In situ coating application expected to extend tube life

Coating applications on heat exchanger carbon-steel tubes can extend the life of the tubes and reduce the number of maintenance downtimes.

Hoping to accomplish that, in December 1997, Citgo's Lake Charles, La., refinery applied an in situ coating to the tubes of six of its propylene exchangers.

The six exchangers are part of the refrigeration section of Citgo's catalytic cracker recovery unit.

All six exchangers have carbon steel tubes in cooling water service. Cooling water runs on the tube side of the exchangers, and ammonia runs on the shell side.

Scope of work

The six propylene exchangers that Citgo coated have fixed tubesheets. Fixed tubesheets are costly to retube because the tubesheets are welded to the exchanger shells.

Because of severe corrosion and pitting on two sets of tubes, however, Citgo retubed two of the exchangers. The company contracted Bob Curran & Sons, Hollywood, Fla., to coat these new tubes.

The remaining four exchangers were relatively new, only 3 years old in December 1997, and experienced only minor corrosion and pitting. Thus, these existing tubes were coated to prevent further pitting and corrosion and to decrease fouling.

Sulfate-reducing bacteria (SRB) causes fouling and underdeposit corrosion of bare carbon steel tubes in cooling water. Fouling increases the pressure drop across the exchanger, and corrosion can cause leaks. These problems often result in costly downtime to clean tubes or to repair leaks.

Fig. 1a [122,890 bytes] shows a sample of one of the tubes from Citgo's exchanger that had been in cooling water service for 3 years. The tube was made of SA214 carbon steel with a 0.75-in. OD. There was about 15-20 mils of corrosion product on the inside surface of the tubes.

Fig. 1b shows the same tube after having been gritblasted for 30 sec. The surfaces were prepared per an SP-5 or NACE-1, white metal specification. The surface profile was 1-1.5 mils, and the pitting was 0.020-0.040 mils deep.

Fig. 1c shows the same tube after its coatings application. The tube received three coats of a polymer coating. The dry-film thickness was 6-7 mils. The surface tension was about 40 dynes/sq cm.

Reason for coated bundles

Citgo made its decision to coat these six bundles on two bases: its past experience with coatings and the cost.

In 1995, Citgo had installed new baked-on epoxy-phenolic coated bundles in its catalytic-cracker recovery-overhead systems. It replaced a bare carbon steel bundle which had experienced fouling and corrosion.

That bundle previously lasted no longer than 6 months before it was removed from service. After 24 months, inspection revealed that no fouling had occurred.

In the past, Citgo had also replaced inhibited admiralty brass bundles with coated carbon-steel bundles. The uninhibited admiralty brass was used because of carbon steel's poor performance in cooling water service.

Despite its poor fouling resistance in cooling water service, carbon steel is desired, however, because of its structural integrity. Coated carbon steel bundles give the corrosion resistance of the coating and the strength of carbon steel.

They protect the tubes from both the shell-side process and the cooling water. Citgo also realized that coated carbon-steel bundles required less cleaning on the water side than did the inhibited admiralty bundles.

The direct results of Citgo's 1995 decision to use coated carbon steel bundles were increased reliability, improved capacity, and higher propylene recovery for the catalytic cracker recovery unit.

In addition to past experience with coated bundles, Citgo performed a quick calculation to make sure that an investment to coat four steel tubes was the best choice.

Although copper-based alloys are often used in water service, the ammonia product on the shell-side of the exchangers precluded its use.

Chlorides present in the cooling water made 300-series stainless steel prohibitive because of the possibility of stress corrosion cracking. Finally, more-expensive metallurgies were considered cost prohibitive.

The two best options were bare carbon steel and coated carbon steel. Simply based on the need to routinely retube exchangers and repair leaks, the use of bare tubes costs more than twice as much to maintain over 10 years than the cost of coated tubes.

Table 1 [74,602 bytes] illustrates this quick calculation based on a 15% rate of return.

The total costs of coated tubes are estimated to be the initial cost to coat the four exchangers, about $100,000, and minor cleaning and coating repairs after 5 years of service.

The estimated costs did not account for process losses as a result of outages during cleaning and repairing, which would be more frequent for the bare-tube option (every year) than the coated option (once in 5 years).

Coatings considerations

In the past, internally coated tubes had limitations. Because the lining had to be done in special workshops, the heat exchangers had to be disassembled, transported, and reassembled. Large units could not be transported.

Today, the coating can be applied in situ. The coatings are matched to the materials of the tubes; they can be novalacs, phenolics, polyamides, and flourinated products.

Although one would expect coated tubes to have a lower degree of heat transfer than uncoated tubes, that is not necessarily true. Fig. 2 [48,687 bytes] shows preliminary observations of the performance of coated tube vs. uncoated tubes over time. Over time, because the coating reduces fouling and corrosion, the coated tubes' heat transfer performance exceeds that of the uncoated tubes.

Before coating application, the surface of the tubes must be prepared to SP-5 white metal. Proper cleaning, surface preparation for coating, and handling during installation and maintenance are required to maintain the integrity of the coating.

The coatings contractor often assists in the tube cleaning and surface preparation.

For Citgo, Bob Curran & Sons first evaluated the service and history of the tubes (water chemistry, process materials, age, temperature). They observed the level of saturation to determine what combination of the following three methods to use to clean the tubes: hydroblasting at 10,000 psi, steam cleaning, or pressure cleaning with a chloride treatment.

If the tubes are new or have been exposed to hydrocarbons, a degreaser is sprayed throughout the tubes and then steam cleaning is done.

The tube coating is applied with a patented nozzle with pressure atomization to evenly apply it throughout the tube. The high pressure (1,800-4,500 psi) drives the coating into the surface, improving bonding and wetting.

Typically, dry-film thickness deviation is 1 mil or less between the top and bottom of the tubes. The total dry film thickness is 6-8 mils.

Coating limitations

Today, applications have only been done to 60-ft long tubes although Ed Curran, president of Bob Curran & Sons, says 100-ft long tube applications are possible. In addition, a 3-ft clearance between the tubesheet to the next wall must be available to perform the work.

Other limitations include the special care needed to prepare the surface for the coating, to handle the coating during installation and maintenance, and to clean coated exchangers. Also, coatings have limited chemical and temperature applications. Depending on the service, the baked-on phenolics can withstand temperatures as high as 400° F. Polyamides and fluoropolymers can withstand up to 550° F.

Before coatings, traditional options to extend tube lives are chemical cleaning and mechanical cleaning.

Chemical cleaning requires proper handling of chemicals, solvents, mineral acids, or foams. Chemical cleaning can also produce toxic H2S gas and hazardous waste.

Also, removing oxides can cause further corrosion and structural degradation of the tubes. In the future, legal pressures will reduce the amount of oxides allowed in plant effluents, thus making oxide removal unattractive.

Mechanical cleaning removes fouling, dirt, and oxide scale. However, constant monitoring is required so that the structural integrity of the tube remains. Again, the removal of oxides can further corrosion.

Results

Before cleaning and coating the six sets of exchanger tubes in 1997, the ammonia coolant pressure was 230 lb. After installation of the new tubes, the pressure dropped to 205 lb. Currently, the heat exchanger's pressure drop runs about 190-200 lb.

In the prior 3 years of service, with bare carbon steel tubes, these four exchangers experienced a pressure drop increase of about 15 lb/year.

For these four sets of exchanger tubes, Citgo expects to get a 10-year coating life on its tubes. Minor touchups on the tubesheet and gasket surfaces will probably be required during maintenance periods.

After 10 years, the tube bundle will probably need to be blasted and coated again. With the new coating, Citgo expects a bundle life of more than 20 years.