Moderate Temperature Process Steam Generators
This section applies to steam generators heated by process fluids with temperatures less than 1000°F.
Many process steam generators operate by natural circulation from an elevated steam drum. Circulation is driven by the density difference between the liquid in the downcomer and the two-phase mixture in the riser (external to the exchanger). A typical steam generator is shown in Figure 300-5. Design guidelines for this type of steam generator are listed below.
The steam/water riser to the steam drum should operate in annular or churn flow.
• Side-to-side flow with 90 degree tube layout is used for good circulation at low pressure drop.
• Target vaporization is usually 5% of total circulation and is obtained by appropriate pipe sizes and drum elevation.
• Blowdown is from the steam drum near the vapor-liquid interface where water is most concentrated.
• Heat flux should be 2000 Btu/hr×ft2 or higher to assure nucleate boiling throughout the bundle.
• A support plate at the steam/water outlet is usually required to avoid tube vibration.
A single steam generator and steam drum are sometimes combined in a kettle-type steam generator as shown in Figure 300-6.
The bundle is elevated relative to the lower part of the shell to provide a minimum bundle-to-shell gap equal to about 10% of the bundle diameter. Cutouts in the lower part of the full tube support plates are required to facilitate free axial flow of water along the bottom of the bundle. Circulation is driven by the density difference between the liquid at the sides of the bundle and the two-phase mixture in the central part of the bundle (internal circulation). Blowdown is from the bottom of the shell near the tubesheet. Liquid level is maintained a minimum of 3 inches above the top tube, plus a control allowance. The kettle should be sized to provide
adequate vaporliquid disengaging space between the normal operating liquid level and the top of the kettle. Horizontal mass velocity of steam approaching the steam outlet nozzle(s) in the disengaging space (Figure 300-7) should not exceed the following criteria:
Operating Pressure, psig Mass Velocity, lb/hr×ft2
The above criteria provide adequate steam quality for most process uses (e.g., stripping steam), but are not adequate for steam turbines. Knockout vessels are occasionally provided immediately upstream of turbines. This arrangement removes condensate formed in the piping as well as that entrained from the steam generator.
U-tube construction described above is acceptable if the difference between process inlet and outlet temperature is less than 200°F. Otherwise excessive thermal stresses develop in the tubesheet. Thin, stayed fixed tubesheets are used when thermal stresses would otherwise be too high. Stayed tubesheets depend on the tubes to contain the shell side pressure. Stayed tubesheets are governed by the rules in Section I of the ASME Code.
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