynamie requirements. A double-aeting thrust bearing may be mounted, if required. Labyrinth seals with eonneetions for seal gas are standard equipment, however, dry gas seals are available. 

Fig ure 5-18. Finish-machined shaft showing integral thrust collar. The rotor is a stiff-shaft design. 

In this automatic thrust balancing system, the pressure behind the compressor wheel is controlled to a value between the compressor suction pressure and the wheel peripheral pressure. As the expander inlet pressure increases above the compressor suction pressure level, the resulting thrust force pushes the compressor wheel, and hence the rotor system, towards the compressor suction. In the reverse situation, when the pressure behind the compressor wheel is reduced below the wheel peripheral pressure level, the rotor system moves toward the expander. 

Figures 6-17 through 6-21 show simplified diagrams of AMB systems. Figure 6-18 depiets two pairs of eoils suspending the rotor about its inertial axis—as opposed to its geometrieal axis as with eonventional oil bearing systems. Thus, the rotor is self-balaneing and vibration eannot oeeur. A third pair of eoils is used for thrust eompen-sation and to determine the preeise shaft position.

Vibration peaks at specific speeds/high axial vibration is often present. Siib-harmomc Usually occurs as a result of loose components or as a result of aerodynamic or hydrodynamic excitations areas to be investigated for coiTection are seals, thrust clearance, couplings, and rotor/stator clearance... 

EiTatic high-frequency vibration amplitude and possibly an audible sound. Rotor mb Labyrinth mbs generally self-comect Disc mbs due to thrust bearing failure often self-comect temporarily through wean steel on steel shrill noise during wear Rotor deflection is critical speed... 

When designing a system for thrust bearing proteetion, it is neeessary to monitor small ehanges in rotor axial movement equal to oil film thiekness. Probe system aeeuraey and probe mounting must be earefully analyzed to minimize temperature drift. Drift from temperature ehanges ean be unae-eeptably high. 

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