Ruhrstahl-Heraeus process with
mechanical vacuum pumps:
absurdity or red-hot technology and
money well spent?
After a hesitant start to the replacement of steam ejectors in steel degassing with mechanical
vacuum pumps, the latter have become state of the art in vacuum degassing and increasingly in
vacuum oxygen de-carburization installations. While there may have been early concerns about
the mechanical vacuum pump’s suitability for production, it has proven reliable in the global steel
industry, many with high production cycles. The advantages are clear: the initial higher investment
is returned rapidly by significantly lower cost of ownership and reduced downtime, and the
lower carbon footprint serves the requirements of modern steel plants in meeting environmental
concerns and legislative requirements.
For Ruhrstahl-Heraeus (RH), the situation is slightly different. The required pumping capacity
of the system is usually very large, and production runs continually 24/7, often serving twin
degassers. RH-treated steel typically comes from an oxygen furnace, which produces steam as a
side product while cooling gases from the furnace. With the availability of steam and increased
performance requirements at low process pressure, a steam ejector may be a good fit. Nevertheless,
the market for dry mechanical pumps for the RH process is growing. This article gives the
background, latest technologies and vacuum concepts for this demanding application.
MILLENNIUM STEEL 2021
Authors: Bill Foote, Anke Teeuwsen and Peter Raynerd
Edwards GmbH
66
STEEL DEGASSING – THE VACUUM TASK
`At process pressure: With the circulation of hot
Vacuum treatments in secondary metallurgy have in
common that low pressure is used to remove impurities
like nitrogen and hydrogen from molten steel, by
lowering the partial pressure. This reaction takes place
at the surface of the bath, sometimes with the help of
oxygen to remove carbon.
The performance requirement of the vacuum pump is
determined primarily by the process gas flow, but also
by supporting gases such as argon for stirring, camera
protection gas, oxygen blowing, if applied, and the leak
rate of the refining station. The second parameter is the
pump downtime to process pressure, to keep the tap-totap times short, maintaining the liquid steel temperature
in the correct range for casting and to maintain
production flow.
The vacuum tasks of a vacuum tank degasser, VD (VOD)
and RH (RH-OB) are in some ways similar (Figure 1). The
process time is typically 20 minutes, the process pressure
is 0.5 Torr (0.67mbar) and the expected pump downtime
would be 5 to 6 minutes with an empty vessel or tank. The
main differences in performance requirements are:
steel through the snorkels in an RH treatment, a large
surface area of molten metal is exposed to the vacuum
during the process. This leads to higher suction speed
requirements at the process pressure as compared with
the VD.
`For pump down: The volume to be evacuated is
smaller on an RH-degasser per degassed ton of steel.
In a VD a ladle with hot steel is placed in a tank,
which is closed with a lid. The ladle itself is oversized,
as it requires a freeboard area for reactions like slag
foaming. The tank volume around the ladle must also
be evacuated. In the RH, the hot steel is lifted into the
cylindric vessel which reduces the empty volume to be
evacuated. Both treatments use the volume of the filter
for pre-evacuation, which leads to shorter pump down
times by equalization of pressures.
VACUUM SYSTEMS WITH MECHANICAL
VACUUM PUMPS
Mechanical vacuum pumps are volumetric conveyers: a
defined gas volume is captured, moved and compressed
PRIMARY PROCESSES
r Fig 1 Schematic of VD (VOD) and Ruhrstahl-Heraeus (RH-OB)
refining station
to a higher pressure. As with steam ejectors in steel
degassing, dry mechanical vacuum systems may consist of
multiple stages.
r Fig 2 Screw vacuum pump mechanisms
r Fig 3 Roots pump mechanism
MILLENNIUM STEEL 2021
Atmospheric stage: Screw vacuum pumps
Like a building, the architecture of a mechanical
vacuum system needs a foundation, formed by the
so-called backing, roughing or primary pumps. This
stage compresses from vacuum to atmosphere and
takes the entire mass flow including incompressible
components like dust and humidity. These pumps do
the main compression work, reflected by a high power
consumption.
Today, vacuum pumps with a dry screw mechanism
(Figure 2) are the state-of-the-art in steel degassing.
The mechanism compresses with two rotating lobes
shaped in the form of a screw. The design of the
screw determines the efficiency and specific power
consumption, as well as the ability to cope with dust
and humidity. Dry screw pumps do not need operation
media, such as water or oil, inside the pump chamber,
for cooling or to avoid back streaming of gas towards
the lower pressure at the inlet of the pump. The screws
are designed with appropriate clearances between the
rotor and stator.
As there will always be a certain amount of fine dust
passing through the filters which will reach the vacuum
system, dry technology offers the huge advantage of
long operational times without the requirement for
frequent oil or water changes with subsequent disposal.
Dust can build up in the pumps in conjunction with high
humidity. Regular solvent or gas flushing will clean out
the dust and can take place with the pump remaining
in situ. The service intervals will depend on usage rate
and the amount of dust and humidity, but they may be
annual or potentially longer.
a
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MILLENNIUM STEEL 2021
r Fig 4 Innovative rotor design
68
Vacuum stage: Roots vacuum booster pumps
Roots pumps (Figure 3) are added in addition to the
screw pumps to increase performance at lower pressures.
