Dynamic compressors

While process instability can affect both the fixed and float bearings, the thrust bearing is more likely to show early degradation as a result of process instability or abnormal compressor dynamics. Therefore, these bearings should be monitored closely, and any degradation or hint of excessive axial clearance should be corrected immediately. 

Many users consider rotaiy compressors, such as the Rootes -type blower, as turbomachines because their behavior in terms of the rotor dynamics is very close to centrifugal and axial flow machineiy. Unhke the reciprocating machines, the rotary machines do not have a veiy high vibration problem but, like the reciprocating machines, they are positive displacement machines. 

Compressors A compressor is a device that pressurizes the air in a gas turbine. It transfers energy by dynamic means from a rotating member to the continuously flowing air. The two types of compressors used in gas turbines are axial and centrifugal. Figures 29-25 and 29-26 depict typical industrial gas turbines. 

Compressor performance can be represented in various ways. The commonly accepted practice is to plot the speed lines as a function of the pressure delivered and the flow. Figure 3-9 is a performance map for a centrifugal compressor. The constant speed lines shown in Figure 3-9 are constant aero-dynamic speed lines, not constant mechanical speed lines. 

The author has a patent for a dynamic surge-detection system, using a boundary-layer probe, presently undergoing field tests. This system consists of specially mounted probes in the compressor to detect boundary-layer flow... 

The use of dynamic pressure transducers gives early warning of problems in the compressor. The very high pressure in most of the advanced gas turbines cause these compressors to have a very narrow operating range between surge and choke. Thus, these units are very susceptible to dirt and... 

Boyce, M.P., Morgan, E., and White, G., Simulation of Rotor Dynamics of High-Speed Rotating Machinery, Proceedings of the First International Conference in Centrifugal Compressor Technology, Madras, India, 1978, pp. 6-32. 

There are at least three reasons for on-stream cleaning. The first is to restore the system s capability. If the unit is a driver, its maximum horsepower will probably drop as it becomes dirty. Cleaning will restore this limit. If the machine is a dynamic compressor, fouling may reduce its head, and therefore, the maximum gas flow rate. Cleaning will restore the capacity limit. 

Compressors using the intermittent compression mode are referred to as positive displacement compressors, of which there are two distinct types reciprocating and rotary. Continuous-mode compressors are also characterized by two fundamental types dynamic and ejector. 

Continuous compression compressors are of two types ejector and dynamic. 

Larger compressors, normally above 150 to 200 horsepower, have forged steel crankshafts. Cast crankshafts are used in medium-size machines. Crankshafts should have removable balance weights to compensate for rotary unbalance as well as reciprocating unbalance. The crankshaft should be dynamically balanced when above 800 rpm. 

Rotary and dynamic compressors have in common the force due to unbalance. As discussed, the objective is to balance a rotor to a level of. Ig or better, which was previously stated to be a force equal to or less than 10% of the rotor weight. 

The dynamic compressors, axial and centrifugal, exhibit one other non-steady force, which is often questioned by foundation designers. This is the torque reaction force at the casing feet and, thus, the foundation due to surge. As a rule of thumb, a value of A the maximum steady-state torque reaction at the casing feet can be used. The frequency is the surge frequency, which is less than operating speed but not readily predictable, because it is heavily dependent on the connected system. 

On those compressors where rotor dynamics can be a problem, which is on all but the standard units (even them sometimes) and reciprocaiiiit compressors, this is the point where the acceptance of the compressor for vibration and critical speed criteria should take place. Finding these in the field later is what the user and vendor want to avoid, and this is why ail the elaborate and careful work is done at the running test time,... 

Another potential problem is due to rotor instability caused by gas dynamic forces. The frequency of this occurrence is non-synchronous. This has been described as aerodynamic forces set up within an impeller when the rotational axis is not coincident with the geometric axis. The verification of a compressor train requires a test at full pressure and speed. Aerodynamic cross-coupling, the interaction of the rotor mechanically with the gas flow in the compressor, can be predicted. A caution flag should be raised at this point because the full-pressure full-speed tests as normally conducted are not Class IASME performance tests. This means the staging probably is mismatched and can lead to other problems [22], It might also be appropriate to caution the reader this test is expensive. 

The reciprocating compressor has several strikes against it when it comes to reliability, if the main consideration of reliability is long run times as would normally be expected from the dynamic type machines. It has a lot of parts and the parts are subject to wear. Before one jumps to the conclusion and totally excludes this compressor from consideration because of these factors, the positive aspects of this machine should be... 

If turndown is a consideration, again the reciprocating compressor, with its many unloading options, does meet the challenge. This flexibility is difficult to achieve reliably in the dynamic machine and may require the compressor to operate at an unfavorable point on its characteristic curve. 

Axial compressors offer a high volume capability in a relatively compact case. As can be said for any of the dynamic compressors, when properly applied, it can be a very reliable compressor. The compressor... 

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