Preventive maintenance blocks blackouts

A change of coupling design on each of its 24 coal pulverisers is helping one Midwestern power station to improve the reliability of its generating process.

More than 50 million consumers and business owners from Ohio to New York and Canada would agree that a consistent power supply is vitally important, not only to the overall economy, but the comfortable, regulated routine of daily life.

The major power outage that struck the USA and Canada in mid-August of 2003 cost New Yorkers alone an estimated $500 million.

Although experts point to an antiquated power grid as the main cause of the recent outage, every portion of the power generation system - down to its smallest component - plays a vitally important role in maintaining the electricity levels that light up the nation's homes, businesses and communities.

About a year ago, a leading coal-fired plant located in the Midwest USA was experiencing gear coupling failure in its coal pulveriser drives.

One plant can have upwards of 25 coal pulveriser drives, depending on its size.

Pulverising coal results in lower emissions and a more efficient use of the fuel.

The heat generated is used to create steam that turns the generators, providing electric power.

Large electric motors drive the coal pulverisers by means of a worm gear arrangement, which also drives an exhauster fan that blows the pulverised coal to the furnaces.

The shafts from the motor, worm gear and exhauster fan must be connected mechanically.

This is where a relatively small, yet vital component, a coupling, comes in.

If this fails, the pulveriser can no longer operate.

The original design of the power plant incorporated the use of "gear" type couplings.

This type of coupling allowed axial movement of the shafts, increasing the wear on the gears and other components, such as the sleeve bearings in the motors.

Eventually, these components would fail prematurely.

Loss of production due to downtime was becoming an issue.

Bearing replacement costs were in the thousands per bearing.

Preventative maintenance costs associated with gear couplings was becoming a burden due to time spent on inspection and lubrication to keep them operative.

The power plant's rigorous preventative maintenance program required regular disassembly of the gear couplings for inspection, cleaning and relubrication of the gear teeth.

Visual inspection of the couplings was a judgment call, unless extensive, time-consuming measurements were performed.

Additionally, heavy machinery was required to move the motors for coupling replacement.

The plant manager needed a solution, and contacted Lovejoy Sales Engineer, Scotty Pucheu, to analyse the situation.

What Pucheu determined was that the current gear coupling design was typical of the technology available when the plant was built in the early 1970s.

The gear couplings did not limit axial movement of motor shaft.

"Sleeve" type bearings were used in the motors.

This fact, thermal growth of the shaft and the position of the shaft at magnetic centre increased wear and shortened life spans of not only the motor sleeve bearings but the gear couplings as well.

"Increased sliding velocity of the gear teeth in the gear coupling", says Pucheu, "creates a lot of heat and wear.

It's like rubbing two sticks together.

The motor moves a bit, the gear on the coupling moves a bit, and this motion could be repeated up to a million times over the course of the day, contributing to premature failure".

A possible solution would have been to install gear couplings with limited end float, but the maintenance issues associated with gear couplings would continue.

Gear couplings are difficult to assess for wear and for inspection and replacement of the gear coupling still required removing the motors.

Gear couplings would still have to be disassembled, inspected, cleaned and regreased every time.

Unwilling to compromise on maintenance costs, the plant manager decided to invest in Pucheu's recommendation to install Lovejoy/Jaure SX disc couplings.

Unlike gear couplings, the Lovejoy/Jaure SX disc coupling design is inherently balanced - less vibration than gear as it wears, and performs limited end float function without special design considerations for the sleeve bearings.

Maintenance free operation is achieved because the Lovejoy/Jaure SX disc coupling has no moving parts and requires no lubrication.

This translates into long coupling life.

Preventative maintenance/inspection of the coupling's flexible elements was now possible using a simple strobe light, even while the pulveriser is running, instead of expensive shutdown and disassembly.

Replacement of the disc packs could now be done predictively and without moving any machinery or disturbing any original alignment.

Another bonus to the customer was that Lovejoy was able to provide, in the same space allowed by the original design, a heavier duty coupling than the gear type.

Nothing had to be moved or modified for installation.

Additionally the component cost was comparable to those of a gear coupling.

"Until our disc coupling came along, it wasn't practical to put a disc coupling in this system", says Pucheu, "but Lovejoy's design allows for more capacity and misalignment capability than any other disc type of coupling available today".

In addition, Pucheu points out that the disc coupling signals the maintenance staff when it needs replacement.

"With a gear coupling, it's a judgment call", said Pucheu, "unless you take it completely off the machine and measure it extensively, no one can tell visually if the gear teeth are acceptable".

With the Lovejoy/Jaure disc coupling, the outer leaf of the disc will crack before the entire disc coupling fails, signalling to the maintenance staff it needs replacing.

Then it simply requires replacing the disc pack.

"The coal pulveriser is critical equipment", says Pucheu, "if you're not blowing coal, you're not generating power and losing revenue".

The plant has been experiencing success for more than a year with the original installation.

"Plant management approved the proposal to convert all gear couplings on 24 coal pulverisers to Lovejoy/Jaure disc couplings", says Scotty Pucheu, "based on the proven benefits and demonstrated cost savings".

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