The mechanism is a two-lobe, figure-of-eight design, with
compression taking place in one stage. The performance of
a mechanical booster is not only described by displacement,
the captured gas volume, but also by the compression ratio
and delta pressure.
The compression ratio influences the effective suction
speed. The lower the compression ratio the lower the
effective suction speed compared with the displacement.
The main governing factor is the clearance between rotor
and stator; the tighter the clearance the less possibility
there is for back streaming of gas. This is advantageous
when pumping light gases like 100% helium, or to achieve
high performance, down to pressures in the range of
10-3 mbar. However, as expected, the same tight clearances
lead to limitations in the thermal resilience of the pumps,
impacting the achievable delta pressure and limiting the
ability of the pump to cope with dust particles. A higher
delta pressure shortens the pump downtime and minimizes
the back-up pump requirement. In addition, a high delta
pressure is particularly significant in improving system
performance during pump down where the pressure is
below ~80mbar.
Some systems have a specially designed booster.
Where this is a pre-inlet cooled booster, it is also called
an exhauster. This design uses a flow of cooled gas into
the pump chamber during the compression phase. These
boosters need an additional gas cooler in the exhaust and
recirculate a part of cooled process gas back into the pump
chamber. Compression to atmospheric pressure is possible
but to handle this task in one stage requires a large motor
with high power consumption.
Any further booster stage added to the system enhances
the suction speed further at lower pressures, resulting
in smaller delta pressures across each stage. For the top
stage, usually called the first stage and used for performing
at process pressure, the trend is for larger boosters to
minimize the number of pumps. This is especially true for
the RH, which has a high capacity requirement.
The performance of large boosters used in steel degassing
today is mostly in the range of 30,000m/h to 42,000m/h,
while the total system suction speed requirement for the
RH goes up to 1,000,000m/h. Traditionally, large vacuum
boosters in the market were designed as compressors for
overpressure applications. These are large, slow running
machines with heavy cast iron bodies and rotors, leading
to some issues, such as difficulty in transportation and
handling when moving the pumps and high inertia for
the motor to overcome. This latter issue leads to a higher
power consumption, as well as slower acceleration and
deceleration of the rotors, where required for speed
control. These heavy-duty designs and very large pump
sizes are somewhat in contradiction to the vacuum task to
be performed, which is the lightest of the process duties,
being a compression from 0.5 to ~3Torr.
Figure 4 shows a new rotor technology developed for the
for the large booster used in steel degassing. The hollow
design reduces inertia significantly and allows for higher
rotational speed, to achieve a higher performance with
lower mechanical losses and lower power consumption.
Three, or four stage mechanical vacuum systems
Where more booster stages are added to increase
performance at lower pressures, the relative size of the
back-up pump reduces. Consequently, a three-stage
system needs more back-up pump capacity to achieve the
same performance at 0.5 Torr as a four-stage system, using
the same type of boosters.
Reducing the number of back-up pumps reduces
the initial cost of the system and will decrease power
and utility consumption, leading to a faster return on
investment. However, having fewer back-up pumps lowers
PRIMARY PROCESSES
the performance of the system at high pressures, which
could impact on pump downtime. Today, three stage
systems are the state-of-the-art in VD, or VOD, perfectly
matching the requirements for performance at the process
pressure in each of the pump down phases, as the volumes
to be evacuated are larger per ton of steel, compared with
the RH.
The world’s first RH system with dry mechanical vacuum
pumps, was a three-stage system, but with fewer backup units, supported by a high delta pressure mechanical
booster and an additional cooler to protect the screw
pump from heat load. This shows that using high delta
pressure boosters can minimize the number of primary
units. However, a four-stage system consisting of the same
type of pumps, requires the same number of large boosters,
properly distributed in the first and second stage, but using
fewer back-up pumps. Starting from a smaller ‘foundation
system’, using the minimal number of back-up pumps, more
back-up performance can always be added as required, to
accommodate oxygen blow requirements. Therefore, today
the trend in RH is for a four-stage system.
Figure 5 shows a comparison between different systems.
All three system shown have 18 x 32,000m/h boosters
in the first stage. The three-stage system requires more
back-up capacity to achieve the same performance at
low pressures. The performance at high pressures is
better and in consequence the power consumption is
relatively higher. While a four-stage system offers lower
performance at high pressures, it consumes less power. If
an increase in performance is required, either for a faster
pump downtime, or to meet a higher specified data point
for oxygen blowing, additional back-up units can be added
as required. The system can thus be adapted to process
requirement, optimizing any investment and consumption
of utilities. In the RH this can lead to significant savings
considering the scale of the system.
r Fig 5 Comparison of pump down performance and power
consumption for three- and four-stage systems
r Fig 6 Mechanical vacuum system performance and throughput
according to pressure
An unpleasant topic to consider is the possible failure
of a mechanical pump. The mindset in the steel world
is widely shaped by experience of steam ejectors. A
steam ejector rarely fails unexpectedly. Its performance
deteriorates in a creeping way over time, as the venturi
profile becomes clogged by dust deposits. An allowance
for this performance decay is accounted for the ejector
design, by an additional performance margin which
extends the interval between maintenance interventions.
