POWER February 2021 - 28

FUNDAMENTALS
■ No extraction steam flow to the LP
FW heaters or deaerator occurs during
the transient.
■ No FW recirculation or high-pressure
heater drain flows enter the deaerator
during the transient.
■ Heat retention of the pump, piping,
deaerator, and FW heater metal mass
is neglected.
2. Pressure decay during a net positive suction head (NPSH) transient event. Cavitation occurs
when NPSH available (NPSHa) is less than NPSH required (NPSHr). Source: Michael F.
Czyszczewski, PE
solution; therefore, the safety valve pressure
is typically used.
The value assumed for the minimum
condensate enthalpy entering the deaerator
(hC
) is critical to the solution. Colder
condensate entering the deaerator will
accelerate the pressure decay. Assuming
that some of the low-pressure (LP)
FW heaters are out of service at the start
of the transient adds some cushion to
the calculation results. Thurston recommended
using the outlet enthalpy of the
lowest-temperature FW heater.
If a transient factor of less than 1.0
is calculated, a more rigorous evaluation
based on fewer simplifying assumptions
may show the design is
acceptable. Some tips on performing
this evaluation follow:
■ Use a pipe flow modeling program to
determine the pipe flow head loss.
■ Use a spreadsheet to collect the data
and perform the NPSHa calculations.
The columns should contain the parameters
needed to calculate NPSHa
over the time of the transient.
■ Spreadsheet rows should represent
an incremental advancement of time
and should cover the entire duration
of the transient. Some flow modeling
software has the capability to simulate
time by combining the results
of multiple steady-state runs. This
software feature would eliminate or
reduce the calculations required with
the spreadsheet.
■ Steam property functions, which are
available for many commercial spreadsheet
programs, are needed to simplify
the work effort.
■ Input data representing the static operating
conditions that exist before
and after the transient should be per
the project heat balances.
28
■ Input a description of how each of
the variables that make up NPSHa
change during the transient. Ideally,
the change rates should be based on
instrument data gathered during an
actual transient. However, if data is
not available, assumptions will need
to be made. This typically includes assuming
that the liquid enthalpy decay
in the deaerator follows an exponential
function.
■ Plot how NPSHa and NPSHr change
over the course of the transient to
demonstrate that sufficient margin exists
at all times.
For reference, the Thurston equation
A few additional words concerning
the last assumption might be helpful.
The metal mass of the pump will
tend to store heat after shutdown. The
temperature (and vapor pressure HVPA
)
of the FW it contains will, therefore,
be at a higher temperature than the
FW in the deaerator. When the pump
is restarted, the pressure at the suction
will suddenly drop and the hot FW
could flash to steam. This differs from
a NPSH transient, as the pressure
change is initiated by the pump and
is, therefore, known as a " hot start "
transient event.
An additional steam flashing situation
could occur if there are long
horizontal runs of suction piping at
high elevations. These pipe runs are
prone to flashing because there is no
increase in static head to compensate
for the pressure decay in the deaerator
and the friction losses that occur as
the FW travels through the pipe. Avoid
routing the pipe this way.
The value assumed for the minimum
condensate enthalpy entering the
deaerator (hC) is critical to the solution.
Colder condensate entering the deaerator
will accelerate the pressure decay.
is based on the following simplifying
assumptions:
■ The volume of FW in the suction pipe
at the start of the transient remains at
a constant vapor pressure as it passes
through the system.
■ All FW leaving the deaerator is a saturated
liquid.
■ Condensate flowrate into the deaerator
equals the FW outflow and is
constant.
■ There is complete mixing between all
fluids in the deaerator.
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FW Pump Transient Solutions
The most commonly recommended options
for preventing FW pump transients
attempt to eliminate the vapor pressure
differential between the pump and deaerator
by either cooling the FW at the
pump inlet or increasing the deaerator
pressure.
Pump Inlet Cooling. NPSHa can be
increased by injecting a small amount of
cooler condensate near the pump inlet.
The most reliable and coldest source of
condensate for this is the condenser hotwell.
A bypass line can be installed to
POWER | February 2021

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POWER February 2021

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