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Contact Section - Blowers
April 24, 2004
Introduction
The blower compresses the gas to sufficient pressure to overcome the pressure
drop through the plant. In a sulphur burning acid plant, the gas handled by the
blower is air. In a regeneration or metallurgical acid plant the gas contains SO2
and traces of acid mist. The gas must be dried in a Drying Tower before passing
through the blower in order to prevent corrosion from acid condensation.
Location of Blower
Metallurgical or Regeneration Acid Plant
In a regeneration or metallurgical acid plant, the blower must be positioned
after the drying tower in order to ensure that moisture, which could cause corrosion due
to acid condensation, is removed from the gas prior to entering the blower.
Sulphur Burning Acid Plant
In a sulphur burning
acid plant the blower can be positioned before or after the drying tower. The positioning will depend on the energy
recovery requirements of the plant
Positioning the blower before the drying tower (‘pusher’ blower)
results in savings in capital cost of the blower because the inlet gas will be more dense
and requires less power to compress. The gas
being handle is simply air so no special materials of construction are required. However, the heat of compression of the gas
is absorbed by the drying acid system thus increasing the heat load on the acid coolers.
Positioning the blower after
the drying tower (‘sucker’ blower) maximizes heat recovery and steam production
and minimizes acid cooling requirements. The
materials of construction for the blower must allow for the fact that the dried air
contains traces of acid mist. Better mist
eliminating equipment is required in the drying tower to protect the blower when the
blower is located downstream. Impaction type
candles or a double mesh pad instead of a single mesh pad should be specified.
Flow Control
The operation of an acid
plant requires that the flow rate through the blower be varied to meet the production
requirements of the plant. There are several
different ways to control the flow through the plant.
Inlet Guide Vanes
Inlet guide vanes provide the most efficient method of controlling the blower
output when the blower is driven by a constant speed device such as an induction motor. Inlet guide vanes are located directly on the
suction flange of the blower and controls the flow through the blower by varying the
position of pie-shaped vanes. In the closed
position, the pie-shaped vanes block off the area for flow resulting in the minimum flow
through the blower. As the vanes are rotated,
more gas is allowed to pass through the blower. Guide
vanes throttle the flow of gas to the blower suction thus artificially lowering the inlet
pressure which lowers the discharge pressure. In
addition to throttling the flow, guide vanes also change the inlet gas angle to the
impeller, thereby modifying the compressor characteristic curve.
The turndown of the blower is limited to about 35% of maximum flow with inlet
guide vanes. Lower flows can be achieved with
the use of a recycle line from the blower discharge to the blower suction.
Variable Speed Motor
A blower with a variable speed motor is more expensive than a blower with
inlet guide vanes. A variable speed motor
offers potential savings in power consumption, particularly if the plant throughput varies
frequently. A variable speed motor driven
blower gives more operating flexibility and lower turndown than a system with inlet guide
vanes.
One disadvantage of variable speed motors is that they may not be available
beyond a certain size (i.e. horsepower).
Variable speed motors are sometimes referred to as variable speed drives
(VSD) or variable frequency drives (VFD).
Steam Turbine
A steam turbine driven
blower makes use of steam generated by the acid plant (particularly sulphur burning acid
plants). As with a variable speed motor
driven system, there are potential savings in power consumption, lower turndown, and
greater operating flexibility than a motor driven blower with inlet guide vanes. Flow through the blower is varied by varying the
speed of the turbine and hence the blower.
Variable Speed Transmission or Gear Box
The flow through the
plant can be controlled by using a variable speed transmission or gear box to vary the
speed of the blower. This option introduces a
complex mechanical device into the blower train. The
variable speed transmission or gear box is generally driven by a constant speed motor.
Electric Motor versus Steam Turbine
The main acid plant
blower is generally driven by either an electric motor or steam turbine. The decision to use one or the other depends on
many factors. Some factors that must be
considered are:
- Availability of high pressure superheated steam
- Cost to purchase electrical power
- Existing co-generation facilities
- Revenue from production of electric power
- Process use for low pressure steam
- Capital cost
Traditionally, sulphur
burning acid plants without co-generation have used steam turbines to drive the main
blower. In this case, high pressure steam has
no value other than to drive the main blower. Generally,
the steam turbine exhausts the steam at a lower pressure that is suitable for use in the
fertilizer complex . Excess high pressure
steam is letdown to a lower pressure, condensed or vented.
