Tube-Sheet Joints

Testing Upon completion of shop fabrication and also during maintenance operations it is desirable hydrostatically to test the shell side of tubular exchangers so that visual examination of tube ends can be made. Leaking tubes can be readily located and serviced. When leaks are determined without access to the tube ends, it is necessaiy to reroll or reweld all the tube-to-tube-sheet joints with possible damage to the satisfactoiy joints. 

For example, unsatisfactory equipment design has caused inlet-end erosion due to improper distribution of water in the water box. Poor design and construction practices have caused erosion-corrosion of the tube sheet due to unsatisfactory water distribution or leakage past pass partitions. Deep channels (wire drawing) or worm holes have been observed in tube holes due to leaking tube-to-tube sheet joints. 

The tube sheet forms the barrier between the shell and tube fluids, and where it is essential for safety or process reasons to prevent any possibility of intermixing due to leakage at the tube sheet joint, double tube-sheets can be used, with the space between the sheets vented Figure 12.16c. 

The high-pressure, feedwater heaters are prone to leaks at the tube-to-tube sheet joints. Lining or over-laying the tubesheet with Alloy 600 and using tube materials with 1% Cr / 0.5% Mo can mitigate this risk considerably88. 

Stress relieve carbon and chrome steel welds and cold bends in amine service regardless of service temperature. For all concentrations of carbonate solutions and in concentrations of caustic up to 30 percent stress relieve for service temperatures above 140 F (60 C). For 30 to 50 percent caustic, the service temperature where stress relief is required decreased from 140 >F(60°C) to 118T (4B C). Welded tubing does not require heal treatment in addition to that required by the ASTM specifications. Rolled tube-to-tube sheet joints do not require stress relief. 

Teflon sealer sheet is used at the tube-to-tube-sheet joint. 

Construction Codes Rules for Construction of Pressure Vessels, Division 1, which is part of Section VIII of the ASME Boiler and Pressure Vessel Code (American Society of Mechanical Engineers), serves as a construction code by providing minimum standards. New editions of the code are usually issued every 3 years. Interim revisions are made semiannually in the form of addenda. Compliance with ASME Code requirements is mandatory in much of the United States and Canada. Originally these rules were not prepared for heat exchangers. However, the welded joint between tube sheet and shell of the fixed-tube-sheet heat exchanger is now included. A nonmandatory appendix on tube-to-tube-sheet joints is also included. Additional rules for heat exchangers are being developed. 

As noted in Table 3 [77], the results of multiple alloy tests in seawater are correlated to the Pitting Resistance Equivalence (PRE) number [47,48,72,77], which also correlates to alloy pterformance in FeCly. Anderson [78] and Streicher [79] used MCA in seawater tests to compare alloy performance. More recently, a simple, flat, plastic (specifically, polymethylmethacrylate, which is often referred to as "perspex ) washer has been successfully used to evaluate a series of alloys in seawater [77]. One application of the ASTM G 48 test has been in simulating leaking tube-to-tube sheet joints in seawater heat exchangers and condensers [87]. When certain highly corrosion-resistant alloys were paired in a dissimilar metal crevice (DMC) with alloys that would be expected to suffer crevice corrosion in the particular test solution, the more corrosion-resistant alloy was found to corrode due to the accelerating effects of the corrosion products from the less resistant alloy. The results of DMC tests in ferric chloride were confirmed by long-term DMC exposures in seawater [82],... 

But welds were used for both the tube-to-tube and the tube-to-tube sheet joints of the steam generator. An automated welding machine was developed, principally for the tube sheet welds. 

Titanium alloys may be subject to localized attack in tight crevices exposed to hot (>70 °C, or 160 °F) chloride, bromide, iodide, fiuoride, or sulfate-containing solutions. Crevices can stem finm adhering process stream deposits or scales, metal-to-metal joints (for example, poor weld joint design or tube-to-tube sheet joints), and gasket-to-metal fiange and other seal joints. 

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