"In the CUI temperature range, if the right coating is just put on properly after excellent surface prep, everything will be fine, regardless of the insulation."

To understand why this statement is a myth, let’s think about a couple of questions:

  • What causes corrosion under insulation (CUI)?
  • Why is insulation specified?

What Occurs Under Water-Absorbent Insulation?

As an example, let's pretend we have outdoor industrial equipment that operates in the CUI temperature range. The coating on the equipment has been selected properly and has been applied by an experienced applicator in accordance with the specification. The location is not in the Sahara Desert, and the right kind of coating has been applied in the best way possible.

But, as we discussed in CUI Myth 1, there really is no such thing as a truly perfect coating, because these systems are installed by humans.

Now, let’s put on a water-absorbent type of insulation (because it’s the cheapest kind).

When water ingress occurs—because it will at some point, either as a liquid or as a condensed vapor—the water will reside against either the steel surface or somewhere within the insulation thickness. The water can't totally get out.

Why can’t the water escape? One would think with heat, the moisture would evaporate. But this is what really happens under that water-absorbent insulation:

When the moderate level of heat (CUI range) coming from the steel eventually vaporizes the liquid water at or near the steel and pushes it outward, the water vapor must travel through the insulation to get to the outside surface. However, when the water finally emerges from the outer surface of the insulation, it contacts the ambient jacket, condenses and is immediately re-absorbed by the insulation.

This whole scenario is occurring for days or weeks, depending on the amount of water that the insulation is capable of holding. That entire time, the heat conservation ability of the insulation is being severely handicapped.

Why was the insulation specified by the engineers who designed the equipment? It may have been for operational stability, temperature control to allow for proper chemical reactions to occur, freeze protection, or to mitigate the risk of internal dew point corrosion.

But when the insulation is even slightly wet, the thermal conductivity is significantly impacted. Even at the upper end of the CUI temperature range, water may be driven away from the steel surface, and CUI stops (for the time being). But by that time, the insulation has been severely thermally prohibited from doing its job.

In addition, that imperfect coating imparts the risk of CUI even if there are only very small areas of imperfection. Wherever hot water exists for long periods, the risk of CUI is there.

So what's the answer? If the insulation doesn’t absorb water, and that insulation system is designed to breathe, and the equipment is designed to allow for proper drainage, this allows for two very important benefits:

  1. Corrosion from long-term wetness is mitigated
  2. The insulation performance is not interrupted and it fulfills the original needs for which it was specified

Testing Insulation Materials for Absorbency

Engineers should always conduct “apples to apples” evaluations of the materials they are considering specifying for a particular project. There are several types of industrial insulation materials that have been used historically in hot service on outdoor equipment in the oil and gas industry, and a couple of newer types as well:

  • Aerogel blanket
  • Expanded perlite
  • Calcium silicate
  • Mineral wool
  • Fiberglass

Expanded perlite and calcium silicate look a lot like chalk. These materials are molded into rigid shapes to match the shape of the equipment. In the case of calcium silicate, it is very water-absorbent.

Mineral wool is a rigid, fibrous batt-style material that becomes water-absorbent when the water-repellent oils used to coat the fibers become degraded by the heat of the service.

Fiberglass has mechanical properties similar to mineral wool, but it typically not oil-coated.

Aerogel blanket is a flexible material, which has been impregnated with aerogel powder.

A truly hydrophobic insulation material is not just coated on the surface with a water repellent, and should remain hydrophobic after short or long-term exposure to elevated temperatures in the desired range of operation.

Here’s how to evaluate for water uptake and water repellency:

These properties are crucial to the long-term functionality of the insulation, and knowing whether the insulation will contribute to the risk of CUI.

Evaluation for water uptake can be a rather simple test that one can do at home with a digital postage or kitchen scale. However, prior to the test, the specimen must first be exposed to the maximum temperature at which the insulation will be expected to do its job.

If you have tested the cooking temperature settings on your kitchen oven, it can be used for the heat-exposure step if you don't mind it radiating quite a bit of heat into your kitchen for a number of hours. The higher the heat exposure, the shorter the time of heat exposure can be.

A better idea is to use some aged insulation that has been on equipment that you know to be at the operating temperature range.

  • After weighing the dry, heat-exposed samples, place them into a pan containing about 1/4" to 1/2" of water. You don't have to hold the insulation down if it floats, as long as it is in real contact with the water.
  • Allow about 15 minutes of contact with water, then remove and weigh.
  • Calculate the amount of water uptake by percent weight gain.

Remember that even very small amounts of weight gain (water absorption) will greatly affect the thermal conductivity. The higher the weight gain, the less chance there is that the water will ever thoroughly escape a water-absorbent insulation system operating in the CUI range.

The Ultimate Test: Hurricane Rita

Although we don't operate our equipment under water, the oddest things can happen!

During Hurricane Rita, there was some double-wall pipe with aerogel blanket inside the annulus sitting in a laydown yard near Beaumont, Texas, awaiting shipment to its subsea destination. Rita brought floodwater that created a full immersion condition for all of that pipe. After the flood subsided, the owner inspected the pipe, and found the insulation fully dry and gave the go-ahead to use the pipe.

I’ve been to trade shows where a mineral wool manufacturer displayed a baby swimming pool with a foot of water in it. Floating there was a miniature boat made of mineral wool. This might impress some buyers, but not an engineer who understands why that mineral wool is floating. The engineer will understand that if that mineral wool is used in hot service, the material would remain hydrophobic for no more than the amount of time it takes the operating temperature to degrade the oil coating. Its original water repellent properties would be irreversibly destroyed.

So engineers and specifiers should understand why the insulation is on the equipment. They should also know how to compare insulation properties to assure they are getting what the equipment truly needs.

It's great to have choices, but we must understand that all insulations are not created equal. Understanding the degradation mechanisms and knowing how to evaluate in an apples-to-apples manner are crucial to protecting our equipment from CUI.

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More in this series:

CUI Myth 1: The Cause of CUI is the Lack of Proper Coating
CUI Myth 2: Shop Coatings are Better Quality
CUI Myth 4: There’s no CUI if the Jacketing isn’t Damaged