Onstream Water Washing
Water washing of convection section tubes externally while onstream is a viable method of removing deposits and improving furnace efficiencies. Good candidates for onstream water washing of convection section tubes are crude heaters, boilers, FCC feed heaters, coker heaters, and fired reboilers.
Why Water Wash
Originally, the reason for developing the technique was the ineffectiveness of early soot blower and convection section designs—fins too dense, too tall, not enough penetration, and so forth. Also, existing gas fired furnaces which are converted to oil firing generally don’t have soot blowers, soot blower lanes, or proper fins.
In the Company’s experience, onstream water washing has produced up to 1 to 2% thermal efficiency improvement in badly fouled sections. Other refineries have reported as much as 50 to 110°F reduction in stack gas temperature, representing a 2 to 4% improvement in thermal efficiency.
The idea is to flow water onto the tube surface. There it immediately vaporizes, spalling soot and other deposits off the tube surface. Steam and most of the deposits are discharged out the stack. The procedure usually produces a visible plume. Lots of water is needed to penetrate beyond the third or fourth row of tubes. If furnaces do not have access openings to the tubes every 5 or 6 rows, you will not be able to get water penetration into the lower rows. A rule of thumb is that the latent heat of the water should be about 40 to 50% of the fired duty of the furnace. This amounts to about 1 gal/min for each 1 MM Btu/hr heat release. However, first attempts should be made at about half that rate.
During washing, someone should observe the bottom of the convection section. The “right” amount of water produces a few drips from the bottom row of convection tubes. This small amount of water dripping into the radiant section has not been a
The object is to wash a cross-section of tubes rather than just one or two tubes. This more evenly distributes the stresses associated with water quenching. Avoid splashing water on refractory. Frequency of washing depends on the extent of fouling. The usual frequency is once or twice per month but one refinery does it daily in a severely fouling service.