A systematic inspection plan for corrosion under insulation (CUI) in process plantsA step by step approach in CUI inspection ensures no area in the plant is left unattended. It saves both money and time required for inspection by allowing the inspector to concentrate his time on most critical locations. Steps followed in the CUI inspection plan of a typical process plant is explained in the following section. I hope this will work as a guide line to develop CUI inspection plans for individual plants.

 STEP 1 : IDENTIFY EQUIPMENT PRONE TO CUI

Not all the insulated equipments are susceptible to CUI. So the first step in any CUI inspection is identifying the potential equipments. Points considered at this stage are the material of construction and operating temperature of equipment:

MATERIAL OF THE EQUIPMENT

In the case of insulated Carbon Steel (CS) equipments, trapped water along with contaminants act as an electrolyte on the surface. As a result general corrosion occurs on equipment surface which slowly lead to the failure of equipment if remained unattended.

 In the case of Austenitic and duplex stainless steel CUI occurs in the form of SCC (Stress Corrosion Cracking) due to increased chlorine concentration.

OPERATING TEMPERATURE OF EQUIPMENT

Insulation makes a closed environment on metal surface. This over saturates the air

trapped inside insulation. Also it is proved that in long term, all equipment works in cyclic temperature range. These aspects widen the temperature range under which CUI happens.

 API 571 - Damage mechanisms affecting refinery equipments gives the temperature range of CUI as

10°F (–12°C) and 350°F (175°C) for carbon and low alloy steels, 140ºF (60°C) and 400ºF (205°C) for austenitic stainless steels and duplex stainless

steels

So in first step, equipments are classified as per material of construction. The equipments falls under above temperature range are susceptible to CUI with temperature range of 49 0C to 93 0C being the most severe environment. Also

equipments working in cyclic temperature range and equipments which are down for long time demand special attention.

 STEP 2 : DETERMINE THE PURPOSE OF THE INSULATION SYSTEM

Insulation systems are provided for heat conservation, process requirement, personal protection and noise reduction. Identify the purpose of insulation. With the help of process and operating department, understand the process environment.

 Insulation for personal safety is applicable where exposed surface temperatures exceed 65 0C in normal or short term operating conditions. As per refinery standards exposed surfaces located within 600mm horizontally or 2100mm vertically of a normal access, walkway work area are to be insulated.

Also contact process department to determine insulation removal is possible in any of the systems. Some time insulation removal can lead to more corrosion (for eg – corrosion due to condensation) or process upsets. But there are instances where entire insulation is removed due to change in process conditions.

STEP 3: IDENTIFY THE COATING SYSTEM UNDER INSULATION

Till 80s, industry did not understand that the environment under insulation was going to be almost like immersion conditions so the correct type of coating was not used. As a result almost none of the surface under insulation older than 15 years is properly protected from CUI. Now it is widely accepted that protective coatings under insulation are the most effective method for preventing CUI.

Different types of coatings are available for different materials and temperature range (Refer Annexure 2). Repair or over coating of an installed system requires evaluation of existing coating that remains on the substrate. Identification of type of coating under insulation will give some idea of CUI happening on metal surface. So proper documentation of type of coating applied is essential for effective inspection against CUI of equipment in future.

Some of the points noted are;

Average life of most of the coatings under insulations is observed to be around 8 years.

Inorganic zinc coatings or galvanizing shall not be used under thermal insulation in the 50 to 150°C (120° to 300°F) service temperature range for long-term or cyclic service. Zinc provides inadequate corrosion resistance in

closed, sometimes wet environments. Thermally sprayed aluminum coatings have performed successfully in marine and

high temperature environments The coating systems recommended for use on carbon steel operating below 150°C

(300°F) under thermal insulation are typically tank lining systems formulated to prevent corrosion.

Austenitic Stainless Steel coating system shall not contain free, soluble chlorides or other halides after curing to prevent SCC. Similarly the coating shall not contain zinc, lead, copper, or their compounds due to the risk of liquid metal cracking.

Aluminum foil wrapping has been used to prevent ESCC of stainless steel under insulation

From the above data it is clear that insulations having inorganic zinc coating need more attention than thermally sprayed aluminum coatings. Similarly austenitic stainless steel systems without aluminum coating require more attention.

STEP 4: ASSESS AGE AND APPEARANCE OF INSULATION SYSTEM

It is observed that average life of most of the coatings under insulation is around 8 years where as a properly maintained insulations lasts for 15 years. So even if insulation appears damage free, corrosion can start on metal surface due to coating damage if the age of insulating system is more than 8 years. At the same time if the insulation cladding is damaged chances are more for water ingress which accelerates CUI. So even though age is the primary concern, appearance of insulation system also needs to be considered during inspection

STEP 5: DETERMINE THE EXTENT OF INSULATION REMOVAL

 From the above 4 steps, systems which are most susceptible for CUI can be identified. Once these systems are identified, next step is the direct inspection of equipments by insulation removal. For this purpose the most critical area of insulation in the equipment should be located.

As per API 570, if external coverings are in good condition and there is no reason to suspect damage behind them, it is not necessary to remove them for inspection of the equipment. But CUI damage is often quite insidious in that it can occur in areas where it seems unlikely. So it is always advisable to remove some insulation from the most critical areas as a part of CUI inspection

Following general steps can be followed to determine the extent of insulation removal for direct inspection of CUI of any insulated equipment.

