Cleaning Up A Maintenance Nightmare

A hydropulper is an industrial blender used in the pulp and paper industry to process fibrous materials into a useable slurry. As shown in Fig. 1, the main parts include: a vessel, a lower chamber containing an agitator or impeller, a rotary drive, motor, gearbox, a tube to re-circulate the slurry and some type of sealing system work to prevent water and other kinds of contamination from damaging the equipment.

In simplest terms, a hydropulper’s tanks are filled with water where agitators mix material into homogenous slurry. Sensors gauge the slurry consistency and make adjustments by adjusting the water to thin or thicken the mix. A rotor or agitator inside the chamber vigorously pulps the fiber while an impeller moves the flow through an outlet and tube back to the vessel. Once the desired consistency is reached, it is pumped out, while a de-watering screen saves the water for re-use.

Hydropulper sealing
During the pulping process, material comes in contact with the rotor, a tremendous shock load is transferred to the shaft and it flexes the shaft of whatever sealing system is being used (contact, lip, labyrinth, etc.). To maintain the integrity of the seal, and keep contamination out, among other things, it must be able to accommodate shaft movement. Over time this action can break down even the best of seals. When a seal breakdown occurs, water runs past the component and down the shaft where it enters and contaminates the gearbox housing. Sealing options that have been tried on hydropulpers include [Ref 1]:

• Lip seals—these dry running devices can wear out, break down or fall apart. Their short service life can be as little as 1800 hours. They actually can do damage by cutting into the shaft at the sealing point. Double lip seals can do twice the damage.
• Sealed bearings—so-called (lubricated-for-life) bearings do not seal out moisture or water.
• Fibrous packing—degrades and can fret the shaft.
• Close clearance designs—still allow for humidity egress/ingress.
• Contact face seals—stop contacting, produce gaps that allow for the movement of air and water across the bearing.
• Flingers—rings that deflect leakage away from packed or sealed equipment are basically ineffective.

In time, using any of these methods, water will be sufficient enough to get past the seal and into the gearbox bearings and cause the bearings to fail. In other words, the root cause of the failure was not addressed.

The hydropulper’s problem 
KTG’s Memphis plant operates five (Voith) hydropulpers that have been in service for approximately 40 years. The units were all retrofitted and modernized in the 1990s, including the gearboxes and motors. Still, they continued to experience ongoing breakdowns—and it was never a pretty sight (see Fig. 2 and Fig. 3).

According to Dave Knox, KTG maintenance planner who oversees maintenance on the plants, refiners, pulpers and paper machines, the main cause of the failure was water contamination in the gearbox. Mounted directly under the hydropulper tank, water entered through the output shaft, entered the bearing housing, contaminated the bearings and the gearbox failed. The problem had been recurring for years and had not been solved by the previous owners.

When the mill restarted in 2003, so did the equipment failures. Although the maintenance team knew that the root cause of the failure was water contamination in the bearing housing, it felt that it just had to live with it. To complicate matters even further, because of the hard-toaccess nature of the components, it was difficult to determine exactly when a contact seal might fail.

The problem continued because the standard overhaul procedure included the use of lip seals. While these components might have been brand new, right out of the box, the gearboxes would be doomed to fail again—it was just a matter of time. In fact, the problem of water contamination hindering the entire system was to continue until the true root cause of the failure was attacked two years ago—when the Memphis facility began to install bearing isolators in its hydropulpers.

Why lip seals fail
To understand the problem at KTG, one has to look at the history of lip seals. At the time they were first made available some 70 years ago, they were the only choice when it came to general-use sealing devices. Because of their inexpensive cost, over the years they became the number one choice for sealing industrial rotating equipment.

Today, according to their own manufacturers, even the best lip seals have a mean life to failure of only 1844 hours—or 77 days of operation. Half last longer than that and half last less than the mean time hours to leakage. This means that lip seals have a guaranteed failure rate of 100%.

