Re: CUI Part 2

From: <wes>
Date: Mon Jun 25 2001 - 17:57:00 EDT

• "SARE, RALPH H." <SARERH@YANPET.SABIC.com> wrote:
  > The following was taken from ASTM STP 880 by Pollock
  > and Barnhart.
  >
  >
  > Very serious corrosion problems can occur to plant
  > equipment and piping
  > components that are thermally insulated if the
  > insulation becomes wet.
  > On carbon steels, the corrosion is usually of genral
  > or pitting type. On
  > stainless steels, the corrosiion is almost always
  > SCC.. The isnualtion
  > usually hides the corrosidng metal and the problem
  > can go undetedcted for
  > years u ntil metal failure occurs.
  >
  > To complicate the problem, the degree of corrosion
  > when insulation gets wet
  > is dependendt on the type of insulation. Inspection
  > for the problem is also
  > difficult Determning the the metal has corroded
  > under the insualtion is not
  > widely available.
  >
  > The STP also touches on the subject related to SCC
  > and the effect of service
  > temp. Here are some.
  >
  > Service TEmp is important in CUI for 2 reasons
  > 1. higher temp allows water to be present againts
  > the steel for less time
  > 2. higher temp make the water more corrosive, and
  > paints and caulking fail
  > sooner.
  > generally, equipment that operates below freezing
  > is protected againts
  > corrosion. Howveer, attachment to the equipment
  > which are not as cold are
  > vulnerable in the transiiton out of the vapor
  > barririer into warm humid air.
  > Cororsion associated with equipment below freezing
  > temp is corrosion outside
  > of, not under insulation
  >
  > Corrosion under warm inuslation is equally difficult
  > to manage due to dry
  > out of entering water. Summary of the study showed
  > that:
  > 1. temp range of 60 and 80C appears to account for
  > the gretaest amount of
  > damage.
  > 2. on very warm equipment even with small
  > weatherporffoing defects,
  > coorosion tends to be at the oint of entry of water.
  > 3. annual corrosion rate amy exceed 1.5 mm per year.
  >
  > Guys, the problem is real and will cost a lot of
  > dollars to detect and
  > repair. The report further stated that there are
  > seven contributing factors
  > to CUI. Top on the list is, now hear this,
  > "equipment design".
  >
  >
  >
  > You might want to try cutting the insulation at the
  weld (if it is possible) and run an ultrasonic scan on it to test for pipe thickness(or corrosion in the pipe)
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