The steam turbine will typically operate at an efficiency of 60 to 65%. Steam turbines that are 70 to 75% efficiency can
be utilized but the capital cost of the unit is more.
If an acid plant has a
co-generation facility, the economics of the situation change. In this case, the high pressure steam has a value
equal to the electric power that is generated. All
the high pressure steam is sent to a turbogenerator to generate electric power. The turbogenerator operates in the region of 80%
efficiency. Low pressure steam
can be extracted from the steam turbine casing for process use. The remaining steam is fully condensed and the
condensate recycled. The blower is driven by
a synchronous electric motor operating at 90 to 95% efficiency.
Overall, when all
factors are considered, the electric motor driven option comes out ahead of the steam
turbine option in most cases.
Surge
Surging or pumping
represents an unstable condition of flow. It
occurs when the inlet volume of a blower is reduced to a value less than the volume
corresponding to the maximum discharge pressure attainable at the particular speed or
inlet guide vane position at which the unit is operating.
Conversely, surging can also occur when restrictions in the system increase
the back pressure to a value greater than the maximum discharge pressure attainable with
the associated inlet volume. When this
occurs, a point is reached where the back pressure is greater than the pressure ration the
blower is capable of developing at that flow. This
causes a reversal in flow through the blower to reduce the blower back pressure. As soon as this reversal occurs, regular
compression is resumed and the cycle is repeated. The
rapid oscillation of inlet flow is known as surging.
Surging is undesirable
since it may result in damage to the blower and downstream equipment. The reversal of flow through the blower may cause
excessive vibration in the unit. Each time
the gas oscillates through the blower, it gets heated and the resulting temperature
increase of the gas and blower may result in damage to the impeller and casing. The oscillations in gas flow also creates pressure
waves in the downstream ducting and equipment. The
high forces created by these pressure waves can damage equipment particularly expansion
joints within the ducting.
If a surge condition
occurs the system must be checked immediately for restrictions in the blower suction or
discharge (i.e. closed damper). Removing this
restrictions should bring the blower out of the surge condition. If the restriction cannot be removed, the blower
must be shutdown immediately to prevent damage to the blower and downstream equipment.
Surge detection systems
can be employed to detect when the blower is approaching a surge condition. Many of these systems work by entering the surge
curve into a program and then measuring blower operating parameters such as flow, pressure
and temperature to determine where the blower is operating relative to the surge curve. If the operating conditions approach the surge
curve, an alarm is raised and if the blower enters the surge condition, the unit can be
shutdown before damage can occur.
Another method of
preventing surge is the use of a recycle line or “blow off”. In an acid plant a “blow off” is not
practical since the gas will contain SO2.
A recycle line from the blower discharge back to the inlet of the drying
tower is a common feature of many acid plant. If
the blower enters a surge condition, the recycle line is opened effectively increasing the
flow through the blower which brings the blower out of the surge condition. The opening of the recycle line can be tied to the
surge detection system discussed previously or can be operator initiated. The recycle line is directed back to the inlet of
the drying tower so the heat of compression can be removed prior to recycling the gas back
to the blower suction. This prevents over
heating of the gas and the blower.
Impellers
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Open radial bladed impellers are ideally suited
for dirty gas, corrosive or high head applications. Their inherent self-cleaning
design permits longer operation on dirt-laden or corrosive gas service without shutdown.
At a given tip speed, higher heads are possible than with other impeller designs.
A wide variety of materials are available to meet special application needs.
Radial bladed impellers have a relatively flat head-volume characteristics,
thus permitting a wide variation in volume with little change in pressure. The
efficiency is almost constant over the normal range of operation. As a result, the
power requirements are proportional to flow. |
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Open impellers with back-leaning blades combine
the features of both the radial bladed type and enclosed type with back-leaning blades.
