Transcript Period 5 - Pipeline Corrosion Control

Period 5
Orac. The cathodic protection
equivalent circuit.
Above ground
Below ground
Orac has been re-made
• The original was stolen by someone in the cathodic
protection industry.
• I made another in the UK that is out on loan, and
forgotten to give back.
• I made another with the help of a trainee technician in
Brazil and that is still at our training centre.
• I made another in South Africa with the help of a trainee
and it is still there.
• The one you see in the pictures in this presentation is
part of my laboratory, here in the UK.
Orac works well.
• ( Named after the portable computer in the
television series 'Blake Seven')
• Orac is a demonstration of the features
encountered during the measurement of
voltages in cathodic protection field work.
• The original Orac was based on the lid from a
biscuit tin, made of a handful of scrap electronic
components, a battery and two multi-meters all
set in a briefcase.
• It was seen by the head of corrosion control at
Shell International in the Hague who wanted to
buy it.
Why is Orac?
• It was build to investigate the claims of
some of the worlds most respected
cathodic protection experts, and proves
that some of their theories simply do not
work in practice.
• This is fine, but destructive criticism is not
much good to anyone.
The ‘IR drop in the soil’ measured
at ground level
The ‘IR drop in the soil’ measured 1
strata down
The ‘IR drop in the soil’ measured 2
strata down
The ‘IR drop in the soil’ measured
close to coating fault.
How does Orac Help?
• Orac was built to show some more likely
explanations for the value of voltages read
during conventional cathodic protection
monitoring. The electronic components
have been constructed to give similar
readings to those actually experienced in
field work. This helps the engineer to build
up a concept of cathodic protection which
enables effective cathodic protection
trouble shooting.
The bits and pieces.
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Meter 1
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This is a typical digital multi-meter such as is used in cathodic protection
monitoring. It measures similar values with similar accuracy to those used
in industry, world-wide. It allows very little current to pass through the
measuring circuit.
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Meter 2
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This is a cheap, analogue multi-meter based on a galvanometer. It is not
particularly accurate as it requires current to pass through the measuring
circuit to activate the magnet which then pulls against the hairspring, to
move the indicator needle across the face of the meter.
Drawing this current reduces the potential difference between the poles of
the meters, and therefore reduces the value of the voltage which is read.
The reduction in voltage, caused by this meter can be measured using the
digital meter and Orac.
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Components
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The battery
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This is simply a method to create an electrical imbalance in the circuit. This
can be replaced with a small 'plug-in' transformer rectifier, such as is readily
available to power a multitude of domestic devices. The demonstration
works equally well with either source of power and will even function when
connected directly to a cathodic protection transformer rectifier in the field.
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The base
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This was a biscuit tin lid, made of tin plated steel sheet to which electronic
components can be soldered to give a good electrical connection. This
base enables a simulation of the 'mass earth' concept which is an axiom in
cathodic protection theory. Both in the field and in Orac there is a virtually
resistance free path for current which has entered 'mass earth'. This is now
a copper sheet from any electronics supplier.
Arrangement
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The input control
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This is a variable resistor which allows control of the current which is fed from the battery to the
base.
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The groundbed resistors
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This is a network of randomly selected resistors soldered together in three dimensions. It is a
concept of any part of the earth in which the current distributes itself throughout the network
according to the basic laws of electricity. This feature is provided to demonstrate the impossibility
of measuring the exact amount of current passing through any particular node in the same way
that it is impossible to determine the exact amount of current passing through any particular point
in the earth in the groundbed area. The whole area is 'charged up' and the current will pass to
mass earth through the least line of resistance.
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Groundbed to remote earth resistances
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Two variable resistors connect the groundbed network to the base. This enables the
demonstration of the effects of a high groundbed resistance, and illustrates the advantages of
seeking to build a low resistance groundbed.
The pipelines … conductors
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The pipeline
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The pipeline is represented by a series of resistors soldered end to end to show the typical resistance in the metal
of a fairly long, large diameter, pipeline. The actual value of the resistance of each pipeline depends of the wall
thickness, the diameter and the length but the values in Orac will be found to be fairly typical, and can be
calculated from the colour coding of the resistors which are used. It was necessary to use some resistor in parallel
during the construction of Orac, but these can be imagined as pipeline manifolds for the purposes of experiments.
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The coating resistance
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The pipeline is connected to the base on 'pillars' which are in fact high resistances and two variable resistances.
Each of the fixed resistances can be conceived as the actual coating resistance and the variable resistors can
represent coating faults.
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It is possible to adjust the total resistance of the coating and the disposition of this resistance along the pipeline to
simulate areas of poor coating . The variable resistors are under the plastic cover and difficult to access in the
same way the it is necessary to excavate to the pipeline to make any effect on the coating.
