SULPHURIC ACID EQUIPMENT
New main blower for
ultra large acid plants
The single equipment size limits for sulphuric acid plants is being pushed to new levels with
the trend towards ultra large sulphuric acid plants, driven by the benefits of economy of scale
and the latest improvements in energy efficiency. Matthias Funk and Markus Hueter of Siemens
Turbomachinery Equipment GmbH report on how Siemens has responded to industry trends
with the launch of a new generation main blower making possible single blower arrangements
for large sulphuric acid plants in the range of 4,000 up to 6,000 t/d.
P
lant designers and operators of sulphuric acid plants around the globe
are facing various challenges due
to recent industry trends, including:
● rising energy costs;
● increased competition;
● rising demand for sulphuric acid.
Licensors, plant designers and key equipment suppliers work on solutions to combine energy-effective operation with high
production output and increased competitiveness of sulphuric acid plants. Economy
of scale together with an overall growing
demand for sulphuric acid, have led to a
move from medium size to ultra large scale
sulphuric acid plants, which benefit from
capex and opex savings. In some cases
these large scale plants are even combined
to provide mega sulphuric acid complexes.
This trend for larger plants applies
not only to grass-root projects but also to
revamp projects.
Figure 1 shows the historic development of this trend.
Fig 1: Development of plant scale
Source: Siemens
Fig 2: Improvement of blower frame-size capacity
Main blower challenges and solutions
All the key components in the acid plant
have to adapt to these trends for larger
acid plants and to support plant designers
and operators with suitable solutions.
Due to its high reliability and availability, the main blower in a sulphuric acid
plant, irrespective of the type of acid plant,
is typically used in a single blower arrangement (without spare) which combines simple
installation with easy operation and lowest
investment costs.
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Source: Siemens
Sulphur 343 | November - December 2012
SULPHURIC ACID EQUIPMENT
Fig 3: Limitation of single blower frame-size capacity
Source: Siemens
Fig 4: Extending blower capacity by parallel operation
Source: Siemens
Fig 5: Dual blowers (2x50%) operating in parallel
Source: Siemens
Sulphur 343 | November - December 2012
In the single blower arrangement,
plant production is directly linked with the
blower frame-size capability. With the rising demand for more production capacity
Siemens has been very successful, over
the years, in improving the maximum flow
per frame size (see Fig. 2).
Continuous R&D improvements in
blower aerodynamics allow substantial
increase of flow capacity within the same
frame size. Compared to the 1970s, nowadays more than 43% higher design flow
rates are offered, providing significant benefits to plant owners such as 40% higher
production with the same blower footprint/
space requirement and a reduction of
blower investment cost. In parallel to providing larger frame-size capacity, aerodynamic blower improvements have delivered
significantly higher blower efficiencies.
Even with the high flow capabilities of the
latest blower series, acid plants have already
reached a level of magnitude which exceeds
the available single equipment size on main
blowers and also some other key components (e.g. boilers, diluters, heat-exchangers,
pumps etc.). Figure 3 shows the limitation of
single blower frame-size capacity.
In order to overcome this design
dilemma the most common approach is
to use a single train arrangement wherever technical feasible and to use a dual
arrangement of two 50% components in
parallel for all cases where this is not possible due to equipment size constraints.
Figure 4 illustrates this concept with
respect to the main blowers. While the largest single blowers are currently limited to
flow rates equivalent to approx. 4,000 t/d,
with parallel operation of two 50% blowers
of the smaller frame-size more than 5,000
t/d are possible. A parallel operation of the
largest size blowers could even technically
push the limit to beyond 6,000 t/d, but only
at an extremely high investment cost.
However, the increased flow capacity
provided by a dual blower arrangement
comes at a price, with a much more complicated piping arrangement, including
shut-off valves, additional monitoring and
auxiliary piping in order to ensure safe
operation under all conditions, including
start-up and shutdown procedures.
Due to the huge scale of the equipment
in acid plants it is vital to locate the components as close as possible to minimise
the interconnecting piping length. Consequently, space availability is always limited.
Although the dual arrangement is based on
two parallel machines of a smaller frame-
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SULPHURIC ACID EQUIPMENT
size, the overall space requirement is about
40% higher than a single blower arrangement. In addition, the extra pipework needs
to be properly engineered to avoid accessibility and maintenance problems.
In a dual blower arrangement, if both
blowers operate in parallel at the same time
they act as one “mega blower” for the plant.
Figure 5 highlights the internal operation
of each individual blower. While the plant is
operating e.g. between 60% to 100% plant
loads, each individual blower is operated
within its own 60% to 100% blower capacity
at the same pressure rise but half of the
total plant flow along the dotted green individual resistance line. However, if only one
blower is in operation there is a dramatic
change (Fig. 6). Under this condition all the
flow into the plant has to be transported
by the one blower in operation. Even if the
blower operates at 120% blower capacity
this is equivalent to only 60% of the plant
flow rate. As the plant and the related
equipment (piping, vessels, converter, etc.)
are designed for the full flow capacity the
relative pressure drop at 60% plant flow
rate is very low, so the blower operates at
the intersection of the yellow dotted plant
resistance line and the blower curve. The
operating point lies in a region far from the
optimum efficiency range. Due to this low
efficiency operation the motor power typically limits the available flow rate to slightly
more than 60% plant flow rate. On the plus
side, the dual arrangement offers some
redundancy, up to 60% plant load is possible. The benefit of this should be evaluated taking into consideration the general
high reliability of main blower systems in all
single train plants.
Fig 6: One blower (1x50%) in operation only
Source: Siemens
As regards investment costs, a larger
single blower offers substantial cost savings
compared to a dual blower arrangement.
