Transcript Period 4 - Pipeline Corrosion Control

Period 4
Instrumentation
Misunderstanding instruments.
• The development of the digital voltmeter
had a considerable influence on those
involved with cathodic protection and has
been a major cause of pipeline failure to
the present time.
• The ‘mechanism’ of the digital voltmeter is
different from analogue meters of the past.
The galvanometer
Energy v elastic memory
• Up until the 1970s, digital instruments were not readily available and
cathodic protection measurements were carried out with a variety of
electrical meters which were based on the galvanometer.
• These instruments work on the reaction of a magnet in an electric
field against the elastic effect of a hair spring. They are less rugged
than digital instruments and more difficult to manufacture. They
also require a considerable amount of current to move the indicator
needle.
• The amount of current drawn by this type of instrument is sufficient
to cause a depression in the potential of the soil in which the
electrode is placed. It can be said that the current flowing through
the measuring circuit causes its own potential gradient (IR drop).
Potential gradient in soil.
Error balances error
• The amount of current drawn by this type of instrument is
sufficient to cause a depression in the potential of the
soil in which the electrode is placed. It can be said that
the current flowing through the measuring circuit causes
its own ‘IR drop’.
• This potential depression can be measured using two
electrodes and a digital voltmeter.
• This 'volts drop' in the measuring circuit caused
substantial inaccuracies in the measurement, and the
readings were shown to be lower than the true voltage.
• This is the opposite effect to that of the 'IR drop in the
soil' which is caused by the passage of the cathodic
protection current.
Accuracy
• During the period in which these meters were
used, the fact that the readings were less than
the criteria, encouraged operators to blame
instrument inaccuracy when protected criteria
could not be achieved.
• All the time that a needle was moved against a
hair spring, at any time in the measuring
process, this required sufficient oppose any
resistance in the measuring circuit.
• Instruments which were extremely sensitive had
to have extremely fragile mechanisms which
were unsuitable for rough field use.
The voltage measurements are
relative to a floating zero
• The error in the measuring system tended to give the impression
that sections of pipeline were not protected and engineers worked to
increase the protection on these sections.
• They found that many sections of pipeline could not be 'protected'
as it proved impossible to increase the output of the cathodic
protection system enough to show the required voltage on the
meter.
• A formula was recommended to correct readings to an increased
value, in proportion to the internal resistance of the voltmeter.
• This formula corrected the readings upwards towards the required
criteria, whereas the correction to present readings, which is
obtained by switching the CP current off, is a correction in the
opposite direction..
Nul-balance meters
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Attempts were made to design specialised instruments which balanced out this error
with an opposition EMF supplied from an internal battery. The idea was that if they
worked on a balancing process when the actual measurement was made, then they
would draw no current. Some of these instruments became quite complex but they
all drew current during the original balancing operation, and hence none reached the
accuracy which is possible with a digital meter.
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The designers seemed to have missed the point that the current drawn, to set up the
balance, created an error which was carried through the whole measuring operation.
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The complexity of operation of the balancing meters required more care and
understanding by the operators, and the instruments were not as rugged as the
simpler ones of the day.
Instruments appeared with more and more dials, switches and adjustments but this
resulted in less and less personnel who understood how to use them. It also had the
adverse effect that the amount of time to take a single reading with such an
instrument, precluded the possibility of taking 'immediate off potentials'.
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Expensive meters.
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There was then the problem of measuring the EMF which had balanced the
measuring circuit. This had to come from a dry cell battery which would not
have a constant potential as the current drawn, to balance the circuit, would
drain the battery each time of use.
Some instruments were then marketed with a more constant (laboratory
tested) battery built in, with which to balance the, commercially available,
dry cell.
The problem that still remained, was to establish a potential value, within
the meter, against which a reliable voltage could be measured.
Sophisticated variable resistors were built into some instruments and it was
claimed that an error free voltage could be measured by a complex
procedure of balancing potentials using two galvanometers, two sources of
DC charge and a variety of switches, variable resistances and a component
called a "potentiometer".
On examination, the "potentiometer" turned out to be a variable resistor
which had been calibrated to give a reading on a mechanically driven digital
dial.
Digital meters
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The availability of digital instruments removed the downward error which had been
caused by the volts drop in the measuring circuit and allowed increased voltages to
be obtained, because they dramatically reduced the amount of current flowing. The
solid state electronics of today's meters only require a tiny amount of current to make
a reading and the resistance in the measuring circuit is greater by an order of
magnitude than the resistance in the circuit to be measured. Using a digital
voltmeter with an impedance of 7 to 10 mega-ohms per volt ensures that the current
flowing through the measuring circuit is not sufficient to cause a potential depression
in the earth in which the electrode is placed.