Increasing the performance of a steam ejector neither
significantly increases investment, nor the cost for
steam, especially when additional steam is available
from BOF. Steam ejectors may also be oversized to
accommodate leak rates that would otherwise endanger
the metallurgical results.
a
MILLENNIUM STEEL 2021
A FEW WORDS ON REDUNDANCY
69
r Fig 7 Simulation of a pump failure, or increase leakage rate, for a four-stage system
MILLENNIUM STEEL 2021
With this background, it is understandable why possible
pump failures are perceived as a major threat. However, the
situation becomes less critical when the actual performance
characteristics of the pump are considered. The main
difference between a steam ejector and a mechanical vacuum
pump is that at higher pressures a steam ejector curve trends
very much towards constant mass flow. The steam ejector
principle works by kinetic energy in the form of vapor particles
hitting gas molecules at higher pressures. Consequently, at
higher pressures, but with the same amount of steam, the
same number of molecules are pumped. This is, of course,
an oversimplified explanation under ideal conditions but can
be helpful to aid understanding. In contrast, the mechanical
vacuum pump is a volumetric conveyer and trends towards
handling a constant volume at higher pressures. Physics
informs us that the number of molecules in a constant
volume is higher at higher pressure, as derived from the mean
free path equation and assuming that the temperature and
molecular diameter of the molecules are constant. Under
ideal conditions Equations 1 and 2 apply.
70
P*V=constant
Eq 1
P1*V1 = P2*V2
Eq 2
Where:
P1 = inlet pressure
V1= inlet volumetric flow
P2= outlet pressure
V2= outlet pressure
For example, a mechanical vacuum pump of 32,000m/h
with an inlet pressure of 0.5 Torr, will compress the gas
into the inlet of a 6,000m/h mechanical pump at a
pressure of 2.6 Torr. This volume contains the same
number of molecules. Therefore, the performance curves
of mechanical vacuum pumps show increasing throughput
with higher pressures, while the performance decreases.
This can be seen in Figure 6.
The pump characteristic, as shown, is helpful when there
is a change in conditions, such as an unexpected pump
failure, or an increased in the rate of leakage. Figure 7
shows a simulation of a pump failure.
The red lines in Figure 7 show the impact on process
pressure if one, or even two, out of the 18 boosters fail
in the first stage. A system with 18 boosters can handle
456kg/h at 0.5 Torr. If one booster fails, the pressure in
the vessel increases to 0.53 Torr, and if two boosters fail
the pressure increases to 0.56 Torr. This is still far below
1 Torr, which is typically acceptable with respect to the
required metallurgical results. The black lines in Figure 7
simulate an additional leak rate of 95kg/h, which leads
to a pressure of 0.6 Torr in the vessel, when using the full
system. Were two boosters to fail, the pressure still remains
below 0.7 Torr.
CONCLUSION
Mechanical vacuum pumps are a valid option for steam
ejectors and have proven reliable in production since 2009.
The main drivers for change are lower cost of ownership and
a lower carbon footprint. This calculation works well for VD
PRIMARY PROCESSES
and VOD plants, where steam for the steam ejector must
be produced by a separate boiler. RH stations, though, are
typically located in converter plants, where steam is often
available as a byproduct of BOF cooling. However, the
quality requirements of the steam are high and sometimes
an additional boiler is still needed to achieve adequate
results. This increases the cost of the steam.
In areas with a high ambient temperature, steam ejector
systems with liquid ring vacuum pumps sometimes have
problems achieving process pressures safely, due to high
cooling water temperatures. This is a critical issue for
production. When using mechanical vacuum pumps the
available steam from the BOF can be converted to power,
which further improves the overall economics. Other
advantages are the immediate readiness of a mechanical
vacuum pump, which does not require the long waiting
time for the boiler to heat up. Pumps also provide the
smooth pressure control during oxygen blowing at high
pressures.
Mechanical vacuum pumps always need a filter and
dust can be reused in production instead of creating
contaminated sludge, as is the case with the cooling
water from steam ejectors and liquid ring pumps.
Finally, all these advantages lead to a smaller carbon
footprint. MS
Bill Foote is Technical Manager, Systems, Anke Teeuwsen
is Global Market Sector Manager Steel and Peter
Raynerd is Business Manager, Custom Engineered
Systems, all with Edwards GmbH
CONTACT: anke.teeuwsen@edwardsvacuum.com
VACUUM SOLUTIONS
FOR STEEL DEGASSING
Pioneers of dry mechanical vacuum
pumps for VD-VOD-RH since 1998
Proven installed base in
the global steel industry
Innovative new rotor technology for
high performance with small footprint
Edwards Steel Degassing System
Large capacity booster for maximised performance
Strong compression booster for fast pump down times
Optimised solutions for quick return
on investment and greener footprint
Robust large screw pump for reliable operation
edwardsvacuum.com