Another example of custom designing is shown in Example 3. In this example, we are asked to design a boiler for a lower pressure of operation for the first few years with the idea of operating at a higher steam pressure after that.

Example 3

An interesting requirement was placed on the design of a boiler. The 175,000 lb/h boiler was to generate steam at 150 psig and 680°F for the first few years and then operate at 650 psig and 760°F. The piping and superheater changes had to be minimal when the time came for modifications.

Operating a steam generator at two different pressures is a challenging task, particularly when a superheater is present. The reason is that the large difference in specific volume of steam affects the steam velocity inside the superheater tubes and the steam-side pressure drop, which in turn affect the flow distribution inside the tubes. The ratio of specific volume between the 150 and 650psig steam is about 4. Hence for the same steam output, we could have a 4 times higher steam velocity at the lower pressure if the flow per tube were the same. Also, if the pressure drop at 650 psig were, say, 30 psi, it would be about 120 psi at the lower operating pressure if flow per tube were the same. Hence it was decided to manipulate the streams and steam flows as shown in Fig. 3.9.

In the low pressure operation, there would be two inlets to the superheater from opposite ends of the headers as shown in Fig. 3.9a. This would make the velocity and pressure drop inside the tubes more reasonable. The total length of tubing traveled by steam in the low pressure option would be nearly half that of the high pressure case, which also reduces the pressure drop. Part of the steam is in parallel flow and part in counterflow. At high gas temperatures, as in this case, the difference in performance between parallel and counterflow superheaters is marginal.

In the high pressure case, all the steam flows through the superheater tubes in counterflow. Because the specific volume is small, the steam can flow as shown with a reasonable steam velocity and without increasing the pressure drop. The performance in both, cases is shown in Table 3.3. Thus with a minimal amount of reworking, the piping could be changed when high pressure operation is begun. The superheater per se was untouched, and only the nozzle connections were redone. This boiler will be in operation for several years. If custom designing were not done, the capacity at low pressure mode would have to be limited to

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Figure 3.9 Superheater piping arrangement for (a) low and (b) high pressure operation.

About 50-60% of the boiler capacity in order to avoid unreasonable steam velocity or pressure drop values. The main steam line has two parallel valves in the low pressure mode and will be converted to single-valve operation in the high pressure mode.

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