10. PERFORMANCE TESTS ON CENTRIFUGAL COMPRESSORS

 Two types of tests are performed on centrifugal compressors; mechanical and thermodynamic tests.

10.1 MECHANICAL TESTS

These can be of various types and complexities depending on the information that it is required to obtain.

The tests conditions ought to be as close as possible to the actual contractual conditions.

During the tests the item of greatest interest is the location of the lateral critical speed.

In general, a running test at maximum continuous speed is carried out (100% of design speed for motor driven units, 105% for steam or gas turbine units); following API standards, this test is completed in 4 hours.

During this test shaft vibration measurements are taken at various speeds, close to the bearings.

An overspeed test is carried out up to the 0verspeed trip setting of the turbine to check the safety of the compressor in the event of control failure of turbine. In this case the speed can reach 10% over the maximum continuous speed at which point the overspeed trip shuts down the machine.

It is known that the coupling has a notable influence above all on the second critical speed; for this reason it is advisable to carry out the tests with the job coupling. If a differentcoupling is used care must be taken to ensure that the overhang (consider weight and axial dimensions) is the same. In particular, the same sleeve weight and position of centre of gravity, same distance of the teeth from the bearing, same weight and flexibility as the job coupling are required.

The connection between compressor shaft and coupling (carried out by means of matching teeth) is considered as a hinge: in fact the bending moment is thus not transmitted to the compressor shaft. In particular, the weight of the spacer is considered divided by two identical forces acting on the end toothings.

Further the test driver ought to be that used for the project, in fact, the driver-coupling compressor group is a whole, the interaction of the component parts of which is not easy to be reproduced. In general, however, the drivers of the test facilities &I€1lS€d; onlyxuith particularly critical machines the tests are made using the project driver.

This is, in general, that which concerns the elastic response of the compressor. One must take into account also the bearings and the seals, since the critical speeds are much influenced by these (type of bearings and seals, their clearances, oil viscosity etc.).

It has already been seen when considering the lateral critical speed and instability problems, that the type of bearing and seal have a fundamental importance in reducing the destabilizing forces that act on the rotor-support system.

It is useful to remember that the case in which asynchronous vibrations occur at speeds which are multiples of the rotation speed, indicates misalignement, bearing failure, or other causes of this kind. Asynchronous vibrations at speeds lower than that of rotation are to be attributed to instability of the oil film in the bearings or in the seals.

The oil used in the test must have the same viscosity as that used in the plant; this can be arranged by adjusting the oil temperature to get the viscosity required at the bearing inlet.

To be able to reproduce the same working conditions in the high pressure seals, one ought to make oil or gas circulate in closed high pressure loop which is complicated and costly.

The old edition of API standards stated to carry out tests without seals installed. The new edition, on the contrary, states to carry out the test with the seals installed. This, in practice, means carrying out the test at a pressure equal to at least a quarter of that required in operation. In fact, the capacity across the low pressure ring is proportional to the pressure: at reduced pressure the cooling is less which leads to an increase in temperature.

During the tests, oil temperature and pressure are measured at inlet and the temperature at the bearing discharge. Sometimes the bearing temperature is measured by embedding a thermocouple into the white metal. Measurements on the lubricating and seal oil capacities are not frequently made.

The measurement of vibrations is carried out both on the shaft and on the case.

The case measurements are made close to the bearing positions in the vertical, horizontal and axialplanes;the measuring instrument consists of an external part joined to the case and one free, practically fixed in space, which has a very low frequency of oscillation and is not affected by the high frequency case vibration. The variation of the magnetic field in the air gap due to the relative movement between the two parts generates electromagnetic forces which, suitably amplified and presented on a monitor, give information on the vibrations of the case. Filters are used to select the various harmonics for monitoring on an oscilloscope.

The vibrations on the shaft are measured in different positions with probes at 900 so that on the oscilloscope it is possible to see the orbits of the points of the shaft axis corresponding to the particular section.

It is also interesting to see the phase variations, that is, to see how the amplitude of vibration moves with regard to a fixed point on the shaft at various rotation speeds (the fixed point on the shaft can be arranged by having a reference mark monitored by a photo-electric ceDJ By observation of the phase it is possible to find between which critical speeds the operating point exists, since in passing across one of these there is a phase change in the vibrations.

For example, before the 1st critical speed, unbalance and vibrations are in phase, beyond the 1st critical speed they are in phase opposition; in reality, these phase changes are never instant but are distributed within a speed range. Naturally, it is necessary that the shaft is perfectly cylindrical and concentric with respect to the supports, otherwise considerable vibrations will be detected even if the shaft does not vibrate.

During vibration measurements, it is necessary to take into account the electric and mechanical run-outs. The electric run-out is a phenomenon due to the fact that during the forging operations magnetic fields are created which subsequentely disturb the measurements.

It is necessary to avoid these difficulties before the tests by de-magnetizing the rotor with a solenoid.

The mechanical run-out due to unavoidable eccentricity and ovality of the mechanical parts can be examined by means of other instruments.

Once trend was to limit vibration amplitude (A), now instead, the trend is to limit the vibration speed (dA/dt) or the vibration acceleration (d²A/dt²) .

The vibration speed is proportional to the product of the amplitude by the frequency and to the dissipated energy; that's why it is an important reference for evaluating vibrations.

In general the vibration amplitudes acceptable on the case are half of those on the shaft; to give an idea of the order of these amplitudes, for a shaft running at about 5000 RPM, the vibration amplitude acceptable is up to about 40 microns.

For the casing, the vibration speed limits are acceptable in the order of 10-20 mm/sec.

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