Chemical Engineering June 2018 - 66

Simplifying Equation (13) yields:
ln [(T1 - ts) / (T2 - ts)] = UA / (WgCpg)
(14)
This equation may be used either
for designing an evaporator or for
checking its off-design performance.
If T1 and ts are known, Equation
(14) may be used to obtain T2,
and from that, duty (Q), as well as
the steam generation in the evaporator,
may be estimated, if the enthalpy
of the feed water entering
the evaporator and the enthalpy of
saturated steam at operating pressure
are both known. The value for
T2 obtained from the previous calculations
may be compared with
the field data to check whether the
evaporator is performing well. This
equation may also be used to arrive
at the surface area A, if T1, T2, ts
and tube geometry and gas flow are
known. The important Equation (14)
also allows users to see what happens
to the boiler duty with steam
pressure changes.
Challenging the HRSG vendor
Savvy process engineers familiar with
such calculations can use the data
provided by the HRSG supplier, along
with field data, to evaluate whether or
not the boiler supplier has overstated
claims regarding steam generation
and temperature profiles. Using the
data provided by the HRSG supplier
(Table 5) we see that the duty shown
by the supplier for the evaporator is
60.37 million Btu/h, while our estimate
(using the field data at a lower
load) for the same guarantee case is
only 58.7 million Btu/h.
Equation (14) comes in handy to
see what is really happening here.
Our estimate of UA for the evaporator
from above is 4.92 × 86,379
= 424,985. Now using vendor data,
we see from Equation (14) that:
ln[(950-492) / (500-492)] = UA/
(500,000 × 0.27 × 0.995) = 4.0474
from which UAvendor = 543,670 Btu/
h˚F], or a 28% higher value of UA
has been used by a vendor for the
same cross-section, tube spacing,
surface area. This means that the
supplier has overestimated the U
value by 28% and submitted guaranteed
results.
Plant engineers can perform such
analyses for each section once they
become familiar with these calculations
and challenge the boiler supplier
if performance is significantly different
from predicted. However, in order to
do this as mentioned earlier, good
field data must be available. They can
measure the gas temperatures at the
evaporator inlet and exit, and prove
that the performance is close to what
is predicted by the vendor.
For example, in the actual operation
case, if the steam temperature
of 863°F, gas temperature at exit of
evaporator and economizer of 485°F
and 350°F and steam/water flow of
55,600 lb/h are accurately measured
and shown to be close, then they
have established their energy balance
by calculation, as well as by
measurements, and the HRSG vendor
will have a difficult time justifying
the design and guarantees.
Plant owners spend millions of
dollars on HRSGs. They should
also ensure the HRSG is well-instrumented
with numerous gas
and water/steam measurement
points. Presently, many HRSG vendors
provide a tapping point at the
HRSG exit alone and plant owners
do not generally think much about
HRSG instrumentation at multiple
locations of the HRSG. This is not a
good practice. There should be multiple
tapping points ahead and after
each heating surface (particularly if
the cross-section of HRSG is large),
well-calibrated instruments and frequent
checks on these readings by
qualified instrumentation engineers
to ensure the thermocouples are
not plugged or damaged. Armed
with such field data, it will be easier
for the plant to simulate the HRSG
performance at any operating point
and extrapolate the results to guarantee
conditions, or vice versa, and
challenge the HRSG supplier if large
deviations in steam generation or
gas/steam temperatures are observed
(Figure 6). Such independent
analysis capability goes a long way
toward ensuring effective HRSG operation
and thorough examination of
proposals from HRSG suppliers who
do not " wing it " when it comes to
stating thermal performance just to
sell the equipment.
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JUNE 2018
65
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Chemical Engineering June 2018

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