Case Study of a Superheater with Tube Failure Problems

Case Study of a Superheater with Tube Failure Problems

An inverted-loop package boiler superheater similar to that shown in Figure 3.27a was having frequent tube failure problems at high loads as well as at low loads. The following information was gathered during the site visit:

1. The superheater was located at the turning section of the furnace without screen

tubes. This is a vulnerable location and should have been avoided for reasons dis- cussed earlier regarding radiant superheaters. The furnace exit gas temperature is difficult to predict accurately, and hence, a large variation from estimated value is possible. Hence, the gas temperature entering the superheater could be ±70°C off from the predicted value. The direct radiation from the furnace will also increase the tube wall temperatures of the tubes close to the turning section, which were failing often. The nonuniformity in gas flow at the turn is also not helping the situ- ation. The plant engineers were wondering if the burner flame shape was causing the problem and the boiler supplier was also supporting this view as he did not want to acknowledge that the design of the superheater was vulnerable. This is a common problem in many plants with radiant superheaters.

2. It was noted that superheater did not have multiple passes but just a single pass with steam entering from top header and leaving at the bottom header. With such

a design, the desired steam temperature is reached at the outlet header more or less in all the tubes. The external furnace radiation plus a high steam temperature

136 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

at the outlet header compounded the problem and resulted in overheating of the tubes close to the furnace exit.

Locating a superheater beyond several rows of screen tubes has its own advantages as discussed earlier. The convective superheater is a proven and safe design. The screen tubes absorb the nonuniformity in gas flow and the external direct radiation from the furnace exit and ensure that a much cooler gas reaches the superheater. The performance is more predictable. The LMTD will be lower, and hence, a little more surface area is required. However, due to the lower tube wall temperatures, such a design will require a lower grade of tube material unlike the radiant superheater. The use of multiple passes and a parallel-flow configuration should have been used for this design as it ensures that the final steam temperature is attained in the tubes further away in a cooler gas zone.

3. In order for an inverted-loop superheater to operate well at low loads continu-

ously, the steam-side pressure drop must be high enough at low loads to overcome gravity loss. However, it was found that the number of streams was too many resulting in low tube-side pressure drop. Hence, the superheater was failing at high loads, and the tubes near the furnace receiving direct radiation from the fur- nace were the worst hit.

Under the circumstances, the only way to improve the situation is to introduce a baffle in the inlet header and make the superheater a two-pass design with parallel-flow con- figuration as shown in Figure 3.36. The final steam temperature will be reached in the tubes in the much cooler gas temperature zone, which will ensure a lower tube wall tem- perature and hence better life. The steam-side pressure drop inside the tubes will be eight times more and was found to be acceptable as a lower steam pressure was required for

Superheated steam out

Saturated steam in Superheated steam out

Mud drum Suggested modification

Saturated steam in

Present design

FIGURE 3.36

Suggested modification for the superheater to improve steam-side flow distribution.

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the process. The steam velocity also was reasonable considering the original design had a very low steam velocity inside the tubes.