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Rubber cuts sewage pumping cost
28
Feature WORLD PUMPS
October 2009
Water & wastewater
Rubber cuts sewage pumping cost In a 2006 experiment, involving pumping sand-laden wastewater at a German sewage plant, a rubberized pump design clearly won an endurance test against pumps made from conventional hard chromium-alloy cast iron, and also against those using ceramic coatings. Dipl.-Ing. Wolfram Kuhn reports.
I
n the Entsorgungsverband Saar (EVS) wastewater/sewage treatment plant in Saarbrücken-Brebach, Germany, an above-average amount of sand and earth has resulted in high wear in the grit channel pumps. These, which used a chromium-alloy cast-iron design, were so worn out after just one year of service that they had to be completely replaced. Long term, this was going to be prohibitively expensive, so two alternative pump systems were proposed and tested - firstly, a pump designed with a hard ceramic coating and secondly, a special pump fitted with a soft inner rubber lining. In a parallel endurance test under realistic conditions, both pumps ran seven months for six to eight hours/ day continuously pumping the same extremely abrasive medium as found in the grit channel of the wastewater/ sewage plant. During this period, the ceramic coating showed definite signs of wear and was completely worn out in places. The rubberized coating, however, was practically untouched and looked almost like new. Neither did the rubber dissolve or separate from the pump's metal core. Not only that, the rubberized pump was also much cheaper than the chromium-alloy pumps used until now.
The test location The EVS, with its 129 sewage treatment plants, is one of the largest disposal www.worldpumps.com
Figure 1: The sewage treatment plant in Saarbrücken-Brebach.
0262 1762/09 © 2009 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature October 2009
and sewage associations in Germany. In the Association's second-largest sewage treatment plant, at Saarbrücken-Brebach, around 25,000 m3 of mixed sewage is cleaned daily. This sewage is transported to the treatment plant (see Figure 1) through the 55 km long main collecting canal. The population of approximately 135,000 people connected to this canal network is located almost exclusively in an extremely erosive area of variegated sandstone. Because of this, the sewage treatment plant receives large amounts of sand - sometimes intermittently. Up to 7 m3/ week is quite usual, though this can sometimes rise to as much as 14 m3 within just three days. This sand is, to a great extent, separated in the plant's two-line grit channel. For the pumps in question this is a particular challenge, especially as the necessary pump capacity results a high flow speed. Because of the large amounts of sand, the pumps have to withstand a highly abrasive sand-water mixture. This literally sandblasts the hydraulic parts. Even so, the pumps - one in each line of the grit channel - have to be reliable.
The original pumping concept Typically, in such abrasive applications, pumps are chosen from materials harder than grey cast iron, mostly from special chromium-alloy cast iron. This was indeed the first choice in the Saarbrücken-Brebach application. The sewage treatment plant was commissioned in March 2001 after being completely rebuilt. In the planning and initial fitting stages, choices were made in favour of proven technology. For the two grit channel lines, submersible pumps were chosen with hydraulic parts that were manufactured using special chromium-alloy cast iron. These should have withstood the constant stress. However, after only a year the pumps were already so worn that they were no longer worth repairing, and the only option, as stated, was complete replacement. For the next few years, this was carried out approximately annually, but in 2005, the EVS decided that the frequent replacement of these extremely expensive pumps would - long term - be too costly. Alternatives were sought.
Researched pump alternatives The market for sewage pumps was researched by a team of specialists. Two possible solutions seemed to promise success. The first was a pump with a very
Figure 2: The pump casing of the chromium-alloy cast-iron pump is already ripped through.
hard ceramic coating. The second was a pump having a special, soft rubber lining – this being a relatively new concept, but one that had already proven itself not only in some sand- and gravel-pits, but also in sewage treatment plants. However, after nearly two years on the market, this technology was still not generally well known in Germany.
