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,
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.