Hot Spots – Internal Causes
Loss of Flow
The most frequent cause of hot spots and resultant tube ruptures is loss of flow and our inability to respond fast enough to prevent the resulting temperature rise. When we lose flow inside one of the tubes in the radiant section, no heat is taken away and the tube wall quickly rises to equilibrium with the flue gas temperature (typically 1500°F). Worse, if we lose flow in many tubes, the flue gas rises quickly toward the flame temperature (typically 3500°F).
Once we lose flow the only hope is instantly to close the main fuel valve (to chop the heat input), but in most cases normal temperature controls will not react quickly enough to prevent tube rupture.
In heaters handling two-phase flow (liquid and gas), the tube wall design temperatures are based on “dispersed” flow in which the liquid and gas phases are thoroughly mixed. For example, the vapor is assumed to be uniformly distributed in small bubbles in the liquid or the liquid thoroughly mixed as a fine mist in the vapor.
Achievement of dispersed flow is strongly dependent on the velocity of the mixture through the tube. When the velocity is too low the gas and liquid phases separate and collect in globs. This in turn increases the resistance to heat transfer at the inner tube wall and inevitably drives the tube wall temperature sharply upward. In services where the potential for coking exists, this increased temperature at the inner tube wall increases the coking rate, driving the tube wall temperature even more sharply upward. Also, the alternate wetting and drying probably encourages coking.
We normally correct phase separation problems by increasing the flow velocity. This can be accomplished by injecting steam, increasing the hydrogen circulation rate (in reforming or hydroprocessing units), reducing the tube size, or reducing the number of parallel passes.
Coking. Process-side deposits on the inner tube walls (coking or other layers) have also caused hot spots and tube ruptures. They increase the resistance to heat transfer at the inner tube wall and drive the tube wall temperature upward.
As the layer builds up, the resistance increases and the tube temperature increases. For a given charge stock with a given coking propensity, the inner surface temperature and the residence time in the tube govern the coking rate and the rate at which the tube wall temperature increases.
The most common cause of increased coking rates is changing to a charge stock with a higher coking propensity or one that requires more heating at the same charge rate. Other causes are reduced pass flow rates due to pass imbalance and simply operating at flow rates that are too low.
We correct for coking by altering the charge mix or returning to the successful one. Successful solutions are those which increase the flow velocity by injecting steam or reducing the tube size or number of parallel passes; increasing the hydrogen circulation rate in reforming or hydroprocessing units; and keeping the pass flows balanced and the charge rates up.