External Mechanical Seal

Externally flushed seals operate similarly to other mechanical seals, but are designed to be installed in the dry space of the volute, just like older style packed seals. Thus, they require external lubrication to prevent overheating from friction between the shaft and rotating and stationary seal parts. This lubrication is provided by capillary type tubes that route water from the discharge of the pump to the mechanical seal, thus making the system closed as opposed to externally lubricated packed seals. There are three downsides to this design.

The first concern is simply the size of the flushing line. These are normally ¼” or 3/8” copper tubing. If the water quality starts to build up sediment in the lines or there is field damage to the lines, there will be a restriction to the flow rate. Reducing the flow rate from what the manufacturer engineered will reduce the seal life.

The second concern is again related to water quality. The seal faces are highly polished surfaces. Any grit that gets between the faces etch the faces which will cause premature failure at best and a leaking seal at worst.

The third concern is the amount of water being flushed across the seal to reduce the heat build-up caused by friction. The amount of flow rate is subject to the pressure difference between the inlet and outlet and the pipe friction loss. The manufacturer designed the pipe size and flow rate to match that needed for the pump. What happens when we put a variable speed drive on the pump? The suction pressure in a closed system remains fairly constant but the discharge pressure drops. The flow varies as the square root of the difference and the flow drops. This may not be much of an issue in a chilled water system but could cause a reduced seal life in a heating system.
The cutaway below shows these tubes on either side of a typical external mechanical seal split-case double suction pump. Notice that they feed directly into the stuffing box where the seal is contained.
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