Progressive cavity (PC) pumps solve many of the issues that occur in demanding pulp and paper processing and production. They are chosen for many applications, including:
- The pulper
- The paper machine
- Coating kitchen
- Waste treatment and recovery
PC Pump Advantages
PC pumps are energy efficient and experience long life in harsh applications. Self-priming and with a low net positive suction head required, their positive displacement flow is proportionate to the speed and independent of discharge pressure. With their large capacity, PC pumps reach flow rates up to 1,500 gallons per minute and have high-pressure capability. With friction losses, pressures as high as 2,000 psi can be achieved.
Though end users should never let a pump run dry, PC pumps can achieve a vacuum of 28 inches. Regardless of fluctuations in media viscosity, temperature or abrasive characteristics, PC pumps provide low pulsation and smooth, repeatable flow, providing precise metering in real-world applications. If the pumped media is shear-sensitive, these pumps are ideal solutions because of their low internal velocities, which add little energy to the media, minimizing the degradation of sensitive products.
Despite their gentle characteristics with sensitive fluids, PC pumps are also non-clogging and can pass solids up to 3 inches in diameter. They also provide reversible flow. Other advantages include:
- Pumping of multiphase media
- Quiet operation
- Simplified maintenance because no special tools are required
- Decreased downtime for maintenance
- Flexible mounting arrangements—such as vertical mounts or mounting a twin shaft grinder directly on the inlet
All PC pumps have quality manufactured rotors and stators, and optimizing these components is important. However, just as important is how each pump handles the eccentric motion and strong radial and torsional loads. Initially, this was accomplished with an unsealed pin on each end of the connecting rod. The connecting rod and pin are typically made from hardened steel or stainless steel and combine to connect the concentrically rotating drive shaft to the eccentrically rotating rotor. The pin is subjected to extreme concentrated load and lubricated by the product, which is often abrasive, resulting in premature wear and failure. The pins and connecting rod become the sacrificial lamb of the pump.
A sealed pin joint was developed and incorporated by many manufacturers. This helped because the pin operated in a well-lubricated bath, but it was still subjected to extreme concentrated loads, and this change was not a substantial upgrade for the tougher applications.
Some companies turned to a crowned gear joint as a solution. This improved the handling of heavy loads but still had disadvantages. The gear joint was protected by a fairly thin rubber boot. When the rubber was damaged or wore, the joint failed catastrophically and without warning. This often proved to be a difficult and expensive rebuild.
One of the better options currently available is the use of a long, flexible shaft as the connecting rod. This shaft and connecting rod have no moving parts and, therefore, no parts to fail. The primary issue for a PC pump with this option is the real estate required that makes retrofit nearly impossible and new installations costly.
What option is available to solve these shortcomings of PC pumps? A PC pump that has a true cardan joint, which uses two sets of sealed perpendicular or diametrically opposing pins, provides many advantages. Most notably, it provides the freedom of angular movement, which facilitates much smoother transmission of angular and axial loads. This PC pump technology eliminates premature failures because of concentrated loads, which can occur with a pin joint pump or catastrophic failures because of failed joint seals as in a gear joint pump, with a much smaller footprint than flexible shaft pumps. In most applications, a seal failure of a cardan joint pump will have little or no effect on the pump’s ability to operate.
Operation in Paper Mill Processes
Understanding the inherent features of a PC pump will help end users understand why a PC pump with a cardan joint is good choice for many applications in the pulp and paper industry. Where is this equipment providing real-world solutions in paper mills?
Stock & Starch Preparation
PC pumps can be automated and controlled by a programmable logic controller to pump diluted paper pulp stock at precise variable flow rates to ensure the continuous feeding of the paper machine with optimum efficiency.
Cationic or anionic starch is cooked, metered and continuously fed. The transfer to the head box or size press requires a solution that can perform at both high and ambient temperatures. Properly applied PC pumps can handle fluids with temperatures from just above freezing to 425 F.
Wet End Chemical Preparation
PC pumps can precisely meter product and process additives—such as pulping and bleaching chemicals, fillers, cationic starch, sizing agents, dyes, fixing agents, defoamers, and retention aids—to the paper stock. This flow remains consistent despite system or temperature fluctuations.
With PC pumps, pigment slurries, latexes, starch, binders (polyvinyl alcohol, carboxymethyl cellulose), biocides and other additives can be successfully metered from storage tanks and containers for dispersion, mixing and cooking. They are equally effective in batch or continuous process operation. PC pumps are key to the preparation of the coating/color before the final station. They can easily handle these abrasive fluids while delivering high dosing accuracy with smooth, pulsation-free flow.
Precipitated & Ground Calcium Carbonate
Mineral pigment dispersing and mixing units rely heavily on PC pumps. Preparing pigment slurries from calcium carbonate or clay requires heavy-duty pumps capable of withstanding these highly abrasive fluids while delivering unmatched metering and repeatability.
Improving the handling of chemicals, soaps and coagulants, PC pumps have been problem solvers in de-inking operations. Since PC pumps do not require check valves, which are prone to failing and sticking, these quiet pumps provide years of hands-free operation. This is important for maintenance staff safety, considering the dangerous and corrosive nature of many of the liquids being moved throughout a paper mill.
In many situations, this part of paper production is a drain on the mill’s operating budget. The treatment of liquid and dewatered sludge, flocculants and effluents in this area are necessary but do not usually add to the mill’s profits. This makes PC pumps—with their limited wearing parts, small footprint and quick maintenance turnaround—a great budget-conscious choice for this area of the mill.