GRAPHS
The program data logs its results and pressing the results button presents the following screen.
Move the mouse cursor anywhere into the white
area under the curve and a time line appears showing the
relevant data about TDS levels and each pump's rate.
DISCUSSION OF RESULTS
The sample the program defaults to shows an
inadequately sized feed tank heater and as a result the
feed tank temperature is somewhere around 68°C (good thing its a simulation)
this causes
Oxygen scavenger dosing pump to running at a high rate during the boilers
production cycle.
This high rate means that the cost of steam is
higher than if the feed heater was increased in size this
would cause less chemical to be dosed and keep the blowdown at a lower level
thus saving fuel.
Other points to notice are that whenever a
blowdown occurs the demand on the other pumps spikes
as make-up water enters the system with the level of alkalinity in the feed
water it can be seen that the
condensate pump is working hard also.
The graphs below consider the effects of blowdown
on the the above boiler set-up with different methods of
control over the chemical dosing.
Graph of Blowdown versus
Time for 1 day with the system using Dosing on Demand (Aquanet method)
Notice how there are 5 blowdown cycles per day, with the low hotwell temperature
the requirement for oxygen
scavenger is high and so the dissolved solids increase
Graph of Blowdown versus Time for 1 day with the system using Dosing on Flow
The way the pumps are set up, because of the low feed water dosing the
chemicals must dose for the worst
case and as a result the TDS is always having to be controlled with excess
blowdown resulting in a waste of
water and increased running costs. There are 9 blowdown cycles with this
configuration.
Graph of Blowdown versus Time
for 1 day with the system using manual control system
(dosing pumps set to average for the day)
This set-up with has the dosing pumps turned down a little and in manual
control. There is an economy of
water and chemicals but when the sequence is running it will be noticed that the
oxygen scavenger is being
quite seriously underdosed for the time the boiler is on peak load. Chemicals
and energy are being saved
but at the expense of the boiler which will be suffering from oxygen damage and
other problems.
FEED TANK HEATER
This graph below has been produced with Dosing on
demand chosen as the dosing philosophy, running the model
in this mode predicts what is ideally required from the dosing system
With the undersized feed heater in this
simulation we notice that at a low steaming rate the heater is not quite
adequate,
as the boiler load increases the feed temperature drops as the system cannot
keep up with demand. The penalty of this
is that the dissolved oxygen in the feed water increases (temperature dependent)
and so extra chemical must be dosed
to counteract this. Notice how the oxygen control pump speed is varying to keep
up with demand, especially as the
Feed Tank temperature drops whenever boiler blowdown occurs.
In this example the load is a simple step function of either 1 or 3 Tonnes/hr.
Run the model with a different load that is varying
throughout the day and it illustrates how difficult it is to maintain manual
control of the boiler water quality.
Changing the setting of just one control; setting
the level switch on the hotwell from 25% to 10% will have a marked
effect on the dosing and economies and life of the boiler system.
Here the heater is coping better but we are still
not reaching the target and the dosing pumps requirement varies
from 60 to 70 strokes per minute but an average of 3.0mg/l of oxygen in the feed
water means a large quantity
of blowdown (when using sulphite for oxygen control) and excessive chemical consumption.
The model allows many different variables to be changed and see the effect they have on a boilers performance..
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or queries from users on this model but any communications
must be directed, in the first instance, through our appropriate agents (see
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Copyright Aquanet International Ltd 2009