Sulzer AHLSTAR Process Pumps for Fertilizer Applications


Research
and development have always played a significant role at Sulzer Pumps. Its main
target is to always meet the customer's needs and processes. This knowledge is
available for clients involved in fertilizer production. Sulzer pumps are
tailor-made for difficult liquids, which means that they are the natural choice
for liquids that are corrosive or both corrosive and abrasive.

AHLSTARtm
Process Pumps are designed for continuous operation in industrial processes,
and the pumps can be used for pumping various kinds of liquids and slurries.
AHLSTARtm pumps are horizontally-mounted with the following
characteristics: single stage, end suction, back pull-out, ROTOKEY impeller
mounting, simplified heavy-duty bearing unit, etc. The AHLSTARtm
Process Pump series has standard (A), wear-resistant (W), non-clogging (N), and
hot liquid (E) hydraulic designs. The series has two solutions for gas
handling: an air-separation (R) design and a self-priming gas removal (S)
design. The gas handling options can be integrated with standard (A), wear
resistant (W) and non-clogging (N) hydraulic designs.

Pumping of slurries, selection of
pump types against erosion

The key
issue when selecting a slurry pump is to classify the type of the pumped liquid
or slurry. The rough selection of whether to use a wear resistant (W design)
pump or an ordinary process pump can be made according to figure 1 below. The
table is then used to define the range of recommended operating conditions.

The above
values may vary, depending on how erosive the particles in the slurry are and
how corrosive the liquid is. Selecting the largest available impeller diameter
(lower running speed) and keeping the duty point as close as possible to the
best efficiency point (BEP) will ensure maximum pump lifetime is achieved. It
is also critical to find the correct balance between a good corrosion-resistant
material and the most suitable hydraulic design of the pump to withstand
abrasion. Firstly, a suitable material should be found to withstand corrosion.
The nest step is to check the material has good resistance against expected
level of corrosion. Combining these factors with the appropriate hydraulic
design ensures the optimum economic solution is achieved.

Sulzer
WPT wear resistant pumps are specially designed for corrosive and abrasive
liquids. The WPP pump is not a heavy-duty slurry design. Having process pump
hydraulics allows efficiencies of the same high level as APT process pump to be
achieved. The key difference is that WPTs' flow patterns are specially designed
to prevent erosion in the hdyraulic parts.

There are
many applications where suspended solids can severely damage the pump in a
short period of time. The pump can be lost in a matter of weeks if the design
and material selection is not correct. If only abrasion has to be considered
(non corrosive liquid), a hard material should be selected. One possible
solution is chromium iron whose hardness is 600 HB. Sulzer has installed
AHLSTARtm WPT wear resistant pumps manufactured from chromium iron
material several applications that are not acidic and have found their life to
be about three times that of a normal process pumps with CD-4MCu material.

Corrosion

Corrosion
is a complicated phenomenon, the result of electrical and chemical effects.
Corrosion is commonly divided into the following categories:

  • general
    corrosion
  • galvanic
    corrosion
  • crevice
    corrosion
  • pitting
    corrosion
  • stress
    corrosion
  • erosion
    corrosion

The
general rule to increase corrosion resistance is to use higher alloyed
stainless steels. Pitting and crevice corrosion resistance in particular can be
improved by the addition of molybdenum and chromium. In addition, duplex
stainless steels are resistant to stress corrosion.

Erosion
corrosion is typically found when pumping liquids containing corrosive solid
particles. Erosion corrosion also happens when pumping clean but corrosive
liquids without solids when the
velocity of flow within the pump is high enough. High velocity of flow is found
within the impeller when a pump operates at high rotational speed.

Corrosion
is a reaction of the material with its environment. the fundamental reason for
corrosion is that the material aims to return to a lower energy level. Chemical
corrosion is the reaction of the material with liquids, gas or solids it comes
into contact with which results in a new chemical compound being formed.  The most common reaction is oxidation; this
results in an oxide layer forming on the surface of the material.
Electrochemical corrosion is affected by acid, alkali or dissolved salts in a
water solution. It is typical for different metals to have different dissolving
pressures, and the electrical potential of materials have different values in
different solutions. electrochemical corrosion always takes place therefore
within electrochemical corrosion pairs. 

