Select the Best Technology for Dewatered Cake Transfer

Moving dewatered cake can be extremely challenging and quite frustrating over time. Progressive cavity (PC) pumps have been pumping dewatered cake for over 70 years, but how do PC pumps stack up against other methods used today? Here is more research on this topic focusing on the advantages and disadvantages given a particular installation.

Most processes ending with dewatering sludge require a method to move the dewatered material to an awaiting truck, a drying bed, incinerator or dryer. To accomplish this, a system must be used to move cake from point A to point B, and this is typically broken down into two groups: pumps and conveyors.

The two groups can be further dissected into the main products in each category:

There are differences in the two groups—advantages between the pump group versus the conveyor group, advantages between PC pumps over piston pumps, and, finally, what differentiates PC manufacturers from one another.

When looking at the two groups, there are differences in cost and space requirements. The products within each group are different but several features stand out.

The most common conveyor systems in cake installations require low angles for elevation change or, at a minimum, uninterrupted runs to get to their destination. This results in considerable floor space either for the conveyor itself or the support structure to get the device off the floor. If there is a need for a large elevation change, conveyors require great distances to accomplish this.

Pumps use piping that is capable of leaving the end of the pump and being routed completely vertical if necessary. Because of this, pipe can be installed overhead, leaving floor space open for other equipment or movement in the area. Pumps require only enough space for the pump footprint.

Based on design, conveyors feature completely open or partially open areas that allow odors to escape. This openness often results in increased housekeeping requirements as well. Cake in an overstuffed auger system can push out from under thin covers. Belt conveyors, by design, require the belt to turn upside down the complete length of travel, spilling any contents that did not drop off at the discharge.

Pumps use sealed piping systems, which can eliminate housekeeping and odor issues.

The cost of the system can vary based on overall distance of travel, elevation changes and product type selected. Simple short runs with no turns (less than 20 feet) are sometimes more economical with conveyors.

For larger systems, a pump system may be more economical, especially with elevation and turns, as a conveyor system requires a new drive system anytime a change in direction is necessary. Pump pricing is basic with the ability to develop a price per gallon; a conveyor increases price with length.

For users needing multiple drop points, both systems can accommodate this with varying degrees of difficulty and cost. With a pump, valves in conjunction with piping are used to allow for selectable discharge points. With a conveyor, either multiple conveyors are used or a reversible conveyor with drop points can be used.

Each option should be evaluated for the given situation to determine the most economical installation.

Some dewatering equipment can output lower solid concentrations of product at times or even have cleaning cycles that discharge a high amount of liquid.

If this scenario is part of the operation, a conveyor cannot move this type of product and must have provisions to drain away from the device. Pumps used to convey cake can move these lower viscosity fluids, making them suitable for systems that are set up to discharge cake as well as lower viscosity media.

Efficiency is dependent on what distance needs to be covered and, in the case of conveyors, the number of changes in direction. In general, a short, single run of a conveyor is going to be more efficient than either pump type. If a longer run or additional motors are necessary, then the PC pump becomes at least as efficient, if not more efficient.

Just like the pumps versus conveyors, when looking at the different pump types there are some distinct differences. Both types of pumps are positive displacement, meaning for each revolution or stroke, a set amount of product is discharged. This is the only real similarity these two pumps share.

Cost is where the two types of pumps vary the most. When pressure requirements increase, a piston pump may only require a larger motor. A PC pump will need to increase the length of the pumping element (stages) to accommodate an increase in pressure.

While this may seem like a disadvantage for the PC pump, the highest-pressure PC pumps are three to five times less expensive than the equivalent flow piston pumps. It is possible to install a PC pump with an integrated hopper for less cost than the power pack on a piston pump.

The typical piston pump is set up with a twin-screw feed hopper (TSF) that feeds the pump dewatered material—both pieces of equipment run off hydraulics. A power pack or hydraulic unit is, therefore, required and needs to be in close proximity to the pump/TSF unit. Thus, there are three pieces of equipment taking up floor space. The PC pump uses a feed hopper integrated with the pump elements, which can increase simplicity of installation, operation and maintenance and save on floor space.

Piston pumps have always been known for their high-pressure capabilities, exceeding that of standard PC pumps. In the past, if the discharge pressure was over 600 pounds per square inch (psi), a piston pump was the only choice. PC pumps were able to combat this by using a boundary layer injection (BLI) setup. This involved injecting polymer (or another lubricant) around the inside of the pipe to reduce the friction, therefore reducing the pressure requirement. This was effective in many cases but not practical for extremely long runs of piping.

To level the pressure playing field, air injection on the pump discharge can be used. This system injects air, pushing plugs of cake at much lower pressures but much greater distances. The combination of BLI and air injection allows for pumping distances of 3,280 feet with reduced pressure at the pump as well as in the pipework. This reduction in power allows for the use of smaller pumps, lowers power consumption and, in some cases, allows plastic piping to be used.

