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Aug 04, 2023

Bearings designed for abrasive liquids

Particulates in the water of a central production plant

Particulates in the water of a central production plant in Kansas City, USA, were causing bushings to fail after 30 months. A natural rubber-based, self-lubricating bearing material was suggested as a replacement. The outcome was a five year period without any performance problems and an estimated saving of approximately US$60,000.

Trigen-Kansas City Energy Corporation is a Veolia company. It co-generates electricity and provides centrally produced steam and chilled water to customers in the Kansas City, Missouri central business district, USA. A Floway VU vertical turbine pump captures up to 24,000 gallons of water per minute from the silt-laden Missouri River to feed chillers and steam generators. In the past, particulate in the water caused the bronze-backed rubber bushings to fail after approximately 30 months.

JCI Industries, a local provider of pump and electric motor repair services based in Kansas City, suggested switching to Graphallast, a natural rubber-based self-lubricating bearing material manufactured exclusively by Graphite Metallizing. Graphallast is especially well-suited for continuous operation when submerged in liquids containing abrasives such as sand and sewage. These new bushings performed without any problems for five years. At that point, they were removed during scheduled maintenance and showed no signs of wear. The decision was made to replace the bushings despite the lack of wear. JCI calculates that the new Graphallast bushings saved US$60,000 by eliminating the need to replace bushings and shafting.

Trigen-Kansas City serves approximately 60 customers in the central business district with more than 4 million ft2 of commercial space. The company's production capacity for Kansas City includes 1.3 million lbs of steam per hour, 10,200 tonnes of chilled water and 5 megawatts of electric generation capacity. The company operates a network of 6.5 miles of steam and 1.5 miles of chilled water distribution pipes.

JCI serves a wide range of markets including municipal, food, chemical, petroleum, utilities and general process industries. JCI maintains more than $2.5 million in its inventory which includes pumps, motors, repair kits, and OEM replacement parts.

When the pump was originally installed, JCI and Trigen-Kansas City engineers specified rubber bushings with bronze backing for the Floway Model VU single stage vertical pumps. The 52 ft VU pump is a close coupled, single turbine pump with a fabricated head discharging above ground. The intake end of the pump is submerged seven feet into the river. Its capacity ranges up to 35,000 gallons per minute (7,950 m3/hr) and it is rated for pressure up to 200 lbs/in2 (13.8 bars).

Originally, this pump was fitted with rubber bearing material in the line shaft and bowl bushings. These bushings are designed for abrasive liquids but still did not provide satisfactory life in this application. JCI uses vibration monitoring equipment to help identify early signs of bushing wear so they can be replaced before they cause extensive damage to the pump. This monitoring equipment consistently indicated that the previously used rubber bushings should be replaced at about 30 months from installation.

Replacing the bushings was a major operation because of the likelihood of damage to other critical components. When the bearings failed, it was necessary to pull the pump, which required removal of the roof from the pump house and hiring a crane to lift the pump. Then, technicians inspected the entire pump for wear and damage and usually had to replace the shafting, machine the flanges and verify the fits. The cost of the entire process was typically around US$60,000.

Replacing the bearings also took the pump out of service for several weeks. Trigen operates a spare pump and, during most of the year, one pump is sufficient to service the company's customers. However, during the peak cooling season in July and August and the peak heating season in January and February, demand may exceed the capacity of a single pump. It was important for Trigen to detect bearing wear problems well in advance so that necessary work would be done outside of peak seasons.

"We have used bushings from Graphite Metallizing in a wide range of applications with excellent results," said a service manager at JCI. "Our first experience with the company was when we used bushings made from their Graphalloy material in pumps in refineries. The refinery occasionally experienced upsets that destroyed conventional metal bearings due to galling or corrosion. Graphalloy provides a constant, low co-efficient of friction, is self-lubricating, non-galling, and provides thermal shock resistance to protect against catastrophic failure. Graphalloy bearings eliminated catastrophic failure and provided much longer life in these applications."

JCI later heard about Graphallast, a natural rubber based self-lubricating bearing material which survives run-dry conditions which destroy conventional plain rubber and plastic bearings. The material permits tight bearing-to-shaft clearances for improved rotor stability and reduced vibration levels. Tough, hard and resilient, the bearing material is specially suited for continuous operation when submerged in liquids containing abrasives.

JCI proposed using the Graphallast GM 801 material in the pump application. Dimensions were obtained and JCI engineers worked with Graphite Metallizing engineers to generate detailed designs. The new material made it possible to tighten up clearances in the bushings, which helps reduce vibration and noise. The material was ordered and installed in the line shaft and bowl section of the pump. The shafts were coated with tungsten carbide.

When the new bushings were installed, the reduction in noise and vibration was immediately noticeable. The bushings were checked with vibration monitoring equipment on a regular basis. Each test indicated that the bushings were in good condition and did not need to be replaced. Despite all indications that the bushings were in operable condition, Trigen asked that the pump be pulled and the bushings removed for inspection five years after they were originally installed.

The inspection showed only minor wear indicating that the bearings clearly had many more years of life in them. The customer made the decision to replace the bushings since the pump had already been removed. The cost of replacing the bushings was much smaller than pulling the pump. In addition, the shafts and journals were recoated with tungsten carbide. This job cost approximately US$60,000 and was the only maintenance expended on the pump bushings in the five years since their installation.

"We have also found Graphite Metallizing to be very easy to work with," says a service manager at JCI. "They provide many different grades with varying levels of hardness and chemical resistance that handle a very wide range of applications. We call their engineering department and give them the specifications of a new application and they recommend the best-suited material. They either provide us with completely machined and ready to install bushings or blocks of material that we machine to final specs in our shop. This makes it possible to keep the semi-finished material on hand so we can quickly finish it in our machine shop to meet our customer's requirements. The result is that our inventory is much smaller."