Pumping abrasive and corrosive media can be a real challenge if you’re not using the right kind of pump, argues Rens Zwart, Group Industrial Sectors Business Development Manager, at Watson-Marlow Fluid Technology Group (WMFTG).
Q. How is it best to specify a pump for abrasive slurry transfer?
A. Let the experts at pump manufacturers provide assistance. Abrasive slurries have many different parameters that can impact on pump selection. These include particle size and distribution (percentage of small particles in the slurry), particle shape, concentration of solids and density of solids. Ultimately, slurry types will behave differently depending on particle size and transport speed, so correct pump specification is important.
Q. What are real costs of using peristaltic pumps for abrasive media?
A. While the initial purchase cost can be slightly higher than other types of positive displacement (PD) pumps, a quick assessment of associated lifecycle costs quickly tips the balance in favour of peristaltic hose pumps. For instance, there are no expensive seals, ball valves, rotors or stators to wear and replace, while hose/tube replacement usually takes only a few minutes. Furthermore, the low cost hose or tube can be replaced in-situ without the need for special tools or skills, making the process extremely economical in comparison with conventional PD pumps where replacement parts can cost up to 75% of the pump’s initial purchase price and take several hours to fit.
Q. How long do hoses last?
A. All hose elements have a serviceable life before fatigue demands replacement; predominantly dependent on the pump speed, but not influenced by the abrasiveness of the fluid being pumped. Depending on the duty and the economics of using a larger and consequently higher flow pump and slowing it down its RPM, operating costs can be much reduced in terms of energy efficiency, hose life and pump maintenance costs.
High quality hose pump manufacturers produce hoses within tight tolerances. They also utilise adjustable shoes to set the perfect compression force for specific process conditions such as suction and delivery pressure and pipework.
Q . Are there any examples of cost savings?
A. Many companies achieve wide-scale savings when switching to peristaltic hose pump technology. The combined heat and power (CHP) plant of Dong Energy in Studstrup, Denmark, is a case in point when it recently sought to switch from coal to biomass. This unusual application involved pumping highly abrasive fly ash slurry and required an ‘engineered’ solution to help maximise hose life. A test facility was built to determine the viscosity of the ash slurry, as well as measure the pressure drop of the pipes at a given water percentage, so that pumps and pipe runs could be dimensioned appropriately.
At Dong Energy, fly ash is mixed with 35% water and fed to the dosing tanks (each with a recirculation line) in the pellet mills through a 650 metre long pipeline. Here, Bredel hose pumps are deployed to avoid settlement. In total, 21 such pumps are now in use at the plant, delivering an optimised solution that has resulted in huge financial savings: fly ash is traditionally handled by large double-acting hydraulic pumps costing circa £500,000 each.
Q. How much downtime can be saved in abrasive pumping/operations?
A. Another example. An APEX 35 hose pump from WMFTG was put to the test in a process- critical abrasive slurry mixing operation at freight carriage component manufacturer, Amsted Rail. The trial showed that in comparison with previously deployed AOD pumps, the APEX pump could extend the time between scheduled maintenance dramatically. While the AO pumps need to be maintained every two weeks, the APEX pump ran continuously for 10 weeks before maintenance was needed. Furthermore, when maintenance of the APEX pump is required, additional savings are apparent as it takes less than 50% of the time needed to reinstate the AOD pump to operational condition.
The same pump model also proved successful at EEW Saarbrücken GmbH, a producer of energy from waste. Here, not only did the APEX 35 pump run for a much longer period when pumping abrasive brine (salt slurry), but the time taken and cost of replacement parts was reduced greatly. EEW had been using a progressive cavity pump to feed the centrifuge for approximately 1.5 hours every 3-4 hours in a 24/7 operation. However, the abrasive nature of the brine demanded the repair of the PC pump stator or rotor every month, along with occasional replacement of the linings. Furthermore, not only would it take a minimum of four hours to perform the repairs (using expensive replacement consumables), but the pump would also have to be removed from the process line.
Q. How complicated is installation?
A. Peristaltic pumps do not require the ancillary equipment commonly used with other positive displacement pumps in abrasive applications, such as double mechanical seals, seal water flush systems, run-dry protection systems, and in-line check valves. Not only does this reduce operating costs, but it also makes installation and maintenance remarkably simple and fast.
For more information contact Watson-Marlow Fluid Technology Group, Falmouth, Cornwall.