DEMM Engineering & Manufacturing
Integrally-geared compressors for industrial and manufacturing units
INTRODUCTION
Integrally-geared compressors are nowadays used in many different industries and wide ranges of plants, manufacturing units and facilities. They have been used in various sizes and configurations in a wide range of services. This article discusses and reviews these widely used compressors which unfortunately were overlooked in many text books and technical articles.
INTEGRALLY- GEARED COMPRESSORS
In an integrally-geared compressor, pinion shafts are arranged around a large central bull-gear. A 3D semi-open impeller can be mounted on each front side of a pinion shaft. There are relatively large aerodynamic exciting forces that are generated in 3D semiopen impellers of integrally-geared compressors. These forces can act as exciting forces that cause high-speed shafts and whole the machine to vibrate. These forces could also induce dynamic stresses. This is one of the reasons that integrally-geared compressors are sometimes (relatively) less reliable and more-noisy.
Another important consideration is the operation of these complex machines at part-load conditions or alternative operating conditions. The aerodynamic exciting forces in 3D semi-open impellers and the complex gear unit behaviours would be different at alternative operating conditions and part-load conditions. Too often, worse cases of dynamic loadings of many gear systems are at part-loads. The behaviour of any rotor varies significantly as the tangential forces produced in the gear system due to operating condition changes. Therefore, it is important to properly analyse and verify the dynamic loads and vibration of different parts of these machines, particularly pinion shafts, at part-loads. This can affect design, selection, operation and reliability of the machinery.
In 3D semi-open impellers of integrally-geared compressors, the height of each vane is usually about 2550 percent greater than the height of vanes on a comparable conventional impeller, in order to increase the capacity and performance of the impeller. In addition, the number of vanes of an impeller is increased to reduce the load on each vane by about 30-40 percent compared to conventional impellers. In this way, the efficiency of 3D impellers under high-pressure ratios is also improved. However, some of these impellers can present new challenges and issues. Too often, different operational, mechanical and dynamic behaviours could be expected from 3D semi-open impellers used in integrally-geared compressors compared to traditional 3D impellers employed in conventional compressors.
For integrally-geared compressors, in addition of common criteria required for high-speed, high-performance gear units, the high axial thrusts coming from 3D semi-open impellers should be taken into consideration. These axial forces are directly transferred from the compressor impellers to the pinion shafts and can cause tilting of the gears. This influences the contact pattern and therefore the load rating of gear teeth as well as the mounting of the bull-gear shaft. In other words, the axial loads of 3D impellers should be managed properly otherwise they can cause a tilt of the compressor shaft on integral gears which could result in operational problems and reliability issues. Single helical gear systems with thrust collars are commonly used. Axial forces from gear tooth systems are usually absorbed by thrust collars (or rider rings). The thrust collar transmits axial forces from 3D impellers to an axial bearing on the main shaft (bullgear shaft).
INLET GUIDE VANE - IGV
Integrally-geared centrifugal compressors are constant-speed machines because their complex dynamic situations (rotordynamics, lateral, torsional, etc.) do not allow variable-speed operation of these machines. The “Inlet Guide Vane” (IGV) is the selected capacity control method for many integrally-geared compressors. IGVs usually consist of a row of aerodynamically-shaped blades placed at the inlet of impeller. The blades can rotate around their aerodynamic centre in order to give the suction flow a pre-swirl that provides an optimum incidence angle with the impeller’s leading edge even at reduced flows.
In an integrally-geared compressor, impellers should achieve high pressure ratios with good efficiency together with operational flexibility, operating at a range. An important aspect is the suppression of surge so that the compressor can operate safely and efficiently at reduced flow-rates. IGVs generate a swirling flow in the direction of rotation of the impeller; this can enhance the underlying stability of the compressor. In compressors with IGVs, the surge usually commences at the peak of the pressure-ratio/mass-flow characteristic. The distance between the surge and BEP (Best Efficient Point) is also relatively small.
It is possible to install IGV for each 3D impeller of an integrally-geared centrifugal compressor. There are compressors designed with IGV systems at the first two stages or more. However, a common configuration is an IGV only on the first stage. There have been compressors with an automatic IGV only on the first stage and a manual IGV (with a limited range) on the second stage.
The operating range of a typical integrally-geared compressor might be increased by using the variable diffuser guide vane, but this can increase cost, complexity and maintenance. This also reduces reliability. This solution is not recommended.
The IGV pre-swirl angle has a notable influence on the performance and operation of an integrally-geared compressor. Negative IGV pre-swirl angles might be adopted for a flowrate that is slightly larger than the rated flow-rate, and positive pre-swirl angles under a smaller flow-rate or part-loads. There is sometimes confusion about negative and positive IGV angles. A negative IGV pre-swirl angle is an IGV angle in the opposite direction of IGV
angles that used for part-load (low flow) operation.
