In previous blogs we have defined Cavitation and reviewed Discharge Cavitation. In this blog I would like to discuss a specific type of discharge recirculation known simply as “Total Discharge Recirculation” or “TDR” for short.
All forms of pump recirculation, as the term denotes, evolve some portion of the liquid not following the desired path. When referencing total discharge recirculation, the difference is that none of the liquid is following the desired path. To be more exact, there is absolutely no liquid leaving the pump enclosure.
TDR is typically the result of a discharge valve accidentally being completely closed during operation or a static head that exceeds the pumps maximum head output (shut off condition). Under these conditions the energy transferred from the motor or engine is being converted to heat energy within the liquid and is not allowed to escape from the pump enclosure. If not rectified TDR can quickly result in highly elevated temperatures within the pump. With the casing pressure at shut off point we effectively now have a pressure vessel and temperatures can continue to rise well past boiling point (100 degrees C).
The build up of heat is conducted through the casing into all parts of the pump. Although the heat can directly damage the seal surfaces, degrade lubricating oil and cause bearing bind or softening of connected piping (plastic), the real damage potential will be the result of thermal shock.
Should the blockage be abruptly removed and cold liquid introduced into a hot pump the sudden temperature change is highly likely to crack a mechanical seal face and may even crack the pump casing.
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- centrifugal pumps
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In the next blog we will discuss “Thermal Shock” and how to avoid damage when cooling a pump down after TDR.
Until Next Time,