Turbomachinery for Nitric Acid Plants - MAN Diesel & Turbo

Turbomachinery for Nitric Acid Plants
Engineering the Future – since 1758.
MAN Turbo

Turbomachinery for nitric acid plants
Nitrid acid, HNO3, is an important intermediate
for a variety of products of the
fertilizer and chemical industries. There
are various processes today for the production
of nitric acid, which all require
specific mechanical equipment. With the
aid of platinum catalysts, ammonia is
oxidized after which the nitrogen oxides
contained in the combustion gases are
almost completely converted with water
in an absorber to nitric acid. The core
equipment of such nitric acid plants is
the turbomachine train, which in most
cases consists of an air compressor, a
nitrous-gas compressor, a tail-gas turbine
and a steam turbine. There are
many good reasons why this equipment
should come from a competent supplier,
such as MAN Turbo.
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Experience
Almost 90 years of turbomachinery
design by MAN Turbo, including
40 years of experience in manufacturing
machine trains for nitric acid plants.
Close cooperation with leading users
and process licensers.
About 460 turbomachines for 170
nitric acid plants using the most commonly
applied processes including the
world’s largest plant (2,000 t/day)
Flexibility and versatility
The supply of complex turbomachine
sets should be entrusted to a manufacturer
capable of designing, manufacturing
and testing all of the machines within
the train – compressors and turbines.
MAN Turbo has the most comprehensive
production and supply programme
in the field of turbomachinery in the
world. This is an asset which guarantees
flexibility and allows the company
to build turbomachines with an optimum
of adaptation to all process requirments.
Another advantage for the customer:
the undivided responsibility for the whole
machine train lies with MAN Turbo.
Quality and economy
The high technical standard and the
economy of operation of MAN Turbo
turbomachines are the results not only
of extensive research and development
activities, modern CAD, calculation and
manufacturing techniques, but also of
the availability of high-speed balancing
and overspeed testing equipment as
well as mechanical and thermodynamic
testing facilities. Dismantling for shipment?
At MAN Turbo's own port – on
the Rhine-Herne-canal close to the port
of Duisburg, the world’s largest river
port – a crane with a handling capacity
of 450 t is available. This allows loading
of complete machine trains, including
their base frames, so that the time needed
for erection work on site is significantly
reduced. Result: considerable cost and
time savings during erection on site.

Processes
For the production of nitric acid there
are four basic processes which are distinguished
by their pressure levels for
combustion and absorption:
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Low/medium and medium/high
two-pressure process
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1 Air compressor
2 NH 3 burner
3 NO compressor
1
4 Absorption tower
5 Tail gas expander
6 Steam turbine
2
1. Low/medium two-pressure process
Combustion at atmospheric pressure,
absorption at medium pressure (4-6 bar)
2. Medium-pressure process
Combustion and absorption at medium
pressure (4-6 bar)
3. Medium/high two-pressure process
Combustion at medium pressure (4-6 bar),
absorption at high pressure (10-12 bar)
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5
4
4. High-pressure process
Combustion and absorption at high
pressure (8-13 bar)
MAN Turbo has supplied machines for
all four processes. These are, depending
on the process:
air compressors (cooled and uncooled)
nitrous gas compressors
tail gas expanders
steam turbines.

medium/high-pressure process
5 1
4
1 Air compressor
2 NH 3 burner
3 Absorption tower
4 Tail gas expander
5 Steam turbine
This means, apart from clearly defined
responsibilities, that the necessary
coordinations are much simpler and
the contract handling procedure runs
more smoothly.
Integration of the machines into the
process is graphically shown using the
medium/high two-pressure process and
the high-pressure process as examples.
In the past 15 years the two-pressure
process has gained general acceptance
in Europe for nitric acid plants with
capacities of 500 t/day and above.
Smaller plants – mainly outside Europe
use the high-pressure process and the
medium-pressure process.
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The air compressor for the two-pressure
process and the medium-pressure process
is an uncooled axial compressor in
most cases, while for the high-pressure
process an intercooled centrifugal compressor
is used.
Whilst steam turbines are used as driver
almost exclusively for large machine
trains for smaller machine sets electric
motors are in service.
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Axial compressor for air
The first axial compressor for a nitric
acid plant was built by MAN Turbo as
early as 1963. For this application the
company developed the type AG, which
has 10-15 axial stages followed by a
centrifugal final stage. MAN Turbo axial
compressors are built from standardized
stage groups with different degrees of
reaction – from 70 to 85 % in the first
stages and to 100 % in the final stages.
The centrifugal stage replaces two to
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three axial stages and the outlet diffuser;
it also contributes to improved efficiency
and an extended operating range of the
compressor. MAN Turbo axial compressors
offer the following advantages:
High efficiency
By the selection of a reduced degree of
reaction in the first stages, flow velocities
can be kept low for high flow rates. Installation
of 100 % reaction blading in
the final stages results in optimal channel
sections and relatively small clearances
due to larger blade lengths.
Wide operating characteristics
MAN Turbo axial compressors with
speed control (e.g., when provided with
steam turbine drive) allow operation
down to a surge point of less than 65 %.
For constant-speed operation it is only
necessary to fit about a third of the

