API Plan 21 is a cooled recirculation flush plan defined by API 682. It draws process fluid from pump discharge, routes the fluid through a restriction orifice for flow limitation, then through an API 682 Plan 21 heat exchanger to reduce fluid temperature, before delivering the cooled flush fluid into the mechanical seal chamber. Fundamentally, it is a cooled derivative of Plan 11, specially engineered for high-temperature process services where the hot fluid cannot sustain stable seal operation without supplementary cooling.
The heat exchanger acts as the core heat-dissipating equipment in this API 682-compliant seal support system, also known as a mechanical seal auxiliary system. We provide custom seal support system solutions with properly sized Plan 21 heat exchangers for various high-temperature pumping services. Its primary duty is cooling the flush fluid and stabilizing the mechanical seal end faces. This function suppresses process fluid vaporization, reduces coking formation at elevated temperatures, and maintains intact lubricating liquid film between mating seal faces — a necessary condition to achieve low-leakage, long-term reliable seal service.

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