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Ceramics and diaphragm pumps – a good match?
feature non-metallic pumps
Ceramics and diaphragm pumps – a good match? Diaphragm pumps work in some of the toughest process conditions in manufacturing, and they have to be up to the task. The gases and fluids that they move are often dangerous and a leak or a spill can cause major damage to the environment or plant personnel. Richard Aerts and Manfred Gut of KNF examine how using ceramics can make diaphragm pumps safer and more environmentally sound.
T
o achieve the highest possible safety in service, pumps employed in the chemical industry must be gas-tight, chemically resistant and maintenance-free. To avoid unwelcome damage due to chemical reactions and to maintain the purity of the gases, contamination by the pumping process must be prevented. Clean vacuum is indispensable for many applications. For these extreme conditions, a special type of positive displacement pump, the diaphragm pump, has become an important asset for many users. The principles of design of the diaphragm pump make it gas-tight and absolutely oil free. The use of PTFE and ceramics ensure excellent chemical resistance.
The first step on the path to diaphragm pumps with almost universal chemical resistance was taken several years ago. Full-PTFE (FT) pumps were designed for small and medium flow-rates and represented a significant innovation. FT offers the best possible chemical resistance. The idea of making all the gas-contact parts of a diaphragm pump from PTFE was a challenge to the design and development engineers. The advantages of this material were common knowledge, but its disadvantages were just as well known. It is a property of PTFE that it deforms or creeps under constant tensile or compressive loads. In manufacture, it is a disadvantage
that PTFE cannot be injection moulded because it has practically no melting point. A primary goal was to devise a diaphragm that fulfilled the following criteria: chemical resistance, elastic de-formability, and low permeability to gases. These conditions can only be met by a PTFE/elastomer combination. The first attempt employed the customary flat diaphragm abut coated with a layer of PTFE. The diaphragm retainer plate was made of steel and coated with a fluorinated polymer to provide the necessary chemical resistance. This design, however, has intrinsic problems. The PTFE layer experiences severe strain because it is rigidly restrained by clamping with the retainer plate. This problem can be relieved by reducing the stroke, but this in turn means that the pump must be larger for a given flow rate. To prevent damage to the PTFE layer, the diaphragm retainer plate must be carefully radiused and the radii must be very well blended into their neighbouring surfaces. This step makes the final product relatively expensive to produce. Even with the best possible design, this concept involves a certain dead volume that has an unfavourable effect on the attainable ultimate vacuum.
Figure 1. Cutaway view of a KNF double diaphragm safety pump clearly showing the top working diaphragm, and the lower safety diaphragm. The volume between the diaphragms is usually monitored for pressure changes.
34
0262 1762/07 © 2007 Elsevier Ltd. All rights reserved
High gas-tightness demands that there is a minimum number of gas seals between the compression space and the world outside. Leaks, when they occur, do so generally at such points,
WORLD PUMPS December 2007
feature non-metallic pumps and so a diaphragm with a central hole was not considered an optimum solution. When operating under extreme conditions, a diaphragm may deteriorate or become damaged. When damage occurs, the diaphragm must be changed and ease of servicing becomes important to reduce downtime. There was no satisfactory solution to this problem with the retainer plate design. If the retainer plate was plastic-coated, the holes in its upper surface could not be used to tighten or loosen it because doing so would immediately result in damage to the plastic coating by the pegs of the spanner employed and loss of protection against chemical attack. To overcome this problem it is necessary to devise some means of clamping that can be tightened from the crankcase side of the diaphragm. This is possible, but the design is decidedly cumbersome. When engineers need to move very toxic, very reactive, or otherwise harmful media, or if they are dealing with a very valuable gas, pump requirements become more stringent. First, gas tightness must be further
About KNF KNF Neuberger Inc. (USA) is part of a multi-national corporation established in 1946, shortly after World War II. The parent company, KNF Neuberger GmbH, is located in Germany’s scenic Black Forest region. The worldwide group of wholly owned operating companies includes Belgium, China, France, Germany, Italy, Japan, Korea, Netherlands, Scandinavia, Sweden, Switzerland, Taiwan, UK and the USA. KNF maintains research and development centres dedicated to their respective disciplines; air and gas pump research in Germany, and liquid pump research in Switzerland. In addition to diaphragm pumps with the double diaphragm system, its product line also contains miniature pumps for instruments, pumps for air or neutral gases, temperature-resistant and heated diaphragm pumps, and diaphragm pumps for potentially explosive areas.
