Explosive expansion of tubes into tubesheets has been used for over 20 years in the fabrication and repair of shell and tube heat exchangers. The use of explosives to perform these expansions has offered several distinct advantages over other methods. First, the process is fast and economical and can be performed with minimal training of personnel. Secondly, explosive forming does not cause the deleterious metallurgical effects which often result from other forming operations. In addition, the process can be performed remotely without the need for sophisticated handling equipment. The expansion of tubes into tubesheets is only one of many possible fastening and joining applications for which explosive forming can be used to achieve highly successful results. The explosive forming process and where it has been used are described. In addition, some possible adaptations to other joining applications are identified and discussed

Schroeder, J. W.

English

NASA Technical Reports Server

THE USE OF EXPLOSIVE FORMING FOR 
FASTENING AND J O I N I N G  STRUCTURAL AND PRESSURF. COMPONENTS 
J. Wayne Schroeder 
Fos t e r  Wheeler Development Corporat ion 
Livingston,  N . J .  07039 
ABSTRACT 
Explosive expansion o i  tubes  i n t o  tubeshee ts  has  been used f o r  over  20 
years  i n  t h e  f a b r i c a t i o n  and r e p a i r  of s h e l l  and tube  hea t  exchangers. 
The use  of explos ives  t o  perform these  expansions has  o f f e r e d  s e v e r a l  
d i s t i n c t  advantages over  o t h e r  methods. F i r s t ,  t h e  process  is  f a s t  and 
economical and can be performed wi th  minimal t r a i n i n g  of personnel.  Secondly, 
explos ive . forming  does n o t  cause t h e  d e l e t e r i o u s  m e t a l l u r g i c a l  e f f e c t s  which 
o f t e n  r e s u l t  from o t h e r  forming opera t ions .  I n  a d d i t i o n ,  t h e  process  can be 
performed remotely without  t h e  need f o r  s o p h i s t i c a t e d  handl ing equipment. 
The expansion of tubes i n t o  tubeshee ts  is only  one of many p o s s i b l e  f a s t e n i n g  
and jo in ing  a p p l i c a t i o n s  f o r  which explos ive  forming can be used t o  achieve 
h ighly  succes s fu l  r e s u l t s .  
This  paper desc r ibes  t h e  explos ive  forming process  and where i t  has  been 
used. I n  a d d i t i o n ,  some p o s s i b l e  adap ta t ions  t o  o t h e r  j o in ing  a p p l i c a t i o n s  
a r e  i d e n t i f i e d  and d iscussed .  
https://ntrs.nasa.gov/search.jsp?R=19860001773 2020-03-23T06:19:33+00:00Z
INTRODUCTION 
The expansion of tubes i n t o  tubesheets of hea t  exchangers is performed f o r  
severa l  reasons. F i r s t ,  the  expansions may provide t h e  pressure seal between 
the  primary' and secondary s i d e s  of the  hea t  exchanger t o  prevent mixing of 
the  two mediums which are being heated o r  giving off heat .  Secondly, the  
expansion provides t h e  a x i a l  s t r eng th  of t h e  j o i n t .  This s t r eng th  requirement 
m y  be q u i t e  l a rge ,  depending upon the  design of t h e  exchanger. Las t ly ,  t h e  
expansions c lose  t h e  annular gap between the  d r i l l e d  hole  and the  tube outs ide  
diameter, preventing the  accumulation of poss ib le  corrodents which may a t t a c k  
the  tube o r  tubesheet ma te r i a l s .  
TYPICAL TUBE-TUBESHEET JOINT DESTGNS 
Tube-to-tubesheet j o i n t s  which requ i re  expansion a r e  general ly one of the  
th ree  following types, s e e  Figure 1. 
The f i r s t  type uses a weld a t  t h e  f r o n t  f a c e  df t h e  tubesheet t h a t  provides 
t h e  pressure t i g h t  s e a l .  The expansion is  done t o  c l o s e  the  annular gap t o  
exclude space f o r  t h e  accumulation of corrodents and t o  provide j o i n t  s t r eng th  
so  t h e  weld is not subjected t o  v i b r a t i o n  o r  a x i a l  loads. This design i s  used 
f o r  high pressure  appl ica t ions .  
For the  second type t h e  tube is  expanded f o r  t h e  f u l l  depth of t h e  tube 
hole. The expansion provides both the  seal and a x i a l  s t r eng th  t o  t h e  j o i n t .  
This design is normally used where the  pressure  d i f f e r e n t i a l  across  the  j o i n t  