If the
pumped liquid leaks out of the pump to the atmosphere there is a high risk of
corrosion, especially when there is a sulfuric acid in the liquid. In
particular, the material becomes weakened and eaten away by corrosion on the
atmospheric side where holes have been drilled and plugged in the volute
casing. This is why drillings e.g. for draining the pump,  are not accepted in the volute casing. When
corrosion and wear take place at the same time in the pump, a metallic material
is usually selected for its construction. Metallic components can resist the
abrasive attack of solids on their surface because their hardness is quite
high. Modern high-tech alloy austenitic cast steels can manage simultaneous
corrosion ad erosion very well.

A good
example of a sucessful trial where AHLSTARtm WPT wear resistant
pumps made from Avesta 654 SMO (Avesta 654 SMO is a trademark owned by Avesta
Sheffield, who has granted Sulzer Pumps Finland Oy a  license to produce the material) successfully
replaced rubber-lined pumps in a number of erosive/corrosive applications.

If
corrosion happens within electrochemical corrosion pairs, all the hydraulic
parts of the pump should be similar to each other. This means that all parts
that are in contact with the pumped liquid must be made of the same material,
especially the nuts and bolts.

The other
parts of the pump, which are not in contact with the pumped liquid all the
time, should have corrosion resistance, especially when leakage from other
process equipment takes place due to operational failures. The baseplate,
bearing unit or other parts will be lost in a very short time if they are made
from cast iron. Sulzer recommends that that they should also be made from
stainless steel for severe services.

It is
always very important to know what kind of liquid is pumped. If it is not
certain what the application is and if there is a clear possibility of chemical
corrosion, loss of material during operation will happen in a very short period
of time. The customer always has to give full information about the liquids
that are in contact with the pump. The level of detail typically required is as
follows.: Density, temperature, pH, chemicals content, contaminants and solids
including content by volume and size. 

Nowadays,
a pump totally manufactured from stainless steel can be supplied. Such pump
provides a longer operating lifetime and thus offers more reliable pumping
solutions.

New features for dynamic shaft
sealing

The shaft
seal is one of the most important factors when attempting to minimize the
lifetime cost of a pump's capital, operating, maintenance and downtime costs.
Experience shows that at least the shaft seal causes 60% of all problems with
centrifugal pumps.

Use of
mechanical seals instead of packed stuffing boxes has brought considerable
improvements but significant operating and maintenance problems remain.
Conventional gland packing and mechanical seals will wear due to mechanical
friction during running and will eventually form a leak path. These seal types
require a supply of sealing liquid for lubrication and cooling, and often this
liquid comes from an outside source. In many applications, this means building
a complete pipeline system to provide clean sealing liquid to the shaft seal.

Mechanical
wear, leakage and the need for external sealing liquid has been practically
eliminated by the dynamic seal. The operating principle of the dynamic seal is
simple. When the pump runs, the expeller generates a liquid ring in the annular
seal chamber and evacuates the liquid pumped from the seal cavity. When the
pump stops, the static seal tightens against the thrust ring, thus preventing
any leakage. Mechanical wear, leakage and the need for sealing liquid have been
practically eliminated and no piping for sealing water is needed. 

In a
dynamic seal the expeller rotates freely in its chamber without any mechanical
contact and therefore without any mechanical wear. When the pump stops, the
liquid fills the seal cavity and presses the flexible disc against a sleeve,
thus preventing leakage.

When pump
starts again, the expeller creates a liquid ring in the expeller seal chamber
and removes the liquid from the seal cavity. When the pump is running, the
flexible disc is no longer presses against the sleeve.