WORLD PUMPS December 2007
improved to ensure no leakage to or contamination from the atmosphere. Second, the medium must not escape from the pump in the event of rare cases of operational diaphragm failure.
Double diaphragm safety system The patented double diaphragm system was developed for special cases where the chance of leakage due to diaphragm failure is unacceptable. As with a traditional diaphragm pump, a pressure-tight elastic diaphragm in the pump head (known as the working diaphragm) is moved up and down at its centre by an eccentric and a connecting rod, thereby providing the pumping action. Below the working diaphragm is a second diaphragm, known as the safety diaphragm. Together with additional sealing rings, this arrangement provides improved gas-tightness, with leak rates as low as < 6 x 10-6 mbar l/s. In rare cases, the working diaphragm may become damaged, but the pumped medium will still not be able to escape. Instead, it will be captured in the intermediate space between the two diaphragms. Because the safety diaphragm is subject to only low mechanical and thermal loads during pump operation, while the working diaphragm is elastically distorted and warmed by the compression process, the safety diaphragm will remain intact if the working diaphragm fails. A rupture of the working diaphragm is easily detected through a sudden and dramatic drop in pumping or compression capacity of the pump.
Figure 2. This pump is equipped with aluminium heads coated with Kynar® PVDF and a PTFE coated diaphragm for resistance to corrosive gases encountered in industrial processes.
Figure 3. Pictured is a custom process pump equipped with ceramic heads and a PTFE-coated diaphragm for resistance to aggressive gases.
Moulded diaphragms In the new range of FT pumps, the flat diaphragm has been replaced by the moulded diaphragm. The moulded diaphragm consists of a neoprene body, which is vulcanized under precisely defined pressure and temperature conditions simultaneously bonded to a chemically treated PTFE film and the steel carrier component, to ensure permanent and reliable bonds.
Figure 4. This 40 lpm gas-tight diaphragm pump is designed for delivering phosphoric acid vapours at 150°C. The heads are made of gold-coated stainless steel and the diaphragm is of Viton®.
www.worldpumps.com 35
feature non-metallic pumps This compact diaphragm element can, after removal of the head, be removed and refitted by simply screwing it into or out of the connecting rod. The curved form of the upper surface of the diaphragm in designed to conform to the shape of the head with the least possible dead volume to provide excellent ultimate vacuum. Because it has no central perforation, this diaphragm provides a hermetic seal between the compression space and the crankcase. The head, the valve discs and the valve bodies of the FT range are made of PTFE. As already mentioned, the major disadvantage of PTFE is its tendency to creep. Over time, the PTFE molecules reposition themselves to relieve internal stresses so that clamping forces between two components reduce with time. The design compensates for this effect by clamping the PTFE in a
‘sandwich’ between a metal plate and the crankcase, and by employing disc springs to maintain clamping force when the PTFE relaxes. The diaphragm pumps of the FT range have flow rates between 10 and 60 lpm, and are used mainly in the laboratory where, because of their versatility, they have found wide acceptance. For chemical plant and preproduction trials, these flow rates are often not sufficient. For removal and circulation of aggressive gases, volume flows in the region of 100 to 250 lpm are required. To reduce the flow losses in the connecting pipe work and to keep the number of connections (and hence potential leaks) as small as possible, diaphragm pumps with high flow rates should not have more than two heads.
Introducing ceramic materials
Figure 5. This standard KNF pump uses solid PTFE heads and a PTFE coated diaphragm. The motor is rated for use in groups C & D environments.
Figure 6. This KNF diaphragm uses a threaded insert that is moulded within the structure, eliminating the clamping disk.
