Chemical Engineering October 2013 - 55

FIGURE 6a (top left) and b (top
right). Shown is a typical arrangement
of the boiler arrangement of a package Dtype
boiler with circulation problems. On
the drum-baffling arrangment shown in
Figure 6b, note the asymmetrical baffling
with respect to gas-fl ow direction. This
baffling system made it hard for many
designated downcomer tubes to act freely
as downcomers
at least 20-30% less than the allowable
rate - to ensure that departure
from nucleate boiler does not occur in
the evaporator tubes. Boiler companies
typically develop their own correlations
based on their experience,
the tube sizes and configuration used,
and they routinely use safety margins
when developing them.
q = 6,330 hfg d-0.1 (G/106)0.51 (1-x)
(2)
where:
q = critical heat flux, Btu/ft2h
d = tube inner dia., in.
G = mass velocity of steam water mixture
through tubes, lb/ft2h
x = steam quality, fraction
Circulation issue, D-type boiler
Figures 6a and 6b show a boiler, fired
with oil and refinery gas, generating
130,000 lb/h of superheated steam
at 630 psig, and 750°F, which had an
interesting problem. Tubes were thinning
and failing at a location in the
boiler bank shown in Figure 6b. There
were two identical units in this plant
and both were having this problem.
Engineers were wondering why this
region alone was facing this problem
- not the hotter zone ahead of these
tubes, or the tubes at the other end of
the same cross-section (that is, in the
same plane perpendicular to the gas
flow direction), where one would expect
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Fluegas out to
economizer
1,050-1,200 °F
Tube failures in this region
Superheater
region
FIGURE 6c. This plan view shows the region where the tubes failed in the convection
bank
the same gas temperature. As a consultant,
I was asked to evaluate the design
and suggest suitable solutions.
Analysis. The first step was to simulate
the boiler performance using the
tube geometry and furnace dimensions
provided, to see if the exit gas temperature,
superheated steam temperature,
and water temperature leaving the
economizer were all close to the measured
field data. This step was designed
to confirm that the model used to carry
out the calculations of gas temperatures,
overall heat transfer coefficients
and actual heat flux were reasonable,
and that the boiler was properly sized,
as that may provide some indication of
other issues such as fouling. The simulation
gave the exit gas temperature
from the evaporator and economizer
as 798°F and 388°F, respectively, which
matched the field data. Hence the simulation
program results were used to
study the circulation issue.
The evaporator section was broken
up into four sections and the gas temperature,
duty and steam generation
in each section was calculated. It was
found that the gas temperatures in
the region where tubes were failing
were ranging from 1,050°F to 1,200°F,
as seen in Table 1b (found in the online
version of this article, at www.che.
com). Circulation calculations were
carried out using these data. Typical
results are shown in Table 2 (online).
The heat flux q in the hottest gas zone
of 1,300°F, given by Equation (3):
U(Tg-Ts)Ao/Ai = 9.5 × (1,300-498) ×
2.5/2.24 = 8,503 Btu/ft2h
(3)
where:
q = heat flux, Btu/ft2h
U = overall heat transfer coefficient,
Btu/ft2hF
Tg/Ts = gas and steam temperatures,
°F
Ao, Ai = the tube outer and inner surface
area, ft2
This value of U is not high enough
to cause DNB in normal boiling situations.
Heat fluxes have to be in the
range of 150 to 200,000 Btu/ft2h before
we can attribute DNB to the tubefailure
problems in natural-circulation
boilers with vertical tubes at this
pressure.
Next, the drum internal arrangement
was reviewed (Figure 6a). This
diagram provided a clue to the problems
that the plant was experiencing.
In a typical D-type boiler with
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Fluegas from furnace

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Chemical Engineering October 2013

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