As alluded to earlier, piston pumps may not be as efficient as PC pumps. The piston pump's hydraulic system takes electrical energy (motor), develops hydraulic pressure via a pump, then delivers the pressurized hydraulic fluid to the pump to do mechanical work. The PC pump takes electrical energy either directly to mechanical work or through a gearbox, which acts as a torque multiplier to do mechanical work.

Taking into account this one component that is not part of the PC pump—the hydraulic motor—there is a power loss of 15%, mostly due to heat.

Most modern-day piston pumps in dewatered applications use two pistons. Two pistons help to minimize pulsing and pressure spikes but are not able to eliminate them completely. The pistons do not run in a continuous feed situation, leaving one piston pushing into the pipe as the other is filling. This scenario results in momentary pressure spikes up to 75 psi every time a piston evacuates, creating pipe movement and general fatigue to piping and accessories. PC pumps form cavities in which the material is moved. As one cavity is closing, another one is opening, giving a linear discharge with no pressure spikes or pipe shake.

Piston pumps require ongoing maintenance to keep seals clean and operating without damage or leakage. Hydraulic lines develop leaks and need to be tightened or changed. Hydraulic fluid and filters have to be changed at regular intervals as well. The PC pump is electrically driven with regular maintenance needed for gearbox oil checks every six to 12 months.

Hydraulic power packs contain large amounts of hydraulic oil that should be changed regularly and disposed of properly. Oil leaks typically develop around fittings and can cause environmental issues in normal operation. PC pumps are electrically driven and do not have those issues.

Piston pumps can be loud as the piston cycle and hydraulic power units deliver pressurized fluid. In some cases, depending on the installation, hearing protection is required. PC pumps are quiet, with a low decibel whine from the motor if a variable frequency drive (VFD) is being used to control it.

Progressive Cavity Pump vs. Progressive Cavity Pump

While it is true that progressive cavity pumps may look similar on the surface, there are differences in how the dewatered cake is delivered to the pumping elements.

Many PC pump manufacturers have an open hopper design with an auger feeding the pumping elements. This design, in general, is not considered a standalone cake pump unless it is mounted under a live bottom hopper, allowing cake to be delivered to the pumping elements without bridging.

To combat bridging in a situation where cake freely drops into the pump hopper, manufacturers set pumps up with near-vertical sidewalls, larger augers and, in some cases, a bridge-breaking device. A larger auger and near-vertical sidewalls allow for low solids cake pumping but are not ideal for cake with a solids concentration greater than 20% to 22%.

When dealing with higher concentrations, manufacturers have a bridge-breaker device that resides in the inlet of the hopper, keeping product from bridging over the auger. This design works well but does require another one or two drives, adding cost and lowering efficiency.

Other designs have been engineered over the years such as an option with a twin-screw feeder (TSF) to feed the pump. One concentrically rotating integrated auger has been developed; other auger systems are eccentrically driven. Running the auger concentrically with vertical sidewalls eliminates the area along the side of the auger where cake can build and eventually bridge over the auger.

The maximum pressure a PC pump can handle with no aids is typically 600 psi, which equates to roughly 300 feet of pipe. By using a BLI setup, injecting polymer (or another lubricant) around the inside of the pipe to reduce the friction, the effective length of pipe can be doubled or the pressure cut in half.

The use of air injection takes this to the next level. The combination of BLI and air injection allows for pumping distances of 3,280 feet with reduced pressure at the pump as well as in the pipework. This reduction in power allows for the use of smaller pumps, lowers power consumption and, in some cases, allows plastic piping to be used. This keeps installation costs and ongoing maintenance low.

Pumps with integrated hoppers are similar in floor space requirements, with the exception of units that have a separate twin-screw feeder (TSF). If the TSF is used, more floor space is required, unless the unit can be mounted above the pump.

Pumps with integrated hoppers are going to be within a certain percentage of each other in cost, considering slight variances in design. The exception again is the twin-screw feeder pump, which tends to be double the cost of a standard hopper pump as there are two pieces of equipment rather than one.

Level control is typically done via load cells or a single measuring device in the hopper. These methods have been problematic as load cells measure a small fraction of weight versus the total weight of the unit, require complex base plates and complex setups. Single level devices require an amount of product above the pump to allow for even speed control, which can result in bridging. They are also prone to blinding from moisture and product buildup as they are positioned in or over the feed hopper.

Users can consider implementing a laser level system that uses three lasers, two for sensing absence and presence of product and the third for level measurement. The level measurement sensor is positioned on the outside of the hopper. This allows for measurement from the side to avoid buildup and moisture, increases accuracy and provides redundancy.

Mark Yingling is the director of product and market management for SEEPEX, Inc. He has a Bachelor of Science in environmental science from the University of Cincinnati and has over 20 years of experience in the grinder and progressive cavity pump industry. He can be reached at [email protected]. For more information, visit www.seepex.com.