'After only a year, the pumps were so worn out that they were no longer worth reparing' When the time came for the next replacement of the existing chromium-alloy cast-iron pump to be carried out, it was decided to carry out a parallel test of both new types under fully realistic conditions. The Brebach sewage treatment plant proved to be ideal, as it features two identical lines in the grit channel, both of them transporting the same amount of sand in the water. In terms of the experimental methodology, note that, while the two different pumps to be tested in parallel were built at the same time, the suppliers knew nothing of the intended comparison. Also, both
pumps were designed and built in such a way as to carry the same amount of material as the original chromium-alloy castiron pump. In addition, it was required that both pumps always had to run simultaneously, so that they had identical operating times. The rubberized pump was installed in Lane One of the grit channel and in Lane Two the ceramic-coated pump was installed. For seven months, from September 2005 until March 2006, both pumps worked in continuous operation and in parallel, averaging six to eight hours running/day. As the test time included the winter months, the sand load was often very high. During the test period, the pumps were left undisturbed. After the test was completed, both pumps were removed and opened up by the sewage treatment plant’s works manager. Even by this stage, neither of the pump suppliers had been informed of the comparison test.
Figure 3: The impeller of chromium-alloy cast-iron is also worn out, if still functional.
www.worldpumps.com
29
30
Feature WORLD PUMPS
October 2009
and the casing channel showed an almost complete absence of wear. Only on two tips of the impeller's blades was the rubber lining damaged, probably by a large jagged stone that attempted to find its way through the pump. However, even here the rubber lining completely retained its resistant qualities. Although the edges of the impacts were ripped, the rips had not spread - the rubber had neither decomposed nor separated from the substrate (see Figure 6).
Physical explanation The sand particles that pass through the pump, and the accompanying small stones, have hard surfaces. When a hard object meets another hard surface (in this case chromium-alloy cast-iron or ceramic coating) both surfaces are affected.
Figure 4: The pump's ceramic coating was also extremely worn – as can clearly be seen here.
The results It was required that both pumps prove themselves against the originally installed chromium-alloy cast-iron pumps. For comparison, see Figures 2 and 3, which show photographs of the condition of a pump of this kind after it had completed about a year’s running. As can be seen, the casing has already been ripped through in more than one place. The impeller vanes were also clearly worn out, if still functional.
The condition of the ceramic-coated pump is illustrated in Figure 4. This shows that after the seven months running time, the ceramic coating was extremely worn. In several places the coating was completely gone, so that the sand was directly attacking the lining material of the grey cast iron. The pump was still functioning, but the coating was so worn that it would not have functioned very much longer. On the rubberized pump (Figure 5), the surfaces of the pump inlet, the impeller,
The bigger the difference between the hardness of the two object’s surfaces, the faster the weaker part wears out. In this way, grey cast-iron pumps wear out much faster than pumps made from harder materials, such as chromium-alloy castiron, or pumps having a hard surface, such as ceramic-coatings. However, even these pumps wear out eventually. There is a very different reaction when a hard object meets a soft and flexible surface. Here, the soft surface (in this case rubber) is partially compressed. The energy that is temporarily absorbed by the rubber is returned to the object such that the object is hurled away. In this way, damage to the surface of the rubber is largely avoided. Only large stones, which have too much kinetic energy, cannot be hurled away and can, therefore, damage the rubber. A further effect is that the hard objects hurled away (sand or small stones) do not slip or slide along the soft surface. Because of this, wear is again largely avoided.
The limits of rubber linings
Figure 5: The rubberised pump is still in good condition.
www.worldpumps.com
As previously mentioned, if any stones passing through the rubberised pump were too big, they might not be hurled away and can cause damage to the rubber. The same effect can be caused by sharp-edged objects. In such cases, it is important that rips in the rubber do not spread. Moreover, it is essential that if any damage goes as far as the ground material (substrate), the rubber around the damaged area remains attached to the substrate. If this is not so, the pump will be destroyed quickly.