is low. 
The t h i r d  design incorporates c i rcumferent ia l  grooves wi th in  the  tubesheet 
hole. The grooves provide add i t iona l  a x i a l  j o i n t  s t r eng th  and a l s o  d i s r u p t  
a x i a l  d r i l l  marks, thus increas ing the  s e a l i n g  a b i l i t y  of the  j o i n t .  This 
design is used f o r  both high and low pressure  appl ica t ions .  
THE EXPLOSIVE EXPANSION PROCESS 
The explosive tube expansion process is  achieved by placing a uniform care- 
f u l l y  s ized explosive charge i n t o  a n  energy t ransmit t ing  medium which i s  s ized 
t o  f i t  the  i n s i d e  of t h e  tube, see Figure 2. The length  of the  chargelmedium 
package, ca l l ed  an i n s e r t ,  i s  t h e  same a s  t h e  des i red  expansion. A 
f lange a t  one end of the  i n s e r t  f a c i l i t a t e s  proper placement with t h e  tube 
bore. The continuing explosive at  t h e  f r o n t  end of t h e  i n s e r t  provides t h e  
means of detonation. 
The explosive general ly used because of i ts  a v a i l a b i l i t y ,  consistency and 
s a f e t y  is detonating cord. The detonating cord, which is  commercially 
ava i l ab le  t o  many charge s i z e s , u s e s  PETN (pentaerythritoltetranitrate) a s  t h e  
explosive core load. 
The body of t h e  i n s e r t  t h a t  p o s i t i o n s  and t r a n s f e r s  t h e  energy from t h e  
de tona t ing  explos ive  is  usua l ly  made from low dens i ty  polyethylene.  .Poly- 
e thylene  has  been s e l e c t e d  because of i ts  low c o s t  and a v a i l a b i l i t y .  I n  
a d d i t i o n  t h e  absence of halogens, s u l f u r  and heavy meta ls  i n  i t s  cons t i -  
tuency make i t  s a f e  t o  use  when expanding m a t e r i a l s  which may be s e n s i t i v e  
t o  s t r e s s  cor ros ion  cracking.  
Se l ec t ing  t h e  proper  charge s i z e  t o  perform t h e  expansion is  achieved by 
conducting a s e r i e s  of expansions using p r o t o t y p i c a l  tubes and t e s t  tubeshee ts .  
Once t h e  optimum charge s i z e , i s  determined f o r  a p a r t i c u l a r  m a t e r i a l  and s i z e  
combination t h i s  d a t a  is  r e t a i n e d  and used f o r  a l l  subsequent expansions having 
t h e s e  parameters.  
The number of tubes t h a t  can be  expanded s imultaneously is  l i m i t e d  only by 
the  no i se  and shock wave c o n s t r a i n t s  a t  t h e  expansion s i t e .  A s  few as one 
and as many as 5000 tubes have been s u c c e s s f u l l y  expanded i n  one de tona t ion .  
ADVANTAGES OF EXPLOSIVE EXPANSION 
The exp los ive  expansion of tubes i n t o  tubeshee ts  o f f e r s  s e v e r a l  d i s t i n c t  
advantages over  t h e  once common p r a c t i c e  of Mechanical r o l l i n g .  
Mechanical r o l l i n g  is accomplished by r o t a t i n g  tapered  r o l l e r s ,  he ld  
wi th in  a meta l  cage, about  t h e  i n s i d e  of t h e  tube. These tapered  r o l l e r s  a r e  
r o t a t e d  and forced  toward t h e  tube  w a l l  by a mandrel, which is a l s o  tapered ,  
t h a t  is  placed between t h e  r o l l e r s .  A s  t h e  mandrel i s  r o t a t e d  and d r iven  
inward t h e  r o l l e r s  gradual ly  f o r c e  t h e  tube  i n t o  con tac t  w i th  t h e  tube  hole .  
Addi t iona l  f o r c e  is app l i ed  u n t i l  t h e  tube  is cold  worked s u f f i c i e n t l y  t o  become 
t i g h t  w i t h i n  t h e  hole .  This  method which can produce s t r o n g  t i g h t  j o i n t s  a l s o  
causes s eve re  l o c a l  s u r f a c e  d i s t o r t i o n  t o  t h e  tube i n s i d e  su r f ace .  This  co ld  
work o f t e n  l eaves  t h e  tube  very  s u s c e p t i b l e  t o  s t r e s s  co r ros ion  caused f a i l u r e s  
i n  s e rv i ce .  
Me ta l lu rg i ca l  examinations have shown t h a t  explos ive  expansion r e s u l t s  i n  an 
almost undisturbed m a t e r i a l  s t r u c t u r e  a s  shown i n  F igure  3. 
Mechanical expansion is a l s o  dependent upon t h e  s k i l l  of t h e  ope ra to r  t o  
achieve  c o r r e c t  and uniform r e s u l t s .  Explosive expansion, because of t h e  