IGV & RELIABILITY
The dynamic disturbances and instabilities in integrally-geared compressors are complicated due to many reasons such as strongly coupled unsteady effects and interactions of IGVs, impellers and diffusers, particularly because of the repercussion of diffusers.
The blade passing frequency is usually the main frequency of the unsteady flow, associated vibration and dynamic forces in an impeller. Regarding the IGV related instabilities, usually three frequencies for instabilities and dynamic force peaks have been measured and reported which are the impeller rotating frequency, blade passing frequency and the half of blade passing frequency (0.5×blade passing frequency). The asymmetric flow pattern in a compressor system usually induces excitations with the impeller rotating frequency. The half of blade passing frequency is usually the frequency of peak instability value. Double harmonic of the blade passing frequency (2×blade passing frequency) might also be notable.
For large positive IGV pre-swirl angles, instabilities are usually very higher compared to ones at zero IGV angle. For example, in some compressors, instability amplitudes at IGV pre-swirl angles of around 40°- 60° could be more than 15 times the values under zero IGV pre-swirl angle. For a large IGV pre-swirl angle, the dominant frequency of unsteady flow is often the half of the blade passing frequency.
For typical positive IGV angles (say around +15°, +20° or +30°), the maximum fluctuation often occurs near the tip of the IGV vane, and the circumferential distribution of pressure fluctuation might be relatively uniform. On the impeller inlet plane, the maximum pressure fluctuation is most often appeared at the pressure side of 3D impeller, and the distribution of fluctuation is more or less uniform along the radial direction. On the impeller outlet plane, maximum fluctuations often occur in the middle part of the passage, and the distribution of pressure is often (relatively) uniform along the span-wise direction. Larger fluctuations are reported at corners of hub/concave surface and shroud/convex surface.
When the IGV pre-swirl angle is much larger (for example, above +35°), the incidence angle of flow occurs at the impeller inlet, the non-uniformity of impeller inlet flow becomes stronger, and the periodic incidence is induced on impeller blades. The periodic change of impeller outlet flow may cause periodic fluctuations at the diffuser inlet flow. The influence of IGV/impeller wake can also cause pressure fluctuations in diffusers. Fluctuations of static pressure usually decrease along the flow direction with the decay of IGV/impeller wake. Usually, the mutual effect of the IGV wake and the impeller wake causes maximum pressure fluctuations to occur at the interface between the impeller and the diffuser for the unsteady “IGV–impeller– diffuser” interactions.
CASE STUDY - SURGE
The case study is about a six-stage integrally-geared centrifugal compressor in a large industrial plant. There was a surge on the 1-stage of this integrally-geared compressor. This was when the plant was working at 97 percent of its rated capacity for the first time, around three months after the commissioning. The plant had many problems and issues and it could reach at 97 percent of its rated capacity three months after the commissioning. The surge was triggered by a dirty gas filter at the upstream in the close distance of the machine; it was higher filter differential pressure than normal. The surge was noticed by the operator because the compressed gas flow to the unit was decreased and the anti-surge systems were activated. The dirty filter was changed when the machine was online. The gas filter change was not so effective to mitigate the issue.
For some integrally-geared compressors (such as this machine), as flow increases, using the first stage IGV, the surge margin decreases which could result in an unexpected surge behaviour in high flow operating ranges, near the rated flow. In other words, for integrally-geared compressors using IGV, the peak efficiency point and the rated point (as well as the operating point of peak head) could be close to the surge point.
For this surge case, the compressor was working near the surge line (or probably at a stall situation or a minor instability) and the surge was triggered by a dirty upstream filter. The dirty filter (higher pressure drop at suction) was just a trigger. In this situation, clean filter might not have a drastic effect on the surge situation. Some instabilities continued after the filter change. To better explain, the operating point required (for 97 percent plant capacity) in those machine settings was already close to the surge.
The focus was on the IGV of the 2-stage. Investigations showed that the IGV of the 2-stage was manually adjusted at the commissioning when the plant was operating at around 60-70 percent of rated capacity. The adjustments might be suitable for the commissioning time, but not proper when the plant was operated at 97 percent of rated capacity. The manual adjustment of the 2-stage IGV from an initial adjusted-point to fully-open (strait vane position) did bring the integrally-geared compressor back to the normal operation.
Based on experiences with this machine during the commissioning and the first year of operation, the following important notes should also be mentioned:
A) There is always a potential to install an upstream gas filter incorrectly. This can bring the compressor to the surge zone and complex operational issues.
B) An error in instrumentations might cause surge. For example, a plugging flow measurement (or dirt inside flow instruments) could cause a higher flow measurement than the actual flow which may result in an undetected surge.