WORLD PUMPS
Feature October 2009
Currently, this same rubberized pump is running once again in test against another pump having a different hard cast iron mould. At the time of World Pumps going to press, the tests results were not yet in.
However, the experience shows, that in most applications, sand or grit causes the abrasive wear, and then the much longer lifetime and the significantly lower initial costs provide clear advantages for rubberized pumps. ■
Conclusion
Figure 6: The rubber on the rubberised impeller had two areas of damage, but these neither spread, nor had the rubber decomposed or separated from the substrate.
What happened after test completion? Upon completion of the experiment, the rubberized pump’s impeller was replaced in June 2007 with an impeller having an enhanced rubber lining. The enhancement was made to avoid those small damages that occurred during the test period. In February 2008, this pump was judged to have proved highly successful. Following eight month’s running time, the rubber lining appeared to be as good as new.
In a seven-month field experiment, it was proven that - in the given case described in this article - the rubberized pump in the grit channel had a much longer serviceable life than a pump made from chromium-alloy cast-iron. It also had a much longer life than a ceramic-coated pump. This greatly reduces the running costs, firstly through a markedly increased life span, and secondly because of much lower initial costs. There are currently no statistics about how much longer the working lives of rubberized pumps are compared with pumps made from hard materials. The experience of the pump manufacturer concerned is that the actual life depends mainly on the type of objects passed through the pump with the water. Large stones or sharp edged objects may damage the rubber, so these should be pumped with pumps made from hard materials.
Contact Dipl.-Ing. Wolfram Kuhn President - Herborner Pumpenfabrik J. H. Hoffmann GmbH & Co. KG 35745 Herborn, Germany Tel: +49 (0) 2772 / 933 102 Email: [email protected] www.herborner-pumpen.de
International Rotating Equipment conference This paper was first presented at the Pump Users International Forum 2008 held in Düsseldorf, Germany in October 2008, and is reproduced with permission from VDMA eV.
www.worldpumps.com
31
Feature WORLD PUMPS
October 2009
Water & wastewater
Rubber cuts sewage pumping cost In a 2006 experiment, involving pumping sand-laden wastewater at a German sewage plant, a rubberized pump design clearly won an endurance test against pumps made from conventional hard chromium-alloy cast iron, and also against those using ceramic coatings. Dipl.-Ing. Wolfram Kuhn reports.
I
n the Entsorgungsverband Saar (EVS) wastewater/sewage treatment plant in Saarbrücken-Brebach, Germany, an above-average amount of sand and earth has resulted in high wear in the grit channel pumps. These, which used a chromium-alloy cast-iron design, were so worn out after just one year of service that they had to be completely replaced. Long term, this was going to be prohibitively expensive, so two alternative pump systems were proposed and tested - firstly, a pump designed with a hard ceramic coating and secondly, a special pump fitted with a soft inner rubber lining. In a parallel endurance test under realistic conditions, both pumps ran seven months for six to eight hours/ day continuously pumping the same extremely abrasive medium as found in the grit channel of the wastewater/ sewage plant. During this period, the ceramic coating showed definite signs of wear and was completely worn out in places. The rubberized coating, however, was practically untouched and looked almost like new. Neither did the rubber dissolve or separate from the pump's metal core. Not only that, the rubberized pump was also much cheaper than the chromium-alloy pumps used until now.
The test location The EVS, with its 129 sewage treatment plants, is one of the largest disposal www.worldpumps.com
Figure 1: The sewage treatment plant in Saarbrücken-Brebach.
