Equistar ends breakdowns in classifying pellets

Equistar Chemical's Bay City, TX high density polyethylene (HDPE) plant is saving $80,000 per year and enjoying higher productivity after replacing four pellet classifiers on two of its three extrusion lines.

Two Vibroscreen 72-inch diameter vibratory circular screeners scalp and dedust 44,000 lb/hour of pelletized HDPE resin.

The plant replaced circular vibratory screeners, which had been breaking down, with 72-inch diameter Vibroscreen^® vibratory circular screeners from Kason Corporation. The new screeners paid for themselves in 2.3 years. The breakdowns ceased while operating and maintenance costs fell.

Milton Hertel, reliability consultant at Equistar, says, "On the previous classifiers, the spacer frames near the discharge spout were repetitively cracking from the machine's vibration. We had to slow the line by 25% or more to take a unit off to change the spacer frame.

"Premature failures of the motors were also occurring repeatedly," Hertel adds. As a correction, he says the Kason Vibroscreen screeners come with a heavy-duty, twice-as-large motor enclosure that does not break down. Although 2 hp compared to the previous classifiers' 5 hp, the Vibroscreen operates more efficiently, using less horsepower.

Because of the previous breakdowns, the plant installed two classifiers on each line to back each other up. The lost production cost Equistar $50,000 per year from not being able to handle the total load when one of the classifiers failed. This added to another $30,000 the plant was losing from higher O&M (including energy) costs and slower line speeds.

No Breakdowns

Equistar installed the new classifiers on the urging of its operations technicians, who gathered good experience with them on the plant's third 60,000-LB/hour extrusion train." The Vibroscreen separators incurred only infrequent breakdowns since the plant Equistar continued was built 10 years ago," Hertel says.

Classifying Pellets

The HDPE extrudate is pelletized into shapes about 1/8-inch long by 1/8-inch in diameter. Following a drying process, the pellets are conveyed to Kason screeners for scalping and dedusting. A top No. 3 mesh screening deck scalps oversize particles while a bottom No. 10 mesh screen dedusts on-size material, with each of the three classifications exiting through side discharge spouts. A rotary valve feeds on-size pellets into a pneumatic conveyor for transfer into a silo.

How the Vibroscreen Separator Works

Bins at floor level collect oversize and undersize particles. On-size pellets are discharged into a pneumatic conveyor.

The Vibroscreen circular vibratory screener is equipped with one imbalanced-weight gyratory motor positioned beneath the screening chamber. The motor imparts multi-plane inertial vibration to the spring-mounted screening decks, causing oversize particles to vibrate across the screen surface in controlled pathways to the screen periphery where they are discharged.

Screening efficiency improves by forcing material to pass over a maximum amount of screen surface. Undersized particles pass rapidly through the screen.

On the first two extrusion trains, Equistar operates two Vibroscreen separators each at a rate of 22,000 LB/hour (although each unit has the capacity of 55,000 LB/hour). The third train produces about 60,000 LB/hour of HDPE pellets, using two Vibroscreen separators. Equistar is building a fourth extrusion train which will also produce about 60,000 LB/hour. The new line, equipped with two Vibroscreen separators, will effect similar savings and productivity gains to meet increased demand for HDPE.