﻿76 Collapse of Tubes by External Pressure. 



Substituting from (18) in (12), we find, as an alternative 

 form of the result, that collapse rings are useless when the 

 collapsing pressure exceeds a limit given by the equation 



Vl 



This limit being of the order 1300 per square inch, the 

 equation suggests that collapse rings may be advantageously 

 employed in all boiler work. But questions other than that 

 of strength may dictate dimensions in practice, and hence 

 (18) is a more convenient form of the result. 



Summary. 



The paper consists of a review, written from the practical 

 standpoint, of recent theoretical and experimental work on 

 the subject of tube collapse. As a conclusion, the following- 

 rules for design are suggested : — 



1. Work in terms of a " collapsing pressure " :J9, con- 

 nected with the (specified) working pressure p by the 

 equation 



$=/■& (20) 



where / is the factor of safety, for which (it is suggested) a 

 value as low as 2 will be sufficient. 



2. When collapse rings are not to he used, fix the proportions 

 of the tube by means of the formula 



3. When collapse rings are to be used, fix the proportions 

 of the tube by the formula 



*~*iy. ( n ) fe 



(making allowance for corrosion, &c, in cases where the 

 thickness suggested by this formula seems insufficient), and 

 use rigid collapse rings spaced at equal * intervals s, given 



* I am indebted to Mr. Cook for suggesting the importance of equal 

 spacing, as a means of realizing the experimental conditions of " en- 

 castred " ends when Adamson's flanged joints are employed. Such joints 

 tend to keep the tube circular, but their influence on the slope of the tube- 

 wall is a feomewhat doubtful quantity. Mr. Cook points out, however, 

 that with equal spacing of the rings, each section of the flue will be kept 

 cylindrical at the ends, just as the ends of each span are virtually 

 " encastred " when a long continuous beam, uniformly loaded, is 

 supported by several equidistant piers. 



