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"The continued operation after all pumping fluid has been transferred will create a dry run condition inside the pump, leading to damage from this lack of lubrication. Such a situation may also occur during pump priming, where the pipe system and pump casing have not yet been filled with fluid and the pump runs dry until the priming period is over.
Many submersible pumps s must remain submerged to insure that the motor is properly cooled. Occasional maintenance to keep the inlet strainer unobstructed is necessary, depending on operating conditions. DO NOT allow a submersible pump to run dry unless it is specifically rated for this.
Some submersible pumps are designed to use water as a coolant, overheating when they run dry and eventually burning out, resulting in mining companies having to incur costly repairs or buy new.
Therefore, a submerged motor should not be run for more than a few seconds without being immersed to prevent over-heating. The point made earlier that a dry pump will over-run and cause damage to the rotating elements (bearings, etc) is also valid." For more queires, checkout tftpumps.com