Page 470 - Compression Machinery for Oil and Gas

Compressor Testing Chapter 12 451

total (stagnation) conditions using average fluid velocity. Pressure sensors will
also usually need to be capable of fast response measurements in order to deter-
mine what dynamic pressure influences are present. Ideally, choosing a sensor
that outputs measurements in absolute pressure is preferred. However, a sensor
that outputs gage pressure may be used so long as the ambient pressure is also
recorded and used to correct the measurement to absolute. The American Soci-
ety of Mechanical Engineers (ASME) recommendations for the location and
orientation of taps for pressure measurement for dynamic compressors (centrif-
ugal and axial) are provided in Fig. 12.1. Circumferential average of static pres-
sure is used for performance calculations. This arrangement is not commonly
encountered in field testing scenarios, but more often used in lab test environ-
ments. Guidance for accurate measurement of pressure is provided in ASME
PTC 19.2 [2].

Temperature
Measurement of temperature can be achieved using a number of devices. Three
of the more common sensors include thermocouples, resistance temperature
devices (RTDs), and thermistors. Generally, the RTD is the most recommended
sensor because of its relatively low-level of drift over time and good linearity.
Thermocouples are good for high-temperature measurements, but proper atten-
tion is required in selecting, installing, and measuring with them to ensure
adequate accuracy. ASME PTC 10 [1] also recommends calculating a circum-
ferential average for temperature measurements for compressor performance
measures (see Fig. 12.1).

Flow Rate
For the lowest level of uncertainty, a physical measurement of flow for the pro-
cess fluid as well as any fuel used is desirable. Some of the more common flow
sensor types include orifice, ultrasonic, turbine, Coriolis, and pitot-tube sensors.
The most common of these sensors in field testing scenarios is the orifice flow
meter. Regardless of type used, adherence to the recommended installation
practices for the sensor (such as those made by the American Gas Association
and those included in ASME PTC 19.5 [3]) is necessary to maintain highest
levels of accuracy.

Vibration
Vibration sensors should be selected based on the frequency range of interest,
amplitudes of motion, and with an understanding of the purpose of the testing.
Most typical vibration sensors are based on piezo-electric crystal technology,
which generates good data for relatively large amplitudes of vibrations occur-
ring up to very high frequencies. Strain-based sensors are also available for
these applications. For very low-amplitude vibrations, and where noncontact
is desirable, proximity probes can be used. The proximity probes can use eddy