cons is tency  of t h e  explos ives  used, produces t h e  same degree of expansion each 
time . 
The explos ive  expansions can be performed e a s i l y  i n  remote o r  r e l a t i v e l y  
i n a c c e s s i b l e  a reas .  Other methods r e q u i r e  more space  and acces s  because of 
t he  s i z e  of t h e  necessary  too l ing .  
F i n a l l y ,  e s p e c i a l l y  when t h e  tubeshee ts  a r e  t h i c k  and t h e  q u a n t i t y  of tubes  
l a rge ,  t h e r e  i s  a n  apprec i ab le  sav ings  i n  t h e  t ime requi red  t o  perform t h e  
expansions. 
OTHER APPLICATIONS OF EXPLOSIVE EXPANSION 
The expansion of tubes i n t o  tubeshee ts  i s  only one of many p o s s i b l e  appl ica-  
t i o n s  of exp los ive  expansion of t ubu la r  components. The process  can r e a d i l y  
be adapted t o  j o i n i n g  t u b u l a r  members i n t o  an  almost endless  v a r i e t y  of 
s t r u c t u r a l  shapes. I d e n t i f i e d  below a r e  only a few of t he  p o s s i b l e  v a r i a t i o n s .  
Tubular p re fab r i ca t ed  s e c t i o n s ,  F igure  4, could e a s i l y  be assembled and t h e  
connections explos ive ly  expanded a t  any l o c a t i o n  t o  form a ladder  o r  o t h e r  
s t r u c t u r e .  Proper ly  s i z e d  explos ive  charges placed i n t o  t h e  ends of t h e  smaller 
tubes would, when detonated,  cause expansion of t h i s  tube  a t  both s i d e s  and t h e  
i n t e r i o r  of t h e  l a r g e r  tube. These expansions w i l l  f i rmly  lock  t h e s e  members 
toge ther .  
F igure  5 shows a method of r e i n f o r c i n g  a  s t r u c t u r e  by adding a  t r u s s .  These 
t r u s s e s  would a l s o  be  explos ive ly  expanded i n  p lace .  S t r a t e g i c  l o c a t i o n s  would 
provide a n  extremely r i g i d  s t r u c t u r e .  
F igure  6 i n d i c a t e s  how t h e  process  can be adapted t o  square  a s  w e l l  as round 
tubing. F i n a l l y  F igure  7 shows how r o t a t i o n  of t h e  expanded tubu la r  connect ions 
can e a s i l y  be prevented by premachining a  key o r  notch i n t o  t h e  d r i l l e d  hole .  
The examples shown a r e  only a  few of t h e  many p o s s i b l e  con f igu ra t ions  f o r  
f a s t e n i n g  s t r u c t u r a l  components us ing  explos ive  forming methods. What t h e s e  
examples do show i s  t h a t  s t r u c t u r e s  could be  e rec t ed  and secu re ly  fas tened  
anywhere wi thout  t h e  need f o r  s p e c i a l  t o o l s  o r  personnel  s k i l l s .  
For a p p l i c a t i o n  i n  remote o r  d i f f i c u l t  environments t h e  explos ive  charges can 
be pre-assembled and secured i n t o  t h e  s t r u c t u r a l  members p r i o r  t o  d e l i v e r y  t o  
t h e  e r e c t i o n  s i t e .  Thus t h e  amount of handl ing r equ i r ed  dur ing  a c t u a l  construc- 
t i o n  is reduced t o  a minimum, 
CLOSURE 
It has  been shown t h a t  explos ive  forming can be used t o  j o i n  and f a s t e n  
tubu la r  members t o  form s t r u c t u r e s .  The a p p l i c a t i o n s  a r e  endless  and wi th  t h e  
knowledge of t h e  process  i n  hand engineers  have another  t o o l  t o  u s e  when design- 
ing  s t r u c t u r e s  f o r  use  i n  t h e  remote a r e a s  of t h e  e a r t h  o r  i n  space. 
I LSUOOTH BORE 
Figure  1. Typica l  h e a t  exchanger tube  t o  tubeshee t  j o i n t s .  
Tube ~olyethylene 
Figure 2.  Explosive i n s e r t  pos i t i oned  i n  a tube  and tubesheet  assembly. 
M E C H A N I C A L L Y  E X P A N D E D  TUBE 
- 
Y E X P A R D E D  TUBE 
gure 3. lOOX photomicrographs showing grain structure and micro-hardness 
values of unexpanded, mechanically expanded and explosively 
expanded type 304 stainless-steel tubes. 
I 
Figure 4. Assembly of tubular components to form a rigid structure. 
Figure 5. Reinforcement of structure by addition of a truss. 
Figure 6. Application using square or rectangular tubing. 
Figure 7. Method of keying to prevent rotation of round structural 
connections. 


The Use of Explosive Forming for Fastening and Joining Structural and Pressure Components

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