Thermal stresses in boiler tubes arising from high-speed cleaning jets

Abstract

Sootblowers are widely used in boiler systems to remove deposits from the hot and fouled heat transfer surfaces. This paper is concerned with a possible scenario where a sootblower jet is placed closer to the heated surface than would normally be the case, possibly because of stubborn deposits or as a result of a more compact plant design. Experimental data are presented for the surface heat transfer distribution when an under-expanded jet from a 12.7 mm convergent nozzle with supply pressure ratio of 5.08 impinges onto a cylindrical surface for nozzle-to-surface spacing of 3, 6 and 10 nozzle diameters. The heat transfer rate is seen to be very high, and to have steep gradients close to the stagnation zone. The experimentally determined heat transfer distribution has been used as the boundary condition for a finite element analysis of the resultant stress distribution. Calculations have been carried out to determine the thermal stresses generated when such a jet, with a stagnation temperature of 300 K , impinges onto a 50 mm diameter steel tube of wall thickness 5 mm and inside temperature 803 K . The effective thermal stress increases during the transient phase of the cooling process and the maximum stress is achieved at the impingement point under steady state conditions. These stresses are well beyond the design limit. Although sootblowing nozzles are not normally placed this close to surfaces, the message for designers of sootblowing systems is clear.