Sagging tubes

Certain corrosive conditions require that special consideration be given to complete drainage when the unit is talcen out of service. Particular consideration is required for the upper surfaces of tube sheets in vertical heat exchangers, for sagging tubes, and for shell-side baffles in horizontal units. 

Oxygen in a large operating boiler may corrode steam-water separators and boiler surface components such as the top drum (especially at the waterline) and tubes. Oxygen corrosion also may occur in superheater and reheater tubes, especially in places where moisture can collect, such as in bends and sagging tubes. 

The best way to find thin, bulging tubes when the heater is off line is to run a ring down the length of a tube. A really severe bulge may be visually observed, even when the heater is in service. Sagging tubes do not represent thin tubes. I have seen radiant-section, horizontal tubes sag by more than one tube diameter and operate for years without failing. 

As the parison is extmded, the melt is free to swell and sag. The process requires a viscous resin with consistent swell and sag melt properties. For a large container the machine is usually equipped with a cylinder and a piston called an accumulator. The accumulator is filled with melt from the extmder and emptied at a much faster rate to form a large parison this minimises the sag of the molten tube. 

To ensure that the centre of the bulb lies on the axis of the tube from which it is made, the tubing, when in the flame, must be rotated continuously as described on p. 52 otherwise the glass will sag to one... 

The unsupported internal tube will sag under gravity during the above operations, but if the rotation of the work is sufficiently uniform it can be centralized as the glass cools by allowing it to drop... 

Put the center of a melting point capillary into a small, blue Bunsen burner flame. Hold it there until the tube softens and starts to sag. Do not rotate the tube, ever. 

After 2 hours, it became clear that the openings in the opposite refractory wall were shrinking. These openings permitted the hot flue gas to exit the combustion chamber and flow into the tubes of a heat-recovery boiler. The apparently melting refractory was sagging and restricting these apertures. 

As indicated earlier, these configurations, almost without exception, are positioned vertically as they are suspended in the radiant zone of the furnace. Horizontally suspended tubes are more subject to buckling and sagging, especially at higher temperatures. 

As the glass sags, remove the tubing from the flame. Gently pull on each end, as straight as possible, until the capillary is as small as desired (Fig. 1.11 B). 

The coils are hung at the top from an articulated rod provided with a counterweight or a system of springs. They are fitted at the bottom with guides enabling them to move in slides prorided in the refractory of the furnace hearth. In this way, the supports are sot directly exposed to the radiant heat and the tubes assume a position in which the stress is a minimum and where sagging cannot cause substantial deformation. Expansion is allowed for, and thermal shocks are easier to withstand, thus lengthening coil life. 

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