AREAS SUSCEPTIBLE TO CUI or CUF DAMAGE

Under the right temperature conditions, CUI or CUF damage can occur at any location that is insulated or fireproofed. CUI and CUF are somewhat insidious in that regard. It is not uncommon to find CUI/CUF damage in locations remote from the more predictable and susceptible locations. However, there are some areas within facilities that  have higher susceptibility for damage. In general, areas with severe CUF damage are easier to identify visually than CUI damage due to cracks and staining of the fireproofing. Certain areas and types of equipment have a higher susceptibility for CUI damage.
1.1 General Areas of DamageThere are a number of locations in oil or chemical processing facilities where CUI damage has a higher likelihood.
Potential locations areas  for CUI throughout Process Facilities common to all equipment type are:

• Areas downwind of cooling towers exposed to cooling tower mist
• Areas of protrusions (i.e. transition points) through the jacketing at manways, nozzles, and other components
• Areas of protrusions through insulation for equipment/piping operating at below ambient or cold service
• Areas where insulation jacketing is damaged or missing
• Areas where caulking on insulation jacketing is missing or hardened
• Areas in the jacketing system are bulged or stained
• Areas where banding on jacketing is missing
• Areas where thickness monitoring plugs are missing
• Areas where vibration has caused damage to the insulation jacketing
• Areas exposed to steam vents
• Areas exposed to process spills, ingress of moisture, or acid vapors
• Areas exposed to deluge systems
• Areas insulated solely for personnel protection
• Areas under the insulation with deteriorated coatings or wraps
• Areas with leaking steam tracing
• Pipe and flanges on PSV’s
• Systems that operate intermittently above 250°F (120°C)
• Systems operating below the atmospheric dew point
• Systems that cycle through the atmospheric dew point
• Ice-to-air interfaces on insulated systems which continually freeze and thaw are susceptible to corrosion damage

All equipment will be shut down at some time or other. The length of time and the frequency of the down time spent at ambient temperature may well contribute to the amount of corrosion under insulation that occurs in the equipment.

1.2 Pressure Vessels

In addition to the areas listed above, there are other areas in vessels, columns, drums, and heat exchangers where CUI may have a higher likelihood..

 Potential locations are:

• Insulation support rings below damaged or inadequately caulked insulation on vertical heads and bottom zones
• Stiffening rings on insulated vessels/columns in vacuum service
• Insulated zone at skirt weld
• Insulated leg supports on small vessels
• Ladder and platform attachments
• Termination of nozzle and saddle insulation
• Fireproofed skirts (CUF)
• Anchor bolts (CUF)
• Bottom of horizontal vessels
• Irregular shapes that result in complex insulation installations (e.g., davit arm supports, lifting lugs, body flanges, etc.)

1.3 Piping

In addition to the areas listed under point no1.1 above , there are other areas in piping where CUI may have a higher likelihood, and includes process piping, refrigerated piping, piping at or below grade, and pipe supports. 

Susceptible Potential locations for CUI and CUF in piping are :

• Deadlegs, vents, and drains
• Pipe hangers and supports
• Valves and fittings
• Bolted on pipe shoes
• Steam/electric tracing tubing penetrations
• Termination of insulation at flanges and other piping components
• Carbon/low alloy steel flanges, bolting, and other components in high alloy piping
• Jacketing seams on the top of horizontal piping
• Termination of insulation on vertical piping
• Areas where smaller branch connections intersect larger diameter lines
• Low points in piping with breaches in the insulation

1.3.1 Cold piping

 Cold piping is piping used to deliver liquid or gases which cools the piping to temperatures below the atmospheric dew point. Cold piping is prone to corrosion due to condensation. The condensation present will most likely freeze in cases where the outside surface of the piping decreases below freezing. In many cases, such as ammonia terminals, piping temperature swings from ambient to -30°F (-27°C) during periods when ammonia is flowing in the piping. This temperature swing leads to continuous freezing and thawing resulting in wet conditions that increases the piping system susceptibility to CUI damage. Additionally, other equipment and components such as tanks, pressure vessels, pipe supports and flanges connected by this piping may be affected by the run off of melting ice.

Ice layers can form on piping operating at temperatures below freezing, and can obscure the view of external surface damage due to a continuous wet environment. In many cases, piping used for these cold temperature applications is insulated. Frequent chilling and condensation accelerates corrosion at points where insulation is breached which exposes the surface of the piping to the atmosphere (i.e., ice-to-air interfaces). Water ingress, due to poorly sealed insulation jacketing, can result in ice build-up causing swelling of the insulation, and create a larger area of damage to the insulation system. This repeated condition creates more and more exposure and susceptibility to corrosion.

Frequent chilling and condensation accelerates corrosion at points where insulation is breached which exposes the surface of the piping to the atmosphere (i.e., ice to air interfaces). Water ingress, due to poorly sealed insulation jacketing, can result in ice build-up causing swelling of the insulation thus creating a larger damaged area to the insulation system; this repeated condition creates more and more exposure and susceptibility to corrosion.

Susceptible locations for CUI/CUF in Piping Operating Below the Dew Point

• Pipe Supports
• Insulation termination areas such as pipe to flanges
• Flanges with stud bolts where insulation bonnets are installed but not sealed
• Piping below flood grade where rising water penetrates the insulation jacketing causing ice lens with swelling which causes jacketing failure
• High foot traffic areas where insulation is degraded by contact with human traffic
• Ice-to-air interface

1.3.2 Pipe supports
The accumulation of water can occur at locations remote from the point of intrusion, especially in services where the surface temperature does not cause the water to evaporate. For example, this can occur on a horizontal line in the middle of a span between pipe supports where the insulation is missing at the supports. Yet, evaporated water may also travel through the insulated system and condense in areas with a lower surface temperature.

1.4 Tankage and Spheres

This includes insulated tanks and spheres in both hot and cold service.
Susceptible locations for CUI and CUF damage in various equipment types are 

• Stairway tread attachments
• Insulation support rings
• Fireproofed legs on spheres (CUF)
• Insulation penetrations such as nozzles, brackets, etc. on shell and roof

1.5 Heat Traced Systems

Heat tracing systems are used to protect pipes from freezing, or to maintain process temperatures for piping that should transport substances that solidify or lose viscosity at ambient temperatures. Heat traced systems are divided between electric and steam traced systems. From a design perspective, electric traced systems with chloride-free (i.e. non-PVC) electrical insulation would be the preferred choice to minimize CUI damage in insulated systems. Though this may be the preferred choice to minimize CUI damage, in reality, the majority of systems in use today are steam traced systems.

When steam tracing  fails, it defeats all CUI barriers. These systems often fail at coupling joints under the insulation. When steam tracing fails under insulation, it introduces moisture, strips away protective coatings, and raises the metal surface temperature within the CUI temperature regime. In addition, the same conditions can potentially cause ECSCC on austenitic stainless steel pipe and instrument tubing under the insulation.

1.6 Shutdown/Mothballing

Equipment or piping systems that are shutdown for extensive periods or mothballed also have higher susceptibility for CUI and CUF damage. During extended idle periods, these weather barriers, i.e. insulation and fireproofing, can deteriorate and lead to increased corrosion . Consideration should be given to removing insulation and fireproofing on equipment and piping systems that are shutdown for extended periods of time or as part of the mothballing procedure, especially in moist and humid climates.