Corrosion monitoring in petroleum refineries

Corrosion Monitoring in Petroleum Refineries

K.R. SONI,SAGA GLOBAL CONSULTANTS

CORROSION MONITORINGCORROSION MONITORING

Corrosion monitoring may be defined as the

systematic measurement of corrosion rate of

equipment with the object of diagnosis and

controlling corrosion.

It can also be used for monitoring efficiency of

implementation of corrosion control system.

CORROSION MONITORING

Provides an early warning that damaging process or

conditions exist which may result in corrosion induced

failures. Indicates the correlation of changes in process parameters

and their effect on system corrosion. Diagnoses a particular corrosion problem, identifying its

cause and the rate controlling parameters such as

pressure, temperature, pH, flow rates etc. Evaluates the effectiveness of a corrosion control

prevention technique such as chemical inhibition and the

determination of optimal applications. Provides management information relating to the

maintenance requirements and ongoing condition of plant.

OBJECTIVES OF CORROSION MONITORING

Quantification of corrosion damage.

Determining the corrosion rates.

Understanding corrosion pattern /

behaviour.

Verifying effectiveness of corrosion

control measures.

OBJECTIVES OF CORROSION MONITORING

Enabling implementation of

corrective actions to avoid

equipment failures.

Assessing control of corroding

environment.

Assisting in selection of materials for

withstanding corrosion

CORROSION MONITORING METHODOLOGY

Actual metal loss due to corrosion can be

assessed by calipers, Ultrasonic thickness

gauge, corrosion coupons, corrosion probes

or radiography. Corrosion rates can then be computed

considering duration of corrosion exposure. Corrosion Coupons are installed in parallel

side streams having isolation facility.

Weight loss exhibited by coupons is

computed to work out corrosion rate. Analysing for Iron ppm in overhead water.

Metal damage assessment Techniques

Corrosion Monitoring : Direct, On-line

Non Destructive Testing: Direct, Off-line

Analytical Chemistry : Indirect, Off-line

Operational Data : Indirect, On-line

CORROSION MONITORING TECHNIQUES

Ultrasonic Thickness gauging Radiographic Examination Electrical Resistance Probes Polarization Resistance probes. Hydrogen Probes Corrosion Coupons

Ultrasonic thickness (UT) measurements:

• Measures time taken by a pulse of “sound” to travel from outside of a

piece of metal, bounce off the opposite wall (or crack, inclusion, etc.)

and return to the surface.

o Knowing the speed of sound through the metal, the thickness of the metal can

be calculated.

• Accuracy of measurements is ± 10 mils for handheld units.

• Skill of the operator very important for the accuracy of

measurements.

ULTRASONIC INSPECTION PRINCIPLEULTRASONIC INSPECTION PRINCIPLE

Ultrasonic inspection involves transmission of very high frequency sound waves through the metal whose thickness is requiredPiezoelectric crystals are used for generation of ultrasonics sound wavesDetection of reflected sound pulses from the front and back faces of metal is done with equipment electronic circuitary. The time taken for the sound to travels the thickness of metal and returned to the probe is given as digital output in terms of thickness of metal

Widely used for thickness monitoring of equipment & piping systems in Refineries. Systematic thickness recording & comparison with previous readings for any deterioration and assessment of corrosion rate. Advantage - Access to one side of object is sufficient for thickness measurement.Disadvantage - Not suitable for pitted or rough surfaces.

High temperature can destroy piezoelectric properties of the probe.

ULTRASONIC INSPECTIONULTRASONIC INSPECTION

Fig. 12 The priciple of time of flight measurement

Fig. 13 Block diagram: Pulse Echo Method

Fig. 18 Backwall echo at the 8th scale graduation

Fig. 58b

Top view with reflector for extension

CORROSION MAPPING

Radiography

• X-rays are beamed through equipment and expose a film on

opposite side of equipment.

• Darkness (density) of the developed film in any given area is

proportional to thinness of the metal.

• Cannot be used easily on large diameter or thick walled

vessels.

RADIOGRAPHY INSPECTIONRADIOGRAPHY INSPECTION

Radiography Technique depends on the opacity of

material to either gamma or X-rays and the passage of

radiation through the component reaching on the

photographic film. The density of the image produced on the film is

related to the thickness and density of the material

under examination.Used in wall thickness measurement / internal

corrosion / deposits assessment. Disadvantages of this method is radiation hazards &

time required to complete exposure.

Electrical Resistance (ER) probes Metal loss measurements are based on the change

in OHMIC Resistance of test probe elements. With corrosion, the cross-sectional area of

corroding element reduces, resulting in increase

in its electrical resistance. The electrical resistance changes are measured

using the wheat stone bridge principle. ER changes are Time Dependant but results can

be assessed faster than corrosion coupons. ER Probe remains Installed for its Operational life Can be Used to Trigger an Alarm

• Gives integrated corrosion rate.

