Energy-Tech Magazine

Introduction
Heater drain systems in fossil and nuclear power plants have proven to be among the most complex systems to design due to the occurrence of two-phase flow phenomena. The overall performance of heater drain systems directly relates to proper sizing and design of the piping and control valves.

Proper sizing is highly dependent on accurate and conservative calculation of two-phase flow pressure losses. Various options for solution methods are available to the engineer. One such method, based on the homogeneous equilibrium model (HEM), is developed which is simple, yet adequate, for the necessary two-phase flow calculations of heater drain systems. This study focuses on plant cycles (both fossil and nuclear) where the feedwater heater drains are cascaded backward (counter to feedwater flow), as shown in Figure 4 for a typical sub-critical fossil plant Rankine cycle.

In such systems, flow is driven by the combination of pressure and gravity forces. As such, the physical location of each feedwater heater, as well as piping arrangement,

References ASME TDP-1, Recommendations for the Prevention of Turbine Water Induction, 2006. Cahn, A. L., “Heater Drain Systems,” 1979 EPRI Feedwater Heater Workshop. Darby, Ron, “Properly Size Pressure-Relief Valves for Two-Phase Flow,” Chemical Engineering, June 2002. EPRI CS-2251, “Recommended Guidelines for the Admission of High-Energy Fluids to Steam Surface Condensers,” 1983. EPRI TR-113189, “Two-Phase Pressure Drop Technology for Design and Analysis,” 1999. EPRI 1011231, “Recommendations for Controlling Cavitation, Flashing, Liquid Droplet Impingement, and Solid Particle Erosion,” 2004. Heat Exchange Institute. Standards for Closed Feedwater Heaters, 2007. International Association for Properties of Water and Steam. IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IF-97), 2007 revision. Kiameh, Philip, Power Generation Handbook, 2002. Landis, Robert L., “Solve Flashing-Fluid Critical Flow Problems,” Chemical Engineering, March 8, 1982. Leung, Joseph C., “Size Safety Relief Valves for Flashing Liquids,” Chemical Engineering Progress, February 1992. Loughrin, Casey J., “Design Methodology and Valve Sizing for Heater Drain Systems,” ASME Paper POWER2009-81116, July 2009.

directly affects the suitability of the heater drain system. The normal drain flow path from each feedwater heater is directed through a heater drain control valve to the subsequent lower-pressure feedwater heater. In the case of the lowest-pressure heater (usually located in the condenser neck), drains are normally directed to the condenser after passing through the heater drain control valve (note that a loop seal might be provided in lieu of a control valve for the lowest-pressure heater). The emergency drain flow path is directed from each heater through a separate heater drain control valve to the condenser, or a flash tank that is vented and drained to the condenser. The use of separate flash tanks generally is dictated by the condenser manufacturer. The control valves, in either the normal or emergency drain lines, maintain the condensate water level in the shell-side of the respective feedwater heater. The normal and emergency drain control valves operate in series, meaning that the emergency drains are used only when the normal drains are not able to maintain the liquid level or when the normal drain destination is unavailable.

Figure 4 shows the lowest-pressure heater drains pumped to the next-higher-pressure heater piping. Plants with heater drains pumped forward to higher-pressure heaters can be problematic as well; but proper design of such systems is outside the scope of this work due to differences in the operations and design considerations involved.

Improper design of the plant heater drains system results in losses in plant cycle efficiency, as well as plant availability, which is why heater drain performance is critical to overall plant operation.

Proper understanding of the two-phase flow phenomena occurring in heater drain systems is essential to proper system design.

System definition: Heater drain system arrangement and design criteria
With proper implementation of the design recommendations presented here, many of the characteristic heater drain system problems can be minimized or avoided completely. The following is an itemized list of design features that have been successfully applied to heater drain systems:
 

Physical location of feedwater heaters

• The lowest-pressure feedwater heater(s) is usually located in the condenser neck beneath the low-pressure turbine section. Since there is little pressure gradient available to pass the flow, the importance of static head is amplified. The greatest feasible elevation difference should be provided. Successive feedwater heaters (excluding the lowest-pressure feedwater heaters) are usually arranged in the following configurations:
• Side-by-side on one or two elevations.
• Vertically stacked in elevated boiler steel or dedicated bay with the deaerator at the top-most elevation due to boiler feed pump NPSH constraints.