They have moderately rising head-volume characteristics, which not only permit a
wide range of flows in a given impeller but also allow a moderate variation in pressure.
Efficiency is greater than that of a radial bladed type. The power-volume
characteristics rises with flow at a rate slightly less than that of the radial bladed
type. |
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Enclosed impellers with back-leaning blades are
extremely useful in applications that require a steep head-volume characteristic as well
as the highest attainable efficiency. Applications include parallel operation with
other compressors, or boosting of another compressor's output. Enclosed impellers
are also used for process recycle service or in applications requiring high efficiency to
reduce overall energy costs. The power-volume
curve will show a self-limiting feature at higher volumes. This feature is very
beneficial when the driver has limited power available but operation throughout the full
capacity range is required. |
Shaft Seals
The shaft of the blower
must be seal at the point the shaft exits the casing to prevent the escape of process gas. There are many different types of sealing
arrangements but the simplest effective sealing arrangement is the split carbon ring type. An air purge may also be employed to minimize the
escape of process gas. The housing of the
shaft seal should be flanged to the rear cover of the casing and be horizontally split to
allow for easy inspection and replacement of the carbon rings.
Bearings
Blowers are equipped with two types of bearings; Radial and
Thrust bearings.
Radial bearings carry the weight of the shaft and impeller
and provides for their free rotation. Radial bearings for sulphuric acid plant
blower applications are generally pivoted shoe oil flooded or sleeve type bearings.
Thrust bearings carry axial forces on the shaft created by
the rotating impeller.
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Pivoted shoe bearings consist of multiple
pivoting shoe pads equally spaced around the base of a horizontally split bearing body.
Each shoe is self-aligning to compensate for shaft angularity. A separate oil
film is formed between each shoe and the shaft journal. The bearings are designed
for easy inspection and maintenance, and can be removed without disturbing the shaft.
Pivoted shoe bearings are used on heavy duty machines where rugged and reliable
service is required. |
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Sleeve type radial bearings are generally for
moderate duty applications. The bearings is horizontally split which allows the
bearings to be removed for inspection and maintenance without disturbing the shaft. |
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Thrust bearings are generally tilt-pad,
double-acting bearings which will accept thrust forces in either direction along the
shaft. The bearing utilizes a number of tilting[pad shoes on either side of the
thrust collar. A separate oil film forms between each shoe and the thrust collar
that is proportional to the speed and loading. The thrust bearing is self-adjusting
so that the thrust load is equally divided among the shoes on either side. The
bearings also compensate for minor misalignment or deflection of the bearing housing or
shaft. |
Gear Box
Monitoring Systems
Large, high speed rotating equipment must be protected from
excessive vibration and temperatures, otherwise they will not give reliable and continuous
service and may fail, sometime catastrophically. A monitoring system provides for
continuous monitoring of vibration and temperature and has the ability to shutdown the
machine if preset limits are exceeded.
Monitoring of bearing temperatures are important since high
temperatures can lead to bearing failure. Typically each bearing on the blower, gear
box and driver will be equipped with an RTD for temperature monitoring.
The following is the minimum requirement for temperature
monitoring:
- One (1) in each blower radial bearing
- One (1) in the blower thrust bearing
- One (1) in each gear box bearing
- One (1) in each driver bearing
- Two (2) in each phase of the motor winding
Monitoring of vibration is also critical. High
vibrations occur because of an out of balance condition somewhere in the equipment.
For the blower it is generally caused by a build-up or deposits on the impeller.
The following is the minimum requirement for vibration
monitoring:
- Two (2) radial vibration probes at 90° apart on the blower
inboard bearing
- Two (2) radial vibration probes at 90° apart on the gear box
high speed radial bearing
- One (1) radial vibration probe on each driver bearing
- One (1) axial position probe mounted on the thrust bearing
- One (1) axial position probe mounted on the high speed shaft
Bentley Nevada produces a monitoring system that is commonly
used for equipment monitoring. For many years the 3300 system was the standard for
the industry. The 3300 is now being phased out in favour of the new 3500 system.
3300 System
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3500 System
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