Axial air compressor for a 1,230-t/day two pressure plant P = 15,435 kW n = 5,152 rpm
stator blades with variable geometry in
order to obtain the same operating
range as with speed control. About two
thirds of the stator blade rows, in the
MP- and HP- sections, are stationary.
Low susceptibility to fouling and
erosion
The MAN Turbo axial compressor is
very tolerant of dirty gases. If there is
any accumulation of dirt in the machine
at all, this occurs in the fixed stator
blades, with fouling decreasing from
stage to stage in the direction of flow.
As the MAN Turbo axial compressor
develops most of the pressure in the
rotor blading, the head generated is not
affected when handling dirty gases;
there will be no change in efficiency
even after long periods of operation.
Gas-borne particles do not present the
MAN Turbo axial compressor with a
significant erosion hazard as the design
of blading used produces a very low
whirl component, allowing dust to follow
the gas paths without being centrifuged
to high impact velocities. For application
in nitric acid plants, the labyrinths at the
shaft outlets of the bearing housings are
provided with an intermediate air-sealed
chamber to prevent entry of NO-gases
into the oil system.
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Centrifugal compressor for air
For smaller plants using the mediumpressure
process with pressures of
about 6 bar, as weil as for highpressure
plants operating in the range between 7
and 15 bar, economically the best solution
today is to use an integrally geared
compressor. Theses multi-shaft compressors
have the advantage over
single-shaft machines, that they allow
direct drive and the integration of a tail
gas expander.
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These compressors of the type “RG”
are built by MAN Turbo with a maximum
number of six stages.
By varying the number of intercoolers,
the gas can be delivered at the temperature
required by a particular process.
For the medium-pressure range an uncooled
version is also available.
The advantage of this type of compressor
is that it can be driven either by an
electric motor or by a turbine or by both
at the same time.
For motor driven units, the bull gear is
driven directly. Whereas for the turbinedrive
version MAN Turbo has developed
a design with an additional pinion shaft,
whose transmission ratio is chosen to
allow the compressor to be driven either
by the tail gas expander or the steam
turbine at its optimum speed. The MAN
Turbo integrally geared compressor
offers the following advantages:
high efficiency due to optimum impeller
design, optimum speed selection,
and interstage cooling
low operating costs due to high efficiency
and minimum maintenance
oil-free compression
excellent part-load performance
high availability
simple operation due to fully automatic
control and monitoring
rugged, compact design.

Integrally geared compressor for a high pressure plant
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Nitrous gas compressor
For the two-pressure process the
nitrous gas compressor is generally of
the centrifugal type. Where the oxidation
pressures are very low, an axial
compressor can be used similar to
low-pressure plants. For nitrous gas
compressors today impellers of high
flowhandling capacity and, if necessary,
with large exit angles (900 impellers) are
also chosen.
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In designing compressors for nitrous
gas, one must take into account that
this gas
forms ammonium nitrite,
nitrate and HNO3 is poisonous,
is highly corrosive,
allows chemical reactions to take
place during compression.
Therefore, specific design steps have to
be taken and special equipment has to
be installed on the compressor.
The formation of ammonium nitrate deposits,
which under certain circumstances
has an explosive effect, is prevented by
avoiding any dead spaces within the
compressor.
Deposits that may form in the flow
channel are removed by “washing” in
regular intervals.
This is done by spraying steam condensate
into the compressor suction
nozzle. The so-called washing process
is performed during the start-up procedure.
In addition, condensate may be
injected into the machine during normal
operation for about five minutes every
eight hours.
Special nozzles spray the water at
high pressure against the gas stream to
give a uniform and precise distribution
guaranteeing quick evaporation of the
water.