36
www.worldpumps.com
The successful path, which employs PTFE as a practically universal chemically resistant material for the head parts of diaphragm pumps, has been extended by the use of ceramics. The excellent chemical resistance, allied to hardness, strength and wear resistance, makes this material particularly attractive for larger diaphragm heads. Due to the limitations that the diaphragm imposes on the stroke, diaphragm compressors and vacuum pumps require a large effective diaphragm diameter, which means that the diaphragm head must fulfil particularly exacting conditions with regard to strength and stability. Tight tolerances characterize this component, which has an important influence on the ultimate vacuum pump. Only tight tolerances can ensure a consistently small dead volume and thus consistent performance in series production. For this reason, PTFE cannot be used for larger diaphragm heads. Up to now ceramic’s brittleness, together with the difficulty of manufacture and machining, have discouraged designers from using
ceramic parts. In recent years, there has been much progress in the technology of ceramic manufacture. Aluminum oxide ceramic manufacture and aluminum oxide ceramic has become particularly significant. By optimizing the time and temperature of sintering as well as the purity and particle size of the raw materials, ceramic quality has been improved and costs reduced. Precision parts, and the diaphragm head is one of them, must be machined with diamond tools after sintering. Because ceramics have great compressive strength but are very sensitive to tensile loads, the designer must take care that the component is practically only subjected to compressive loads. The moulded diaphragm used with the FT head is also used for the ceramic head pump. Even in this much larger version, it can be made to conform to the head shape. The valve discs and valve bodies are again made of PTFE. Ceramic combined with PTFE has made possible the development of a pump with a high flow-rate and first-class resistance to chemicals. This new development of single and twin-headed pumps with ceramic diaphragm heads has extended the performance range of chemical resistant pumps by a factor of three, to 130 lpm for single or 230 lpm for twin heads respectively, and has thus opened these products to applications that were closed to the FT range. ■ CONTACT Manfred Gut Gas Pump Product Manager KNF Neuberger GmbH (World Headquarters) Alter Weg 3 D-79112 Freiburg-Munzingen Germany Tel: +49 (07664) 59090 www.knf.de Richard Aerts Process Products Manager KNF Neuberger, Inc. (USA, Canada, Mexico) 2 Black Forest Road Trenton, NJ 08691 USA Tel: +1 609 890 8600 www.knf.com/usa.htm
WORLD PUMPS December 2007
Ceramics and diaphragm pumps – a good match? Diaphragm pumps work in some of the toughest process conditions in manufacturing, and they have to be up to the task. The gases and fluids that they move are often dangerous and a leak or a spill can cause major damage to the environment or plant personnel. Richard Aerts and Manfred Gut of KNF examine how using ceramics can make diaphragm pumps safer and more environmentally sound.
T
o achieve the highest possible safety in service, pumps employed in the chemical industry must be gas-tight, chemically resistant and maintenance-free. To avoid unwelcome damage due to chemical reactions and to maintain the purity of the gases, contamination by the pumping process must be prevented. Clean vacuum is indispensable for many applications. For these extreme conditions, a special type of positive displacement pump, the diaphragm pump, has become an important asset for many users. The principles of design of the diaphragm pump make it gas-tight and absolutely oil free. The use of PTFE and ceramics ensure excellent chemical resistance.
The first step on the path to diaphragm pumps with almost universal chemical resistance was taken several years ago. Full-PTFE (FT) pumps were designed for small and medium flow-rates and represented a significant innovation. FT offers the best possible chemical resistance. The idea of making all the gas-contact parts of a diaphragm pump from PTFE was a challenge to the design and development engineers. The advantages of this material were common knowledge, but its disadvantages were just as well known. It is a property of PTFE that it deforms or creeps under constant tensile or compressive loads. In manufacture, it is a disadvantage
that PTFE cannot be injection moulded because it has practically no melting point. A primary goal was to devise a diaphragm that fulfilled the following criteria: chemical resistance, elastic de-formability, and low permeability to gases. These conditions can only be met by a PTFE/elastomer combination. The first attempt employed the customary flat diaphragm abut coated with a layer of PTFE. The diaphragm retainer plate was made of steel and coated with a fluorinated polymer to provide the necessary chemical resistance. This design, however, has intrinsic problems. The PTFE layer experiences severe strain because it is rigidly restrained by clamping with the retainer plate. This problem can be relieved by reducing the stroke, but this in turn means that the pump must be larger for a given flow rate. To prevent damage to the PTFE layer, the diaphragm retainer plate must be carefully radiused and the radii must be very well blended into their neighbouring surfaces. This step makes the final product relatively expensive to produce. Even with the best possible design, this concept involves a certain dead volume that has an unfavourable effect on the attainable ultimate vacuum.
