Page 473 - Compression Machinery for Oil and Gas

Compressor Testing Chapter 12 453

currents, capacitive, or inductive technologies. Additional technologies that are
available for vibration, and displacement in general, include laser and ultrasonic
sensors.

Other Measurements
Some other common measurements made in dynamic and performance testing
include gas composition, rotational speed or position, noise, strain, and force
measurements.
Gas composition is critical for performance testing; however, getting a live
measurement is not always possible. In some instances, a gas chromatograph
may be made available that can provide a live and real-time measurement,
but that is usually used just in research or test facilities. More often, if an accu-
rate gas sample is needed, it will be necessary to collect a gas sample that has to
be sent off for analysis. Proper sampling procedures and techniques are avail-
able in the guidelines.
It is also of common interest, especially in performance testing, to know the
rotational speed and/or position. Probably the most common means of taking
this measurement is an encoder. An encoder can provide an absolute position
or the relative change of position as a shaft rotates. If only the speed of rotation
is desired, a key phasor that provides a once-per-revolution signal can be used
with a DAQ capable of defining timing to determine the shaft rotating speed.
Noise, or sound, measurements can use any level of microphones depending
on the accuracy and purpose for testing. Often, when taking noise measure-
ments to determine ambient sound pressure levels, calibrated sound-level
meters are used. When making noise measurements where the frequency of
the noise is the desired output, almost any microphone with adequate output
for a DAQ system is acceptable.
Strain measurements are typically made using strain gauges that are applied
to a surface with a bonding compound. The proper preparation of the surface
and installation of the gauge are critical to the accuracy of the measurement.
Strain gauges can also be welded onto the surface of interest, or an optical mea-
surement can be used to determine strain fields on the surface of a material.
Selection of force measurement sensors are based on the measurement
needed. For impulse testing, instrumented hammers are available. Selection
of the size of hammer and the impact surface of the hammer should be selected
based on the article being tested. For reaction forces subjected to static or
dynamic loads, varieties of load cells are available to fit the location and size
of the test location.

Data Acquisition
There is a wide selection of DAQ systems available on the market today. Choos-
ing the correct one is based on matching the type of sensors to be recorded, the
frequencies of interest, and the processing that is required on the data.