Electromagnetic Testing Chapter 3- Electromagnetic Testing

The speed of testing is also an important consideration affecting probe
selection. Annular probes, with absolute and differential coils, have been
designed to be pulled through the bores of heat exchanger tubing at a
constant rate of speed (up to 3 feet per second—fps). Since heat exchangers
may contain hundreds of small diameter tubes, some probes have been
designed so that they can be blown through the heat exchanger tubes and
withdrawn at a constant rate of speed or pushed through the tube at a
constant rate of speed. When used with a strip chart recorder, indications of
flaw location are easily obtained. For example, if recorder chart speed is 1
in./s, and a cable puller is used to withdraw the probe at the rate of 1 fps, a
30-foot-long heat exchanger tube would be represented by a 30-inch chart
trace. In addition to accurately recording the position of baffles and tube
sheets, a carefully calibrated system can actually detect defects near and
under baffles and tube sheets in addition to defects between baffles and tube
sheets.
Charlie Chong/ Fion Zhang