Extending the Life of a Rotary Lobe Pump: Wear Factors

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Extending the Life of a Rotary Lobe Pump: Wear Factors

How long does a pump last? That’s a loaded question. Does it depend on the pump? Yes. Does it depend on the conditions? Yes. Could it be something else? Maybe.

Although there is no definitive answer to the question of how long a pump lasts, you can identify the biggest culprits of pump failure — and the solutions to extend your pump life. Vogelsang identified the four main causes of pump failure typically seen in the field.

Abrasion

Problem: Abrasion is the sandpaper effect that solids in the medium have on the pump. Things like food waste, grit, sand and rags are common in wastewater applications. Abrasion damage is amplified by the size and configuration of opening clearances in the wet-end of the pump. Abrasion can also affect seals. Abrasives get caught between the seal faces, causing a buildup of heat that leads to seal failure.


Solution to abrasion: If you’re using a pump that is made of “traditional” materials, you will most likely get traditional abrasion. That’s why Vogelsang offers the widest variety of materials in the industry, from cast iron to super duplex. Lobes are available in both metal and elastomeric materials. Tungsten carbide coating of wear plates, housings and lobes is available for those extremely tough applications. To address the problem of chemical attack, soak testing is available to test elastomeric compatibility.

Slip

Problem: Slip is defined as the difference between the theoretical displacement of the pump and the actual displacement of the pump. This disparity causes wear. As the material slips around the lobe, it causes wear on the lobe surfaces. There are various causes of slip in pump operation. General fluid characteristics can negatively or positively affect the amount of slip in a pump. Lower viscosity fluids see an increased amount of slip. Higher viscosity fluids see a reduced amount of slip. Higher discharge pressures cause a greater amount of slip.

Solution to slip: To combat a number of these slip issues that were prevalent with early rotary lobe pump design, Vogelsang developed the Hi-Flo lobe. The Hi-Flo lobe has four tips and is convoluted. This greatly extends the sealing lines in the pump — a 100 percent increase compared to bi-lobes and a 33 percent increase compared to a three-tip lobe. The design reduces slip due to the extra sealing and extends pump life due to lessened effects of wear. Another benefit of the four-tip lobe is that as a geometrically balanced design, it virtually eliminates pulsation.

Solution to both abrasion and slip: Injection housings help with both abrasion and slip. The injection housing design (see a video here) features a bevel on the housings to direct the flow to the middle of the lobe. This eliminates solids recirculation at the inlet of the pump, which allows each lobe to interact with the solids once, not several times.  There will be little effect on the wear characteristics of the housings but wear on the lobes will be greatly reduced.

Shaft Deflection

Problem: Shaft deflection is the distance the shaft deviates from horizontal. As discharge pressure increases, shaft deflection does as well. This results in a shaft that rotates in a small circle around its true axis. This exacerbates the wear that is caused by other sources because of the increased interaction of parts. The telltale sign of shaft deflection is a conic wear pattern to the lobes. 


Solution to shaft deflection: The key to minimizing shaft deflection is go big or change to a non-overhung load. Vogelsang pumps feature some of the largest shaft diameters in the industry, giving them superior strength to minimize deflection. Additionally, Vogelsang builds pumps up to 6,000 gpm — it can move to a larger pump and slow it down. To further solve the problem of shaft deflection, the VX-QD pump design features an outboard bearing, turning the overhung load to a between bearing load, virtually eliminating deflection.

Seal Issues

Problem: Most seals fail due to heat that comes from friction. The seal chamber is a relatively low-pressure area in a pump where material tends to collect. While grit and hard particles are tough on seals, fibrous material is more problematic due its tendency to wrap around the seal faces and shaft. As the fibers become packed, they force the seal faces apart, and can even cause them to fracture. Grit gets between the seal faces and causes heat buildup. The use of the typical component seal is problematic due to the inability to ensure the faces are properly set. Set too tight and the result is higher heat. Too far apart and you’ve got a leaky seal, as well as particles getting between the faces that cause additional wear. With most lobe pumps, the first indication of a leak is sludge spewing out of the pump.

Solution to seal issues: To combat these issues, Vogelsang developed its own cartridge mechanical seal, the Block-Ring Cartridge Seal. As the name indicates, the seal features a large block ring that functions as a large heat sink to dump heat buildup into the pumped liquid. It also serves as a breakdown bushing by closing the gap between the seal, wear plate and lobe. The cartridge design ensures that the seal faces are in the proper position during installation and replacement. One of the biggest advantages of Vogelsang’s pumps is the seal bottle that acts as an early leak detection device, as well as providing a means of balancing the pressure differential across the seal faces. This causes pumped material to follow the path of least resistance, the discharge of the pump. The seal chamber also has provisions for an external seal flush.

A wastewater operation is at the mercy of the medium it pumps. Since pumpage cannot be changed, the pump design must be improved to overcome the problems of abrasion, slip, shaft deflection and seal issues. When looking to replace a pump that has failed, look at the engineering to make sure the design addresses these problems.

Want to learn more about this topic? 

Read more about Vogelsang pump features here. 

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