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It is appreciated that the pipeline can be short-circuited to other buried metal but this can be simulated on Orac by
direct connection between the 'pipeline' and a suitable variable resistor connected to the base. This is one use of
the extensions on the right hand side of Orac.
Test posts
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The test post positions
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These are represented by access holes through which the meter connections can be made. This is the same as
making a direct connection to the pipeline metal through a test facility in field work, and the same affect is
achieved on Orac.
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Earth contact positions
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These are provided as bare ends of the wires that poke through the plastic cover of Orac. These are displayed in
this way so that the connection can be made using the meter probes or by making contact with the porous plug of
a standard copper/copper sulphate electrode.
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Model extensions
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A red and a green wire are provided at the right hand side of Orac to enable the pipeline and the groundbed to be
connected to other demonstration pieces.
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Electrolytic capacitor
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This is included to demonstrate the including it in the circuit can reproduce the effect of the so-called decay in
cathodic protection which is manifest when the cathodic protection is switched off. Orac can therefore
demonstrate 'immediate off' readings using a recording voltmeter or data-logger.
What can Orac demonstrate?
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Orac was built to test the truth of statements made in leading text books
relating to cathodic protection practice. Many of these quote 'equivalent
circuits' and have illustrations similar to those that are seen in electronic
drawings. It is therefore logical to build the arrangement and apply the
potentials to become familiar with the behaviour of the meter when used for
monitoring the electrical behaviour. Theory has to be correct for the model
to work, but when it does it then makes the theory easy to understand.
Orac enables all experiments to become readily repeatable and facts to be
properly established.
Orac can demonstrate the difference between an analogue meter and a
digital meter.
The meter is placed between the two potentials and the current passing
through the meter allows the potentials to equalise, to a limited extent.
The analogue meter continues to demand energy to balance the elastic
memory of the hair spring, whereas the digital meter can be likened to filling
up a number of containers in a row and displaying the amount of the fill on a
screen powered from an independent source of energy.
One of the demonstrations
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Orac can demonstrate the difference between analogue and digital meters by the
following procedure.
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Connect the battery terminals to the two clips.
Connect the digital meter to test post 1 and a half cell position in normal soil.
Switch on the meter and adjust the output until the meter reads 1.5 volts.
Set the analogue meter to the 10 volts DC range.
Connect the negative pole of the meter to the green extension lead from the pipeline.
Watch the digital meter reading, while touching the positive analogue connection to
the same half cell position connection as that connected to the digital meter.
Note that the reading reduces from about 1.5 volts to about 0.72 volts.
Watch the dial of the analogue meter while touching its positive connection to the
same half cell position on Orac as that connected to the digital meter.
Note that the meter needle shows about 0.5 volts.
Connect the analogue meter permanently across the same connections and
disconnect the digital meter positive connecting.
Note that there is little detectable movement to the analogue meter caused by the
disconnection of the digital meter. Touching the digital meter lead to and from the
connecting points will confirm this.
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Important basic understanding.
• This experiment proves that the meter simply short
circuits the two subject potentials and that the higher the
resistance in the meter the lower the ratio of error that it
introduces.
• Applying this principal to cathodic protection monitoring it
becomes apparent that the introduction of digital meters
increased the apparent success of applying cathodic
protection, by increasing the value of the voltages that
were measured.
• Cathodic protection engineers were able to attain the
criterion of -0.850 volts and pipelines were considered
to be protected, but some time later failed due to
external corrosion.
The position of the electrode is
critical.
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Orac can allow engineers to measure voltages and current measurements
that can be computer analysed. These can be compared directly to
measurements made in the field during CIPS surveys
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How to do this using Orac.
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By leaving the negative terminal of the digital meter connected to the
pipeline at test post 1 and moving the positive connection to a variety of
connecting points, it can be seen that the resistances between the
groundbed and the electrode have a major effect on the readings.
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Four connections are provided and labelled 'half-cell positions in normal
soils'. These connections are extensions of a network of resistors which are
connected to the base. This is the same concept as connecting the
electrode to a random point on earth while taking a conventional 'pipe to soil
potential reading' during cathodic protection monitoring. The connection is
at the earth surface to a finite point of resistance as represented on Orac.
Equivalent CIPS survey
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Connect the DC V pole of the digital meter to the pipeline through test post
1 and the common pole of the digital meter to the left hand wire of the 'half
cell positions in normal soils'.
Switch on the meter and connect the battery.
Adjust the output to get a steady reading of 1 volt.
Move the positive meter connection to the second position from the left,
then to the third and then the fourth.