As was experienced at KTG, no one can determine when the time is up for a lip seal. There simply is no advance warning. The only way to tell is after the equipment stops working and the lip seal has burned to a crisp and probably grooved the shaft.

Contact vs. non-contact 
While a lip seal or contact seal operates with contact, the bearing isolator, a non-contacting labyrinth-type seal, makes no contact. It never wears out and can be used over and over for many years. With this in mind, it may not make sense to protect rotating equipment that is designed to run uninterrupted for years, with a product that could experience a 100% failure rate in a relatively short period of time.

Bearing isolators
In the late 1970s, an alternative to contact/lip seals was made available with the invention of the Bearing Isolator, a noncontact, non-wearing, permanent bearing protection device [Ref. 2].

The bearing isolator consists of two parts, a rotor and stator, which are unitized so they don’t separate from one another while in use. Typically, the rotor turns with a rotating shaft, while the stator is pressed into a bearing housing. The two components interact to keep contamination out of the bearing enclosure and the lubricant in—permanently.

Today, bearing isolators are used to protect motor and pump bearings, machine tool spindles, turbines, fans, gearboxes, paper machine rolls and many other types of rotating and related equipment. Though the end-user has a choice, the best bearing isolators are made of metal, usually bronze, manufactured to specification, with a vapor-blocking feature to inhibit the free transfer of contamination (see Fig. 4).

The hydropulper solution 
When Dave Knox approached Mike Perkins, his Chesterton distributor, about the Memphis mill’s ongoing hydropulper breakdown problem, Perkins suggested trying Inpro/Seal branded bearing isolators. Following this recommendation, Knox met with Joe Klein, Inpro/Seal’s regional manager. Working together, Knox and Klein developed a plan of attack.

Bearing isolators were engineered and manufactured to the hydropulper drives’ exact needs and specifications. Between 2005 and 2006, these new devices were installed on two of the five hydropulpers as part of the overhaul program. For the last two years, the Memphis KTG site has not experienced a single hydropulper failure. That’s because the reason for their previous ongoing failures— water entering the gearbox housing—was totally eliminated. In the future, this type of bearing protection is expected to be applied to the remaining three hydropulpers.

The rest of the story
In addition to bearing isolators on its hydropulper drives, KTG also uses PMR bearing isolators on its paper machines. The PMR (an acronym for paper machine roll) bearing isolator was specially engineered for the size, speed, alignment and operating conditions of wet and dry ends of machine rolls.

As with the hydropulpers, before the availability of bearing isolators, end users had to contend with sealing methods that allowed roll bearings to fail. The leading cause of this failure also was contamination entering the bearing housing—contamination from heat, humidity, paper stock, water and oil leakage.

The bottom line 
K.T.G. (USA) LP – Memphis cleaned up the problem with its hydropulper breakdowns by keeping water out of the units’ bearing housings—the root cause of the failures. Key to this was replacing outdated sealing methods with state-of-the-art non-contacting technology.

Since it began installing Inpro/Seals two years ago, the Memphis operations have yet to experience a single breakdown on any bearing isolator-equipped hydropulper. Once the facility installs these devices on its other hydropulpers, breakdown due to water contamination should be totally eliminated.

One thing is certain—the installed bearing isolators will not experience unexpected breakdown [Ref. 3]. These well-engineered components should run maintenancefree throughout their intended design life, which could be 20 years or more.

References
1. Before the advent of the bearing isolator 
2. David C. Orlowski holds the patent for the “bearing isolator,” a term he coined when he founded Inpro/Seal Company in 1976. 
3. The first bearing isolators, installed in a process plant in Iowa over 20 years ago, are still operating. In addition, Inpro/Seal offers a full no-questions-asked warranty. 
4. Based on available statistics.

Dave Orlowski is founder, president and CEO of Inpro/Seal Company. E-mail:dco@inpro-seal.com

(Editor’s Note: This article is based on one that first appeared in the December 2007 issue of Maintenance Technology magazine.)