 

Remove all the inspection windows are access the condition of the surface under the window. Special consideration to be given on the condition of the coating under insulation. If the coating under the inspection window is good then it is an indication of less CUI in the system.

If there is indication of coating damage or starting of general corrosion under inspection window some insulation can be removed from the most critical areas. Effect of design, external factors, site condition and CUI history of the equipment (or similar equipment) shall be considered to identify the critical areas of CUI. As per various refinery standards and observations, locations in insulated equipments where CUI can mostly be found are enlisted below.

EFFECT OF DESIGN

1. Equipments with vapor barriers, weatherproofing or protrusions through the insulation or at insulation termination points such as flanges.

2. Equipment designed with insulation support rings welded directly to the vessel wall; particularly around ladder and platform clips, and lifting lugs, nozzles and stiffener rings.

3. Locations where moisture/water will naturally collect (gravity drainage) before evaporating (insulation support rings on vertical equipment) and improperly terminated fireproofing.

4. Dead legs (vents, drains, and other similar items). 5. Pipe hangers and other supports.

6. Valves and fittings (irregular insulation surfaces). 7. Bolted-on pipe shoes. 8. Steam tracer tubing penetrations. 9. Termination of insulation at flanges and other piping components.10. Insulation jacketing seams located on the top of horizontal piping or improperly

lapped or sealed insulation jacketing. 11. Termination of insulation in a vertical pipe. 12. Low points in piping systems that have a known breach in the insulation system,

including low points in long unsupported piping runs. 13. Carbon or low-alloy steel flanges, bolting, and other components under insulation in

high-alloy piping systems. 14. The first few feet of a horizontal pipe run adjacent to the bottom of a vertical run

CONDITION OF INSULATION SYSTEM

1. Locations in the piping system/ equipment where insulation is damaged2. Piping or equipment with damaged/leaking steam tracing.

3. Localized damage at paint and/or coating systems

4. Piping or equipment with damaged/leaking steam tracing.

5. Vibrating piping systems that have a tendency to inflict damage to insulation jacketing providing a path for water ingress.

6. Caulking that has hardened, has separated, or is missing.

7. Bulges or staining of the insulation or jacketing system or missing bands. (Bulges may indicate corrosion product buildup.)

8. Locations where insulation plugs have been removed to permit piping thickness measurements

9. Locations having soil to air interference

EFFECT OF EXTERNAL FACTORS

1. Rainfall2. Drift from cooling towers

3. Condensate falling from cold service equipment

4. Steam discharge

5. Process liquids spillage

6. Spray from fire sprinklers, deluge systems and wash-downs

7. Marine environment/ proximity of sea

8. Proximity of chlorine containing process units

These points should be considered while doing inspection removal. As per OISD standards insulation removal can be carried out on sample basis in such a way that all the identified locations get inspected at least once in 12 years. Inspection frequencies can also be followed as per annexure 1. Inspector can decide on the extent of insulation removal to carry out thickness survey and to assess the pipe surface condition as per the frequency stipulated for the pipeline based on its service.

As a general practice for piping, insulation is opened at the support locations and at elbows, tees and every ten meters interval if operating temperature is below 150 0C. If operating temperature is above 150 0C insulation pockets are removed for ultrasonic thickness gauging.

Similarly for Columns / vessels / heat exchangers;

Insulation is opened at the top dish and at insulation support rings in top zone in CUI prone zone.

Insulation is removed from top shell up to operating temperatures 150 0

In all the vertical columns and vessels, insulation is opened at the nozzles/ manways and at sample Pockets for UT (Ultrasonic thickness) measurements

In horizontal vessels, insulation is removed at sample areas where detachment is seen.

In the equipment sections operating above 150 0C, the insulation pockets are removed for ultrasonic thickness gauging.

STEP 6: INSPECTION PLAN FOR AREAS WHERE INSULATION REMOVAL IS NOT PRACTICAL

When insulation removal is not practical, suitable NDT methods can be used. Piping under non accessible culverts, long stretch of pipe lines where insulation removal is not cost effective etc includes in this list. Some of the NDT methods that can be used are Long Range Ultrasonic Testing (LRUT), Pulsed Eddy Current Technique (PEC) and profile radiography.

Long Range Ultrasonic Testing (LRUT) can be used for pipeline inspection where operating temperature is less than 125 Deg C. A small band of insulation needs to be removed for mounting array of UT transducers band in LRUT technique. It scans the pipeline longitudinally on both sides of transducer ring using guided ultrasonic waves. This technique gives the cross-sectional metal loss of pipelines. Technique is suitable for long straight length pipe.

Pulsed Eddy Current (PEC) may be deployed without removal of insulation on both equipment & pipelines and average metal wall thickness of the location below the insulation can be measured. PEC technique may also be used for inspection of fire proofing, skirts/pipelines. Necessary caution to be taken when PEC is used at projections like nozzle, stiffener ring etc as the projection also generates additional eddy current.

Profile radiography may be used for measuring thickness without removing insulation. Cordoning of the area for radiography is the main disadvantage.

 STEP 7 (FINAL STAGE): Further action can be taken after accessing the condition of metal surface exposed by the above guidelines. If the metal surface is found to be in good condition there is no need for further removal of insulation. But if there is indication of CUI,

inspector should access the extent of CUI damage and if necessary, action to be taken for extensive removal of insulation.