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Mechanical Seals are a vital pump component which prevents fluid from escaping a pump chamber through the port where a shaft drives the pump. When these seals fail, they need to be replaced quickly to avoid excessive downtime.

 

In recent years mechanical seals have replaced many stuffing boxes which was the original sealing method. With the introduction of mechanical seals, pumps could be made smaller, and innovations such as jet pumps became a reliable product. Mechanical seals, however, remain a delicate component of the pump. Even small scratches on the sealing surface can create a leaking seal.

 

The mechanical seal is generally a long-lasting pump component. But, of course, they can fail for various reasons. So, how do you reduce mechanical seal failures? Here are some of the common causes:

 

Running Dry - Mechanical seals rely on two mating surfaces compressed by a spring and elastomers surrounding them to seal the fluid. Surfaces must be lubricated while pumping by creating a water film between them. If the seal is not lubricated the faces will gall and the seal will be destroyed, causing a leak from the shaft area.

 

Abrasives or Chemical Compatibility - As in the case of running dry, abrasives can enter the seal cavity and grind between the seal faces. As before, when the sources fail to mate up properly, the shaft area will leak. Since elastomers are part of the seal, chemical incompatibility can cause the elastomer to deteriorate. Fluid will then flow around the outside sealing area.

 

Insufficient NPSH - When the NPSHa (available) does not exceed the NPSHr (required by the pump) cavitation will occur in the impeller eye causing vapor bubbles to implode within the impeller. These implosions can occur between the sealing faces and have the effect of running dry within the seal. This can be eliminated by altering the conditions of the suction side of the pump or by reducing the flow on the curve to also reduce the NPSHr.

 

Pumping Too Far to the Right or Left on the Curve - Pump curves have what is called “Best Efficiency Point” (BEP). This point is also where flow and pressure delivered are best balanced for the impeller design. Causing the pump to produce too far to the left of BEP causes the pressure to build near the outlet of the volute. This pressure is concentrated on one side of the impeller, which then puts pressure against the shaft, causing some deflection. Since the seal is on the shaft, some deflection can occur within the seal itself causing the seal faces to not align properly. When operating too far to the right of BEP, there is little to no pressure on the volute discharge, and again, imbalance on the shaft occurs.

 

Heat - Mechanical seals have a temperature rating, and pumps are designed with fluid temperatures in mind. If a fluid temperature reaches higher than the rating, elastomers can fail and water can even boil between the seal faces. This will cause failure due to no lubrication. Another heat related cause is pumping against a closed valve. Without flow, the fluid in the pump will heat and eventually boil which will lead to seal failure.

 

Excessive Suction Pressure or Working Pressure - Pumps have a Maximum Working Pressure and, depending on design, a Maximum Suction Pressure. The mechanical seal also has a Maximum Pressure Rating after which fluid can be forced past the seal and leaking occurs. The seal must be capable of handling the pressure limits of the pump whether it is the suction side or the discharge side.

 

Considering all the potential problems a seal can have, it is also important to select a seal based on the product to be pumped. Surfaces, elastomers, and spring tension are all variables in seal selection along with the physical size restrictions. If care is taken in the selection process, the seal may last the life of the pump.