Sealless Pumps - Best industry practice for Concentrated Solar Power (CSP)
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Sealless Pumps – Best industry practice for
Concentrated Solar Power (CSP)
Matthew Thompson BEng (Hons)
Author affiliation:
Renroc Group (Pumps & Engineering) Australia
Factory 6/126 Merrindale Drive, Croydon Victoria 3136
matthew.thompson@renrocgroup.com
ABSTRACT
Concentrated Solar Power (CSP) will form a part of the Australian energy makeup in
the years to come. Whether Linear Fresnel, Power Tower, Trough or Dish technology,
the most important element of these systems are the pumps used to circulate the heat
transfer fluid (HTF) around the solar field. For maximum availability of the plant and
for the best reputation of the industry, it is imperative that the pumps used are reliable
and fit for this purpose.
Initially mechanical seal pumps were used on Spanish plants. This lead to fires and
multiple seal failures resulting in reduced availability of the plant and high operating
expenditure. These problems are caused by the inherent limitations using traditional
mechanical sealing technology.
It is important that the CSP industry in Australia take on board the lessons learned from
these problems and use sealless pumping technology. This technology gives total leak
free operation as well as total containment of environmentally hazardous fluids, is
capable of handling high temperature fluids (hot oil at up to 400°C, water 300°C and
above) and provides high reliability leading to maximum availability of the plant.
This paper will outline CSP technology and the pumps required, explain traditional
mechanical sealing technology and its limitations, and explain sealless pumping
technology and its advantages leading to best industry practice.
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
INTRODUCTION
Centrifugal pumps are used by most industries for transporting all types of fluids. In
concentrated solar power pumps are used to circulate the heat transfer fluid (HTF)
around the solar field (see Figure 1). These pumps are crucial to plant operation.
Without the pump the plant shuts down because the HTF cannot be circulated and
therefore solar energy will not be collected.
Figure 1: Schematic of a Trough CSP Plant (Image courtesy of Abengoa Solar)
Centrifugal pumps are typically driven by an electric motor and use centrifugal force to
impart energy on a fluid and allow it to be circulated through a system.
A key element in any pump is how drive is transmitted from the electric motor to the
pump impeller. Traditional arrangements use mechanical seals to seal the area where the
shaft protrudes through the casing. This technology has inherent limitations particularly
when pumping high temperature fluids such as those used in CSP. Mechanical seals will
always leak under normal operation which means as fluids become more hazardous
containment becomes more problematic and more expensive.
There are, however, sealless options using either permanent magnets or electromagnetic
forces to drive the pump impeller. This technology is not hampered in the same way
traditional mechanical seals are on high temperature applications and is now becoming
best industry practice on CSP plants. Sealless pumps do not leak as there is no leak path
for the fluid; this also means that zero emissions energy generation can use zero
emissions pumping technology.
MECHANICAL SEALS
A mechanical seal is used to seal the area where a pump shaft protrudes from the volute
casing where the fluid is energised by the pump impeller (see Figure 2).
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
Mechanical Seal to prevent
fluid escaping to
atmosphere.
Fluid in this section
of the pump
Figure 2: Mechanical seal pump (image courtesy of PumpWorks 610)
A mechanical seal has a moving face and stationary face which provides the seal. A
single spring or multiple springs provide the closing force which keeps the seal faces
together, as is shown in Figure 3.
Figure 3: Simple mechanical seal (image courtesy of Goulds Pumps)
Mechanical seals require lubrication at the seal face to remove heat generated friction
and prevent high wear; this means that there is always a leak path for the pumped fluid
to atmosphere. These seal emissions are coming under increasing environmental and
government scrutiny, for example, the TA Luft legislation in Germany.
Mechanical seal pumps will always have a use within industry. However, fluids become
more dangerous or extreme temperatures are used. They are inherently limited. For high
temperature fluids seals require extensive cooling systems so that the faces do not crack
from the extreme heat and thermal forces associated with this. Also, often double seals
have to be used adding to complexity and cost.
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
These limitations on high temperature fluids were seen clearly at CSP plants in Spain.
Hot oil pumps where the oil is up to 400°C experienced persistent seal failures.
It would seem that mechanical seal development is reaching, or possibly has already
reached, a design plateau with further developments likely to be rare and highly
expensive.
CANNED MOTOR PUMPS
There are alternatives to using traditional mechanical seals. These are termed sealless
pumps and one such construction is the canned motor pump. This technology combines
the centrifugal pump with an electric motor in one compact unit. The pump impeller is
directly connected to the motor rotor and drive is transmitted (see Figure 4).
Figure 4: Working principle of a canned motor pump (image courtesy of Hermetic
Pumpen GmbH)
A leak free seal is provided by the stator lining with secondary containment then
provided by the motor housing. These pumps are not hampered in the same way as
mechanical seal pumps. Cooling fluid for the motor is provided by an external heat
exchanger which results in no problems being experienced with high temperatures.
There are also no mechanical seals or roller bearings which are considered the two
highest failure components in centrifugal pumps.
Zero emissions and full secondary containment mean the plant is safer for personnel as
they are protected from 400°C+ fluids and the environment is kept cleaner. The
reliability of these pumps results in plant availability being kept at a maximum.
CANNED MOTOR PUMP USE IN CONCENTRATED SOLAR POWER
The most advanced solar concentrating technology is the use of parabolic troughs. It is
on these plants that canned motor pumps were initially trialled. Two examples of plants
are, an all-solar thermal power station with a capacity of up to 50 MW (CSP) in Spain
(schematic shown in Figure 5) and as a hybrid solar thermal power station with ISCCS
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
(Integrated Solar Combined Cycle System) in Egypt, with a capacity of up to 150 MW.
