Originally by Neal Gunn – Technical Manger
Anyone familiar with industrial magnetic drive (mag drive) pumps has been asked about dry running the pump. After all, a dishwasher and washing machine at home do that all the time. There are bearing materials that allow that, right? The answer is a qualified, “Yes,” but let’s take a closer look at dry running.
Dry running, except in the literal sense, is a misnomer. Industrial pumps are mechanically sealed or sealless and can suffer from the results of what the industry has dubbed “dry run operation.”
Silicone Carbide (SiC) Bearings
Let’s examine the mag drive pump configuration with silicone carbine (SiC) bearing components, one of the two sealless types of rotodynamic centrifugal pumps.
Mag drive pumps have product lubricated bearings that require liquid to be present for proper operation. These hydrodynamic (plain) bearings depend on a fluid barrier separating the bearing components that becomes a wedge when there is relative motion between the two surfaces. When this fluid barrier is disrupted, the two surfaces will touch and can lead to component damage. This effect can be cumulative, resulting in component failure when limits are reached.
If the SiC surfaces touch during operation, the components can, depending on several factors, break in a short time. This does not mean the pump immediately stops operating. The broken SiC pieces may still support the shaft well enough to allow rotation of the pump rotor. Broken SiC can be detected by increased vibration and often a power increase. The condition of the pump will only get worse if operation is continued.
When the pump loses suction, it should be shut down immediately. Damage to the SiC components can occur in a matter of seconds. Using a power monitor to detect when the motor load has suddenly changed, such as when the pump loses suction, will protect major pump components. Continued operation without adequate liquid at the suction can quickly lead to severe damage of the SiC components.
For long life, the pump requires that there be liquid available and that it be operated within the pump flow range of the curve and within the pump design limits. Please note that it is best to start the pump against a partially closed (about a quarter open) discharge valve. The suction valve should always be open when operating and only closed when the pump must be isolated for maintenance reasons.
So SHOULD a pump be dry run? Ideally, no… but CAN it survive under upset conditions?