Progressive cavity and peristaltic pumps are used in many industries. Part One of this two-part series discusses progressive cavity pump selection, mechanical seal maintenance, how spiral stator technology can improve the performance of these pumps and how the increase in hydraulic efficiency means less maintenance and a longer service life. Part Two will cover why peristaltic pumps are a good choice in many industries for ease of maintenance and long life.
Progressive cavity pumps are used in several processes within the paper industry—such as pumping calcium carbonate, clays, titanium dioxide, and many chemicals. They also appeal to other industries—including oil and gas, mining and chemical. They are cost-effective, generate significant pressure and produce high flow rates.
Progressive Cavity Pump Selection
Sizing and selection of progressive cavity pumps can be difficult for end users. A smaller pump costs less but will need to operate at a higher speed, which increases the wear rate. The mean time between failure will increase with the selection of the smaller pump.
These pumps require regular maintenance. The seven-year operating maintenance costs make the initial pump investment cost seem small. If end users choose the smaller pump and the high-speed option, they will likely increase their consumption of spare parts by as much as 50 percent. Therefore, during the seven-year operating span of a typical pump, the maintenance costs could be double the costs of selecting the properly-sized pump.
End users are conscious of capital investment costs, but the desire to get the order completed and a manufacturer’s desire to optimize capital investments are, at times, in conflict with selecting the longest-lasting pump with the least maintenance costs. Pump selection is one situation in which the more expensive option may be a better investment. With time, that decision may be the most beneficial and cost-effective for end users.
Simple Mechanical Seals Replacement
End users of progressive cavity pumps should consider mechanical seal maintenance. Many pump users have plant requirements to reduce inventory and promote standardization, and most progressive cavity pump manufacturers can incorporate many manufacturers’ mechanical seals. Using a specific brand is typically not a problem for progressive cavity pump manufacturers.
Advancements in mechanical seals for progressive cavity pumps have been made, and a few manufacturers’ designs allow for mechanical seal maintenance without disassembling the frame and motor or the rotor and stator. These designs allow the mechanical seals to be removed and replaced in 30 to 60 minutes. With this capability, the pump stays in line, providing for a quick restart.
Prior to this advancement, the entire pump was removed for repair and/or seal replacement. Often, a full eight-hour day was required to remove the pump, repair it and place it back in service. Approximately 25 percent of progressive cavity pump repairs are mechanical seal repairs. Therefore, these advancements significantly improve the pump’s usage rate and uptime.
Progressive cavity pump users would further improve their operational efficiency by selecting pumps that have greater hydraulic efficiency, which allows for slower operation and an overall smaller pump footprint.
New Spiral Technology
In general, two types of stators can be used in progressive cavity pumps—a standard, round stator and a spiral-designed stator (see Image 1). Standard, round stators have been used for 80 years. Spiral designs are a recent development, and only a fraction of progressive cavity pump manufacturers offer this option.
Spiral technology often offers higher efficiency and higher pressure and flow capabilities. This technology’s stator has a much thinner rubber lining. A standard round stator has a much thicker layer of rubber inside. The lining of the spiral stator pump is even all around the stator. Standard stator pumps require much lower tolerances than spiral stator pumps—also called even-wall progressive cavity pumps. Lower tolerances result in less efficiency and more wear in round stator designs.
Traditional, round stators also suffer extreme wear during startup. For instance, highly abrasive materials will slip or leak backward because of the cold rubber, which has not heated or expanded. This expansion of the thicker rubber in the stator is required to tighten the tolerances to allow more efficient pumping. During this time, the pump will suffer more wear or damage. With even-wall technology, these extreme tolerances are not required. The even-wall technology is more efficient at startup and during normal operation.
Spiral Stator Technology Compared to Conventional Round Stator
The spiral stator progressive cavity pump’s even-wall technology allows for a more rigid and tighter pumping unit. With the enhanced compression of rotor and stator friction, back slip and wear are minimized so that the pump can be used with the same rotor in higher temperatures. When compared with the conventional stator design, the even-wall pump experiences less interference between the rotor and the stator, which results in a lower starting torque and higher efficiency.
Improved Hydraulic Performance
When comparing the hydraulic performance of an even-wall progressive cavity pump to a conventional progressive cavity pump, the even-wall technology results in higher pressure, lower backflow and higher efficiency. Figure 1 compares the relative flow (cubic meters per hour) versus the counter pressure (bar).
The relative flow rate drops as the pumps’ counter pressures increase. This trend reflects how the pumps’ hydraulic efficiency decreases because of the counter pressure increase. When comparing the hydraulic efficiencies of both pumps at 6 bar (88 psig), the conventional pump results in an efficiency of 81.6 percent while the spiral stator progressive cavity pump has an efficiency of 93.6 percent. The results at a counter pressure of 10 bar (150 psig) are similar, resulting in a higher pressure with lower backflow for the spiral stator technology.
Figure 2 shows the total efficiency difference measured from pump power demand. The conventional pump’s maximum efficiency is 54 percent, while the even-wall progressive cavity pump has a significantly higher maximum efficiency of 63 percent.
Higher hydraulic efficiency reduces the energy consumed, and it facilitates a longer pump service life. Backflow can cause a high degree of wear. The even-wall progressive cavity pump will have a longer service life because of the minimized backflow between the rotor and stator.
All the features discussed in this section improve pump performance but also increase pump costs. Depending on an application’s parameters and conditions, end users must select the features that are required for their process. Progressive cavity pumps offer many advantages. Some are high flow delivery in a small package, the ability to handle significant slurries and relatively low cost.
Part Two of this series discusses peristaltic pumps and how they can be a good choice for handling solids in many industries.