Using galvanometers, the error in measuring had helped to balance out the IR drop in
the soil which, at present, causes optimistic readings. In fact it was the advent of
digital metering which bought the question of the cathodic protection criteria sharply
into focus.
Pipelines began to fail in areas where digital voltmeters had shown that the pipeline
had achieved a voltage difference of -0.850 mv with respect to an electrode placed
immediately above the pipeline on the ground surface. It is doubtful if this value
would have shown on a galvanometer which often required the ground contact to be
watered to allow sufficient current to pass through the metering circuit to activate the
magnet against the hairspring.
This inaccuracy was not an advantage, but did have some tendency to balance the
inherent error in the measuring technique.
Laboratory work
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An example of laboratory discipline can be seen in the British Standards Institute,
Code of Practice for testing paints and coatings for their cathodic disbondment
potential. This code of practice specifies that the reference electrode is placed
exactly 10mm from the metal to electrolyte interface, at a coating fault which is
deliberately made on a test piece. The pH, temperature and pressure at the time of
the test are specified, as are the period of time and the stages of voltage increase.
Gross errors can be readily demonstrated during this procedure by simply moving the
reference electrode.
Another example is shown in the book 'ELECTROCHEMICAL PRINCIPLES OF
CORROSION' A Guide for Engineers, by Dr LL Shrier for the Department of Industry
of Great Britain.
This shows clearly that the reference electrode has the sensitive end of its glass
capillary at the immediate interface between the metal and the electrolyte.
The reason for this is that current passing through an electrolyte meets with a
resistance which causes a voltage drop. If the electrode is placed randomly, then an
undefined variable is introduced into the calculation. The use of an inert gel in the
capillary prevents the introduction of a chemical disturbance of the subject interface.
This feature is subject of most of the experiments shown in the web pages and video
clips that are the IP of Cathodic Protection Network.
Daniell’s Cell
Care of copper/copper-sulphate
electrode
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There is much confusion amongst field operatives in cathodic protection, as to the properties and
usage of the 'half-cell'. One engineer wrote that it is capable of 'scanning a section of pipeline'.
It is quite clearly not capable of scanning anything, it simply makes contact with the ground,
forming one link in the measuring circuit.
A lot of fuss has been made by some engineers about the cleanliness of the copper sulphate
solution and the purity of the metal within the electrode but this only has 1% to 2% effect on the
readings which is an acceptable error by comparison to that caused by moving the location of the
electrode, which can introduce an error of 100% or more of the actual value. It is worth making
sure that the copper rod is pure and clean and that the solution of copper sulphate is saturated
and pure. Distilled water should be used to dissolve crystals of copper sulphate making sure that
there are always some crystals remaining in the clear plastic tube.
A matter which is rarely bought to question is the nature of the porous plug which might introduce
other components into the solution. Laboratory electrodes will have neutral plugs but those used
in cathodic protection field work have wooden or plaster plugs which can contain either acids or
alkali in their composition. There seems to have been little attention paid to this and it would
probably not cause a substantial error into the measurement.
Errors can be introduced by poor contact between the electrode and the electrolyte. In very dry
conditions on hard concrete or dry hardened sand there might be such a high electrical resistance
that it has a significant effect on the reading, even when a very high impedance digital meter is
used.
Poor contact between the conductor lead and the pole of the meter can cause errors and
fluctuations if there are metal oxides and damp associated with these connections. These
phenomena will be caused by the micro-reactions taking place between the films of moisture and
the variety of salts and metals.
Much ado about nothing
• Many specialists claim that there are significant errors in
measurements made due to impurities in copper-sulphate solutions
and cleanliness of the copper rod.
• They even specify that the copper/copper-sulphate electrode should
be ‘calibrated’ , this is nonsense. It cannot be calibrated. The
reaction between copper and a saturated solution of it’s own salts is
determined by the laws of nature.
• The value of each individual electrode can be compared with that of
another electrode but this is not calibration and every electrode
reaction is altered every time it is used.
• The difference in reaction potentials can be determined but this is of
little value because the error caused is exceeded by an order of
magnitude by the misunderstanding of the measurement itself.
• The difference in voltage between electrodes is in micro volts and
the errors caused by movement of the electrode can be in volts.