Metal Samples MS2500L, Operator'S Manual

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11
Proper probe location is the first and one of the most important considerations in obtaining pertinent
data with the corrosion rate instrument. Any system can be expected to have corrosion occurring at
several rates as a function of location within the system. The different rates may be due to changes of
temperature, changes in velocity, impingement effects, differences of metallurgy, variations in character
of fluid stream, etc. It is therefore unlikely that a single probe location will be able to produce quantita-
tive corrosion rate data for the overall system. Each probe does, however, provide valuable information
by reading the corrosion at the particular location, and by indicating the general range of overall corro-
sion by following changes in corrosion rates caused by adjustments in inhibitors, oxygen level or process
variables.
The change in corrosion rate of a system due to altering a variable should be in direct ratio for all parts even
though the absolute rates are widely different. For instance, if the addition of an inhibitor will reduce the
instrument’s reading by 90 percent, all normal surfaces of the same metal contacted by the inhibitor should
have their corrosion rates similarly reduced. Usually, a probe location is selected to read on the high side of
a system’s average rate in order to accentuate changes.
It should be emphasized that a corrosion rate reading is the actual corrosion rate taking place on the
surface of the Test electrode at the time of measurement.
The condition of the probe’s Test electrode surface due to past service, as well as current process condi-
tions, may have an important bearing on the readings.
The corrosion engineer is challenged to adapt the above to his situation and interpret the results In light of
the location and operational factors involved.
E. Probe Installation
Probe installations should generally be selected on the following basis:
1. Locate the probe as close as possible to the warmest section of the system. If vaporization is
experienced in the process, the probe should be installed immediately after condensation.
2. If the fluid is normally flowing, it is recommended that the probe be inserted in such a way as to
experience approximate pipe wall velocity. The probe should not be installed in a stagnant section of
line. Some turbulence at an elbow or tee may result in slightly high, but acceptable, corrosion rate
readings.
D. Probe Placement