Note that this produces little change, in spite of the slightly different
resistances in the measuring circuit.
Move the positive meter connection to the wire labelled 'dry tarmac'.
Note that the meter reading reduces from 1 volt to about 0.95 volts. This is
due to the increased resistance between the base and the connection point.
It reduces the amount of current which is available to be measured by the
meter and thereby reduces the reading.
Move the positive meter connection to the wire labelled 'dry concrete' and
the reading reduces to about 0.666 volts for the same reason that it reduced
as described above. Note that the resistances in steps 5 and six are
introduced in the measuring circuit.
CIPS continued
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Move the positive connection of the digital meter to the position labelled
'half-cell positions at the top of the excavation'.
Note that the reading is about 1 volt and that the arrangement of resistors is
similar to that of the 'normal soil' positions. This section has been
constructed to show the reasons for the lower readings obtained when
moving the electrode closer to a coating fault immediately before it has
been exposed by excavation. Of course, once the fault is uncovered it has
no connection to the electrolyte and the pipeline is effectively 'coated’ by the
air.
Move the positive connection of the digital meter progressively down the
row of wires labelled 'moving half-cell down excavation', noting each
reading. Note that the readings reduce as the resistance between the
electrode and the pipeline reduces. This can be regarded as a different
effect from that of a resistance in the measuring circuit. Here we are
looking at the true voltage measurement and not an error due to the
inefficiency of the meter.
The potentials at each pole of the meter are of closer value due to the
passage of current through the cathodic protection system. This is the
improperly named 'IR drop in the soil'.
CIPS continued
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In the field this voltage is due to the resistance of the soil where the cathodic
protection current passes onto the metal. Where there is no coating fault, this voltage
is high, as demonstrated on Orac at the 'top of the excavation', which is out of the
area of influence of the low resistance path to the
pipeline. As the contact
point moves closer to the lower value of resistance between itself and the pipeline the
voltage between this point and the pipeline is reduced, as seen on the meter.
Typical readings on Orac during this procedure are as follows:Top of excavation
-1.034 volts
Position 2
-1.034 volts
Position 3
-1.059 volts
Position 4
-1.048 volts
Position 5
-0.453 volts
Position 6
-0.342 volts
Position 7
-0.159 volts
Position 8
-0.007 volts
Similar values will be obtained when moving a 'half-cell' electrode down the sides of
an hole which is being excavated towards a coating fault. Indeed this is a method of
guiding an excavation to the coating fault following a condition monitoring survey.
Contact with the pipeline.
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Moving the contact point along the pipeline has undetectable effect on the reading.
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During the massive voltage survey conducted by North Thames Gas, during the 1980's I was part
of a team of technicians working for the Slough Office in the High Wycombe area. We were
measuring voltages between the pipeline and an electrode which was stepped along the ground
above the route of a coated, steel, high pressure gas pipeline. This was done at 10 meter steps
while the impressed current cathodic protection was automatically switched on and off at the
closest transformer rectifier.
In order to carry out this survey we had to use a long thin wire (actually armature winding wire) as
a connection to the pipeline through the normal cathodic protection test post facilities. These test
posts were positioned at access points such as road crossings, and were up to 1 mile apart. We
would connect the wire to the test post and unreel it from a spool which we carried, the other end
of the wire being connected to the positive pole of the high resistance digital voltmeter.
On one occasion I was instructed to chose two test posts approximately 1 mile apart and to take
two voltage readings at each end. The thin wire was to be connected to the local test post and
then to the test post one mile away.
The readings showed that connecting to the pipeline one mile distant made no detectable
difference to the voltage shown on the meter from that shown on the meter when connected
locally. However, the reading was altered considerably by moving the location of the electrode a
few meters. We carried out several tests to prove these facts and submitted a report, but got no
feed back or further information.
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This feature can be demonstrated
on Orac by the following procedure.
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Connect the battery and adjust the output to achieve a voltage of 1.500v
when the negative terminal of the meter is connected to test post
position 1 and
the positive connected to 'half-cell in normal earth'.
Connect the negative terminal of the meter to the pipeline at test post
position and note that the voltage has not changed.
Connect the negative terminal of the meter to test post 3 position and note
that the voltage has not changed.
Calculate the total resistance along the 'pipeline' on Orac and you will find
that it is the equivalent to the total resistance in the metal along several
miles of large diameter steel pipeline.
Movement of the position of the negative connection produces little or no
change in the voltage but movement of the positive connection produces
significant variations. This is the most significant fact in understanding
monitoring of cathodic protection and the measurement of voltages in the
study of the corrosion reaction.
Using Orac can give an engineer a good concept of the electrical balances
involved in the application of cathodic protection in the field.
Orac 1984