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1800 Vickers

Note 1: in/in/°F X 10-6 (multiplied by 1.411 to get cm/cm/°C) Note 2: BTU/hr/ft2/°F (multiply by 0.488 to get calories/hr/cm2/°C) Note 3: RC X 25 = Vickers; BHN X 10 = RC

Note 1: in/in/°F X 10-6 (multiplied by 1.411 to get cm/cm/°C) Note 2: BTU/hr/ft2/°F (multiply by 0.488 to get calories/hr/cm2/°C) Note 3: RC X 25 = Vickers; BHN X 10 = RC

for a bearing with grooves. Polymers and graphites are much softer than silicon carbides or hard coatings, and they are not recommended for liquids with hard particles. Silicon carbide versus silicon carbide will grind up most particles. For particle concentrations of 50 parts per million (ppm) or less, particle hardness is generally not a factor in bearing performance.

Strainers of 100 mesh are recommended to reduce the amount and size of particles going through the flow path. These strainers are installed for internal or external injection. Remember: Particles that will pass through a 100 mesh screen may be as large as 0.006 in (0.15 mm), whereas nominal diametral bearing clearances of 0.002 in (0.05 mm) are common.

Note also that the bearing material combination of silicon carbide against silicon carbide or against carbon is electrically conductive.

Running Dry Most sealless pump failures occur because the pump system is not monitored and the pump is allowed to run dry. When this happens, the bearings will run dry. When the bearings run dry, hard, brittle bearing materials such as self-sintered silicon carbide will fail within minutes. Graphite silicon carbide may run dry for as many as 10 to 20 minutes with no damage. The polymers, which are poor heat conductors, expand inwardly and seize against the mating surface. The graphites will also move inwardly, but they tend to wear rather than seize, which results in excessive clearances when the pump is stopped and cooled. One optional design uses Teflon strips that expand inwardly when the unit runs dry so the shaft runs on the Teflon rather than on the silicon carbide, thus helping to avoid bearing failure.

Another option is to employ an external circulating tank, with no external running parts, that will provide the bearings with external lubricating liquid should dry operation occur. This type of external tank system has allowed pumps to operate for two hours or more without incurring damage to the bearings.

Flow Path The amount and direction of flow for cooling of the magnets and lubrication of the bearings is critical to the operation of a sealless pump (Figure 16). It is preferable for the liquid to lubricate the bearings before being heated by the magnets. This reduces the possibility of vaporization of the liquid occurring at the thrust-bearing faces. The amount of liquid circulated through the system is usually between 1 and 8 gpm (4 and 30 l/min). It is usually channeled to the front and back bearings, the thrust bearing face, across the magnets, and to the impeller hub. Some manufacturers have computer programs that calculate the flow, pressure, and temperature of the cooling/lubricating flow at various critical locations along the flow path. These programs can also account for the effects of the liquid specific gravity, specific heat, and viscosity. The local pressure and temperature is used to determine the vapor pressure at that point to ensure that the liquid is not flashing. The programs can also calculate axial thrust and determine if the lubrication at the thrust bearing face is hydrodynamic or boundary. This is done at various flow rates, impeller diameters, and pump speeds.

The temperature rise of the liquid as it travels through the cooling-lubricating flow path depends to a great extent on the liquid's characteristics, such as specific gravity, specific heat, vapor pressure, and viscosity. With a nonmetallic shell on ambient water service, a typical temperature rise might be 1 to 2°F (0.5 to 1°C). For a metallic shell, with its much higher eddy current losses, the temperature rise could be as much as 8 to 12°F (4 to 7°C).

internal internal

FIGURE 16 Magnetic sealless pump components for the internal flow system
FIGURE 17 Typical performance for a 1.5 X 1.0 X 6 pump at 3550 rpm, comparing characteristics for sealless versus mechanical seal construction

Performance The head capacity curve of a typical two-pole speed sealless pump matches that of a conventionally sealed pump; however, the overall efficiency is lower. With a nonmetallic shell, the efficiency may be only about two points less at the best efficiency point flow rate, but as much as six points less at one-half the best efficiency point flow rate. When a metallic shell is used, the efficiency at best efficiency point flow rate may be as much as 8 to 12 points lower than a comparable pump with a mechanical seal (Figure 17).

APPLICATION ADVANTAGES OF SEALLESS PUMPS:

• No leakage to the environment

• No loss of valuable liquids

• Lower noise levels

• High suction pressure does not affect the axial thrust

• Can handle liquids from 0 to 4 toxicity rating

• Because of no leakage, there is much less chance of a fire

• Easier to obtain construction permits and permits for continued operation

• Less external piping required

APPLICATIONS THAT SHOULD BE REVIEWED BEFORE SEALLESS PUMPS ARE APPLIED:

• Dirty liquids

• High temperature

• Liquids that solidify

• Viscous liquids above 200 centipoise

• Oversize drivers that can cause decoupling during acceleration

• Cavitation of liquid in the impeller eye that can result in excess thrust

• Excessive entrained gas

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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