In times of tight budgets this clearly is a
major benefit for a single blower arrangement (and together with the easier operation)
the main reason for the general preference
of this concept in sulphuric acid plants.
Table 1 summarises the pros and cons of
single and dual blower arrangements. Due to
its many advantages a single blower arrangement has generally become the industries
first choice for sulphuric acid plants.
However, for high end plants exceeding 4,000 t/d this preferred solution has
not previously been possible with existing
blower sizes.
Siemens has now officially launched
the next generarion of main blowers for
ultra large sulphuric acid plants – the
STC-SO (SFO 22). The larger frame size
of the new main blower closes the gap in
single blower arrangement for acid plants
with a production from 4,000 t/d up to
6,000 t/d, whilst maintaining the superior
efficiency and rugged design of the field
proven STC-SO (SFO 18) series.
As plants have increased over the
years, the related blower design features such as flow rate and power have
also increased to new levels (e.g. current drive power already exceeds 9 MW).
The key design figures for Siemens’ new
frame-size continue this trend:
Acid plant capacity:
4,000-6,000 t/d
Design flow rate: 330,000-490,000 Nm3/h
Drive power range: 8,000 to 14,000 kW
Impeller diameter:
2,240 mm
Table 1: Single vs dual blower arrangements
Single blower arrangement
Dual blower (2 x 50%) arrangement
Arrangement
Simple, non-redundant arrangement
More complicated (partly redundant) arrangement
Operation
Simple operation
More complicated operation/start-up and
shutdown procedures
Capital expenditure (capex)
Solution with lowest capex on blower and
associated plant components and accessories
Higher capex due to duplication of smaller units and
costs for additional equipment (e.g. valves, piping, etc.)
Operating costs (opex)
Optimum aerodynamic selection possible
Lowest operating costs
Increased margin to surge due to mandatory parallel
operation with slightly lower efficiency
Higher mechanical losses
Higher power consumption
Miscellaneous
Motor start-up more difficult especially for
large drive/high power units in weak grid
Detailed engineering of drive system mandatory
Minimum space requirement
Emergency operation with 60% plant load possible
Motor power only 50% of single arrangement with
easier start-up of motor
Higher space requirement
Result
Preferred choice blower arrangement
Arrangement for plants exceeding single blower limits
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SULPHURIC ACID EQUIPMENT
The new scroll housing measures about 6 m
x 6 m (236” x 236”) and is 25% larger than
the previous largest blower series. Suitable
drives - either conventional electric motors at
fixed speed, or VFD systems and multi-stage
steam turbines - are available in the required
power range and already operate blowers
with drive powers in the 9 MW range.
Drive power has grown with increasing
plant capacities as the drive power is proportional to the handled flow rate/plant capacity
(Fig. 7). For ultra large acid plants in a single
blower arrangement drive powers of over 10
MW will apply. In remote location, in particular, the start-up of large electric motors is
already an issue that needs to be addressed
at an early engineering stage. The optimum
solution from various control methods (IGV
control vs speed control) and drive options
is determined on a case by case basis,
depending on the site specific conditions
Inrush currents on fixed speed motors
can be successfully reduced by specially
designed motors, auxiliary start-up methods
(e.g. autotransformer) down to wound round
motors with a liquid starter. In addition to
offering low starting currents VFD drives also
offer potential savings during operation via
speed control. The same applies to steam
turbines as a driver. As most of the ultra
large plants are sulphur burning plants with
excessive heat recovery and steam production the direct use of steam as the drive
media seems to be a likely option.
Nowadays, in addition to equipment
challenges for larger plant sizes, sulphuric
acid plants are also faced with continuously rising energy costs. The main blower
is by far the largest consumer of electric
energy in sulphuric acid plants, typically
about 70% of the energy costs are related
to the operation of the main blower. So the
plant operational costs are closely linked to
the main blower and its efficiency (Fig. 8).
Over the years substantial improvements to the blower efficiency levels have
been achieved through various improvements to the aerodynamics and mechanical layout of blowers. Compared to
standard equipment a substantial saving
in the range of 5 to 10% can be achieved
by using a high-efficiency blower series.
Due to the continuous long-term operation
of the blower, especially in combination
with the high level power demand, this
results in a substantial energy saving.
In times of high energy costs it should
be a routine procedure for new investments to evaluate the total costs (capital
and operating costs) in the engineering
Sulphur 343 | November - December 2012
Fig 7: Drive power − plant size correlation
Source: Siemens
Fig 8: Capital and operating costs of main blowers
Source: Siemens
and procurement phase in order to determine the most cost effective solution in
the long run, as the plants are designed
for a lifetime more than 30 years. Focusing on investment costs alone will lead to
shortsighted decisions, especially in times
of rising energy costs.
Both new and existing plants can benefit from the latest improvements in the
blower sector as illustrated in the following
case study.
In line with an energy saving programme, a European copper smelter
approached Siemens to evaluate the
potential energy savings for replacing the
existing blower with high efficiency equipment. The project was performed in the
following steps:
● Analysis of future and existing operating conditions.
● Case study was made indicating a sub-
stantial energy saving >11% with new
blowers.
● Existing drive systems could be used,
foundation and piping to be adapted.
● New machines to be erected within
given annual plant downtime period.
● Energy savings could also be used to
increase production by 4% (plus additional margin for future operations).
● Management approval of blower
replacement on basis of energy savings
and short term return on investment.
● Blower replacement was completed on
time, within the given shutdown period
and even exceeded the predicted
energy saving.
When revamping or plant debottlenecking,
the use of high-efficiency equipment should
be assessed with a view to reduce the
impact of high energy costs and improve
the overall competiveness of the plant. ■
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