This procedure ensures optimum,
gentle cleaning of the flow channel and
has little influence on the process mass
flow rate as only small quantities of
water are injected.
The poisonous effect of nitrous gas
makes it necessary to prevent any escape
of gas from the compressor. This
requirement is met as folIows:
Double sealing chambers are provided
at the main shaft in the casing.
The inboard chambers are fed with
sealing air (from the discharge of
the air compressor), while the outboard
chambers are provided with a
connection to atmosphere. This is led
outside the machine house in case
any leakage does arise. The ingress
of NO-gas into the lube-oil circuit is
prevented by an additional sealing
chamber in each bearing housing
of the machine train.
A special sealing agent is applied
at the horizontal joint flanges of the
casing. In addition, the casing joint
bolts are tightened at an exactly
defined force by means of a hydraulic
tightening device.
All parts in contact with the gas are
made of acid resistant CrNi steel to
counteract the corrosive effect of the
nitrous gases. The chemical reactions
that occur during compression have to
be taken into account when designing
such a compressor. These reactions
are the dissociation of N2O4 to NO2 and
to a much lesser degree and the
oxidation of NO to NO2. The latter
reaction causes heat to be released,
whereas for dissociation considerable
amounts of heat are needed. In
designing the compressor, these heat
quantities/temperature changes are
determined stage by stage and considered
when sizing the flow channel
sections.
Nitrous-gas compressor
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Tail gas expander
Utilization of the energy still contained in
the tail gas after the absorption process
allows 35 - 100 % of the compressor
drive power to be recovered.
MAN Turbo builds tailgas expanders for
a wide range of conditions, both impulse
and reaction turbines, of singlestage
and multi-stage design. For con-
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trol purposes a system of adjustable
inlet guide vanes has been developed
over the past few years which gives
some advantages over the more traditional
nozzle group control arrangement.
This allows stepless control over the full
operating range and improved efficiency
through the use of a purely reactiontype
blading.
All components in contact with the gas
are made of acid-resistant CrNi steel
to avoid corrosion. The outboard labyrinths
of these machines are also provided
with two intermediate chambers
and are purged with sealing air from the
same source as the seals of the nitrousgas
compressor.

The rotor blades are provided, where
possible, with integral shrouds to give
better sealing between the blades and
the stator. This design also increases
damping to prevent blade vibration
and allows the efficiency to be improved.
The stator blades are fitted in a blade
carrier which is fixed in the turbine
casing in a thermally flexible connection.
The adjustable stator blades can be
brought into the desired position
by means of an electric motor or a
hydraulic actuator.
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Steam turbine for a 1,230-t/day two pressure plant P = 9,410 kW n = 7,973 rpm
Steam turbine
The steam turbine is normally fed with
steam generated in the process. In a
few cases this process steam is utilized
for other purposes and the steam turbine
is replaced by an electric motor.
In nitric acid plants the steam turbine
not only has to provide the normal driving
power but also the startup power of
the HNO3 turboset as initially the tail gas
expander does not deliver any power.
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The start-up power may be more than
twice the normal power.
There is also an increasing demand for
turbine designs which allow extraction
of a certain steam quantity for process
purposes or for an additional generator
drive.
For these applications MAN Turbo provides
condensing steam turbines of
the reaction type with and without extraction
or steam admission.

