Replacing leaded bronze in pumps with Graphalloy®

An engineer writes:

Asarcon 520 was a lead impregnated bronze used for sleeve bearing in horizontal split pumps. I have not seen it in use since my days at Worthington Pumps in the late 70’s. It was a material highly specified by the marine industry for on board ships, boiler feed pumps and fire pumps. As you can imagine, lead impregnated bearings were capable of running for short periods of times un-lubricated without the rotor disintegrating. The material was readily available in bar stock, however with the issues associated with lead contamination today, I do not believe it is available in the market any longer. To make sure, I would call the local bearing distributor or and bar stock suppliers and see if a similar material is available under some other name.

Years later, while I worked for Exxon as a Rotating equipment engineer we began using graphite/metal Graphalloy (graphalloy- http://www.graphalloy.com/html/products.html) sleeve bearing for pumps and Thorlon for the slower running centrifuges. We retrofitted some old packing pumps with mechanical seals that operated in hot oil services. These pumps, by design, depended on the packing for shaft support. In order to support the shaft and allow the mechanical seal to live, I installed steel encased Graphalloy (with spiral grooves to convey cooling pumpage) between the bearing housing and the old packing box. This eliminated the run out and allowed the seals to run true and consequently not leak. The Graphalloy material can be used in plain stock or encased in a steel sleeve to add strength, depending on need. In either case you must calculate the loads to assure that the bearing is appropriately designed, can support the shaft loads and you do not located your support at a node in the flexural curve.

Luis

How will Graphalloy handle our jet fuel pump application?

The principal bearing issue for Jet fuel is its very low specific gravity which makes jet fuel a poor lubricant.

Graphalloy bearings have many applications in very low gravity pumpages (lng, ethane, propane, benzene, CO2 etc.). Graphalloy is excellent for these applications because in these low specific gravity fluids Graphalloy provides a margin of safety allowing moving parts to contact without galling or seizing.

We have a history with aircraft fuel gear pumps (from memory I think these go back to the 60's). Our primarily pump business is in high HP centrifugal boiler feed and process pumps (vertical and horizontal) where low lubricity, flashing or run dry conditions are a possibility. Graphalloy bearings will allow the pump to survive an upset. So we have lots of related experience and are confident that the materials can handle a jet fuel pump but we encourage you to do you own testing in as close to real world environment as possible.

We mostly supply finished bearings/bushings and generally these are custom sized for a specific manufacturer's equipment. Pumps are a large part of our bushing applications. The key Graphalloy material advantage is the ability to run dry or in very light fluids (jet fuel for example) without galling. This permits the pump to run dry for a period and then re-establish pumping without failing. (FYI http://www.graphalloy.com/html/pump.html ).

For a metal pump, the usual design is to install, with a press fit, directly into the housing of the pump. The bushings can be supplied with a metal backing but this is normally not necessary. One advantage to the press fit installation directly into the housing of the pump is that as the temperature of the housing changes the bearing can be designed to maintain both secure mounting and a constant clearance to the shaft over a wide range of operating conditions.

In order to proceed to a design, our engineers will need: pump cross sections and dimensions on housing and shafts, materials of construction, rpm, expected loads and pumpages.

How does Graphalloy compare to Tungsten Carbide

A reader wrote to ask about the comparison of Tungsten Carbide to Graphalloy.

As you know Tungsten Carbide is an extremely hard, brittle material. As a bearing, it resists wear from abrasives.

Graphalloy is much softer, but can last longer running dry (without lubrication). However, it doesn't stand up to abrasives as well.

Which is better, depends on the application. Go to www.graphalloy.com/html/inquiry.html and give the engineering department a chance to review your application.

Can you give an example of a high temperature use?

GRAPHALLOY has recently been used in a high temperature steam (up to 540C) and inert atmosphere environment to convert wood chips and used cooking oils into biodiesel fuel. Because standard bearings were not an option in this environment, Graphalloy provided a solution. The production of biodiesel fuel is important because it is biodegradable, nontoxic, clean-burning and renewable.

Additionally, it burns much cleaner than petroleum-based diesel, creating about 60% less net carbon dioxide emissions. These factors make it an ideal fuel for buses, trucks and farm equipment. This application design uses several standard bushings as well as custom-sized bushings for their prototype machines.

Graphalloy’s diverse mountings and extensive experience proved valuable in developing a solution for this difficult environment. Alternative fuel manufacturing is a growing field due, in part, to efforts to reduce dependence on petroleum-based fuels. Graphalloy will continue to be a valuable resource for these high-temperature applications.

Why do GRAPHALLOY bushings have oversize OD and ID dimensions relative to my housing and shaft diameters?

A. A GRAPHALLOY bushing needs to be retained in its housing via an interference fit. The coefficient of thermal expansion of GRAPHALLOY material grades is low relative to steel and other common housing materials. In order to maintain retention at elevated temperatures, the initial press fit needs to be sufficient to overcome the difference in size change as temperatures rise.

Graphalloy is also quite elastic and the interference between the bushing OD and the housing ID will close up the bushing ID upon installation in the housing. The bushing also needs to have a clearance with the rotating shaft to allow movement.

If the housing and shaft are similar materials and are heated to similar temperatures, the clearance between bushing and shaft will remain nearly constant.

What is GRAPHALLOY and when should I use it?

GRAPHALLOY(r) is a solid, uniform combination of graphite and metal. Molten metal is impregnated into a graphite substrate under high temperature and pressure. Used as a bushing or wear surface, GRAPHALLOY survives dusty atmospheres, high temperature, submersion and corrosive environments. GRAPHALLOY does not swell, shrink, or cold flow.

Graphalloy is self-lubricating and used in difficult applications when a rolling element bearing would fail. It deposits a graphite layer on the inner race which is not normally removed by high temperatures or submersion in a fluid. It is not normally a competitive replacement for a ball bearing where there are no environmental complications.

Hard carbons for bearings and bushings

Q. Hard carbon materials are available; will they perform better GRAPHALLOY(r) for bushings?

A. Carbon materials can be made over a wide range of hardnesses - from soft brush materials to nearly ceramic hard mechanical seal face materials. Hard carbons can be much harder than usual shaft materials. These hard carbons can score shafts to the point where the shaft is actually cut.

Bushings, especially those internal in pumps, are intended to be wear parts, absorbing the shocks, rubbing and abrasion of normal use without damaging the shafting. GRAPHALLOY is chosen for pumps to provide non-galling, self-lubricating bearings.

GRAPHALLOY with a moderate hardness is used in most pump applications to ensure that expensive shaft materials will be protected.