• Corrosion rate measurement in liquid & vapour phase.

• Corrosion measurement made without removal of test sample.

• Suitable for measurement of uniform corrosion.

• Can detect low corrosion rates.

• Not suitable when pitting or stress corrosion attack are likely.

• Errors introduced in measurement with metallurgical condition of probe element & temperature changes.

ELECTRIC RESISTANCE METHODELECTRIC RESISTANCE METHOD

Electrical Resistance (ER) Probes

• Variety of probes available

• Probes measure change in electrical resistance of a metal specimen and convert into a corrosion rate.

• The probes are available in a variety of metallurgies and corrosion sensitivities.

• ER probes give essentially instantaneous corrosion rate measurements.

• Meaningful data can be obtained in a few days if corrosion is severe or the probe is sensitive to corrosion.

• Probes show corrosion at the location of the probe.

Presented by : Dr Mabruk Issa Suleiman

Electrical Resistance (ER) probes


Electrical Resistance (ER) probes – its elements

ER probes - wire loop probe

•Change in resistance of a thin wire is proportional to the corrosion

rate.

•Measurements are sensitive to rapid changes in temperature.

•Thin, sensitive elements can be damaged by high velocity streams or

particulate matter in streams unless properly shielded.

Electronics

for

measuring

resistance

ER probes - wire loop probes• As corrosion occurs, diameter of the wire decreases and its resistance increases.

• Wire loop probes are very sensitive to pitting corrosion.R = low R = high

R = low R = high

ER probes - cylindrical probe

• The same measurement method as wire loop except a thin metal cylinder is the sensor.

• The cylinder is more mechanically robust than the wire loop.

• The cylinder is less sensitive to pitting corrosion.

Electronics

for

measuring

resistance

HAC INHIBITOR PROGRAM MONITORING

Probe Fittings

Fixed (Threaded) Removable (Under Pressure)

• Special High Pressure Access System used

Corrosion Monitoring Probes

Monitoring with ER Probes

Linear Polarization Resistance (LPR) Probes

Corrosion Rate indicated / measured

Instantaneously A Small Voltage is Applied to an

Electrode in Solution Measurement of Current indicates

the Corrosion Rate Can only be Used in relatively clean

water / liquids / eletrolytes. Will not Work in Gases or Water/Oil

Mixtures

LINEAR POLARIZATION RESISTANCE METHODLINEAR POLARIZATION RESISTANCE METHOD

This technique is based on the measurement of apparent “resistance” of a test cell when it is polarized by a small voltage of the order of 5 to 20 mV.

Icorr is the corrosion current (amp/m2) which is readily converted to metal loss (mm/year).This method gives instantaneous corrosion rate.More sensitive to small variation in corrosion rate.Not suitable for low conducting media.Not suitable for localised form of corrosion.

K Rp =

Icorr

Hydrogen Probes

• Hydrogen is a by-product of the

corrosion reaction in acidic solutions.• Hydrogen probes detect hydrogen

permeating through the steel – an

indication of corrosion rate.• Hydrogen probes are surface

mounted.• Detects rate of hydrogen penetration

/ diffusion through pipe wall.

CORROSION MONITORING INSTRUMENTATION

Portable Meters Data Loggers Continuous Single Channel Continuous Multi-channel Remote Monitoring Units


Installation of probes

CORROSION COUPON METHODCORROSION COUPON METHOD

Weighed specimens are exposed to the environment for a

specified period and the weight loss of metal is measured

thereafter.

From the weight loss, corrosion rate is determined by the

following relationship.

22.3 x Wt loss (mg) Corrosion rate Mils / year (mpy)

= Sp. gr. of metal x exposed area (in. sq) x time (days)

DETERMINING PITTING RATE

Max. pit depth x 365 Pitting rate (mpy) Mils/penetration Year

= Time of test (days)

Types of coupons

Strip Coupons

Disc Coupons

Rod Coupons

Coupons with applied stress

Coupons with residual stress


The advantages of coupons includes: Visual interpretation Deposits can be observed and analyzed and layer effects

studied. Weight loss can be determined. The degree of localization of corrosion can be observed &

measured. Inhibitor film effects can be observed. It gives average corrosion rate.

Used for detection of corrosion in gas / oil pipelines, vessels,

tanks & cooling water system.


CORROSION COUPONS

CORROSION COUPONS

Corrosion Coupon Racks At Inlet Header Of Cooling

Towers

CORROSION MONITORING - Conclusions

Corrosion monitoring offers an

answer to the question of whether

more corrosion is occurring today as

compared to yesterday ! It is possible to qualify the cause of

corrosion and quantify its effect. Corrosion monitoring remains a

valuable weapon in the fight against

corrosion.

THANK YOU

Technology

Corrosion monitoring in petroleum refineries