Page 261 - Compression Machinery for Oil and Gas

246 SECTION II Types of Equipment

Torsional field testing:
Representative field data
N1 = first natural frquency = 4620 rpm = 77 Hz
During startup, 5× and 6× energy excite the first critical speed, even though 3× is
the largest energy order 3×
1× 2× 4× 5× 6× 7× 8×
980 rpm

5× Excites N C 1
(924 rpm)

6× Excites N C 1
(770 rpm)

0 rpm
0
PWR SP A
200 Hz
FIG. 5.55 Representative torsional field data. (Courtesy of SwRI.)

FIG. 5.56 Strain gage telemetry system components. (From SwRI photograph of Wireless Data
Corporation system.)

A torsiograph may be used to measure oscillations in rotational velocity.
This type of test is normally performed on the outboard ends of the equipment
due to shaft mounting requirements (a threaded pilot hole and/or stub shaft must
be used). The data are most useful for determining torsional critical speeds. The
instrument is best suited to short-term measurements. Similar data can be
obtained with an encoder, although the signal must be appropriately processed,