Plugged tubes

Figure 13.5 Plug-type dezincification on the internal surface of a brass condenser tube. Note the extreme porosity of the copper plugs. Tube wall thickness was 0.040 in. (0.10 cm). Compare to Fig. 13.13. (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)...

Surfaces of plugged tubes were carefully compared to surfaces of tubes from the same condenser that were never plugged. Only plugged tubes showed denickelification. Stagnant conditions, as well as the deposits and chloride, caused the dealloying. The deepest dealloying was up to 5% of the remaining wall thickness. 

Before the exchanger left the plant, the free tubes v. ere cleaned with high-pressure water jets. The plugged tubes were opened up by drilling >s-in. holes through the plugs to relieve any trapped pressure. But these holes were not big enough to allow the tubes to be cleaned. 

A certificate was attached to the exchanger stating that welding and burning were allowed but only to the shell. The contractor, having removed most of the tubes, decided to put workers into the shell to grind out the plugged tubes. He telephoned the plant and asked if it would be safe to let workers enter the shell. He did not say why he wanted them to do so. 

The certificate attached to the exchanger when it left the plant should have contained much more information. It should have said that the plugged tubes had not been cleaned and that they contained a chemical that gave off fumes when heated. Better still, the plugged tubes should have been opened up and cleaned. The contractor would have to remove the plugs, so why not remove them before they left the plant ... 

Check the recirculating CW pressure. An increase in pressure drop indicates fouled or plugged tubes, or a fouled tube sheet. 

Tube-to-tube interactions. The problems of velocity profile elongation and thermal runaway can be eliminated by using a multitubular reactor with many small-diameter tubes in parallel. Unfortunately, this introduces another form of instability. Tubes may plug with pol5nner that cannot be displaced using the low-viscosity inlet fluid. Imagine a 1000-tube reactor with 999 plugged tubes ... 

To summarize, some of the tubes in the bundle continue to foul until they plug off. These tubes get hotter until they reach the temperature of the hot oil circulating through the shell. Other tubes continue to receive more and more flow. The velocity in these tubes increases to the point where the rate of fouling becomes inconsequential. This velocity is called the terminal-tube velocity. These tubes may run 50 to 150°F cooler than the plugged tubes. 

Flow measurements showed flow rates lower than expected and backwashing was observed. The backwash contained brownish siliceous matter and other solid impurities. All three heat exchangers had silt and dirt deposits and partially plugged tubes. The tube ends were corroded and thinned down. Perforation of the tubes originated from inside (cooling water side). Pitting shows it to be underdeposit corrosion. 

If the heater is conservatively designed, as is usually the case, the initially plugged tubes cause small, if any, deterioration of performance. Eventually however, film buildup and plugged tubes increase to the point that the heater can no longer operate near design conditions. As the feedwater heater efficiency drops off, it affects the overall cycle efficiency, which causes an increase in operating costs and, at some stage, the heater should be replaced. 

Other workers had made similar observations earlier, but had discarded their plugged tubes and reported the experiment as a failure. Hogan and Banks did not discard their plugged tubes, but worked up the polymer to see what they had. What they had was linear polyethylene of almost perfect linearity (9). 

Fouling. Reboiler fouling is common in services handling imstable or corrosive chemicals. Foulants either form scale on heat transfer surfaces, or plug tubes, or both. Increased fouling is often accompanied by increased degradation of material. 

The material of construction of the tubular reactor can also influence fouling and plugging. Tubes made from polymeric materials, especially fluoropolymers, have been more successful than those made of glass and metals. Fouling and plugging can also be minimized by only carrying out the early part of the reactions in the tube, such as done with tube-CSTR systems [20-22]. 

Thermal overperformance one stream cold exit temperature > expected instrument/ plugged tubes/inlet velocity < design, fouled screen on pump suction/pump problems, see Section 2.3/increased heat load. 

Columns are commonly made of stainless steel. The stationary phase is retained by porous, stainless steel frits at both ends of the column. The entrance frit acts as a filter for particles and helps distribute liquid evenly over the column diameter. Samples should be passed through a 0.5- to 2-pim filter prior to injection to prevent contaminating the column with particles, plugging tubing, and damaging the pump. Deterioration of peak shape is a common indication of clogging of the frit, which can be reverse-flushed with solvent or replaced. 

Amuay HYAY-2 9.5 mmscf/d Hydrogen Plant (Reformer) Furnace F-SSl Explosion- Plugged tubes, not dried out, exploded and ruptured adjacent tubes. 

Back-flushing is an effective method to open and clean plugged tubes. Figure 16-1 shows a properly installed back-flush connection. Note that the back-flush nozzle is the same size as the water-inlet nozzle (see Chapter 12). 

Biological fouling Hardness deposits Plugged tubes Cycles of concentration Cooling tower wet bulb temperature Hydrocarbon leaks... 

An exchanger with a high water exist temperature is suffering from either plugged tubes, a low water pump discharge pressure, or increased exchanger duty. On the other hand, a high water-inlet temperature is indica-... 

The mechanical design had allowed for a pressure drop through the tube side of 50 psi the operating pressure drop was 70 psi. Partially plugged tubes had greatly increased the normal crude AP. The pass partition had to withstand the high pressure differential. When it finally failed, it was pushed against the channel-head outlet nozzle. 

Plugging tubes in heat exchangers causing shutdowns and loss of production in power utilities ... 

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