Figure 1. Cutaway view of a KNF double diaphragm safety pump clearly showing the top working diaphragm, and the lower safety diaphragm. The volume between the diaphragms is usually monitored for pressure changes.
34
0262 1762/07 © 2007 Elsevier Ltd. All rights reserved
High gas-tightness demands that there is a minimum number of gas seals between the compression space and the world outside. Leaks, when they occur, do so generally at such points,
WORLD PUMPS December 2007
feature non-metallic pumps and so a diaphragm with a central hole was not considered an optimum solution. When operating under extreme conditions, a diaphragm may deteriorate or become damaged. When damage occurs, the diaphragm must be changed and ease of servicing becomes important to reduce downtime. There was no satisfactory solution to this problem with the retainer plate design. If the retainer plate was plastic-coated, the holes in its upper surface could not be used to tighten or loosen it because doing so would immediately result in damage to the plastic coating by the pegs of the spanner employed and loss of protection against chemical attack. To overcome this problem it is necessary to devise some means of clamping that can be tightened from the crankcase side of the diaphragm. This is possible, but the design is decidedly cumbersome. When engineers need to move very toxic, very reactive, or otherwise harmful media, or if they are dealing with a very valuable gas, pump requirements become more stringent. First, gas tightness must be further
About KNF KNF Neuberger Inc. (USA) is part of a multi-national corporation established in 1946, shortly after World War II. The parent company, KNF Neuberger GmbH, is located in Germany’s scenic Black Forest region. The worldwide group of wholly owned operating companies includes Belgium, China, France, Germany, Italy, Japan, Korea, Netherlands, Scandinavia, Sweden, Switzerland, Taiwan, UK and the USA. KNF maintains research and development centres dedicated to their respective disciplines; air and gas pump research in Germany, and liquid pump research in Switzerland. In addition to diaphragm pumps with the double diaphragm system, its product line also contains miniature pumps for instruments, pumps for air or neutral gases, temperature-resistant and heated diaphragm pumps, and diaphragm pumps for potentially explosive areas.
WORLD PUMPS December 2007
improved to ensure no leakage to or contamination from the atmosphere. Second, the medium must not escape from the pump in the event of rare cases of operational diaphragm failure.
Double diaphragm safety system The patented double diaphragm system was developed for special cases where the chance of leakage due to diaphragm failure is unacceptable. As with a traditional diaphragm pump, a pressure-tight elastic diaphragm in the pump head (known as the working diaphragm) is moved up and down at its centre by an eccentric and a connecting rod, thereby providing the pumping action. Below the working diaphragm is a second diaphragm, known as the safety diaphragm. Together with additional sealing rings, this arrangement provides improved gas-tightness, with leak rates as low as < 6 x 10-6 mbar l/s. In rare cases, the working diaphragm may become damaged, but the pumped medium will still not be able to escape. Instead, it will be captured in the intermediate space between the two diaphragms. Because the safety diaphragm is subject to only low mechanical and thermal loads during pump operation, while the working diaphragm is elastically distorted and warmed by the compression process, the safety diaphragm will remain intact if the working diaphragm fails. A rupture of the working diaphragm is easily detected through a sudden and dramatic drop in pumping or compression capacity of the pump.
Figure 2. This pump is equipped with aluminium heads coated with Kynar® PVDF and a PTFE coated diaphragm for resistance to corrosive gases encountered in industrial processes.
Figure 3. Pictured is a custom process pump equipped with ceramic heads and a PTFE-coated diaphragm for resistance to aggressive gases.
Moulded diaphragms In the new range of FT pumps, the flat diaphragm has been replaced by the moulded diaphragm. The moulded diaphragm consists of a neoprene body, which is vulcanized under precisely defined pressure and temperature conditions simultaneously bonded to a chemically treated PTFE film and the steel carrier component, to ensure permanent and reliable bonds.
