Get more life out of heat exchangers: putting sleeves into the tubes of tubular exchangers can reduce erosion, enable reactivation of tubes disabled by perforations or worn walls, or bridge discrete points damaged by cracks.(Operations & Maintenance)

Heat exchanger sleeving, a money-saving technology generally associated with the electric power industry, is drawing increasing interest from engineers in the chemical process industries (CPI), as well.

Sleeving consists of expanding thin tubes (sleeves) into the tubes of a heat exchanger. The expanding process produces a residual, interfacial fit pressure between the outside surface of the sleeve and the inside surface of the tube. The sleeves may be short, for instance about 6 to 16 in., or may extend for the full straight length of the tubes. Typical sleeve thicknesses are 0.01 to 0.03 in., depending upon material of construction and thickness of the original tubes. In addition to the expansion step, sometimes the inner end of the sleeves is welded to the inside wall of the tube.

In tubular heat transfer equipment in power plants, sleeving has long been used for one or more of these purposes:

* To reduce the prospect of inlet-end tube erosion (short sleeves for this purpose are also called ferrules, and their use is called ferruling)

* To restore tubes to service that had been plugged by plant personnel because of known perforations in discrete, identifiable locations

* To restore tubes to service that had been plugged because their walls had become excessively thin

* To bridge failures in discrete locations of tubes that are otherwise intact; for example, if a tube has a circular crack just beyond the inner face of the tubesheet

Before applying sleeving to similar problems at CPI plants, it is useful to: be aware of the sleeving methods and equipment available, be able to determine how sleeves affect the heat transfer performance of heat exchangers, and be able to calculate the changes in pressure drop through the tubes.

Utility-plant roots

It is understandable that sleeving first made its name at power plants, where planned feedwater-heater and steam surface-condenser life may be as long as 40 years [1]. Power stations defer replacing feedwater heaters as long as they can because it is costly and time-consuming, and they seldom replace steam surface condensers at all. Full-length sleeving in those condensers is an alternative for powerplant management, as are the options of continuous cleaning of the tubes, cleaning during outages, replacement of individual tubes, full retubing, and ferruling the inlet ends of the tubes.

Admittedly, planned heat-exchanger life in the process industries is seldom as long as ten years. So, many heat exchanger installations are designed with ease of replacement and/or onsite retubing in mind. Whereas it would not be economical for a power station to keep spare feedwater-heater bundles on hand, many petroleum refineries and other CPI plants keep spares in inventory; they replace bundles in which the tubes have deteriorated with those spares, and then retube and store the original bundles. Furthermore, the life of the tubes in a CPI heat exchanger might be so short that the user considers it to be expendable if the cost of retubing approaches the exchanger's replacement cost.

Despite these facts, the …