Shock Pulse Analyzing Case Study

The Northampton Generating station is a 120MW solid fuel power generation plant. The facility moves solid fuel with conveyors into silos for storage. Conveyor reliability is critical to the operation of the entire plant. No fuel, no power. There are about a dozen critical belt conveyors that, upon failure, could starve the plant of fuel resulting in an outage. The main bearings on the conveyors were not monitored, leaving the facility unaware of pending bearing failures. Several failures occurred that resulted in load reductions. However, the failures could have easily resulted in a total plant shutdown at a cost of more than $100,000 per day.

Management decided to begin a regime of condition monitoring on the many large, slow-turning bearings. Vibration monitoring was feasible but cumbersome and impractical because of the low speeds involved. Infrared temperature measurement was considered to detect a failure-in-progress, but this would not provide the desired advance warning.

Various vendors offered suggestions, including the use of shock pulse monitoring (SPM) for lubricant and bearing condition assessment. SPM is a well-established technique. Initially, SPM was launched to enable the technician to cost-effectively track bearing condition. The tool proved effective as a method to assess general lubrication requirements as well.

Shock Pulse Basic Principle of Operation

SPM stands for shock pulse monitoring. The term “shock pulse” describes what is produced when a ridge or asperity from one surface contacts a ridge or asperity of another surface when the lubricant film, designed to separate those surfaces, is lost.

The pulse is a high-frequency signal that is collected with a sensor tuned to measure noise in the 32 kHz range. Research done by engineers within SPM has led them to recommend sensors fixed to this specific frequency. High-frequency signals are subject to damping from various influences, including thickness of the lubricant film, the number of layers of machinery that the signal must travel through, the thickness of the layers, bearing looseness, bearing condition, sample location, sample collection repeatability, sensor condition, etc. Consequently, precision in planning, data collection and interpretation is needed.

The SPM signal is collected as close as possible to the center of the loaded surface. A visual indication of the extent of asperity contact is provided to the analyst at the moment of data collection, and the signal can be captured for trending. The analyzer alerts the user by giving color-coded warnings - red, green and yellow alarms - plus, A, B, C and D grades. These colors and grades enable a technician to act immediately or to continue to investigate the problem.

The SPM data collector also provides a method for collecting an overall vibration reading in inches per second (IPS) for the analyst. Data is collected in vertical, horizontal and axial planes. The data can then be stored for trending or can be reported at the moment of analysis and either recorded by hand or not recorded.

Application of the Technology

While the original intent of the shock pulse program was only to assess the health of low-speed bearings, it has been a valuable tool in checking the proper lubrication of other systems as well.

By using the Ultra-Lube acoustic lubrication device, the technician was able to find those bearings that previously were ignored because they were hard to grease. The Ultra-Lube is a maintenance tool, manufactured by UVLM, Inc., that attaches to an ordinary grease gun. It allows the user to listen to the actual sound of the bearing in the sonic frequency range (20 Hz to 20 kHz) as it is being lubricated. The device also saved grease on the easy-to-reach bearings that were frequently overlubricated.

With early successes, the scope of the program and the list of monitored equipment grew rapidly. The program now includes motors, pumps, conveyor bearings, fans and critical rotating equipment throughout the plant. If equipment has a grease fitting, it is monitored as well.

Steps in the Process

After management selected the method, the SPM technician provided assistance in identifying the proper location to mount adapters (to improve the quality of the collection process and the data itself). Next, he began to collect initial baseline data, such as: bearing speeds and numbers, nameplate information, asset numbers, etc. Once the baseline data was established, a route was constructed and the technician began to collect data and modify the work practices.

Additionally, the lubrication strategy changed dramatically. Greasing is no longer the responsibility of both operations and maintenance personnel whom often had conflicting priorities. The previous strategy of greasing was to have several people perform what was perceived as a simple job. Technicians would overgrease, undergrease and simply not grease at all.

Now lubrication is a responsibility delegated to one person to be fulfilled in a proactive manner. Additionally there is one person in charge of all vibration, oil analysis and shock pulse monitoring activities. This change was a pleasant one for those who previously performed the greasing responsibility.

There were many surprises for the technician as the program unfolded. He discovered motors for which each bearing had received a different lubricant. Over time, it was discovered that bearing manufacturers used one type of grease in the shielded bearings, the equipment rebuild shop would use a different type of grease, and the plant would use a third. A lot of time was spent talking with bearing supply houses and equipment rebuild facilities, and searching for OEMs to determine the original practices. Were the greases compatible? No one knew.

Currently, the technician knows the bearing type and what, if any, grease is supplied by the OEM or the rebuilder following a maintenance event. This enables the plant to dictate what type of grease to use in the equipment and to limit the types of grease used, reducing confusion and possible contamination of the bearings.

Results

The biggest challenge was just getting the program started. All the equipment needed to be identified, categorized by greasing and data-collecting capabilities, and simplified. The process took five months of dedicated effort from start to full operation. The plant technician had assistance from a couple of vendors, including the SPM vendor, which provided valuable help and information. It was also helpful to read in Lube-Tips how other companies solved their problems.

Northampton Generating is still repairing some equipment, but has the bases covered with shock pulse, oil analysis and vibration. Previously unmonitored critical equipment was not sufficiently maintained through other PM tasks. Every time data is collected, some piece of equipment is found in an early stage of wear. When wear is found, the technician adds some grease, which adds more life.

The program generates savings through failure avoidance, particularly for critical unspared equipment. The saves are countless. In the fuel-handing building alone, 90 bearings are under shock pulse analysis. In the first week, two bearing failures were detected, 15 bearings on the verge of serious trouble were discovered and more than three dozen needing lubrication, from three shots to three tubes of grease were identified.

It is difficult to quantify precisely how many failures were avoided. However, any one failure that would have resulted in a plant shutdown pays not only for program capital and implementation costs, but also for long-term program support. Each shutdown costs between $100,000 and $150,000. The program exceeded plant expectations regarding cost avoidance, problem solving and learning about the plant and about the science of preventive maintenance.

Keys to implementing a successful program include:

1. The person who runs the program needs to have a thorough understanding of almost every facet of the plant.

2. Education occurs or is acquired through training and on-site trial and error. Most of the training took place at the plant. The most important part of the training (learning the data collector) occurred at the vendor’s facility.

3. Management needs to support the program.

4. Strict limits for oil analysis, vibration or shock pulse must be set. The author sets the standards and accepts nothing less each time. Each technology platform has its own support software.

5. Be patient. Be consistent. Grab the ball and run with it.