|In most seal designs, the heat generated at the faces enters the fluid surrounding the seal. This often results in an increase in temperature for the fluid unless it is cooled or continuously replaced or the fluid may become too hot for reliable seal performance. For this reason, most seals operate with continuous replacement of the surrounding fluid - a process known as "seal flush."
Under certain conditions, some seals can operate satisfactorily without a flush, however, the standard practice today involves use of a flush system. Applications without a flush are called dead-ended seals. These usually involve non-volatile fluids at low speeds and low pressures. For these seals, heat is transferred from the seal faces through the liquid and into the metal surrounding the seal chamber.
In scenarios that will benefit from a flush system, however, there are many choices. The American Petroleum Institute (API) has categorized and labeled the common systems as API Flush Plans and flush requirements are often given in minimum or recommended flow rates. A certain minimum flush is needed to obtain the performance of the seal. Heat generated at the seal faces is absorbed by the flush through ideal mixing. This raises the temperature of the flush. Typically, increases of 15-degrees-Fahrenheit for water and low volatility fluids, 30-degrees-Fahrenheit for lube oils and 5-degrees-Fahreneht for volatile hydrocarbons are allowed. Seal performance generally improves when the flush rate is greater than the minimum. As heat transfer improves and the average temperature around the seal decreases with an increased flush rate, face temperature and wear rate decreases. Therefore, the recommended flush rate should be based on experience with similar applications. Considerations should include performance goals, fluid properties, seal chamber design and seal design. In the absence of specific experience, the rule of thumb is 1 gpm/inch of seal size as the minimum flush rate.
For more information, consult with John Crane Inc. to ensure that all of the aforementioned issues are addressed and the mechanical seal is engineered properly for the application before installing any hardware.