Mechanism of Thermal Tube Impact

Although extensively researched and straightforward to predict, the effect that relatively cold water impacting on hot metal furnace tubes still raises concerns amongst boiler owners and operators for a variety of reasons. For this reason, this issue is given detailed discussion here, with frequent reference to institutional, independent and Clyde Bergemann texts on the subject. The Clyde Bergemann SmartCannons™ when used as part of the SmartClean™ process significantly reduce these concerns.

The picture above shows a hypothetical model of a crack with depth “a” growing through a tube of wall thickness “S”. Each time the material around the crack is subjected to stress through the impingement of cooling water. As the area over which the water impinges on the tube metal (from the cleaning device) moves over this point, it experiences a cooling period from its original steady state temperature, typically about 700-800 degrees Fahrenheit for a typical water wall tube. A thermocouple installed within the metal wall of the tube will record a cooling effect, down to a lower level, say 70 degrees lower. The magnitude of this “thermal excursion” from the original temperature is the key determinant in quantifying the actual incremental tube crack growth da / DN noted above. The fundamental relationship underlying this was set forward in the original 1963 hypothesis by Paris and Erdogan:

The subject of thermal crack fatigue was analyzed by the Electrical Power Research Institute (EPRI) culminating in the publication of a report dated 1986, report CS-4914 prepared by Babcock and Wilcox. In this report the empirical relationship noted above was again asserted in practice through the gathering of actual field data from many hundreds of installations. Tube crack depths were measured, thermal impacts (Delta T) were recorded and the empirical relationship was determined to hold. The final conclusions relate to the EPRI making recommendations for using water cleaning devices as long as the parameters for cleaning are correctly applied. Such parameters include pressures, speeds (referred to as progression velocity) and nozzle sizes. EPRI’s recommendation indicated that selection of a site specific Delta T should be done to ensure an expected tube life of over 40 years (presuming a certain number of cleaning cycles per day).

Comparisons of the cleaning performance and thermal impact of water lances has been conducted. An example of this is in the testing done at a site in west Kansas, USA. Some results of this testing are shown below. It can be seen that for each year of operation (given a certain total number of cleaning cycles per year), the expected tube crack depth increases for every cleaning device. This is in the nature of the accumulation of crack depth with time.

The Water Lance: Although several models of water lance are noted, the common models typically achieve thermal impacts which induce a crack depth of 31% to 51% within 40 years.
The Water Cannon: The water cannon tested induced a thermal impact in the tube to indicate a predicted average crack depth of 16% after 40 years.

Due to the shallow impact angle of water on the wall with water lances, it has been noted at several installations, including during the testing noted above, that typically four to five cooling cycles are imposed on the tube for each cycle of cleaning. This factor is a significant contributor to the increase in the tube crack propagation rate for water lances. However, it can be seen that these multiple impacts are not visible for the water cannon technology. A second small dip is observable indicating a possible slight tightness in the cleaning pattern.