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PRESSURE + LEVEL MEASUREMENT
Engineering considerations
For the engineering plant under consideration the main focus for steam
production is to ensure that plant process steam demands are satisfied
and excess steam available for power co-generation is an additional
advantage. Sufficient steam productions are therefore not measured in
terms of available steamfor power generation. In a fluctuatingoff-gas pro-
duction environment that results in fluctuating steam flow productions,
it is to be expected that steam availability for power generation will not
be constant and steam shortages will occur that may lead to unforeseen
turbine trips. It should be noted that a turbine is designed to operate
withinmaximumandminimumallowable steamflow limits. Steamflow
above the maximum capacity is not allowed and cannot be utilised by
the turbine. However, when the steam flow drops below the minimum
allowable limit, protectionmeasureswill immediately shut down the inlet
steamvalves to the turbine tripping themachine instantly. Theminimum
steam flow limit helps to protect a turbine against damaging vibrations.
Figure 2
demonstrates hypothetical steam flow productions from
fluctuating off-gases over an arbitrary 100 hours’ time interval, where
the fluctuating curve represents steam flow production (m.
total
) in the
boiler houses and the constant 50 ton/h line denotes plant usage
(m.
usage
) demands that need to be satisfied. The area between the two
curves denotes excess steam available for power co-generation. As
mentioned, plant steam demands are not constant and only chosen as
such for illustrative purposes. If all of the excess steam from
Figure 2
are utilised for power co-generation, the maximum average rate of
power co-generation can be calculated by integrating the product of
the area between the two curves with a conversion factor (C
f
) and given
by (1). This conversion factor is a function of the isentropic efficiency
of the turbine, which in turn is a function of the steam flow.
Some investment questions need to be addressed regarding the
quantity of turbines and at what capacities need to be procured for
power generation purposes. As mentioned above, this article does
not address any investment questions and will only demonstrate the
potential power co-generation effect if off-gas flaring is regulated.
Figure 2: Hypothetical steam flow production over time.
The hypothetical residual steam from
Figure 2
is taken and plotted in
Figure 3
with the addition of a turbine’s operating limits. The two con-
stant lines represent the minimum and maximum allowable steam
flow limit of the turbine. All steam flow above the maximum limit
(30 ton/h) cannot be utilised by the turbine and for every instance
that the steam flow drops below the minimum limit of 10 ton/h, the
turbine will trip if in operation. It should be noted that when power
co-generation takes place in such fluctuating steam flow conditions,
protection measures are in place to protect a turbine from tripping
in close proximity. Common engineering practice is to allow for a
time of steam stability, i.e. a continuous time interval before start-up
where steam flow must be sufficient to have kept the turbine opera-
tional. Under such protectionmeasures
Figure 3
does not necessarily
imply that six turbine trips would have been experienced within the
100 hour time period. Note that when a turbine is not operational,
all power generation capability of the steam goes to waste, since it
cannot be stored and utilised at a later stage.
Under fluctuating steam availability steam shortages are ex-
pected, however, trips should be limited as far as possible, since a
turbine is designed to operate continuously and each trip occurrence
depletes the life expectancy of the machine.
The scenario as depicted in
Figures 2
and
3
displays a situation
where plant steam usages cannot be altered to assist turbines dur-
ing low steam flow availability. The generic layout given in
Figure 1
shows that parallel to off-gas steam production in boiler houses,
off-gas flaring takes place. Thus, if plant steam usages cannot adapt
to assist power co-generation, an investigation into off-gas flaring
might prove to be advantageous.
Figure 3: Hypothetical steam flow available for power co-generation over
time, with turbine operating limits.
The following section displays simulation results for the engineering
plant where co-generation exists under conditions described in this
section. Measured steam productions and plant usages are used to
display power co-generation capabilities. An investigation is then
launched into the off-gas quantities that were flared, from obtained
measurements, and how the regulation of this flaring could poten-
tially have impacted power generation at this plant. As mentioned,
power co-generation is only selected to stipulate the effect of off-gas
flaring and therefore simulations where boiler house capabilities are
hypothetically increased to observe an even more positive power
co-generation effect will not be discussed. All simulations are per-
formed with the optimisation model from [2].
120
100
80
60
40
20
0
120
100
80
60
40
20
0
0 2 0 4 0 6 0 8 0 1 0 0
0 2 0 4 0 6 0 8 0 1 0 0
Time (h)
Time (h)
Total steam production (ton/h)
Excess steam (ton/h)
W
max
= ▒〖
C
ƒ
(m
total
– m
usage
)dh〗
(1)
∫
100
0
•
•
5
June ‘17
Electricity+Control