Steam turbine
The MAN Turbo modular system
For the important subassemblies of
MAN Turbo steam turbines, such as
bearing housings, control-valve/nozzle
casing, control-stage casing and exhaust
casing, as well as for the connecting
elements, a modular system of
geometrically graded component sizes
has been developed. Combining the
various modules permits flexible adaptation
of the machine to multifarious appli-
cations through the selection of optimum
design features. In order to keep
the blading free from vibration resonance
throughout the speed range, a thick
dropshaped profile with an integral
shroud segment was developed for the
rotor blades in the reaction part of the
turbine. This is a very rugged profile;
which can be used within a wide stagger
angle and blade spacing range and is
available with different chord lengths.
The blade selection procedures ensure
resonance free operations. The tapered
twisted rotor blades of the exhaust
stages and the corresponding tapered
stator blades are again selected from a
standard blade range. The rotors are
forged from the solid and are balanced
fully bladed under vacuum conditions at
operating speed.
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Control and safety equipment
A complex machine train of the kind
used in a nitric acid plant makes very
great demands on the control and protection
systems in terms of quality and
flexibility. Such systems must consider
the requirements of the process and
provide all functions from start-up to
emergency shutdown of the machine
train and process. MAN Turbo has developed
electronic control and protection
systems which interface with
the mechanically/hydraulically actuated
final control elements.
Anti-surge control, surge detector
In the performance graph of a turbo
compressor the surge line is the dividing
line between ranges of stable and
un-stable operation. Reduction of the
volume flow rate beyond this line causes
separation of the flow from the
blading, the gas flows periodically from
the discharge end to the suction end –
a phenomenon referred to as compressor
surging. The resultant extreme
load reversals and temperature fluctuations
in the gas-handling components of
the machine must be avoided. This is a
task of the anti-surge control, a system
which, by controlled opening of the
blow-off or by-pass valves, always
keeps the compressor flow rate in the
stable range of the performance graph.
A special development of MAN Turbo
for this purpose is the electronic control
and protection system called “Turbolog
S7-400”.
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Speed control
To allow speed and therefore process
pressures and flow rates – to be controlled
as well as the machine train to be
protected against overspeed, MAN
Turbo has developed the turbine control
and protection system “Turbolog S7-
400”. This system by far surpasses the
requirements of NEMA specification SM
23, class D. Depending upon specific
requirements, it is designed for any of
the following functions: (Refer also to
the special brochure “Turbolog S7-
400”.)
redundant turbine speed control
redundant overspeed protection
complete functional testing of all trip
circuits during normal operation
speed-controlled starting of the
turbine from standstill
combined control of speed and
extraction pressures
back-pressure control
control of turbines with sidestream
admission
inlet pressure control
power output control
frequency control

1 Trip valve with steam strainer
2 Live-steam control valves 3 Contral oil
pressure accumulator
4 Speed controller with positioner (Turbolog)
5 Sealing-steam control 6 Shaft barring gear
7 By-pass valve
8 Surge detector
9 Non-return flap with closing device
10 Water injection
11 Blow-off valve
12 Anti-surge control 13 Surge detector
14 Non-return flap with closing device
15 Labyrinth sealing air for NO-gas compressor
and tail gas expander
16 Labyrinth sealing air for bearing pedestals
17 Quick-closing inlet flap 18 Quick-opening
by-pass flap
19 Adjustable stator blades
20 Axial-displacement measurement
21 Bearing-temperature measurement
22 Shaft-vibration measurement
23 Speed monitoring
Quick shutdown
Quick opening of blowoff valve
Control of shaft barring gear
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Components
Baseframe/steel foundation
If allowed by the local conditions at site,
the entire machine train can be shopmounted
on a common baseframe or
on a steel foundation; this offers the
following great advantages:
all installation and assembly work, including
piping and cabling, can be
done in manufacturer’s works
easy handling during testing and
transport
less time for erection
simplification of site work.
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Oil supply
For the supply of oil to the machine bearings
and the hydraulic control devices,
oil supply units conceived by MAN
Turbo are used. These are designed
either to MAN Turbo standard or API
Standard 614; customer specifications
can be taken into account if desired.
Gearbox, couplings
A gearbox in the train allows optimum
speeds to be selected for the individual
machines within the train. This gearbox
is normally of the single-stage spur-gear
type. The individual casings of the machine
set are connected by gear type or
diaphragm couplings. Also each individual
casing has its own thrust bearing.
This flexible separation has the following
advantages:
minimal effect from adjacent machine
rotors over wide operating conditions
flexibility in selection of machine
arrangement in the train
optimal clearance between stator and
rotor (thereby ensuring best efficiency)
greater inter-casing angular displacement
allowed, e.g., after a long period
of operation.

