A collaboration between Cambi and NOV Inc. using the latter’s Flexishaft design resulted in a new generation of progressive cavity pumps for thermal hydrolysis, with improved maintenance intervals and performance.
Cambi’s thermal hydrolysis process (THP) systems, much like many sewage and sludge processes that require pumps, have traditionally been installed at wastewater treatment plants with progressive cavity (PC) pumps of the universal pin-joint type. Though widely considered state-of-the-art pumping solutions that work well with digester feeds, these pumps have empirically been challenged upstream of the thermal hydrolysis component of a sludge line.
A positive displacement pump, i.e., a pump that displaces a fixed volume of liquid with each rotation.
In a progressive cavity (PC) pump, a rotor (single helix) rolls eccentrically in the stator (double helix), creating pockets (or cavities) that move toward the discharge end of the pump as the rotor turns.
French mathematician René Moineau developed the concept for the pump in the 1930s in his PhD thesis “A New Capsulism”. He granted licenses to four companies to manufacture and sell progressive cavity pumps.
This pump is also known as a helical screw pump, progressing cavity pump, PC pump, or PCP.
There are many contributing factors to the difficulty of using this type of pump with the thermal hydrolysis process. THP units typically deal with very viscous sludge, often secondary/waste activated sludge or a mixture of different kinds of sludge, and so they tend to require pumping mechanisms that are more robust than those that are used for primary or more conventionally viscous sludge.
Gear joints normally present maintenance difficulties due to having multiple parts in a complex arrangement. As these parts are designed to be constantly in motion, there is a consequent drive train backlash that increases with time and wear.
It is also typical of sludge pumps to need consistent repairs due to problems caused by sludge particles such as sand, grit, stone, glass, and fibres. Sludge temperature and chemistry, including pH, also affect pump reliability and durability. Hence, these pump mechanisms are routinely inspected or maintained around every 4,000 hours of operation.
The challenges experienced with combining the conventional pin-joint type PC pumps and the thermal hydrolysis process are shared across various pump suppliers. In some cases, these have led to high maintenance costs, unpredicted failures, and solutions that require additional personnel or expertise. In response to these concerns, Cambi decided to trial a new design of a different type of pump through collaboration with NOV. The company is a leading engineering solutions provider, supplying industrial pumps and mixers as part of their portfolio, competing with companies like Seepex and Netzsch.
NOV owns the MONO and MOYNO lines of progressive cavity pumps that accompany several Cambi THP systems around the world. In collaborating with Cambi for a modification that would address the drive train wear and maintenance problems, NOV looked to a design that they have been manufacturing for over half a century, the MONO Flexishaft™.
The standard version of a Flexishaft progressive cavity pump consists of a high-tensile strength shaft fixed at the drive and rotor ends. This shaft continuously flexes to accommodate the eccentric motion of the rotor and eliminates the need for universal joints and split coupling. NOV’s Flexishaft progressive cavity pumps use a low inclusion grade of stainless steel for the shaft, coated with an ECFTE (Halar™) coating for impact and corrosion protection. It is also possible to create the shaft out of a shorter titanium rod. The shaft has tapered friction joints pulled into a stub shaft at a specified torque by high-tensile cap head screws. These joints on both sides of the shaft avoid “backlash” in the drive train.
The Flexishaft special titanium rod assembled with tapered friction joints (top right) is able to absorb alternating stresses as it flexes (bottom). The curving motion in the photo is augmented for visual purposes only.
The original MONO progressive cavity pumps for Cambi had to fit within a limited footprint and were designed with the universal pin-joint coupling. To modify the MONO PC pump into a Flexishaft PC pump within the same space, NOV and Cambi used the titanium Flexishaft, which is two-thirds of the length of the standard Flexishaft version, as well as a hollow pump driveshaft.
The design of the conventional PC pump (top left) VS a Cambi Flexishaft PC pump (top right). The shorter titanium rod allows the Flexishaft to use the same pump housing (bottom).
In June 2019, two trial pumps were put in operation in two different sewage sludge treatment plants using the Cambi B6 system for regular inspection by Cambi and NOV engineers.
The first plant in Hengelo in the Netherlands only processed waste activated sludge (WAS) in its THP system. Even with this especially viscous sludge, the Cambi Flexishaft PC pumps revealed no issues during the first inspection after around 2,500 hours of operation. The second inspection at about 5,500 hours of operation revealed only slight wear on the ECTFE coating, which is applied on the titanium only for abrasion protection and not for chemical resistance. It was only after around 8,500 hours of operation that a high power alarm and low flow indicated stator wear. The coating wear on the shaft had also increased, but with no damage to the shaft.
The trial pumps at the second plant in Seafield, Edinburgh, also performed impressively. A significant difference was observed in terms of ease and frequency of maintenance versus the older pumps. The older PC pumps would need replacement parts due to rapid drops in flow every 4-5 months. The new Cambi Flexishaft PC pumps only had a worn stator after around 7,000 hours of operation. All pumps for the plant’s THP pulper unit have now been upgraded to Flexishaft. The utility has also expressed its intention to use the same pumps for the digester feed in the future due to the marked difference in maintenance requirements.
Even while trials were ongoing, both plants already ordered additional Flexishaft PC pumps for their THP systems.
The Flexishaft piece (shaft only) has a two-year warranty.
Being the new standard pump for Cambi thermal hydrolysis units, the Cambi Flexishaft pump design has effectively replaced the pin joint pumps in all new projects since the second half of 2020.
The demand for the equipment has led to almost 70 Cambi Flexishaft pumps either ordered or installed as of mid-2021. The 16 Cambi B6 THP systems utilising 25 Flexishaft pumps now have at least 130,000 accumulated operating hours, and one B2 system using the same pump has over 8,000 operating hours. All these pump inspections have revealed no significant issues to date.
Should more mature THP plants wish to switch from the conventional Cambi pin-joint pump to Flexishaft, minimum site works are needed as the pumps have the same flange-to-flange dimensions, motor, gearbox, housing, and stator. Plants utilising pumps from other manufacturers will need piping and possibly electrical modifications, besides removing the old pump and installing the new Flexishaft pump.
01 July 2021
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