POWER March 2013 - 16

How to Avoid Feedwater Heater Drain
Design Pitfalls
Feedwater heaters are used to preheat boiler feedwater by
condensing steam extracted from several stages of the steam
turbine. Feedwater heaters enhance the thermal efficiency of
the power plant by reducing the amount of fuel burned in
the boiler to produce a specified power. At the same time,
the steam energy extracted from the turbine by the feedwater
heater helps to reduce the rate of energy rejection to the environment
via the condenser.
Steam extracted from the turbine for feedwater heating is condensed
on the shell side of the feedwater heater. The hot condensate
collects in the shell and drains to the next lower pressure
heater or condenser. A level control valve and piping maintains
the proper condensate level in the shell. The condensate level
control in the heater shell is very important. High condensate
levels can adversely affect steam turbine operation, while low
levels can cause steam blow-through, damaging the heater internals
and drain piping.
This article explores the complexities and key issues associated
with selecting the proper pipe sizing/layout and control
valve design that must control hot condensate flow from the
feedwater heaters. The biggest challenge occurs when hot condensate
flashes to steam as pressure decreases in the drain piping.
The flashed steam produces a two-phase flow mixture that
can restrict flow in the drain system and thus upset operation of
the heater shell level control. The flashed steam can also erode
the control valve internals and drain piping.
Complex Heater Design
The feedwater heater shell can comprise up to three separate
zones within the shell: the desuperheating, condensing, and
drain cooling zones. First, the incoming superheated steam enters
the optional desuperheating zone, where it is reduced in
temperature until reaching saturated conditions. Next, the steam
enters the condensing zone, where the saturated steam changes
state, at the saturation temperature, to become saturated liquid.
Finally, steam condensate enters the optional drain cooling zone,
where it is subcooled below the saturation temperature by the
incoming feedwater.
In multizone heaters, the normal heater drains are connected
to the outlet of the drain-cooling zone and therefore
are capable of handling subcooled condensate. The normal
drains are routed through a level control valve to the next
lower pressure heater, which also improves the cycle efficiency.
In contrast, emergency drains are typically connected
to the condensing zone, where they discharge condensate at
saturated temperature and pressure conditions through a separate
line and level control valve directly to the condenser.
The drains from the lowest high-pressure (HP) heater in the
typical Rankine cycle are routed to the deaerator.
Many Complex Calculations
Maintaining the proper condensate level in the heater shell
is critical. Therefore, the heater drain level control valve and
piping system must be adequately designed to discharge the
hot condensate flow across the specified operating range of
the plant. Guidance and commentary on how to complete a
successful design is provided in the remainder of this article.
16
See the sidebar " Step-by-Step Calculation Procedure " for a
summary of the discrete calculation steps described in the
article.
Review Drains from Feedwater Heater to Level
Control Valve
The drain piping upstream of the level control valve should be
designed to handle single-phase condensate without steam
flash. For proper operation, the drain should be adequately
subcooled to prevent steam flashing when line pressure decreases
due to frictional pressure drop or elevation change
(upward-rising pipe). The frictional pressure drop is minimized
by using guidelines such as the Heat Exchange Institute
(HEI) criterion of heater nozzle velocity not exceeding 4
ft/sec at operating temperature. However, the velocity-based
Step-by-Step Calculation Procedure
The main text describes the calculations and their sequence
in much detail. The following is a step-by-step guide that
will be useful to those wishing to develop an Excel spreadsheet
or for those who merely desire an overview of the calculation
process.
Step 1. Gather the required inputs:
■ Upstream/downstream heater pressures
■ Condensate flow rate and supply temperature
■ Pipe size and length/orientation for piping upstream/downstream
of control valve
Step 2. Calculate critical pressure at the exit of the control valve.
Step 3. Check critical pressure against the saturation pressure
and the downstream heater pressure to establish the correct
exit pressure for pressure gradient calculations and to
establish single-phase or two-phase flow in piping downstream
of the control valve.
Step 4. Calculate piping resistance (K + f L/D) for the piping
segment downstream of the control valve, working backwards
from the downstream heater.
Step 5. Assume value for control valve outlet pressure (P2
)
downstream of control valve with the calculated exit pressure
and input the associated fluid properties in the modified Bernoulli
equation.
Step 6. If the right-hand side of the modified Bernoulli equation
matches the left-hand side, then the assumed value of P2
is correct. Otherwise, change the value for P2
the correct value for control valve outlet pressure is obtained.
Note that the value for P2
and repeat until
will change as the downstream piping
resistance (K + f L/D) changes.
Step 7. Ensure that control valve inlet pressure is calculated
using conventional single-phase flow line pressure drop.
Step 8. Once control valve inlet/outlet pressures are established,
the control valve datasheet can be filled out for
the vendor to supply a suitable valve for flashing/cavitation
service.
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POWER | March 2013
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POWER March 2013

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