0262 1762/09 © 2009 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature October 2009
and sewage associations in Germany. In the Association's second-largest sewage treatment plant, at Saarbrücken-Brebach, around 25,000 m3 of mixed sewage is cleaned daily. This sewage is transported to the treatment plant (see Figure 1) through the 55 km long main collecting canal. The population of approximately 135,000 people connected to this canal network is located almost exclusively in an extremely erosive area of variegated sandstone. Because of this, the sewage treatment plant receives large amounts of sand - sometimes intermittently. Up to 7 m3/ week is quite usual, though this can sometimes rise to as much as 14 m3 within just three days. This sand is, to a great extent, separated in the plant's two-line grit channel. For the pumps in question this is a particular challenge, especially as the necessary pump capacity results a high flow speed. Because of the large amounts of sand, the pumps have to withstand a highly abrasive sand-water mixture. This literally sandblasts the hydraulic parts. Even so, the pumps - one in each line of the grit channel - have to be reliable.
The original pumping concept Typically, in such abrasive applications, pumps are chosen from materials harder than grey cast iron, mostly from special chromium-alloy cast iron. This was indeed the first choice in the Saarbrücken-Brebach application. The sewage treatment plant was commissioned in March 2001 after being completely rebuilt. In the planning and initial fitting stages, choices were made in favour of proven technology. For the two grit channel lines, submersible pumps were chosen with hydraulic parts that were manufactured using special chromium-alloy cast iron. These should have withstood the constant stress. However, after only a year the pumps were already so worn that they were no longer worth repairing, and the only option, as stated, was complete replacement. For the next few years, this was carried out approximately annually, but in 2005, the EVS decided that the frequent replacement of these extremely expensive pumps would - long term - be too costly. Alternatives were sought.
Researched pump alternatives The market for sewage pumps was researched by a team of specialists. Two possible solutions seemed to promise success. The first was a pump with a very
Figure 2: The pump casing of the chromium-alloy cast-iron pump is already ripped through.
hard ceramic coating. The second was a pump having a special, soft rubber lining – this being a relatively new concept, but one that had already proven itself not only in some sand- and gravel-pits, but also in sewage treatment plants. However, after nearly two years on the market, this technology was still not generally well known in Germany.
'After only a year, the pumps were so worn out that they were no longer worth reparing' When the time came for the next replacement of the existing chromium-alloy cast-iron pump to be carried out, it was decided to carry out a parallel test of both new types under fully realistic conditions. The Brebach sewage treatment plant proved to be ideal, as it features two identical lines in the grit channel, both of them transporting the same amount of sand in the water. In terms of the experimental methodology, note that, while the two different pumps to be tested in parallel were built at the same time, the suppliers knew nothing of the intended comparison. Also, both
pumps were designed and built in such a way as to carry the same amount of material as the original chromium-alloy castiron pump. In addition, it was required that both pumps always had to run simultaneously, so that they had identical operating times. The rubberized pump was installed in Lane One of the grit channel and in Lane Two the ceramic-coated pump was installed. For seven months, from September 2005 until March 2006, both pumps worked in continuous operation and in parallel, averaging six to eight hours running/day. As the test time included the winter months, the sand load was often very high. During the test period, the pumps were left undisturbed. After the test was completed, both pumps were removed and opened up by the sewage treatment plant’s works manager. Even by this stage, neither of the pump suppliers had been informed of the comparison test.
Figure 3: The impeller of chromium-alloy cast-iron is also worn out, if still functional.
www.worldpumps.com
29
30
Feature WORLD PUMPS
October 2009
and the casing channel showed an almost complete absence of wear. Only on two tips of the impeller's blades was the rubber lining damaged, probably by a large jagged stone that attempted to find its way through the pump. However, even here the rubber lining completely retained its resistant qualities. Although the edges of the impacts were ripped, the rips had not spread - the rubber had neither decomposed nor separated from the substrate (see Figure 6).
Physical explanation The sand particles that pass through the pump, and the accompanying small stones, have hard surfaces. When a hard object meets another hard surface (in this case chromium-alloy cast-iron or ceramic coating) both surfaces are affected.
Figure 4: The pump's ceramic coating was also extremely worn – as can clearly be seen here.