Quality assurance
MAN Turbo’s QA programme is the
basis for the reliability of the company’s
high-quality products. The QA division,
which reports to the Chairman of the
Executive Board, initiates, coordinates,
and monitors all activities affecting quality,
while also issuing, and controlling
the compliance with, QA Manuals and
Procedures, such as:
Design
Material procurement
Production planning
Fabrication
Functional tests
Procedural qualifications
Calibration of gauges and test
equipment.
The main features of an inspection plan
for a nitric acid machine train are:
Destructive testing of specimens
taken from workpieces of the following
components:
- machine casings
- inlet rings
- volutes
- nozzle segments
- stator-blade carriers
- shafts
- impellers
- rotor and stator
- blades
- labyrinth seals
- control-valve/nozzle
- chests
- trip valve internals
Ultrasonic examination after rough
machining and finish-machining of
- cast steel casings
- impellers
- shafts
- rotor and stator blades
- inlet and control-valve/ nozzle
chests
- nozzle segments
Magnetic-particle testing of
- machine casings
- rotor and stator blades
- stator-blade carriers
Due penetrant examination of
- machine casings
- shafts
- impellers
- rotor and stator blades
- stator-blade carriers
Radiography of machine casings
Dimensional and visual examination of
- machine casings (pre-machined and
finish-machined)
- shafts and rotors
Heat stability test of shafts
Hydrostatic testing of casings
Overspeed testing of impellers
Run-out measurement on rotors
Balancing of rotors at operating speed
Clearance measurements on assembled
machines.
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Workshops
To meet today’s demands for high
quality, economic production and short
delivery, advanced workshops qualified
to the latest standards are required.
The new fabrication shops and test
facilities of MAN Turbo’s turbomachinery
sector satisfy these requirements in
every respect. High-capacity working
facilities have been combined in fabrication
isles for an optimum flow of
materials, including equipment for the
machining of casings, blade fabrication,
impeller fabrication, rotor machining,
and assembly.
The main machining operations are
performed by CNC machine tools. The
machine tools incorporate the latest
technology.
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1 Materials receiving
2 Diaphragms
2a Small parts stores
3 Shipment
4 Rotor
4a Materials/toolshop
5 Small parts & blade fabrication
6 Casing fabrication
6a High-bay warehouse
7 Assembly
8 Balancing
9 Stores
10 Gas turbine service shop
11 Large machinery test stand
12 Development test stands
13 Training Shop
14 Screw compressor service shop
15 Turbomachinery service shop
16 Free-field test stand
17 Gas turbine test stand
18 Piping shop
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Test facility
MAN Turbo owns and operates different
testbeds for turbomachinery in order to
execute factory acceptance tests. One
test centre for compressors, steam,
process-gas and industrial-gas turbines
as well as for individual components has
been integrated into the production
shops.
Machines/machine trains are completely
assembled in the assembly bay and
subsequently transferred to the adjacent
test stand by two 80 t cranes.
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After erection on the 44 m long steel
foundation and connection to the permanent
piping systems (steam, air, water,
oil), all kinds of tests to DIN/VDI/ASME/
API can be conducted:
Mechanical testing of the individual
machines
Mechanical testing of a complete
machine train
Thermodynamic testing with air and
other gases in open or closed loop
systems.
The drivers used during testing are
either those supplied under a particular
contract or the various test stand
drivers which are available, i.e.:
An 18 MW condensing steam turbine
One 6.5 MW and two 2.5 MW
variable-speed motors which can,
if necessary, be coupled together.
In 2004 an additional packaging and
test centre for large strings was commissioned.
It is now possible to work on up to three
machine trains at the same time, on
an area of over 4400 square meters.
Modules up to 600 tonnes are by no
means an exception.
Two cranes, each capable of lifting
150/180 tonnes, and with a crane hook
height of over 17 metres, guarantee
the necessary flexibilty in the machine
assembly and testing.
This test bed includes a cooling water
circuit with three cooling towers, a 20 MW
electrical power supply in addition to the
steam supply (50 bar, 500 °C, 55 t/h).

Erection and service
All erection and service activities for
MAN Turbo machine trains are carried
out by MAN Turbo’s centralized machinery
service group. The service group’s
activities ensure the greatest possible
availability and operational reliability of
MAN Turbo turbomachinery even after
many years of operation.
This objective is pursued through
the use of meticulous erection
procedures,
thorough commissioning/trial
operation,
regular inspection,
preventive maintenance,
fast repairs,
readily available spare parts and
extensive advisory services.
The company’s service consultancy
team further ensures expert feedback
on a regular basis to promote the consistent
development of MAN Turbo’s
products and services. The following
provisions have been made to ensure
comprehensive service to our customers:
a close-knit network of representatives
in many countries of the world
service outlets at strategie points
an independent service workshop at
the company’s Sterkrade works.
Emergency service is available at
Sterkrade
hours.
outside normal working
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MAN Turbo AG
Steinbrinkstrasse 1
46145 Oberhausen/Germany
Phone +49. 208. 6 92-01
Fax +49. 208. 6 92-20 19
www.manturbo.com
MAN Turbo – a member of the MAN Group
In the interests of technical progress,
subject to change without notice.
Printed in Germany. December 2008
Turbo 1003 e 1208 1.5 ba