CompressorTechTwo - 2010 (Sample)

Figure 3. Accumetrics transmitter, with Kevlar strap attachment
(shown for illustration on a small glass shaft).

made specific recommendations to increase the margin of safety on the compressors. As with any computer simulation, the quality of the results and the subsequent analysis are highly dependent on the accuracy of initial data used to model the rotor using the analysis software.

To confirm that the operational changes were producing the desired results, AP Dynamics decided to instrument the now-repaired compressor package with a rotor telemetry system to measure torsional vibration during operation. An AT-5000 EasyApp rotor telemetry system from Accumetrics Associates was chosen as the vital link between theoretical calculations and real-world results.

The AT-5000 EasyApp is a miniature, battery-powered

TORSIONAL ANALYSIS METHODOLOGY

For torsional analysis purposes, a compressor/drivetrain represented by a lumped parameter model, in which inertia of the drivetrain is represented by a finite number of rigid disks of inertia connected by inertia-less torsional springs. Each spring represents the stiffness, which is defined as the angular deflection for a given applied torque — between the two associated disks.

Figure 6. A model for drivetrain torsional vibration analysis.

A time transient simulation is used to predict the drivetrain response (the angular deflection at each disk and the torque at each spring) to the applied torques at the engine and the compressor as the system evolves through time.

Figure 4. Installation of
Accumetrics AT-5000.

digital telemetry system that can be applied to shafts of various diameters using Kevlar straps and connected to a strain gauge (Figure 1) applied directly to a shaft where torsional vibration is suspected.

The small transmitting module (Figure 2), which includes a lithium battery, mounts opposite an adjustable yoke to maintain balance.

The combination requires only 0.7 in. ( 18 mm) of radial clearance around the shaft. The Kevlar strap (Figure 3) has a tensile strength of 3000 lb. ( 13. 62 kN) and the overall system operates to over 10,000 g’s — far beyond the centrifugal force expected for this application.

Battery life can be as much as 150 hours on a single lithium battery when used with high-resistance strain gauges. Other components of the telemetry system include a receiving antenna and a receiver (Figure 5). By gathering set shows the system response while running in resonance with one of the system’s torsional natural frequencies at 913 rpm. The second set shows the system response while running outside resonance at 1000 rpm. Notice the large vibratory torque, which develops when the system operates at resonance. Magnitude of angular and torque vibration at resonance can be as high as two to three times that found at nonresonant speeds.