Pumps & Systems, April 2008

When handling difficult liquids like sludge, restaurant grease or solids-laden fluids, the challenge is continuing flow through the entire operation. When debris or large solids clog a pump or pipeline, the entire process can stop within minutes. The shutdown, cleanout and restart are time consuming, and the costs frightening. It's a nightmare scenario.

It's also avoidable. Several facilities which process difficult liquid/solid combinations now rely on dual-shafted grinders on the inlet side of pumps to ensure continuous flow and prevent blockages. These powerful shredders apply several tons of cutting force at peak loads, ripping apart large solids or rags into smaller particles. The smaller particles flow harmlessly through pumps and pipes. These grinders are even used to protect a wide variety of pumps including centrifugal, chopper and progressive cavity pumps.

The grinders were originally developed for wastewater treatment, including sludge pumping. Since then, grinders have been installed in a variety of difficult applications such as grease, pulp, manure, refining and biofuels processing.

Operation

These grinders are specialized for liquid handling-they must be powerful, handle pressure and allow maximum flow. One type of grinder uses low-speed, high torque grinding to handle troublesome solids and was first developed to help wastewater treatment operators cope with an onslaught of rags and trash in the sewers-"muffins" in industry lingo. Grinders have gradually grown bigger and more powerful.

Figure 1. The grinders have two rows of sharp, steel cutters which rotate toward one another. The teeth actively grab solids and pull them through the cutters to shred them into smaller pieces. The shafts also rotate at different speeds so the cutters interact like a pair of scissors-slicing the solids, rather than crushing them. This helps ensure a small, consistent particle size of about 0.5-in or smaller.

To simplify the installation process, the grinder comes in two parts-the steel housing is bolted into the pipeline at both ends and a removable cutter cartridge holds the two shafts of cutters, the gear reducer and motor and slides down into the housing. This streamlines maintenance inspections and repairs since the cutter cartridge can be removed, and the pipeline put back into service.

Grinders can operate 24/7 and are lubricated by the flow so they require very little maintenance. The largest grinder can be installed in pipelines up to 20-in in diameter and process up to 5,000-gpm. For maximum efficiency the grinders use only a 3-, 5- or 10-hp motor depending on the amount of flow.

A control panel monitors the unit to ensure it does not try to shred anything potentially damaging-such as a wrench or rebar. When the controller sees an amperage spike, the cutters will stop for several seconds, reverse and then try grinding again. The unit will automatically try the forward and reverse process three times in order to clear a stubborn solid.

Biodiesel

Managers of a waste-to-energy plant in southern Utah installed two such grinders to eliminate blockages in two manure pipelines that connect concentrators with digesters.

"During the start-up phase, we wanted to do everything we could to decrease the amount of maintenance that's going to be needed to keep the plant running effectively," said B.J. Moore, general manager for Best Biofuels. The plant is designed to convert swine waste from 257,000 animals into 25 tons of methanol per day.

Moore said each grinder, installed upstream of the concentrator in an 8-in line, sees 400-gpm of raw manure-a total of 500,000-gpd. Concentrated manure is now moved by progressive cavity underflow pumps at 200-gpm from the concentrators through 4-in lines to the digesters without any interruptions or blockages. He added he also saw some benefit in the concentrators.

 "We were very concerned about blockages taking down our manure delivery system for an hour and a half every other day," he continued. "Two guys were needed to remove a blending unit from each of two lines, back flush it and replace it-a very unpleasant task we didn't want when the plant was past start-up."

Their unit can grind a wider variety of solids than single-shafted macerators, while its low-speed operation results in higher torque and fewer interruptions. Design flow rate is 800-gpm, while pressure drop is 1.60-psi.  The units adapt to most applications with little or no modification to piping, channel or power, and offer high-pressure capability up to 90-psi with no seal flush required and no packing gland to adjust.

"In a 60 day, side-by-side test against an alternative unit, [this] line never had to be taken down. But the line with the other unit went down three times during the trial, so we are going with [this grinder] in both lines," said Moore.

Grease Applications

In recent years the appeal of restaurant grease as a biofuel has grown. Nutrient rich, the material can enhance cogeneration systems.

Figure 2. Several cities feed restaurant grease right into sludge digesters at wastewater treatment plants-the grease mixes with sludge, increases the amount of methane produced and, consequently, the power output of their cogeneration plant. It is like pumping a digester full of caffeine-engineering studies show the grease seems to speed up the process. Vacuum trucks scoop up the grease at restaurants and deliver it to the plant.

Problems occurred at several facilities when rocks, rags, kitchen knives, clothing and other debris plugged the receiving and pumping equipment. Truckers were delayed during unloading, and the pumps needed expensive maintenance. Three sites are testing new grinders in order to tame the grease chaos.

In addition, several large grease receiving projects are installing pre-engineered systems which include all piping, plumbing and controls needed to unload, measure and pretreat the grease prior to pumping it into their system. The pre-engineered system incorporates a grinder to shred solids and a rotating auger screen to remove the trash and debris from the grease. As the trash is lifted, it is washed and compacted before dropping into a dumpster for landfill disposal.

The system removes the trash and ensures the smooth flow of grease into the processing system. That is just the way plant managers like it-maximum flow, minimum problems.