Hybrid solar thermal power plants generating some of their electricity using natural gas
or coal have been proposed in Australia.
Figure 5: Schematic of a parabolic trough solar plant (Image courtesy of Solar
Millennium AG)
Parabolic trough solar thermal power stations use a pipe (receiver) with a HTF in the
focal line of the parabolic trough. One axis of the receiver tracks the sun's axis in such a
way that the sunlight is always concentrated onto the heat transfer pipe. A photo of a
parabolic trough collector showing the receiver pipe is shown in Figure 6.
Figure 6: Parabolic trough collector (image courtesy of Wikimedia.org)
The concentrated solar radiation heats the HTF, in some cases synthetic oil, to
approximately 400°C. Individual troughs are connected to one another using
distribution lines. To pump the oil at this temperature requires a specialised pumping
solution. The now proven technology for this application is the use of Hermetic CNPK
pumps.
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
Hermetic CNPK canned motor pumps (as shown in Figure 7) are used to circulate the
400°C synthetic thermal oil. In these plants they can be used with either tube heat
exchangers (this arrangement is used on trough plants in Spain) and also using air
cooled arrangements (this is used in Egypt). All pumps are operated using a frequency
converter to facilitate the handling of the varying flow rates during the course of the day
while also providing maximum efficiency.
Figure 7: Canned motor pump using external heat exchanger (Image courtesy of
Hermetic Pumpen)
The canned motor pump success story is not only limited to oil pumping. Canned motor
pumps have been used successfully on direct solar steam generation as well. Hermetic
pumps have been used on the Plataforma Solar de Almeria (PSA) test field in Spain
with great success.
On this plant, a Hermetic CAMKT 30/6 (PN 100) high pressure pump, operating at 100
bar system pressure and 400°C with an external heat exchanger, is used to circulate the
water. A photo of the pump is shown in Figure 8.
Figure 8: Hermetic CAMKT 30/6 in operation in Spain (Image courtesy of
Hermetic Pumpen)
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, Australia
M. Thompson
This high pressure pump is a multistage canned motor pump in a barrel design. Thanks
to the barrel construction the pump requires only one static gasket instead of 6 static
gaskets as would be required in a ring section type pump. A cross sectional diagram for
a barrel canned motor pump is shown in Figure 9.
Figure 9: Cross section of barrel design pump for high pressure (Image courtesy of
Hermetic Pumpen)
Asia’s first parabolic trough power station, using direct steam generation principle and
located in Kanchanaburi Province in Central Thailand, also uses a Hermetic Type
CAMKT 44/3 (PN 100) high pressure pump with an external heat exchanger. Based on
the experience gained in Almeria, this pump is also a barrel construction design.
Hermetic canned motor pumps are now also proposed for use on Dish Collector
Systems and Linear Fresnel Systems. Their reliable low maintenance operation, as well
as the ability to withstand high temperatures and pressures, makes them the best
industry practice for use in Concentrated Solar Power Generation.
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, AustraliaM. Thompson
MAGNETIC DRIVE PUMPS
The final option for sealing pumps is the use of a magnetic coupling; this uses
permanent magnets to transmit drive from the electric motor to the pump impeller (see
Figure 10).
Figure 10: Magnetic coupling working principle
With the drive transmitted by permanent magnets a leak free seal is then provided by
the containment shell. This results in high safety for plant personnel as with the canned
motor pump.
With the use of high temperature magnets these pumps can run with product
temperatures of up to 450°C without external cooling. This fits well for pumping HTF’s
in solar plants. Magnetic drive pumps are now being used on CSP plants in Spain.
CONCLUSION
One of the most important aspects of any CSP system is the pump used to circulate fluid
around the solar field. Without this the solar energy will not be able to be collected and
converted to electricity. Mechanical seal pumps have inherent limitations when
pumping high temperature fluids. As a result the CSP industry is moving more towards
sealless pumps because they provide the highest reliability for the plant and also the
added benefit of zero emissions. This results in higher plant safety and environmental
protection. Zero emissions power is best served by zero emissions pumps.
REFERENCES
ITT - Goulds Pumps, New York, viewed 27/5/2010
http://www.gouldspumps.com/pag_0012.html
Hermetic Pumpen GmbH 2009, Convincing worldwide: Hermetic pumps in the
chemical industry, Hermetic Pumpen GmbH, Gundelfingen, Germany
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, AustraliaM. Thompson
PumpWorks 610, 2009, PWH API610 Single Stage OH2, PumpWorks 610 a division of
Best Pumpworks, Houston, TX, USA
BRIEF BIOGRAPHY OF PRESENTER
Matthew Thompson is a Mechanical Engineer who graduated with first class honours
from Monash University in 2007 completing a final year thesis project on “Pumping
Environmentally Sensitive Liquids: a study of sealless pumping technology”. This study
also examined improvements to the efficiency of magnetic drive pumps using titanium
containment shells.
He has worked with BP before moving to the Renroc Group to work in sealless
pumping technology. He has completed training with Hermetic Pumpen, Klaus Union,
Sero and Dickow Pumpen, all sealless pumping companies, and Nova Magnetics
Burgmann, a magnetic coupling manufacturer, and is considered an expert in sealless
pumps and sealless technology in Australia.
Currently he advises companies on the best pumping technology for their requirements
and speaks with companies about the benefits of sealless pumps.
Solar2010, the 48th AuSES Annual Conference
1-3 December 2010, Canberra, ACT, AustraliaYou can also read