A
conventional dynamic seal needs three operational factors:

  • positive
    incoming head
  • liquid
    temperature less than boiling point
  • limit
    for maximum incoming head

There
are, of course, many applications where one or more of these requirements are
not fulfilled. for example, in evaporator applications there might be negative
suction head or high operating temperature. As the dynamic seal has many
benefits, there has been great demand to expand its operational limits. the
latest dynamic seal construction allows for low or high liquid level in pump
suction and even light vacuums can be handled. The new design also allows
liquid temperature that exceed boiling point to be pumped.

Sulzer
has developed new dynamic seal designs. Use of these seals requires a full
understanding of the pumping application and process conditions. In addition to
the requirements of the pumping application, the pressure conditions inside
pump and pump duty point also affect the operation of the seal. Sulzer Pumps
has solved these problems and is the first company in the world to provide
these new advanced dynamic seal designs.

Dynamic seal for negative suction
head

If there
is negative suction head to the pump, a conventional dynamic seal leaks air
from the atmosphere into the pump. But in Sulzer's new dynamic seal, there are
recirculation holes in the casing cover near the impeller edge. Pressure
created by the impeller will also increase pressure in the seal chamber. The
expeller is then able to operate on a conventional dynamic seal. When pump is
stopped, the static seal works as normal.

If there
is a constant vacuum on suction side, then two static seals are required. One
works as with a conventional seal and the other prevents outside air entering
the process when the pump is at standstill. With this seal arrangement,
impeller balancing holes are not generally used, because vacuum pressure
towards the static seal is not permitted when the pump is running.

Dynamic seal for high incoming
heads

In case
there is a temporary high inlet head to the pump, a normal dynamic seal cannot
be used. But with the new design, Sulzer combined a normal dynamic seal and
grease lubricated gland packing. In this case, gland packing replaces the
static seal.

Dynamic seal for liquids above
their boiling point

One
limitation for normal dynamic seals has been the liquid boiling point.
Normally, when over pressurized liquids enters a seal chamber at atmospheric pressure
it vaporizes. In Sulzer Pumps' new dynamic seal, there is a throttling ring in
the neck of the seal chamber that limits liquid flow into the chamber itself.
From the chamber, there is a connection that leads the remaining exhaust steam
to an area of low pressure in the process. By taking pressurized steam away
from the seal chamber, stress caused to the static seal is minimized.

These new
dynamic seal designs expand operational possibilities greatly. But the use of
these seals is highly process dependant. All applications require knowledge of
process and systems. But with all seals described above, the benefits of
dynamic seals are available where none were previously available.

Costs of shaft sealing

When
examining sealing costs, the costs have to be calculated over a sufficiently
long period of time. Calculations below have been made over a 10-year period,
which gives a comprehensive idea of the structure of a lifetime costs of a
pump. As far as all sealing costs are concerned, the dynamic seal is the most
inexpensive sealing method. The dynamic seal also gives excellent reliability,
meaning that there are no costs resulting from seal damage.

The
following variables have been used in calculation of the graphs below:

Operational
costs are highest with seals that require a sealing liquid. The costs of
sealing liquid vary greatly between various mills, but the essential things to
consider are the production, investment and disposal costs of sealing liquid as
they are actually accrued.

If
maintenance costs are concerned, it should be remembered that cord packings
require adjustments and many changes of parts during their lifetime. This
increases the cost of packing stuffing boxes.

Significant
costs are created in continuous processes when faults occur in equipment
critical to the main process. In the worst case, the entire plant has to be
stopped during production. When calculating costs for unexpected shutdown, at
least the following costs should be considered:

  • Production
    losses
  • Product
    waste as a result of fault
  • Spare
    parts and replacement work
  • Line
    start-up costs to achieve normal production

The final
cost of a shut down becomes very high if it is difficult to obtain spare parts
for some reason, consequently extending the shutdown.

The
dynamic seal essentially improves operational reliability and costs accrued
during the operation of a pump. Mechanical wear 
and leakage of the seal has been practically eliminated. The Sulzer
Pumps' dynamic seal is suitable for almost all pumping applications in the
fertilizer industry.

For more
information, contact Fluid Engineering at 800-841-994 today.

Sakari
Hänninen

Sulzer
Pumps Finland
Oy



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