Seals and gaskets: New methods cut water, energy consumptio

Pulp & Paper, Mar 1995 by Glowacki, Jeremy

During the past several years, the dry-running mechanical seal has gained considerably on compression packing as the most environmentally acceptable and water/energy saving method for sealing rotary shaft equipment in the pulp and paper industry. However, recent innovations have allowed the compression packing industry to take a huge leap forward with its own dry-running designs.

Timothy Suprise of Durametallic Corp. says that while compression packing still holds up to 65% of the pulp and paper market, in the long run, installation of mechanical seals can provide a more efficient and less expensive operation than packing.

With new cartridge and split seal designs, Suprise points out that mechanical seals have become much more "user-friendly," meaning that they no longer require the time and effort once required to disassemble the equipment for installation. "Often times, these seals can be installed on large shaft equipment in less time than it takes to remove existing packing and repack a pump, agitator, or hydropulper," he says.

Mechanical seal manufacturers also solved water consumption and effluent problems that had pulled some pulp and paper manufacturers away from compression packing. With what is referred to as a dry-running technology, Durametallic, for one, has designed a gas barrier technology utilizing instrument air or a nitrogen gas source.

Gas barrier technology enables a mill to operate double mechanical seals with zero emissions at drastically reduced energy requirements, according to Suprise. An added bonus to this technology is its ability to enhance the capability of off-design operations, especially with pumps that have had a tendency to cavitate when handling fluids, while also reducing 25 to 40% of a mill's water requirements.

In the pulp and paper industry, Suprise adds that dry-running mechanical seals are used mostly in the chemical services applications, particularly as a way to add value to bleaching operations as well as the more demanding acid pumps in the pulp mill.

COMPRESSION PACKING. According to Al Harrelson, manager of engineering at Garlock Inc.'s mechanical packing div., compression packing manufacturers noticed that they were losing business to the more environmentally friendly and economical mechanical seals. After considering energy consumption and effluent concerns, he says, compression packing producers realized they needed to develop their own dry running technology to remain competitive.

"With traditional compression packing braids, a certain amount of lubricant is always required to leak out of the seal to act as a binder between the packing rings and the shaft," Harrelson explains. "This used to be our only way to prevent frictional heat buildup."

Harrelson says that compression packing had managed to remain competitive prior to a dry running design due to several factors. The first factor was the higher product and inventory costs associated with mechanical seals.

Although a mechanical seal manufacturer would argue that the device would eventually pay for itself in water and energy cost savings, he points out that a mill's mechanics required special training to be able to properly maintain and install the mechanical seals. In addition, the runout, shaft finishes, and bore finishes of the pumps usually had to be upgraded to like-new conditions to work properly with a new mechanical seal.

With this in mind, Harrelson explains that compression packing manufacturers such as Garlock developed a technology utilizing high-temperature, low-friction graphite die-formed tape rings instead of traditional braided packing.

"Pure graphite is one-to-one with steel in its thermal expansion and con. traction properties," he says. He adds that the thermal conductivity properties of graphite allow the material to conduct any heat that is generated between the rings and shaft to the outside box bore walls where it can be given off. "This means that the material doesn't require leakage to occur to keep frictional heat down," he says.

Although he says that he believes there will always be a need for braided packing in certain areas of the mill, the most recent developments in this dry running technology have helped the compression packing business to suddenly appear more revolutionary than its rival mechanical seal competitors.

NEW SEALING COMPOUND, Even more revolutionary, perhaps, is a technology from Tom-Pac Inc. of Montreal, that has recently begun to take hold in the pulp and paper industry. According to Thomas Daly of Northeast Mill Supply, the self-lubricating sealing compound called TP-4000 offers an alternative to conventional compression packing or mechanical seals. TP-4000 is a colloidal mixture of lubricants, amalgamated with Aragraphe fiber, then pressure treated to ensure uniform dispersion. The compound can evenly surround a shaft and act as a lubricated plug, eliminating pressure points. With minimal friction, it also offers significant energy savings in pumps, agitators, and other rotating or reciprocating equipment.