Page 180 - CW E-Magazine (7-11-2023)

Special Report

patible with the metallurgy of the heat (phenol cooler) in seawater service the problem. In addition, a comparative
exchanger. from a refinery is discussed below: study was also made between two para-
llel phenol coolers (having one stand-
If scaling or other fouling is expec- Copper and its alloys have long by unit in the process stream), where
ted, provisions in the piping could be been used to fabricate heat exchanger the cooling seawater was in the shell
made to allow connections for flushing tube bundles exposed to seawater and side in one and tube side in the other.
out or chemical circulation cleaning. other marine environments. For in-
In large plants, it may be profitable to stance, copper-nickel alloys containing Failure history of phenol coolers
have a tank of cleaning fluid available 10-30% nickel have been extensively Phenol is used in a lube refinery
for periodic flushing of shells and/or used in coastal petroleum refineries for to improve the viscosity index of the
tubes. Makers of commercial cleaning the tubing in sea water-cooled heat ex- lubricating oil by removing the aromatic
products would be able to advice in changers. These Cu-Ni alloys have gene- contents present in the lube distillates.
this. rally provided excellent service, partly Initially, when the Phenolifiner plant
because of their good resistance to gene- was commissioned in the refinery, there
Maintenance of heat exchangers ral corrosion and partly because of the used to be only one cooler (E-202 A).
Planned shutdown of the heat ex- inherent resistance of Cu-based alloys Later, after about 10 years, a second
changer should be taken at regular in- to bio-fouling and erosion corrosion. parallel cooler (E-202 B) was installed
tervals (decided by experience or equip- Based on many years of experience, as a standby, since the first cooler used
ment manufacturer’s recommendation) design velocities below which erosion to fail very frequently.
and thorough inspection of the interior corrosion is not a problem have been
and the exterior of the units should be established as 1.5-2.1 m/s for 70-30% The first phenol cooler (E-202A)
carried out. Neglect in keeping all tubes Cu-Ni alloys. This velocity is below was commissioned in 1999, handling
clean may result in complete stoppage that which can lead to erosion corro- phenol in the tube side and cooling
of flow through some tubes, causing sion failures at tube inlets and bends. salt water in the shell side. Originally
severe thermal strain, leaking tube It is generally accepted that in portions of the metallurgy of the bundle was
joints, or structural damage to other the tube where flow is fully developed, cupronickel (90-10 type) and later on
components. When sacrificial anodes velocities can be somewhat higher, per- changed to Cu-Ni (70-30 type) because
are provided, they should be inspected haps 3-4.6 m/s, before erosion corro- of very high erosion-corrosion tak-
at regular intervals to determine whether sion becomes a problem. ing place in the heat exchanger tubes.
they should be replaced for giving conti- Even after changing the metallurgy, the
nued cathodic protection to the shell. However, most Cu-Ni alloys suffer bundle used to give a maximum life
greatly accelerated corrosion both in the of 1-2 years. The first leak in a brand
Exchangers subjected to fouling or presence of crevices and when exposed new bundle appeared within 3 months
scaling should be cleaned frequently. to seawater polluted with sulphides. and the entire bundle was condemned
A light sludge or scale coating on Several recent surveys document the within 6 months. Similarly, again a
tubes greatly reduces their efficiency. adverse effects of localised corro- new fabricated bundle leaked within
A marked increase in pressure drop and/or sion caused by dissolved sulphides on few months and the entire bundle was
reduction in performance usually indi- Cu-Ni alloy tubing in heat exchanger retubed in less than a year.
cate that cleaning is necessary. The unit services. The localised corrosion rates
should first be checked for air or vapour as high as 19-mm/year of Cu-Ni alloys Therefore, a new tube bundle par-
binding to conform that this is not the were reported in the presence of tially retubed with titanium tubes and
cause for reduction in performance. crevices and dissolved sulphides(15-17). cupro-nickel tubes (70-30) were in-
Intervals between cleaning should not stalled in the phenol cooler, based on
be excessive, as it increases the thick- Problem the feedback received from the OEM
ness of deposits on heat exchanger A phenol cooler tube bundle of (70- in 2004. The tube bundle leaked again
surfaces, causing difficulty in cleaning 30 type) Cu-Ni alloy tubes used to fail within 28 days of operation and the
process(13, 14). very frequently causing huge loss of leaky tubes were found to be of 70-
phenol, incurring very high down-time 30 Cu-Ni metallurgy. The bundle had
Case study and maintenance cost in a lube refinery. to be immediately pulled out, causing
A real life case study of failure of Detailed investigations were carried very high down-time cost and loss of
cupro-nickel tubes in a heat exchanger out for identifying the root cause(s) of phenol. The leaky bundle consisted of

180 Chemical Weekly November 7, 2023

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