The results It was required that both pumps prove themselves against the originally installed chromium-alloy cast-iron pumps. For comparison, see Figures 2 and 3, which show photographs of the condition of a pump of this kind after it had completed about a year’s running. As can be seen, the casing has already been ripped through in more than one place. The impeller vanes were also clearly worn out, if still functional.
The condition of the ceramic-coated pump is illustrated in Figure 4. This shows that after the seven months running time, the ceramic coating was extremely worn. In several places the coating was completely gone, so that the sand was directly attacking the lining material of the grey cast iron. The pump was still functioning, but the coating was so worn that it would not have functioned very much longer. On the rubberized pump (Figure 5), the surfaces of the pump inlet, the impeller,
The bigger the difference between the hardness of the two object’s surfaces, the faster the weaker part wears out. In this way, grey cast-iron pumps wear out much faster than pumps made from harder materials, such as chromium-alloy castiron, or pumps having a hard surface, such as ceramic-coatings. However, even these pumps wear out eventually. There is a very different reaction when a hard object meets a soft and flexible surface. Here, the soft surface (in this case rubber) is partially compressed. The energy that is temporarily absorbed by the rubber is returned to the object such that the object is hurled away. In this way, damage to the surface of the rubber is largely avoided. Only large stones, which have too much kinetic energy, cannot be hurled away and can, therefore, damage the rubber. A further effect is that the hard objects hurled away (sand or small stones) do not slip or slide along the soft surface. Because of this, wear is again largely avoided.
The limits of rubber linings
Figure 5: The rubberised pump is still in good condition.
www.worldpumps.com
As previously mentioned, if any stones passing through the rubberised pump were too big, they might not be hurled away and can cause damage to the rubber. The same effect can be caused by sharp-edged objects. In such cases, it is important that rips in the rubber do not spread. Moreover, it is essential that if any damage goes as far as the ground material (substrate), the rubber around the damaged area remains attached to the substrate. If this is not so, the pump will be destroyed quickly.
WORLD PUMPS
Feature October 2009
Currently, this same rubberized pump is running once again in test against another pump having a different hard cast iron mould. At the time of World Pumps going to press, the tests results were not yet in.
However, the experience shows, that in most applications, sand or grit causes the abrasive wear, and then the much longer lifetime and the significantly lower initial costs provide clear advantages for rubberized pumps. ■
Conclusion
Figure 6: The rubber on the rubberised impeller had two areas of damage, but these neither spread, nor had the rubber decomposed or separated from the substrate.
What happened after test completion? Upon completion of the experiment, the rubberized pump’s impeller was replaced in June 2007 with an impeller having an enhanced rubber lining. The enhancement was made to avoid those small damages that occurred during the test period. In February 2008, this pump was judged to have proved highly successful. Following eight month’s running time, the rubber lining appeared to be as good as new.
In a seven-month field experiment, it was proven that - in the given case described in this article - the rubberized pump in the grit channel had a much longer serviceable life than a pump made from chromium-alloy cast-iron. It also had a much longer life than a ceramic-coated pump. This greatly reduces the running costs, firstly through a markedly increased life span, and secondly because of much lower initial costs. There are currently no statistics about how much longer the working lives of rubberized pumps are compared with pumps made from hard materials. The experience of the pump manufacturer concerned is that the actual life depends mainly on the type of objects passed through the pump with the water. Large stones or sharp edged objects may damage the rubber, so these should be pumped with pumps made from hard materials.
Contact Dipl.-Ing. Wolfram Kuhn President - Herborner Pumpenfabrik J. H. Hoffmann GmbH & Co. KG 35745 Herborn, Germany Tel: +49 (0) 2772 / 933 102 Email: [email protected] www.herborner-pumpen.de
International Rotating Equipment conference This paper was first presented at the Pump Users International Forum 2008 held in Düsseldorf, Germany in October 2008, and is reproduced with permission from VDMA eV.
www.worldpumps.com
31