Figure 4. This 40 lpm gas-tight diaphragm pump is designed for delivering phosphoric acid vapours at 150°C. The heads are made of gold-coated stainless steel and the diaphragm is of Viton®.
www.worldpumps.com 35
feature non-metallic pumps This compact diaphragm element can, after removal of the head, be removed and refitted by simply screwing it into or out of the connecting rod. The curved form of the upper surface of the diaphragm in designed to conform to the shape of the head with the least possible dead volume to provide excellent ultimate vacuum. Because it has no central perforation, this diaphragm provides a hermetic seal between the compression space and the crankcase. The head, the valve discs and the valve bodies of the FT range are made of PTFE. As already mentioned, the major disadvantage of PTFE is its tendency to creep. Over time, the PTFE molecules reposition themselves to relieve internal stresses so that clamping forces between two components reduce with time. The design compensates for this effect by clamping the PTFE in a
‘sandwich’ between a metal plate and the crankcase, and by employing disc springs to maintain clamping force when the PTFE relaxes. The diaphragm pumps of the FT range have flow rates between 10 and 60 lpm, and are used mainly in the laboratory where, because of their versatility, they have found wide acceptance. For chemical plant and preproduction trials, these flow rates are often not sufficient. For removal and circulation of aggressive gases, volume flows in the region of 100 to 250 lpm are required. To reduce the flow losses in the connecting pipe work and to keep the number of connections (and hence potential leaks) as small as possible, diaphragm pumps with high flow rates should not have more than two heads.
Introducing ceramic materials
Figure 5. This standard KNF pump uses solid PTFE heads and a PTFE coated diaphragm. The motor is rated for use in groups C & D environments.
Figure 6. This KNF diaphragm uses a threaded insert that is moulded within the structure, eliminating the clamping disk.
36
www.worldpumps.com
The successful path, which employs PTFE as a practically universal chemically resistant material for the head parts of diaphragm pumps, has been extended by the use of ceramics. The excellent chemical resistance, allied to hardness, strength and wear resistance, makes this material particularly attractive for larger diaphragm heads. Due to the limitations that the diaphragm imposes on the stroke, diaphragm compressors and vacuum pumps require a large effective diaphragm diameter, which means that the diaphragm head must fulfil particularly exacting conditions with regard to strength and stability. Tight tolerances characterize this component, which has an important influence on the ultimate vacuum pump. Only tight tolerances can ensure a consistently small dead volume and thus consistent performance in series production. For this reason, PTFE cannot be used for larger diaphragm heads. Up to now ceramic’s brittleness, together with the difficulty of manufacture and machining, have discouraged designers from using
ceramic parts. In recent years, there has been much progress in the technology of ceramic manufacture. Aluminum oxide ceramic manufacture and aluminum oxide ceramic has become particularly significant. By optimizing the time and temperature of sintering as well as the purity and particle size of the raw materials, ceramic quality has been improved and costs reduced. Precision parts, and the diaphragm head is one of them, must be machined with diamond tools after sintering. Because ceramics have great compressive strength but are very sensitive to tensile loads, the designer must take care that the component is practically only subjected to compressive loads. The moulded diaphragm used with the FT head is also used for the ceramic head pump. Even in this much larger version, it can be made to conform to the head shape. The valve discs and valve bodies are again made of PTFE. Ceramic combined with PTFE has made possible the development of a pump with a high flow-rate and first-class resistance to chemicals. This new development of single and twin-headed pumps with ceramic diaphragm heads has extended the performance range of chemical resistant pumps by a factor of three, to 130 lpm for single or 230 lpm for twin heads respectively, and has thus opened these products to applications that were closed to the FT range. ■ CONTACT Manfred Gut Gas Pump Product Manager KNF Neuberger GmbH (World Headquarters) Alter Weg 3 D-79112 Freiburg-Munzingen Germany Tel: +49 (07664) 59090 www.knf.de Richard Aerts Process Products Manager KNF Neuberger, Inc. (USA, Canada, Mexico) 2 Black Forest Road Trenton, NJ 08691 USA Tel: +1 609 890 8600 www.knf.com/usa.htm
WORLD PUMPS December 2007