Pumps & Systems, June 2008
The rotary lobe pump belongs to the positive displacement pump family. It is a dual shaft pump design with external timing gears, allowing two intermeshing rotors to operate synchronously. The pump operation is similar to the function of a positive displacement blower: Minimized clearances between rotors and the pump casing allow pumping action by forming a seal with the liquid between the suction and discharge side. Most positive displacement pumps need to be operated with elastomers (progressive cavity pumps, hose pumps, diaphragm pumps, plunger pumps, etc.). Since the rotary lobe pump incorporates a timing gear, non elastic rotors like steel or stainless steel rotors can also be used. Since the rotary lobe pump has no preferred rotation direction, it can be operated in a reversible mode.
Industries and Markets
Rotary lobe pumps are utilized in almost every industry with the major distinction existing between the sanitary and non-sanitary markets.
The sanitary market needs lobe pumps with CIP (Cleaning In Place) and SIP (Sterilization In Place) capabilities. Materials of construction for product wetted parts are stainless steel and higher grade materials with highly finished surfaces. The all metal rotors are machined to allow only the smallest clearances to the casing. To prevent zones where fluid can accumulate in the pumps, the number of parts in contact with the fluid is minimized. For example, no wear plates or pump casing protection plates are incorporated in sanitary lobe pump designs.
The non-sanitary lobe pump market includes many industries: for example, water and wastewater, oil and gas, biofuels, pulp and paper, marine, sugar and chemical industries. For these markets, the pumps are often exposed to harsher, more abrasive fluids. There is little need for CIP or SIP, but many non-sanitary lobe pumps are MIP (Maintenance In Place), which allows the replacement of all fluid wetted parts onsite without the removal of any pipe systems or drive mechanisms. This often allows the use of sacrificial wear plates and other features for easy maintenance and cost effectiveness over the entire life cycle.
Applications and Troubleshooting
This article focuses on non-sanitary lobe pumps. To ensure a successful rotary lobe pump installation, many factors play a role. To name a few: expectations of customer and vendor, fluid characteristics, flow and pressure, process and pipe design, space constraints, maintainability, availability of spare parts, downtime, etc.
Wastewater Treatment Plants (WWTP) require lobe pumps for various sludge applications. They are used for handling abrasive and viscous fluids, often feeding thickening and dewatering devices.
A recent addition for rotary lobe pump applications has been filtration, especially membrane bioreactors utilized for wastewater treatment. Submerged micro- or ultra-filtration membranes are installed upstream from a rotary lobe pump, on average one pump unit per train. The rotary lobe pump pulls the filtered water through the membranes (permeation); as necessary, the reversible operation capabilities are used to back-flush the membranes for a cleaning cycle. The key here is SIMPLICITY. The reversible operation of the rotary lobe pump simplifies the pipe design and the PLC programming of an MBR system in comparison to a centrifugal pump. Also, a wide flow and differential pressure range can be targeted with variable speed operation.
Wastewater Treatment Plants
Under abrasive conditions, ease of maintenance of a rotary lobe pump is important for operations of WWTPs. With many rotary lobe pumps, all fluid wetted parts are replaceable through the front cover, allowing repairs and/or modifications onsite, minimizing downtime. The wide variety of available rotor designs and materials in the rotary lobe pump market allows troubleshooting and reduction of life cycle cost through optimization in the aftermarket. Modular pump designs allow various rotors to fit in the same pump casing.
Example: Initially, a rotary lobe pump equipped with a dual lobe rotor is installed to feed a dewatering belt press. The pump operates according to the performance curve, but pulsation and vibration damages the pipe system. The solution is the screw rotor: The low pulsation characteristic of this rotor designs allows a smooth flow compared to a centrifugal pump, eliminating the vibration problems. The screw rotor design also accounts for success of rotary lobe pumps in the MBR Market. With the high flows required and no abrasion being present, the pump units are operated at speeds up to 500 rpm, where the screw rotor eliminates pulsation problems.
Variable Frequency Drives
By incorporating a variable frequency drive (VFD), the metering capabilities of rotary lobe pumps (flow is proportional to operation speed) can be utilized. The rotary lobe pump has a constant torque characteristic, required torque is unchanged at different operation speeds with constant discharge pressures. Most VFDs can be used for dynamic and constant torque operation, the constant torque operation generally reduces the HP rating of the VFD. Because of the reduced HP rating, some rotary lobe pump units could not be operated because the VFD was calculated for dynamic torque operation so it was undersized for the rotary lobe pump.
Renewable Energy Market
A relatively new market is the biofuels industry, especially ethanol and biodiesel production. New process solutions often demand new developments from pump manufacturers.
Biodiesel plants require various pump applications. While soybean oil is often used as primary feedstock, the use of various additives throughout the biodiesel production process requires high rate materials of construction. Rotary lobe pumps utilized in this industry usually are equipped with pump casings from stainless or duplex steel. With rotor elastomers, various kinds of FPM have been used, tested and often failed.
The solution is rotary lobe pumps equipped with PTFE or stainless steel rotors, dynamic o-rings from Chemraz 505, static o-rings from PTFE. As a rotor material, PTFE is often not possible with other rotary positive displacement pumps since it is a cold flowing material. The timing gear with a non-contacting rotor synchronization allows the use of PTFE for rotary lobe pumps.
Specialty Chemical Plant
In a specialty chemical plant, sanitary lobe pumps were used to pump a mixture of water and crushed plastics material in various sizes with abrasive characteristic for a protection foil production line. The lobe pumps were used in the front end of the process and the high wear rate required constant rebuilding of the pump units. Rotors and pump casings were constantly shipped to the manufacturer for re-machining. In addition, tight tolerances in the area where the shaft extended to the front cover caused accumulated material, closed clearances, and increased friction, creating high, problematic temperature build-up.
The sanitary lobe pumps were replaced with a non-sanitary lobe pump design. To reduce the life cycle cost, the new units are equipped with axial and radial pump casing protection plate, eliminating any re-machining. Also, a set of protection plates only costs a fraction of a new stainless steel pump casing. When the customer installs new protection plates and rotors, the pump unit is back to factory tolerances.
To deal with material build up, a lobe pump design with a flat front protection plate was chosen. A heavy duty design was chosen with shafts not extending into the front cover area, eliminating material and temperature build-up. The customer increased safety and reduced life cycle cost.